Article

Developing a Map of Geologically Defined Site-Condition Categories for California

Authors:
  • California Geological Survey, Sacramento CA
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Abstract

Consideration of site conditions is a vital step in analyzing and predicting earthquake ground motion. The importance of amplification by soil conditions has long been recognized, but though many seismic-instrument sites have been characterized by their geologic conditions, there has been no consistent, simple classification applied to all sites. As classification of sites by shear-wave velocity has become more common, the need to go back and provide a simple uniform classification for all stations has become apparent. Within the Pacific Earthquake Engineering Research Center's Next Generation Attenuation equation project, developers of attenuation equations recognized the need to consider site conditions and asked that the California Geological Survey provide site conditions information for all stations that have recorded earthquake ground motion in California. To provide these estimates, we sorted the available shear-wave velocity data by geologic unit, generalized the geologic units, and prepared a map so that we could use the extent of the map units to transfer the velocity characteristics from the sites where they were measured to sites on the same or similar materials. This new map is different from the California Geological Survey "preliminary site-conditions map of California" in that 19 geologically defined categories are used, rather than National Earthquake Hazards Reduction Program categories. Although this map does not yet cover all of California, when completed it may provide a basis for more precise consideration of site conditions in ground-motion calculations.

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... A regional map that integrates geologic-based V S30 and measured V S30 is generated for Les Cayes using various geostatistical techniques and data from the whole country. Following similar efforts in California, Central and Eastern U.S., and Texas (e.g., Wills and Clahan, 2006;Thompson et al., 2014;Parker et al., 2017;Zalachoris et al., 2017) measured V S30 from Les Cayes, and other regions in Haiti were matched to geologic units based on their geographic location. Available measured values represented three main regions, with most of the data coming from Port-au-Prince, followed by the region of Les Cayes (found on the southern west side of the country, see Fig. 2), Ganthier, and select data points from Cap-Haitien (both found on the southern west end of the country, see Fig. 2). ...
... Finally, two of the measured values were also from the Cretaceous period, but corresponded to limestone and chert deposits. A set of four of the measured values plot outside of any area mapped as a geologic unit, which was an issue also found in Wills and Clahan (2006) study. These values plot within water bodies based on the geologic map that was employed in this study. ...
... The average V S30 for each geologic unit was obtained along with the standard deviation, assuming a log-normal distribution following Wills and Clahan (2006) and Parker et al. (2017) (see Table 1). When the sample size allowed for a normality check, this assumption was verified. ...
Article
The Mw 7.2 Nippes, Haiti, earthquake occurred on 14 August 2021 in Haiti’s southwest peninsula and in the midst of significant social, economic, and political crises. A hybrid reconnaissance mission (i.e., combined remote and field investigation) was coordinated to document damage to the built environment after the event. This article evaluates two ground-motion records available in Haiti in the context of the geology of the region and known areas with significant damage, such as Les Cayes. We also present a new map of time-averaged shear-wave velocity values to 30 m depth (VS30) for Les Cayes and Port-au-Prince based on the geostatistical approach of kriging and accounting for region-specific geology proxies and field measurements of VS30. Case studies of ground failure observations, including landslides and liquefaction triggering, are described as well as the intersection of social and engineering observations. Maps depicting this important intersection are provided to facilitate the assessment of how natural hazards and social conflicts have influenced the vulnerability of Haiti’s population to earthquakes.
... Tinsley and Fumal (1985) classified the mean V S by geologic age in the Los Angeles earthquake hazard evaluation. Wills and his coworkers developed geologicbased correlations for California by grouping geologic units according to comparable V S30 values (Wills et al., 2000;Wills and Clahan, 2006;Wills et al., 2015). One important limitation of geological-based techniques is that, being ordinal data but not interval data, geologic units unavoidably have "boundaries," so the estimated V S30 is also given artificial boundaries. ...
... The estimation process of the model is separated into two steps: one is an external drift model that uses variables such as topographic slope and geologic groups as the parameters of the "drift," and the other is a simple Kriging (SK) process. In the California application, the drift of the model is Wills and Clahan (2006) model plus topographic slope that is applied to certain geologic groups. Therefore, it also inherits the artificial boundaries of Wills and Clahan (2006) model. ...
... In the California application, the drift of the model is Wills and Clahan (2006) model plus topographic slope that is applied to certain geologic groups. Therefore, it also inherits the artificial boundaries of Wills and Clahan (2006) model. Moreover, because the determination of the drift plane is through a nonspatial method, the spatial structures of the topographic slope and geologic age are still not considered. ...
Article
Full-text available
We propose a new framework of VS30 proxy based on Cokriging method and apply the framework to build a VS30 map for mainland China. This framework utilizes the VS30–topographic slope correlation in the cross-semivariogram to benefit VS30 estimation and has the following benefits: (1) the estimation results are consistent with the measurement data; (2) the estimation uncertainty can be represented by error variance at each unsampled location according to the spatial structure of VS30 and topographic slope; (3) the result map does not have artificial boundaries; and (4) the estimation results can reflect the spatial relation between VS30 and topographic slope and the spatial relation of local spatial environment of topographic slope. We quantify the performance of this framework and compare it with that of three other topographic slope-based VS30 proxy models, including original models developed from exogenous data and models developed from China local data. The result shows that the framework proposed in this article has the best performance. The framework is applied to 7797 borehole VS30 measurements to build a VS30 map for mainland China. The map can capture the high VS30 values in regions where the geological conditions are dominated by flat-lying rocks. Moreover, we consider the effect of sample bias that comes from oversampling of borehole profiles in flat terrain regions when applying borehole data in the proposed framework. We utilize the relation of VS30 and topographic slope to quantify this bias, and use a distance-related data spatial declustering method to eliminate it.
... The mean values for the rock units are 670, 810 and 905 m/s, respectively and the coefficient of variation are 0.169, 0.087 and 0.227, respectively from younger to older rock units. These COV values represent relatively acceptable variability, which indicates the credibility of the validated results when compared with the other statistical studies of V S30 by Wills and Clahan [65]. This study by Wills and Clahan [65] indicates that COV values of [65] and Kockar et al. [48] because the methodology of this study also focuses on geologic units that are geographically constrained. ...
... These COV values represent relatively acceptable variability, which indicates the credibility of the validated results when compared with the other statistical studies of V S30 by Wills and Clahan [65]. This study by Wills and Clahan [65] indicates that COV values of [65] and Kockar et al. [48] because the methodology of this study also focuses on geologic units that are geographically constrained. Finally, the calculated V S30 results have been combined together with the directly determined seismic measurements of the field survey to develop an extended, consistent and well-distributed database to prepare a regional V S30 model. ...
... These COV values represent relatively acceptable variability, which indicates the credibility of the validated results when compared with the other statistical studies of V S30 by Wills and Clahan [65]. This study by Wills and Clahan [65] indicates that COV values of [65] and Kockar et al. [48] because the methodology of this study also focuses on geologic units that are geographically constrained. Finally, the calculated V S30 results have been combined together with the directly determined seismic measurements of the field survey to develop an extended, consistent and well-distributed database to prepare a regional V S30 model. ...
Article
Assessment of potential seismic risk and losses in urban environments is necessary for several purposes ranging from risk mitigation to city and regional planning. It is well known that loss estimation should be performed within an interdisciplinary setting involving earth sciences and engineering. Field experience from recent events worldwide shows that the spatial variability of seismic damage is due to the combined effects of earthquake source properties, local site conditions and structural characteristics. In this study, a scenario-based multi-input damage estimation framework in an urban region was utilized for the Gaziantep city center (southeastern Turkey) that is located in a region of high seismic hazard with no large events in the instrumental era. Initially, a thorough geological and seismo-tectonic assessment of the area was performed followed by estimation of two critical scenario events with moment magnitude (Mw) of 6.5 and 6.6 on nearby active faults. Then, a regional velocity model was compiled from regressions of existing regional geotechnical and seismic data in terms of the VS30 parameter. As the next step, field surveys for the assessment and classification of buildings in the study area were performed followed by vulnerability analyses. As the last step, the mean damage ratios were computed at 198 neighborhoods within the city center. The results indicate not only a high hazard but also high risk in the Gaziantep area due to the combination of close proximity to the faults along with local site effects and building fragilities.
... Although V S30 is a simplified index for the characterization of soil properties, it is impractical to conduct the requisite densely spaced direct measurements for large areas. Therefore, in terms of the applications to hazard mapping, earthquake fatality/loss impact estimation, earthquake insurance models, and so forth, which are designed to use at regional or even national scales, it is sometimes necessary to estimate V S30 through correlations with geological, geomorphologic, or topographic proxies (Wills et al., 2000;Wills and Clahan, 2006;Wald and Allen, 2007;Yong et al., 2012), or to estimate it from shallow borehole profiles (i.e., boreholes less than 30 m deep) at the same site (Boore, 2004;Boore et al., 2011;Dai et al., 2013;Midorikawa and Nogi, 2015;Wang and Wang, 2015). ...
... Tinsley and Fumal (1985) used the surficial geologic age and grain size to map V S in the Los Angeles region. Wills and his coworkers (Wills et al., 2000(Wills et al., , 2015Wills and Clahan, 2006) applied geologic units from the geologic map as predictors for V S30 in California. Li et al. (2019) considered the geologic age and grain size in developing a site classification map of China. ...
... Fumal and Tinsley (1985) showed that, in the Los Angeles region, the difference in V S between medium-grained strata (sand) and fine-grained strata (silt and clay) is small (approximately 15%) and that between fine/medium and coarse (gravel)/very coarse (boulders and cobbles) is large (approximately 60% on average). Wills and Clahan (2006) result shows that, in California, for Quaternary alluvium, the mean V S30 of coarse grain strata is approximately 50% larger than that of fine grain strata. ...
Article
Full-text available
Employing extrapolation models to estimate the time-averaged shear-wave velocity to 30 m (VS30) from a shallow borehole profile is an effective way to expand the VS30 data volume and a necessary initial step for developing VS30 proxy models. Past extrapolation model studies have relied only on shallow borehole shear-wave velocity (VS) profiles to estimate VS30. In this study, we enhance the model by accounting for additional parameters including location slope, geologic age, and geotechnical class. We first compile a new borehole profile database (BPDB) that contains information about 8831 boreholes in China. Using this BPDB, we analyze the VS characteristics of strata for various location slopes, geologic ages, and geotechnical classes. The result shows that the location slope and geologic age have significantly different effects on the VS characteristics, whereas the differences for sand, silt, and clay are small. We build a parameter classification scheme that classifies the location slope into six groups, the geologic age into seven hierarchical types, and the geotechnical class into four hierarchical types. This scheme ensures that each group/type has its distinctive VS characteristic. We evaluate five existing VS models and choose the model that shows the best performance on the BPDB as the “prototype” model. We classify the BPDB boreholes by the parameter classification scheme and use the form of “prototype” model to develop a parametrical model that consists of 33 single-parameter-value models. For a shallow borehole (<30 m), if its location slope, geologic age, or geotechnical class is available besides its VS profile, applying the parametrical model will get more accurate estimated VS30 and model uncertainty estimation than those of existing models that only utilize the information of the VS profile.
... A horizontal-to-vertical response spectral ratio (HVSR) method can be used to estimate the site condition for strong-motion stations (e.g., Yamazaki and Ansary, 1997;Zhao et al., 2006;Di Alessandro et al., 2012;Ghofrani and Atkinson, 2014). Commonly, for regional site classification, proxies such as topographic slope (Wald and Allen, 2007), geomorphometry (Yong et al., 2012;Yong, 2016;Iwahashi et al., 2018), geology (rock or sediment type and age) (Park and Elrick, 1998;Wills et al., 2000;Holzer et al., 2005;Wills and Clahan, 2006), or combinations of factors Thompson et al., 2014;Wills et al., 2015;Ahdi et al., 2017;Parker et al., 2017) are used. These proxies are developed statistically by correlating V S30 to the proxy parameters in areas where V S30 data are available. ...
... We use NEHRP site categories rather than the Chinese code standards, because the geological and geographical proxies for NEHRP classes were better constrained at present. We follow the geology-site condition associations of Park and Elrick (1998), Wills et al. (2000), Holzer et al. (2005), Wills and Clahan (2006), and Wills et al. (2015) and apply the associations to mainland China. For glacial deposits and loess, we apply the associations based on studies on Chinese loess (Wu et al., 2012) and late Quaternary geomorphology in the southeastern Tibetan plateau (Shi, 2002). ...
... ASCE (2016) specifies that if site category B is established without the use of on-site V S30 measurements (as is the case here because we use the geology proxy), then that category is considered to have an amplification equal to 1.0. Similarly, we lack the site-specific evaluations required to We follow the geology-site classification associations by Park and Elrick (1998), Wills et al. (2000), Shi (2002), Holzer et al. (2005), Wills and Clahan (2006), Wu et al. (2012), and Wills et al. (2015). We use the age, genesis, and lithologic descriptions of the units in the final geology and Quaternary maps to assign the NEHRP site categories to each geologic unit (Table 4). ...
Article
We use geology as a proxy to produce a site condition map of mainland China for seismic hazard assessment. The geology is from a 1:1,000,000‐scale digital database and a 1:2,500,000‐scale Quaternary map. We use the lithological descriptions and ages to assign the site condition for each geologic unit. Published site condition data are used to evaluate our mapping results. Our results agree well for the strong‐motion stations in Gansu–Sichuan area, except in the mountainous area. There, the mismatches are due to our geological map scales missing narrow river terrace deposits. Our results also match well in the Shandong and Beijing Plain areas, except in thin fluvial and alluvial deposits fringing bedrock mountains. Our site‐response categories match well‐published average VS–depth profiles from microzonation studies of 14 major urban areas.
... The correlation between Vs 30 and surface geology is due to the dependence of seismic velocity on physical properties such as porosity, density, cementation, and fracture spacing (Vilanova et al., 2018), which are properties that are generally consistent within geological units defined by lithology and age. Surface-geology-derived Vs 30 maps have been deemed usable for seismic hazard assessments (e.g., Wills and Clahan, 2006;Forte et al., 2019) with the caveat that they are not substitutes for geotechnical techniques for local site-specific studies. ...
... Our methodology showcases the contribution of geological information in seismic hazard studies. Given the known correlation between Vs 30 and the underlying geology (Wills and Clahan, 2006;Vilanova et al., 2018;Forte et al., 2019), we were able to extrapolate Vs 30 values from the Metro Cebu area to the entire central Cebu. This method was applicable to this area because Metro Cebu is located on one side of an anticlinorium, and the same geological units were exposed on the opposite side. ...
Article
Full-text available
The heavily populated Cebu Island is cut by the Central Cebu Fault System (CCFS). While the CCFS has not produced any MW > 5.0 earthquakes in the past century, recent strong earthquakes in the adjacent islands have brought attention to the seismic hazards in the region. Fault properties such as strike, dip, slip direction, and surface trace length were determined based on literature review, fieldwork, and analysis of geomorphic features. Empirical relations were utilized to estimate down-dip width and magnitude. The gathered data were used to create a three-dimensional model of the four major faults in the CCFS: Balamban Fault, Central Highland Fault, Uling-Masaba Fault, and Lutac-Jaclupan Fault. The 3D model was used to generate peak ground acceleration maps of central Cebu, should an earthquake occur along any of the major faults. Site corrections were made based on the seismic velocity of the upper 30 meters of the subsurface. The major faults are estimated to be capable of generating MW 6.4 to 7.1 earthquakes. Worst-case scenarios in densely populated areas show 0.40 to 0.70 g of PGA, suggesting the potential for severe damage in central Cebu. We explored the advantages of using raster mathematics in a GIS platform for calculating and presenting ground motion. These advantages include rapid calculations for tens of millions of points, reducing the effects of interpolation artifacts in final map products. This study emphasizes the importance of detailed structural, geological, and geomorphological data in modeling seismic hazards. Further investigations on the seismogenesis of the CCFS segments are recommended.
... The seismic response of 2D sites with spatially variable VS is evaluated using linear elastic 2D and 1D SRAs (Fig. 1). The sites consist of 30 m-deep VS correlated random fields developed using the variance-covariance matrix approach (Vanmarcke 1983), based on a "seed" 1D VS profile generated using the relationship by Kamai et al. (2016) for conditions consistent with sites in California. In reality, there are no 2D sites, but rather 3D sites that unavoidably encompass a wide range of site conditions (e.g., variable VS, inclined bedrock, inclined wave propagation) affecting the seismic response. ...
... The slnVs determines how variable VS is within the modeled space, the correlation model determines the rate at which the VS correlation decays with distance, and the qhor and qver determine the span within which VS values are similar. For the baseline sites, slnVs values commonly observed in nature are selected: 0.2, 0.3, 0.4, and 0.5 ln units (e.g., Wills and Clahan 2006). It is worth noting that these target slnVs values are for the entire model, but only 75 to 98% is achieved within the recording zone. ...
... In the United States, a first effort was made by Wills et al. (2000) who prepared a site-category map of California by classifying geologic units based on V S30 s measured in 556 profiles. This study was followed by Wills and Clahan (2006) who used 19 geologically defined categories to develop maps of Vs characteristics in California and the recent update by Wills et al. (2015) who characterized with greater accuracy the V S of the young alluvial deposits. Kottke et al. (2012) used a similar approach and proposed a geology-based proxy method for CENA. ...
... The map and associated digital databases are the result of compilation and interpretation of published and unpublished 1:250,000-scale and limited 1:500,000-to 1:63,360-scale maps (Wilson et al., 2015). We used the geologic formation on which each recording site is situated and assigned a NEHRP site class based on the work of Wills and Clahan (2006;WC06) for California, adapting it to the Alaska geologic conditions. Generally, the site classes we assigned are one unit lower (i.e. ...
Article
The Alaska Regional Network and Transportable Array provide an invaluable waveform data set for studying ground motions in Alaska. However, the data set is useful only after the site effects at each station are well understood. Considering the large number of stations associated with these networks, it would be onerous to measure the sub-surface velocity structure beneath every station using geophysical exploration techniques involving arrays, such as active-source or passive-source non-invasive array methods. Instead, it is more economical to estimate the site conditions using waveforms recorded at the seismic stations. Most of the methods for estimating site response from recorded waveforms use the frequency-dependent ratio between the horizontal and vertical component of either ambient noise or S waves from earthquakes. We instead use the horizontal and vertical component of P waves to infer the sub-surface velocity structure. It has been demonstrated that the ratio of radial to vertical P waves is mostly sensitive to sub-surface shear velocity. Therefore, the sub-surface velocity structure can be estimated using an approach similar to teleseismic P receiver functions, but at much smaller scale and higher frequency. The results from this method are in good agreement with results from active-source or passive-source non-invasive array methods and have been widely used in the Central Eastern United States. The Alaska Regional Network and Transportable Array have recorded numerous earthquakes in the magnitude range of interest and at shallow depth, which provides an ideal opportunity for this study. V S 30 is used to represent the site amplification of ground motions in all ground motion models that are used in generating the US Geological Survey’s National Seismic Hazard Maps, as well as in the professional practice of seismic hazard analysis. The results of this work provide a basis for improved site-specific hazard estimates in Alaska.
... Crustal Vs is parameterized with 10 cubic B-splines with asymmetric density higher in the shallower crust (Berg et al., 2018). We perturb eight free parameters (Table S1) (Wills & Clahan, 2006) with station RUS marked as a star and main geological features and major faults labeled, including the San Andreas (SAF), Garlock (GF), San Jacinto (SJF), and Elsinore (EF) faults. (d) Scatter plot of each station's H/V at 7 s period and receiver function delay time (s) from (a and b), colored according to the Vs30 (m/s) nearest to that station. in the top linear layer. ...
... As in previous work (Berg et al., 2018), as 7 s period ( Figure 1a) we observe high H/V ratios in sedimentary basins including the Los Angeles, Central Valley, Salton Trough, and Ventura basins; we observe low H/V ratios in mountainous regions such as the Sierra Nevada and Peninsular Ranges. The surface patterns of soft sediment compared to hard bedrock are also evident from the Wills and Clahan (2006) Vs30 map of the region (Figure 1c). ...
Article
Full-text available
Plain Language Summary Our study focuses on finding a new model to accurately image the near‐surface and upper crust of Southern California, as this structure is critical in amplification of ground motion during large earthquakes. To accomplish this, we uniquely combine seismic data from hundreds of Southern California stations to retrieve surface waves and body waves, including from basins where body‐wave data is typically discarded for being too great a nuisance. By employing a revolutionary processing technique after obtaining these datasets, we are able to test the robustness of our model by quantifying its uncertainty and sensitivity. Our new model includes fluid‐saturated sediments in the Los Angeles, Salton Trough, Central Valley, and Ventura basins. Additionally, we image hard, crystalline rock in the Peninsular and Sierra Nevada Mountain Ranges, and see evidence for rock origins in marine or continental environments, respectively. We are also able to see changes in structure across major faults, and areas of high‐fracturing. Outside of major basins, our overall results suggest widespread shallow fracturing and/or groundwater undersaturation.
... These maps are prepared for several active regions of the world (e.g. Petersen et al. 1997;Park and Elrick 1998;Lee et al. 2001;Lee and Tsai 2008;Wills and Clahan 2006;Wills et al. 2015;Kanlı et al. 2006) and are further utilized for generating the ground motion models (e.g. Boore and Atkinson 2008;Stewart et al. 2015;Bozorgnia et al. 2014;Boore et al. 2014;Goulet et al. 2014), probabilistic seismic hazard assessment (e.g. ...
... Wald et al. 1999). A similar approach has also been applied by Wills and Clahan (2006) to prepare a site characterization map of the California region. ...
... Although values of 30 % coefficient of variation can be observed in sediments (e.g. Wills & Clahan 2006;Sato 2019), we decide to use models MR3 and MR4 in our analysis to study the effects for extreme cases of velocity perturbations in sediments. The smoothing of the average amplification in these media suggests that the average TF between the stochastic realizations of the random models could be biased due to the presence of extreme results for a given realisation. ...
... The choice of the parameters of the ACF in this analysis is mainly based on values reported in the literature for sediments (e.g.Wills & Clahan 2006;Thompson et al. 2009;Sato 2019). Model MR5, with a cv of 10 %, was chosen as an intermediate case which still preserves the deterministic feature of velocity model. ...
Article
Some geological configurations, like sedimentary basins, are prone to site effects. Basins are often composed of different geological layers whose properties are generally considered as spatially homogeneous or smoothly varying. In this study, we address the influence of small-scale velocity fluctuations on seismic response. For this purpose, we use the Spectral Element method to model the 2D SH wave propagation on a basin of 1.1 km long and ≈ 60 m deep, representing a 2D profile in the city of Nice, France. The velocity fluctuations are modeled statistically as a random process characterized by a Von Karman autocorrelation function and are superimposed to the deterministic model. We assess the influence of the amplitude and correlation length of the random velocities on the surface ground motion. We vary the autocorrelation function’s parameters and compute seismic wavefields in 10 random realizations of the stochastic models. The analyses of our results focus on the Envelope and Phase differences between the waveforms computed in the random and deterministic models; on the variability of ground motion intensity measures, such as the peak ground velocity (PGV), the pseudo-spectral acceleration response (PSA); and the 2D basin response (transfer function). We find that the amplitude of fluctuations has a greater effect on the ground motion variability than the correlation length. Depending on the random medium realization, the ground motion in one stochastic model can be locally amplified or de-amplified with respect to the reference model due to the presence of high or low velocity contrasts, respectively. When computing the mean amplification of different random realizations the results may be smaller than those of the reference media due to the smoothing effect of the average. This study highlights the importance of knowing the site properties at different scales, particularly at small scales, for proper seismic hazard assessment.
... Although values of 30 % coefficient of variation can be observed in sediments (e.g. Wills & Clahan 2006;Sato 2019), we decide to use models MR3 and MR4 in our analysis to study the effects for extreme cases of velocity perturbations in sediments. The smoothing of the average amplification in these media suggests that the average TF between the stochastic realizations of the random models could be biased due to the presence of extreme results for a given realisation. ...
... The choice of the parameters of the ACF in this analysis is mainly based on values reported in the literature for sediments (e.g.Wills & Clahan 2006;Thompson et al. 2009;Sato 2019). Model MR5, with a cv of 10 %, was chosen as an intermediate case which still preserves the deterministic feature of velocity model. ...
... Finally, Vilanova et al. [69] observed that the correlation between slope and V S30 is weak in Portugal. Due to the reasons stated above, in this study the derivation of V S30 values was performed following the Wills and Clahan [70] methodology. In this method, a number of geologic units are defined for the region of interest, and estimates of V S30 values are defined based on measurements. ...
... The variation of the CTI depends on the spatial structure of the DEM variation, and for this reason this value was estimated using ArcGIS [84]. Finally, the variation of the V S30 values is obtained from the database of Wills and Clahan [70] and Wills et al. [85] (see Table 4). ...
Article
For seismic risk assessments, ground shaking is generally considered the primary cause of building damage. However, earthquake-induced liquefaction can also create significant structural damage and losses. Predicting the expected degree of damage in structures exposed to ground failure due to liquefaction is thus essential for seismic risk assessment. There are well-established tools and methods to evaluate the seismic risk due to ground shaking. However, despite the large number of studies related to liquefaction assessments, modelling ground failure due to an earthquake is far less mature. The incorporation of liquefaction into a seismic loss estimation framework presents several challenges. The use of simplified procedures requires a multitude of assumptions, which increase the uncertainty in the loss assessment, whilst detailed approaches based on in-situ data are resource-demanding and impractical for large-scale risk analysis. To address these shortcomings, a fully prob-abilistic hazard and risk assessment due to liquefaction has been employed using the OpenQuake-engine and demonstrated through a case study applied to mainland Portugal. The liquefaction hazard and risk assessment framework followed in this study allows for the incorporation of both aleatory and epistemic uncertainties, which are herein explored through a sensitivity analysis.
... The geologic categories from the source region should be appropriate for the target region. Prior to NGA-Sub, the primary example of Approach II was the use of a California geology-based model (Wills and Clahan, 2006) for alluvial sites in Italy (Scasserra et al., 2009). Approach II was used in NGA-Sub for some geology groups in Alaska, with the PNW taken as the source region (described in Ahdi et al., 2017b). ...
Article
NGA-Sub data resources are organized into a relational database. We describe the Site table within that database structure, which contains metadata for 6502 stations that have recorded earthquakes incorporated into the database. Critical site parameters for ground motion modelling are time-averaged shear-wave velocity ( V S ) in the upper 30 m of the site ( V S 30 ) and depths to various V S horizons (1.0 and 2.5 km/s). We compile V S profiles for the global study regions and use these data to compute V S 30 where profile data is available from reliable sources. When this is not the case, which is commonly encountered in many regions, we adopt a proxy-based V S 30 -estimation framework whereby estimates are provided, in order of preference, from locally derived models, models derived for a source region and applied to a different target region with some degree of validation, and global or source-region models applied to a target region without validation. Epistemic uncertainties in category median V S 30 are provided when proxy-based models are required but their validation is not possible. Sediment depth terms are also evaluated using measured V S profiles that exceed measured velocity thresholds where available or are estimated for regions having 3D seismic velocity models (e.g. Cascadia, Japan, New Zealand, and Taiwan).
... The underlying, seminal concept-that flat ground tends to be soft and steep ground tends to be hard-is quite useful but also often inefficient and/or insufficient for predicting V S30 . Several regional models have thus aimed to improve on this approach, generally using higher resolution elevation models, more advanced statistical schemes, and/or by binning the data on mapped geology (e.g., Wills and Clahan, 2006;Thompson et al., 2014;Ahdi et al., 2017;Li et al., 2020). Considering the growth of community geophysical data sets, satellite remote sensing, and algorithmic learning, more advanced and accurate solutions may yet be achievable, both at national and regional scales. ...
Article
The shear-wave velocity time averaged over the upper 30 m (VS30) is widely used as a proxy for site effects, forms the basis of seismic site class, and underpins site-amplification factors in empirical ground-motion models. Many earthquake simulations, therefore, require VS30. This presents a challenge at regional scale, given the infeasibility of subsurface testing over vast areas. Although various models for predicting VS30 have thus been proposed, the most popular U.S. national, or “background,” model is a regression equation based on just one variable. Given the growth of community data sets, satellite remote sensing, and algorithmic learning, more advanced and accurate solutions may be possible. Toward that end, we develop national VS30 models and maps using field data from over 7000 sites and machine learning (ML), wherein up to 17 geospatial parameters are used to predict subsurface conditions (i.e., VS30). Of the two models developed, that using geologic data performs marginally better, yet such data are not always available. Both models significantly outperform existing solutions in unbiased testing and are used to create new VS30 maps at ∼220 m resolution. These maps are updated in the vicinity of field measurements using regression kriging and cover the 50 U.S. states and Puerto Rico. Ultimately, and like any model, performance cannot be known where data is sparse. In this regard, alternative maps that use other models are proposed for steep slopes. More broadly, this study demonstrates the utility of ML for inferring below-ground conditions from geospatial data, a technique that could be applied to other data and objectives.
... These include the widely used Wald and Allen (2007) model, based only on slope criteria, or the Iwahashi and Pike (2007) model based on multiple geomorphological criteria. Site condition models from extrapolations of surface geology data (lithology and/or stratigraphy) were also proposed for different countries, such as that of Wills and Clahan (2006) in the United States, Lee and Tsai (2008) in Taiwan, McPherson and Hall (2013) in Australia, Foster et al. (2019) for New Zealand, Vilanova et al. (2018) for Portugal, Di Capua et al. (2016) and Forte et al. (2019) in Italy and Panzera et al. (2021) for Switzerland. Finally, some hybrid methods combine different information, such as terrain classes, lithostratigraphic criteria and DEM information (e.g., Stewart et al. 2014;or Ahdi et al. 2017). ...
Article
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Quantitative estimation of seismic risk over a region requires both an underlying probabilistic seismic hazard model and a means to characterise shallow site response over a large scale. The 2020 European Seismic Risk Model (ESRM20) builds on the 2020 European Seismic Hazard Model (ESHM20), requiring additional information to firstly parameterise the local site condition across all of Europe, and subsequently determine its influence on the prediction of seismic ground motion. Initially, a harmonised digital geological database for Europe is compiled, alongside a model of topographic/bathymetric elevation and a database of 30 m averaged shearwave velocity measurements ( $$V_{S30}$$ V S 30 ), in order to produce separate 30 arc-second maps of inferred $$V_{S30}$$ V S 30 based on topography and on geology. We then capitalise on a large database of seismic recording stations in Europe for which site-to-site ground motion residuals ( $$\delta S2S_{S}$$ δ S 2 S S ) have been determined with respect to the shallow crustal ground motion model used in the ESHM20. These residuals allow us to incorporate site amplification functions into the European GMM calibrated upon either observed or inferred $$V_{S30}$$ V S 30 , or on the European geology and topography models. We present the resulting pan-European seismic site amplification model and assess its impact on seismic hazard and risk compared against other approaches. The new site amplification model fulfils the requirements of the ESRM20 and, providing uncertainty is fully propagated, yields estimates of seismic hazard and risk at a large space scale that may be comparable to other methods often applied at local/urban scale where better-constrained site information is available.
... The 3D velocity model used for the CS15.4 simulations is CVM-S4.26-M01, obtained from full 3D waveform tomography (F3DT) (Lee et al. 2011;Small et al. 2017), supplemented by borehole information where available, and using a tapering scheme (Ely et al. 2010) to create a geotechnical layer that reflects the near-surface velocities from Wills (2006). The shear-wave velocity (V S ) has a floor value of 500 m/s. ...
Article
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The development of site- and path-specific (i.e., non-ergodic) ground-motion models (GMMs) can drastically improve the accuracy of probabilistic seismic hazard analyses (PSHAs). The varying coefficient model (VCM) is a novel technique for developing non-ergodic GMMs, which puts epistemic uncertainty into spatially varying coefficients. The coefficients at nearby locations are correlated by a prior distribution imposed on a Gaussian Process. The correlation structure is determined by the data, and later used to predict coefficients and their epistemic uncertainties at new locations. It is important to carefully verify the technique before its results can be accepted by the engineering community. This study used a series of simulation-based controlled ground-motion datasets from CyberShake to test a modified VCM technique, which partitions the epistemic uncertainty into spatially varying source, site, and path terms. Because the simulation parameters (inputs) are known, verification of what is recovered by the VCM from CyberShake simulations is straightforward. We found that the site effects in CyberShake datasets can be reliably estimated by the VCM. However, the densely-located self-similar events in CyberShake datasets along pre-defined faults violate the isotropic assumption underlying the VCM, thus preventing the VCM from capturing the genuine source effects. For path effects, cell-specific attenuation approaches fail to recover the anelastic attenuation pattern of the 3D velocity model, which is most likely due to other unmodeled effects and inappropriate assumption of wave-propagation path. Instead, a midpoint approach that only considers the aggregated path effects can better recover the strong anelastic attenuation within basins by fixing the correlation length of path effects. Lessons learned in this study not only provide guidance for future applications of VCM to both simulation and empirical datasets, but will also guide further development of the technique, with emphasis on the recovery of path effects.
... parameter in building codes to quantify the local site effect. To this end, several researches have been attempted to develop procedures for estimation of average shear velocity in upper 30 meters of surface (V S30 ) [2,[27][28][29]. Here, the prominent approach developed by [27] is adopted for estimating V S30 . ...
Article
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Providing appropriate near real time ground motion shaking map is a critical requirement to effectively manage the consequence of an earthquake. In the present study, the standard procedure adopted by USGS ShakeMap to develop the ground motion shaking map is calibrated to implement in Iran. Selecting appropriate ground motion predictions equation and properly modeling of the local site condition are two important parameters that should be properly modeled to provide an appropriate ground motion shaking map. Here, a set of local, regional and global GMPEs that show good performance in the previous studies are adopted. Besides, the approach developed by Borcherdt [1] is used to take into account the local site condition. The VS30 of the region exploited from the proxy approach proposed by Wald and Allen [2]. The study evaluates the potential applicability of this method by compiling a database of measured and estimated VS30. The results indicate that the method outperforms than random selection of the site class. The calibrated model implements to generate the ground motion shaking map of the Sarpol-e Zahab, Iran earthquake (2017). The result shows that the approach performs better than employing GMPEs alone. The calibrated model can be used to generate the database of ground motion shaking of past earthquakes in Iran, which is an important requirement to develop empirical fragility or vulnerability models.
... Proper choice of training covariates for ML predictive models is key to obtaining reasonable and accurate estimates in DSM. In this work, we chose the covariates shown in Table 2. Punctual covariates such as terrain slope, topographic elevation, and geological typology are included, based on previous work, that showed an improvement in the performance of predictive models in predicting V s30 (Wills and Clahan, 2006;Wald and Allen, 2007). Slope and elevation were obtained from a digital elevation model (DEM) of 12.5 m resolution available from public satellite data (https://asf.alaska.edu), ...
Article
Seismic site amplification and seismic hazard maps are crucial inputs for decision making and risk evaluation in places where seismicity imposes a significant risk to human life and infrastructure. In this work, we propose a novel machine learning (ML) based methodology to integrate qualitative and quantitative data to map the degree of seismic amplification in an area of Chile, one of the most seismically active countries on Earth. Our method uses measurements of surface shear wave velocities (Vs30) and predominant frequencies (f0) combined with gravity anomaly maps to update the geographic extension of seismic amplification classes. Additionally, we trained the predictive models to interpolate and extrapolate Vs30 and f0 to the unsampled sites. Applying this method to the Santiago basin resulted in (i) a refined seismic amplification map, and (ii) maps of Vs30 and f0 estimated with improved accuracy. The best predictions, obtained by ML techniques and validated through cross-validation, are possibly due to the inclusion of spatial covariates for algorithm training, enhancing the ability of the model to capture the spatial correlations of geological, geophysical and geotechnical data. The estimation of predominant frequencies (f0) is improved considerably by including gravity as a covariant. The accuracy of the f0 predictions apparently depends more on the choice of covariates than on the algorithm used, while the Vs30 predictions are more sensitive to the chosen algorithm. These results illustrate the great potential of machine learning predictive algorithms in digital soil mapping, which surpass traditional geostatistical techniques. The major contribution of this work is to introduce a novel methodology, based on artificial intelligence models, to extend local measurements of site-specific dynamic properties. This information can be used to quantitatively estimate seismic hazard over a regional scale.
... Geological units in Nador give a first knowledge on the shear wave velocity. Many studies (Pitilakis et al. 2013;Wills and Clahan 2006) and several Building codes (NEHRP,EC8) give correlation between shear wave velocities and geology. The soil in the basin contains soft sediments of fluvial and littoral origin dating from a recent quaternary age down to a depth of 20 m. ...
Chapter
Local seismic hazard is influenced by the variation of soil parameters which modulates the propagation of seismic waves through soil. Nador is one of the cities in Morocco which is located in a seismically hazardous region. Investigations using ambient noise and SPT-N values were conducted in order to characterize site effects in the urban area of Nador. The predominant period values obtained vary between 0.2 and 0.5 s; whereas shear wave velocity in superficial soft sediment varies from 156 to 212 m/s. The geotechnical conditions are also used to identify the areas of high impedance contrast. It is found that soil with high predominant period undergoes significant amplification. This is observed in the center and near to the shore of Nador city which is underlaying deep soils with clay and sandy clay deposits. The provided results can be useful for seismic risk studies in the region.
... Such site-characterization parameters can be extracted from seismic inversion results. However, it is worth noting that other studies estimate these site parameters directly, based on empirical relationships for surfacewave data at particular wavelengths (e.g., Martin and Diehl 2004;Albarello and Gargani 2010), or other proxies such as surficial geology (e.g., Wills and Clahan 2006) and topography (e.g., Yong et al. 2012). As these approaches do not formally represent inversion, they are not considered further in this review, but see Yong (2016) and Savvaidis et al. (2018) for further discussion on this topic. ...
Article
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Seismic site characterization attempts to quantify seismic wave behavior at a specific location based on near-surface geophysical properties, for the purpose of mitigating damage caused by earthquakes. In recent years, techniques for estimating near-surface properties for site characterization using geophysical observations recorded at the surface have become an increasingly popular alternative to invasive methods. These observations include surface-wave phenomenology such as dispersion (velocity-frequency relationship) as well as, more recently, full seismic waveforms. Models of near-surface geophysical properties are estimated from these data via inversion, such that they reproduce the observed seismic observations. A wide range of inverse problems have been considered in site characterization, applying a variety of mathematical techniques for estimating the inverse solution. These problems vary with respect to seismic data type, algorithmic complexity, computational expense, physical dimension, and the ability to quantitatively estimate the uncertainty in the inverse solution. This paper presents a review of the common inversion strategies applied in seismic site characterization studies, with a focus on associated advantages/disadvantages as well as recent advancements.
... This geological classification combines lithology and age. − The work by Sá et al. [17] requires the estimation of the V S30 for the SW of the Iberian Peninsula in a study of loss assessment and, in the absence of a local V S30 model, they employ two generic proxy-based models based on geology [18] and on topographic gradient [8], finding a better performance of the geologically based model. ...
Article
The time-averaged shear-wave velocity in the upper 30 m of the ground, VS30, is a key soil descriptor for estimating site response despite its recognized limitations. It is employed in both, site-specific probabilistic hazard assessments (PSHAs) and regional seismic codes. This work presents a model for estimating VS30 in the Iberian Peninsula as a function of three proxies: topographic slope, geological age and lithology at each site. Tasks accomplished include: 1) gathering existing Vs profiles and calculating their VS30; 2) defining an adequate set of representative age and lithological groups; 3) classifying the available VS profiles according to these groups; and 4) carrying out a regression analysis between VS30, slopes, age and lithological groups. Based on the regression analysis and the dependency on the slope, some of the initially proposed groups were amalgamated, before proposing the final model. This model considers topographic slope values extracted from a digital elevation model (DEM) with 200 m horizontal resolution, plus six geological age groups and four lithological groups. It provides an estimate of the mean and standard deviation of logVS30 (and hence VS30), which can be used for sites without direct estimates of velocity profiles (and VS30) in the Iberian Peninsula.
... The 2D random field models are generated for target s lnVs = 0.2, 0.3, 0.4, and 0.5, commonly observed in nature (e.g. Holzer et al., 2005;Wills and Clahan, 2006). Figure 2 shows a sample 2D random field model for a target s lnVs = 0.2, and Figure 4 shows the correlation functions for ln(V S ) in the horizontal direction. ...
Article
One-dimensional site response analyses (1D SRAs) with shear-wave velocity ( V S ) randomization are commonly performed to estimate median site-specific amplification factors (AFs) under the implicit assumption that this approach yields a realistic response. In this work, an investigation is conducted to determine the appropriate amount of V S randomization (σ lnVs ) needed to capture a median response that accounts for 2D V S spatial variability effects. Results from 2D SRAs and 1D SRAs with V S randomization show that the median 2D seismic responses are generally higher than 1D responses at the site’s fundamental frequency, and that higher V S variability has a mild impact on the median 2D seismic response amplitude at the fundamental frequency, whereas it significantly reduces the median 1D response. Findings indicate that the 84 th percentile AFs based on 1D SRAs conducted with V S randomization using σ lnVs = 0.25, approximate well with the more realistic median 2D SRA-based AFs around the fundamental frequency, while the 70 th to 60 th percentiles might be more appropriate at higher frequencies. The benefit of using percentiles of the 1D SRA-based AFs higher than the median is shown for different site conditions and supported by comparisons against empirical data from four downhole sites.
... However, regional risk evaluations might include different earthquake scenarios defined by ruptures having different locations, different geometries and different magnitudes, considering their corresponding probabilities of occurrence. Fig. 8a shows the median PGA, P GA, affecting every house in San Carlos in the earthquake scenario computed using the Boore et al. [47] GMPM, where the average shear-wave velocities at the top 30 m, V S30 , were estimated using the procedure developed by Wills and Clahan [48]. As shown in Fig. 8a, the eastern region of San Carlos is affected by median PGAs ranging from approximately 0. ...
Article
When performing a regional seismic risk assessment, one is interested in estimating the damage, losses, and, in general, the seismic performance of several structures spatially distributed within a region. In this type of probabilistic risk analysis, it is common to consider the spatial correlation between the ground motion intensities affecting different locations. However, the correlation of damage experienced by different structures conditioned on their levels of intensities, herein referred to as structure-to-structure damage correlation, is usually neglected. This study evaluates the effect of the structure-to-structure damage correlation and proposes a simplified model for incorporating it into regional seismic risk assessment through the use of copula functions. In order to illustrate the influence of the structure-to-structure damage correlation on the regional seismic risk, a case study in San Carlos, California, is analyzed considering and neglecting the structure-to-structure damage correlation. Results show that neglecting the structure-to-structure damage correlation may lead to significant underestimations of large-consequence risks.
... Thus one may argue that a c v of 30 % is unapplicable for the current velocity model because of the low-velocity contrast between the layers. In reality, c v s of this order are not unrealistic but rather commonly observed in sediments (e.g.Wills and Clahan, 2006;Sato, 2019). ...
Thesis
When an earthquake occurs, the wave field generated by the fault propagates through the earth crust then with in surfaces geological layers before reaching the earth's surface. In order to understand how these seismic waves propage from the fault till surface buildings or structures, it is possible to use numerical simulations of the wave propagation. This research project focuses particularly on the physical phenomena that take place within the superficial geological layers while taking into account the spatial variability of soil properties, non-linear behaviors and complex basin geometries
... As a consequence, there is not a homogeneous quality information for site characterization among strong-motion web sites. When geophysical measurements are not collected, terrain-based site condition proxies can be used integrating surface geology, topographic slope and terrain class (Wills and Clahan 2006;Wald and Allen 2007;Yong et al. 2012;Yong 2016;Kwok et al. 2018), or geotechnical or geomorphic categories, such as done in the NGA-West2 PEER ground motion database (http:// ngawe st2. berke ley. ...
Article
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Many applications related to ground-motion studies and engineering seismology benefit from the opportunity to easily download large dataset of earthquake recordings with different magnitudes. In such applications, it is important to have a reliable seismic characterization of the stations to introduce appropriate correction factors for including site amplification. Generally, seismic networks in Europe describe the site properties of a station through geophysical or geological reports, but often ad-hoc field surveys are missing and the characterization is done using indirect proxy. It is then necessary to evaluate the quality of a seismic characterization, accounting for the available site information, the measurements procedure and the reliability of the applied methods to obtain the site parameters.In this paper, we propose a strategy to evaluate the quality of site characterization, to be included in the station metadata. The idea is that a station with a good site characterization should have a larger ranking with respect to one with poor or incomplete information. The proposed quality metric includes the computation of three indices, which take into account the reliability of the available site indicators, their number and importance, together with their consistency defined through scatter plots for each single pair of indicators. For this purpose, we consider the seven indicators identified as most relevant in a companion paper (Cultrera et al. 2021): fundamental resonance frequency, shear-wave velocity profile, time-averaged shear-wave velocity over the first 30 m, depth of both seismological and engineering bedrock, surface geology and soil class.
... For seismic risk modelling purposes, site effects due to surface geology are generally mapped at large scales, using either the EC8 soil classes or the Vs30 proxy parameter [37]. These site condition maps can be computed by (1) using geological parameters [38][39][40][41], (2) using morphological parameters (e.g., slope) [42][43][44], or (3) using hybrid methods combining both geological and DEM information [45]. Once the site condition map is built, the information can be derived, in terms of amplification factors, through the EC8 soil coefficient S or empirical equations correlating ground motion to site condition. ...
Article
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This article studies the effects of the soil data and exposure data of residential building inventories, as well as their spatial resolution, on seismic damage and loss estimates for a given earthquake scenario. Our aim is to investigate how beneficial it would be to acquire higher resolution inventories at the cost of additional effort and resources. Seismic damage computations are used to evaluate the relative influence of varying spatial resolution on a given damage model, where other parameters were held constant. We use soil characterization maps and building exposure inventories, provided at different scales from different sources: the European database, a national dataset at the municipality scale, and local field investigations. Soil characteristics are used to evaluate site effects and to assign amplification factors to the strong motion applied to the exposed areas. Exposure datasets are used to assign vulnerability indices to sets of buildings, from which a damage distribution is produced (based on the applied seismic intensity). The different spatial resolutions are benchmarked in a case-study area which is subject to moderate-to-average seismicity levels (Luchon valley in the Pyrénées, France). It was found that the proportion of heavily damaged buildings is underestimated when using the European soil map and the European building database, while the more refined databases (national/regional vs. local maps) result in similar estimates for moderate earthquake scenarios. Finally, we highlight the importance of pooling open access data from different sources, but caution the challenges of combining different datasets, especially depending on the type of application that is pursued (e.g., for risk mitigation or rapid response tools).
... Numerous proxies have been developed in the absence of sitespecific V S profile information to correct for site response (Wills and Clahan, 2006;Wald and Allen, 2007;Yong et al., 2012;Parker et al., 2017). Recent advances have included high-frequency P-wave receiver function techniques, which use weak-motion earthquakes recorded on network seismograph stations to estimate shallow bedrock depth and overlying V S . ...
Article
We analyze multimethod shear (SH)-wave velocity (VS) site characterization data acquired at three permanent and 25 temporary seismograph stations in Oklahoma that recorded M 4+ earthquakes within a 50 km hypocentral distance of at least one of the 2016 M 5.1 Fairview, M 5.8 Pawnee, or M 5.0 Cushing earthquakes to better constrain earthquake ground-motion modeling in the region. We acquired active-source seismic data for time-averaged VS to 30 m depth (VS30) at 28 seismograph stations near the Fairview, Pawnee, and Cushing epicentral areas. The SH-wave refraction travel times coupled with Rayleigh- and Love-wave phase velocity dispersion were extracted and modeled in a nonlinear least-squares (L2) joint inversion to obtain a best-fit 1D VS versus depth profile for each site. At a subset of sites where the preferred L2 inverse model did not optimally fit each of the Love, Rayleigh, and SH travel-time datasets, we explore application of simulated annealing in a joint inversion to find a more global solution. VS30 values range from 262 to 807 m/s for the preferred measured (in situ) VS profiles, or National Earthquake Hazards Reduction Program (NEHRP) site class D to B, and are broadly comparable with estimates from previous data reports in the region. Site amplification estimates were calculated next from 1D SH transfer functions of the preferred VS profiles and then compared against observed horizontal-to-vertical spectral ratios (HVSRs) from nearby seismograph stations. We generally see good agreement between the predicted in situ model and the observed HVSR resonant frequencies, with nominal amplifications between 2 and 10 within the 2–15 Hz frequency band. Next, using 40 known in situ VS30 measurements in the region, we demonstrate that the in situ VS30 values improve the fit for selected suites of ground-motion models (GMMs) for M 4+ earthquakes within a 50 km hypocentral distance when compared with proxy methods, arguing for future development of GMMs implementing in situ VS profiles.
... As reported by various authors (Kramer 1996;Hashash et al. 2010;Kaklamanos et al. 2013) the parameters need to be determined in ground site response analysis of ground motion are earthquake magnitude, local geology, surface topography, fault mechanism, path between source and site, and dynamic properties of the soil. Evidence from past global earthquake events (e.g Phillips and Aki 1986;Wills and Clahan 2006;Semblat et al. 2000;Slob et al. 2002;Stewart et al. 2003;Topal et al. 2003;Pitilakis et al. 2004) shown that amplification of ground motion is extremely dependent on local geology, topography and geotechnical conditions. Various numerical methods for 1D site response analysis including time-domain nonlinear (NL) method (e.g., Kramer 1996;Kramer and Paulsen 2004;Adel and Cortez-Flores 2004), cyclic wise equivalent linear method (Kramer 1996) and frequencydomain equivalent-linear (EQL) method (Schnabel et al. 1972a(Schnabel et al. , 1972b(Schnabel et al. , 1972cKramer 1996) have been compared based on the merits and demerits of analysis. ...
Article
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Earthquake is a sudden release of energy due to faults. Natural calamities like earthquakes can neither be predicted nor prevented. However, the severity of the damages can be minimized by development of proper infrastructure which includes microzonation studies, appropriate construction procedures and earthquake resistant designs. The earthquake damaging effect depends on the source, path and site conditions. The earthquake ground motion is affected by topography (slope, hill, valley, canyon, ridge and basin effects), groundwater and surface hydrology. The seismic hazard damages are ground shaking, structural damage, retaining structure failures and lifeline hazards. The medium to large earthquake magnitude (< 6) reported in Ethiopia are controlled by the main Ethiopian rift System. The spatial and temporal variation of earthquake ground motion should be addressed using the following systematic methodology. The general approaches used to analyze damage of earthquake ground motions are probabilistic seismic hazard assessment (PSHA), deterministic seismic hazard assessment (DSHA) and dynamic site response analysis. PSHA considers all the scenarios of magnitude, distance and site conditions to estimate the intensity of ground motion distribution. Conversely, DSHA taken into account the worst case scenarios or maximum credible earthquake to estimate the intensity of seismic ground motion distribution. Furthermore, to design critical infrastructures, DSHA is more valuable than PSHA. The DSHA and PSHA ground motion distributions are estimated as a function of earthquake magnitude and distance using ground motion prediction equations (GMPEs) at top of the bedrock. Site response analysis performed to estimate the ground motion distributions at ground surface using dynamic properties of the soils such as shear wave velocity, density, modulus reduction, and material damping curves. Seismic hazard evaluation of Ethiopia shown that (i) amplification is occurred in the main Ethiopian Rift due to thick soil, (ii) the probability of earthquake recurrence due to active fault sources. The situation of active fault is oriented in the N-S direction. Ethiopia is involved in huge infrastructural development (including roads, industrial parks and railways), increasing population and agricultural activity in the main Ethiopian Rift system. In this activity, socio-economic development, earthquake and earthquake-generated ground failures need to be given attention in order to reduce losses from seismic hazards and create safe geo-environment.
... Our main motivation in this study is to understand the causes underlying the skewed distribution of V S30 measurements for a large, real dataset and use that understanding to reduce uncertainty in proxy-based estimates of V S30 . Wills and Clahan (2006), Xie et al. (2016), Vilanova et al. (2018) Topographic gradient and indices Global Gallant and Dowling (2003), Wald and Allen (2007), Allen and Wald (2009) ...
Article
The time-averaged shear-wave velocity in the upper 30 m depth from the ground surface, or VS30, is often used as a predictor to describe local site effects in ground-motion models. Although VS30 is typically determined from in situ measurements, it is not always feasible to obtain such measurements due to project restrictions or site accessibility. This motivates the development and use of proxy-based VS30 predictions that leverage more readily available secondary information such as surface geology, topographic slope, or geomorphic terrain classes to estimate the mean VS30 and associated uncertainty. Traditionally, empirical distributions of VS30 have been observed to have long right tails, leading to high levels of associated uncertainty. In this study, we present a physical framework that is grounded in fundamental principles of geostatistics and probability to explain the uncertainty and skewness associated with VS30 measurements. Specifically, by invoking Lyapunov’s central limit theorem, we hypothesize that the distribution of VS30 can be theoretically approximated by a reciprocal–normal distribution. We show that a non-normal and skewed distribution of VS30 is to be expected and is not a sign of measurement error or sampling bias, although sampling bias can exaggerate such skewness. Our framework also enables us to propose the mode as a characteristic value of VS30 measurements, as opposed to the mean or median, which can overestimate the most probable value.
... To address this issue, various inference approaches have been proposed relating V S30 to other easy-to-obtain ground features, often topographic and/or geologic proxies (e.g. Wald and Allen, 2007;Weatherill et al., 2020;Wills and Clahan, 2006;Yong et al., 2012). Meanwhile, although V S30 has been incorporated into many seismic regulations (e.g. ...
Article
This article describes an open-source site database for a total number of 1742 earthquake recording sites in the K-NET (Kyoshin network) and KiK-net (Kiban Kyoshin network) networks in Japan. This database contains site characterization parameters directly derived from available velocity profiles, including average wave velocities, bedrock depths, and velocity contrast. Meanwhile, it also consists of earthquake horizontal-to-vertical spectral ratio (HVSR) and peak parameters, for example, peak frequency, amplitude, width, and prominence. In addition, the site database also comprises topographic and geological proxies inferred from regional models or maps. Each parameter is derived in a consistent manner for all sites. This site database can benefit the application of machine learning techniques in studies on site amplification. Besides, it can facilitate, for instances, the search of the optimal site parameter(s) for the prediction of site amplification, the development and testing of ground-motion models or methodologies, as well as investigations on spatial or regional variability in site response. All resources (the site database, earthquake HVSR data at all sites, and the MATLAB script for peak identification) can be freely accessed via: https://doi.org/10.5880/GFZ.2.1.2020.006
... The input data required is thus only V s30 , z wt , a max , and M w . The V s30 data was taken from Wills and Clahan (2006); z wt was obtained by spatially interpolating over data from California Department of Water Resources (2019); and a max and M w are from the earthquake scenario. ...
Article
This paper introduces the multihazard optimization-based probabilistic scenario (multihazard OPS) method to create an ensemble of multihazard scenarios that can be used for efficient evaluation of spatially distributed infrastructure. Each multihazard scenario is a map depicting a possible realization of the co-occurring multiple hazard effects associated with a single earthquake, including ground motion intensity contours, liquefaction potential contours, and locations of surface fault rupture. Together, when the small set of multihazard scenarios are combined with their computed weights, they represent the probabilistic hazard in a way that captures spatial correlation, includes multiple hazards, and is computationally efficient. In demonstrating the method for Los Angeles, California, we find a set of 350 multihazard scenarios matches the regional hazard and damage with errors small enough for most practical purposes. Further reduction is possible depending on the desired tradeoff between acceptable errors and computational efficiency. A sensitivity analysis suggests it is important to consider each hazard type in determining the multihazard scenarios, although the outcome is not sensitive to the precision of the weights.
... Average S-wave velocity to a specific depth is widely used in engineering seismology for Uniform Building Code (UBC) site classification, commonly to depth of 30 m (V S30 ) (Building Seismic Safety Council, 2001). Maps of V S30 (average V S to a depth of 30 m) have been prepared for California for earthquake hazard site assessment (Wills and Clahan, 2006;Wills et al., 2015). Seismic velocity normally increases with depth, because geologic materials become stronger with depth due to increased pressure; older sediments are better compacted and better cemented. ...
Article
Seismic surveys were performed using a novel application of combined active and passive surface wave methods to evaluate the integrity of levees protecting islands in the Sacramento‐San Joaquin Delta, California, USA, from flooding. Delta islands have been undergoing rapid subsidence during the past century due to farming practices that have led to the loss of a surficial peat layer. A large earthquake on any one of several active faults in the region could cause multiple levee failures and extensive flooding in the Delta. Surface wave surveys were carried out along the crest of levees using the active method (2D MASW) and along the base of levees using the passive method (Microtremor Array Method or MAM). 2D seismic S‐wave velocity (VS) profiles were prepared for each site by combining shallow data from the active survey along the crown of the levee with deeper data from a passive survey along the toe of the levee. VS profiles reveal a low‐velocity peat layer beneath the levee body that was confirmed by geotechnical borehole logs. Lateral variability of the levee was evaluated using average velocity to the base of the levee versus along‐levee distance. This article is protected by copyright. All rights reserved
... However, the V s30 is related to the type of rock mass and soil mass, which is difficult to obtain with a large number of field measurements on the regional scale. According to the relevant codes (Eurocode 82,003; The Ministry of Construction of the People's Republic of China 2010), previous research results (Wills et al. 2000, Wills 2006Rapolla et al. 2010Rapolla et al. , 2012Paoletti et al. 2013;Caccavale et al. 2017) and public and private technical reports, we obtained the V s30 of four lithological groups, as shown in Table 3. Previous studies have developed an empirical equation to obtain the amplification effect using V s30 and PGA information (Choi and Stewart 2005;Bhatt et al. 2019). ...
Article
Full-text available
Critical acceleration is an inherent property of a slope and determines the slope stability under seismic action. The critical acceleration model is a core element of regional seismic landslide hazard assessment. Therefore, the purpose of this paper is to reveal the influence of different critical acceleration models on assessments of potential earthquake–induced landslide hazards. Traditionally, the Newmark critical acceleration model has commonly been used to evaluate the potential earthquake–induced landslide hazard. This method needs to assume the failure depth of the slope, which leads to an underestimation of the predicted displacement of the seismic landslide. Recently, the prediction equations of critical acceleration based on a parametric study of the limit equilibrium method overcomes the limitation of Newmark critical acceleration model and has been applied to assessments of co-seismic landslide hazards. In this study, we use Newmark critical acceleration model and prediction equations of critical acceleration to obtain the distribution maps of potential earthquake–induced landslide hazard in Shimian County, with peak ground acceleration of 10% and 2% exceeding the probability in 50 years. In addition, the nonlinear effect of site and topographic effects on peak ground acceleration were considered. The results show that Newmark critical acceleration model underestimates the area and value of the predicted displacement, while prediction equations of critical acceleration produces seismic landslides in a wider range of mountainous areas. This indicates that the critical acceleration model has a significant influence on assessments of potential earthquake–induced landslide hazards. In addition, the study not only provides valuable reference for assessment of potential earthquake–induced landslide hazard, emergency response of seismic landslides, and land planning in the study area, it also provides a useful demonstration for the selection of a critical acceleration model in seismic landslide hazard assessments for future researches.
... The choice of a unique soil of type A might lead to unconservative results and estimated losses. However, while there exist some proposals to use VS30 values derived from topographic slope (Wald and Allen 2007) or geological maps (Wills and Clahan 2006;Forte et al. 2019) for rough consideration of soil effects at a site, in this study it was preferred to avoid introducing soil models which might have serious limitations and can produce some bias when comparing risk of areas where rapidly changing soil conditions occur, as it frequently happens in Italy. Indeed, in the near future more accurate micro-zonation studies, that are being produced throughout Italy, will be available and will be employed in the next risk assessment for a more reliable estimation of site effects. ...
Article
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The last National Risk Assessment NRA for Italy was developed at the end of 2018 by the Department of Civil Protection (DPC) in response to the specific requirement of the Sendai Framework for Disaster Risk Reduction 2015-2030 to periodically adjourn the assessment of disaster risk. The methodology adopted to perform seismic risk assessment and build national seismic risk maps was specifically developed to comply with the recent Code for Civil Protection, issuing that, in addition to a solid scientific base, risk assessment should be characterized by a wide consensus of the scientific community. As a result, six research units belonging to two Centers of Competence of the DPC, namely ReLUIS (Network of university laboratories for seismic engineering) and EUCENTRE (European Centre for Training and Research in Earthquake Engineering), collaborated under the guidance and coordination of DPC to produce the recent updating of national seismic risk maps for the residential building stock. This paper describes the methodology adopted to develop the consensus-based national seismic risk assessment and presents the main results in terms of expected damage and impact measures (unusable buildings, homeless, casualties, direct economic losses).
... The proxy methods have been basically developed to address this requirement. Many studies such as Wills and Silva (1998) and Wills and Clahan (2006) correlate the V S30 with geologic units. In these studies, it is assumed that V S30 is dependent to the physical properties of soil. ...
Article
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This paper provides a review of the procedure of customizing ShakeMap V4.0 provided by U.S. Geological Survey based on seismic characteristics of Iran. Selecting appropriate GMPEs, adopting a suitable spatial cross-correlation model and proper modeling of site condition are important factors in the context of the ShakeMap algorithm. The present paper technically reviews the aforementioned parameters. In addition, a number of statistical tests were performed to provide the optimum configuration. The initial prototype of the configured ShakeMap has been used to provide the ground motion shaking map in the aftermath of a great earthquake in Iran (since Aug, 2021).
... For seismic risk analyses at a regional scale, the use of VS30 in ground-motion modellings necessitates its characterization in locations where site-specific VS30 measurements are unavailable. To address this issue, various inference approaches have been proposed relating VS30 to other easy-to-obtain ground features, often topographic and/or geologic proxies (e.g., Weatherill et al., 2020;Wald and Allen, 2007;Wills and Clahan, 2006;Yong et al., 2012). Meanwhile, although VS30 has been incorporated into many seismic regulations (e.g., European code EC8), disputes over its application have never ceased (e.g., Castellaro et al., 2008). ...
Preprint
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This paper describes an open-source site database for a total number of 1742 earthquake recording sites in the K-NET (Kyoshin network) and KiK-net (Kiban Kyoshin network) networks in Japan. This database contains site characterization parameters directly derived from available velocity profiles, including average wave velocities, bedrock depths, and velocity contrast. Meanwhile, it also consists of parameters obtained from earthquake horizontal-to-vertical spectral ratio (HVSR), e.g., peak frequency, amplitude, width and prominence. In addition, the site database also comprises topographic and geological proxies inferred from regional models or maps. Each parameter is derived in a consistent manner for all sites. This site database can benefit the application of machine learning techniques in studies on site amplification. Besides, it can facilitate, for instances, the search of the optimal site parameter(s) for the prediction of site amplification, the development and testing of ground-motion prediction models or methodologies, as well as investigations on regional variability in site response. All resources (the site database, earthquake HVSR data at all sites and the MATLAB script for peak identification) can be freely accessed. Supplemental contents include three tables (Table S1, S2 and S3) and two figures (Fig. S1 and S2) and a word file (“Comparison with Data in the Literature”). Table S1 lists KiK-net profiles with an incomplete entry of velocity data. Table S2 gives the 14 KiK-net sites without DAT profiles but with image profiles. Table S3 tabulates values of the Pearson’s correlation coefficients between site parameters, which are visualized in a 2D matrix in Fig. S1. Fig. S2 compares slope estimates using different algorithms. The word file displays the comparisons of data in this database with those in the literature. The site database, earthquake HVSR data (metadata and plots), as well as the MATLAB code for automatically picking peaks (frequency, amplitude, prominence and width) from HVSR curves can be freely accessed.
... Where improved data are available, regional V S30 maps have begun to include other proxies for V S30 in their calculations (e.g. Thompson et al., 2014;Wills and Clahan, 2006). Higher-resolution digital elevation models, as well as the inclusion of location-specific lithologies and geomorphic constraints, can accordingly inform a ''hybrid'' V S30 map, one which incorporates several inputs to estimate V S30 values. ...
Article
Time-averaged shear wave velocity over the upper 30 m of the earth’s surface ( V S30 ) is a key parameter for estimating ground motion amplification as both a predictive and a diagnostic tool for earthquake hazards. The first-order approximation of V S30 is commonly obtained through a topographic slope–based or terrain proxy due to the widely available nature of digital elevation models. However, better-constrained V S30 maps have been developed in many regions. Such maps preferentially employ various combinations of V S30 measurements, higher-resolution elevation models, lithologic, geologic, geomorphic, and other proxies and often utilize refined interpolation schemes. We develop a new hybrid global V S30 map database that defaults to the global slope-based V S30 map, but smoothly inserts regional V S30 maps where available. In addition, we present comparisons of the default slope-based proxy maps against the new hybrid version in terms of V S30 and amplification ratio maps, and uncertainties in assigned V S30 values.
Article
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The time-averaged shear wave velocity of top 30 m (V S30) is the most commonly used parameter to classify a site and evaluate its amplification characteristics for the seismic design. The in-situ seismic tests must be performed up to a depth of 30 m for obtaining the shear wave velocity (V S) profiles to estimate V S30. It is intimated that, in most of the cases, the measured V S profile does not extend up to 30 m due to numerous reasons including limitation of testing techniques and unfavorable field conditions. Since, the measurements of V S30 are unavailable for the majority of Pakistan and the world, the local geology and topographic slope or its combination are used to estimate V S30. However, there is no field-based validation of the estimated V S30 is performed in Islamabad-Rawalpindi region, the proxy-based estimation may lead to unrealistic results. To accommodate this, region specific extrapolation methods are developed. This study develops an empirical data-driven function of V S30 from shallow V S profiles by correlating V S30 with the time-averaged V S to depths less than 30 m. In this regard, 85 V S profiles are used from Rawalpindi-Islamabad region. A comparative analysis of the proposed procedure is carried out with the published methods. It is revealed that V S30 predicted by the proposed function results in close matches with the data measured in the western United States. In addition, the results indicate that the local geology and topographic slope proxies may not be acceptable for usage in the region due to their greater uncertainty. Finally, a procedure for extrapolating the V S profile from available shallow depth measurements up to 30 m is proposed.
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This study uses a database of 21 vertical borehole arrays in California to examine whether a two-dimensional (2D) site response analysis framework that accounts for soil heterogeneity via spatially correlated random fields can explain misfits observed from prior one-dimensional (1D) ground response modeling. The main hypothesis is that the overprediction of ground motion at site modal frequencies, consistently observed in many site response validation studies, is caused by soil heterogeneity and 2D/three-dimensional (3D) wave propagation effects that cannot be captured by 1D analyses. We apply classical “within” boundary conditions for borehole input motion along with equivalent incident wave motions derived using a framework developed here to help elucidate the effects of the down-going wave on observed fundamental mode resonances. Results from 2D and 1D analyses are compared to observations using a transfer-function-based taxonomy and residuals of other intensity measures (IMs) including response spectra. The uncertainty in predicted IMs is estimated from the many realizations of 2D models. This 2D approach was found capable of scattering seismic waves and producing transfer function variability resembling the observed event-to-event variability in empirical transfer functions (ETFs). For several sites that exhibit less down-going wave effects in ETFs (i.e. flatter peaks) and/or higher variability in ETFs, median transfer functions from 2D analyses provide a significantly better estimate of the median ETF than conventional 1D deterministic analyses, especially at fundamental modes. In contrast, for some of the sites that are well represented by 1D methods (e.g. Wildlife Liquefaction Array and Treasure Island), 2D methods with generic levels of spatial variability may over-represent the heterogeneity and consequently underpredict amplifications at higher mode frequencies.
Chapter
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The time averaged shear wave velocity to 30-m depth (VS30) is used for seismic hazard assessment for National Earthquake Hazard Reduction Program (NEHRP) site classification. There are several models to predict VS30 from average shear wave velocity to a depth (z) less than 30 m (VSz). This study evaluates the prediction capability of the existing models in the United Arab Emirates (UAE). The analyses show that there is a significant prediction biases in the existing models developed in different regions. By reviewing borehole data from the selected sites, it reveals that these biases are mainly because of the characteristic geology of the UAE where weak bedrock is encountered at shallow depths. This bedrock mostly shows a considerable variation in both clay content and the porosity which also have a direct effect on the prediction uncertainty.KeywordsVS30VSzRegression modelUAE
Chapter
The P-wave seismogram method estimates the average shear wave velocity over a representative depth (\(V_{SZ}\)) from earthquake recordings at a site. The \(V_{SZ}\) is computed from the amplitudes of the radial and vertical P-wave arrivals on the earthquake recordings and an estimate of the seismological ray parameter (\(p\)). The ray parameter is estimated from the depth of the event, the epicentral distance, and the regional crustal velocity model. We evaluated the P-wave seismogram approach to estimating \(V_{SZ}\) at 153 seismic recording stations in California for which shear wave velocity profiles are available and tested the effect of different crustal models on the estimated ray parameter and the resulting \(V_{SZ}\). Across all the sites, the estimated \(V_{SZ}\) values were, on average, about 24% larger than the measured \(V_{SZ}\), although the difference was negligible for softer sites and as large as 45% at stiffer sites (>1000 m/s). Two crustal velocity models for California were considered: a simplified four-layer crustal model for the entire state and a set of more detailed crustal models used for different parts of the state. The effect of the assumed crustal velocity profile was not significant for earthquake events with focal depths greater than about 3–5 km, but for shallower events the detailed crustal velocity model produced \(V_{SZ}\) values significantly smaller than the \(V_{SZ}\) from the simplified crustal velocity model due to the effect of the shallow low velocity layers and large gradient on the ray parameter.Keywords \(V_{SZ}\) Shear wave velocityP-wave seismogram
Article
The time-averaged shear-wave velocity in the upper 30 m (VS30) is widely used as a proxy for site characterization in building codes. Regional estimations of VS30 often use either slope-based, terrain-based, or geological approaches as a proxy. This technique has proven useful at a number of locations globally, and slope-based estimates formed the basis of the original global VS30 model implemented by the U.S. Geological Survey. Geostatistical models involve the study of potentially spatially correlated parameters. Modeling challenges arise when parameters are scarce or uncertain, and traditional geostatistical workflows cannot be implemented in all settings. In this study, the benefits of the spatial extents of VS30 proxies are used to supplement local data to implement a methodology for improving estimates using a multi-Gaussian Bayesian updating framework. This methodology is presented in the context of a data-scarce region, specifically, the Kathmandu Valley in Nepal. Using geostatistical approaches typically used by the petroleum industry, this article develops a novel practice-oriented framework for VS30 estimation that can be adapted for use on a region-by-region basis. This framework provides an informed estimate and assessment of the uncertainties in which quantification of VS30 is required in geotechnical earthquake engineering applications.
Article
The P-wave seismogram method is utilized to estimate the V S30 at 194 ground motion recording stations in California. Comparison with the measured V S30 at these sites shows an average overestimation of 9%, which is similar to values reported by other studies for the P-wave seismogram method. The estimated V S30 are within ±50% of the measured V S30 for 85% of the sites. The V S30 residuals are interpreted with respect to the peak frequency obtained from horizontal-to-vertical spectral ratio (HVSR) measurements, and the results indicate that stations with estimated V S30 greater than 500 m/s may be systematically overestimated by the P-wave seismogram method when a distinct peak is identified in the HVSR. Thus, the peak frequency from HVSR can be used to identify sites with potential bias and also to correct this bias. The P-wave seismogram V S30 estimates from this study are compared to estimates from a geology/slope proxy, and the results suggest that the P-wave seismogram method achieves similar accuracy and less variability relative to the measured V S30 compared with the geology/slope proxy-based method.
Article
The near-surface seismic structure (to a depth of about 1000 m), particularly the shear-wave velocity (VS), can strongly affect the propagation of seismic waves, and therefore must be accurately calibrated for ground motion simulations and seismic hazard assessment. The VS of the top (<300 m) crust is often well-characterized from borehole studies, geotechnical measurements, and water and oil wells, while the velocities of the material deeper than about 1000 m are typically determined by tomography studies. However, in depth ranges lacking information on shallow lithological stratification, typically rock sites outside the sedimentary basins, the material parameters between these two regions are typically poorly characterized due to resolution limits of seismic tomography. When the alluded geological constraints are not available, models, such as the Southern California Earthquake Center (SCEC) Community Velocity Models (CVMs), default to regional tomographic estimates that do not resolve the uppermost VS values, and therefore deliver unrealistically high shallow VS estimates. The SCEC Unified Community Velocity Model (UCVM) software includes a method to incorporate the near-surface earth structure by applying a generic overlay based on measurements of time-averaged VS in top 30 m (VS30) to taper the upper part of the model to merge with tomography at a depth of 350 m, which can be applied to any of the velocity models accessible through UCVM. However, our 3D simulations of the 2014 Mw 5.1 La Habra earthquake in the Los Angeles area using the CVM-S4.26.M01 model significantly underpredict low-frequency (<1 Hz) ground motions at sites where the material properties in the top 350 m are significantly modified by the generic overlay (“taper”). On the other hand, extending the VS30-based taper of the shallow velocities down to a depth of about 1000 meters improves the fit between our synthetics and seismic data at those sites, without compromising the fit at well constrained sites. We explore various tapering depths, demonstrating increasing amplification as the tapering depth increases, and the model with 1000 m tapering depth yields overall favorable results. Effects of varying anelastic attenuation are small compared to effects of velocity tapering and do not significantly bias the estimated tapering depth. Although a uniform tapering depth is adopted in the models, we observe some spatial variabilities that may further improve our method.
Article
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This paper considers the uncertainty in the shear wave velocity (Vs) of soil and rock profiles for use in earthquake site response calculations. This uncertainty is an important contributor to uncertainty in site response, which in turn is an important contributor to uncertainty in earthquake ground motions and in seismic hazard. The paper begins with a discussion of the different types of uncertainty and how they are characterized in probabilistic seismic hazard analysis, and how this differentiation is particularly ambiguous in the case of soil properties. This is followed by a description of the probabilistic models of Vs that are most commonly used in engineering practice, for both generic and site-specific applications. In site-specific applications, the uncertainty in Vs (which is measured by the logarithmic standard deviation or by the coefficient of variation of Vs) is lower than in generic applications, but other elements of the profile model are also different. Next, the paper discusses the issues that arise in characterizing the uncertainty in Vs in site-specific applications using non-invasive surface wave methods and summarizes the insights obtained by comparing the results from multiple blind studies in which the same surface-wave data (and no other site-specific data) were provided to multiple teams of analysts. Finally, the paper provides recommendations on how to characterize uncertainty in Vs for both generic and site-specific applications.
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This study describes an approach for modeling wave scattering and the spatial variability of ground motion in geotechnical site-response analysis by modeling soil heterogeneity through 2D correlated random fields. Importantly, the required site-specific inputs to apply the proposed approach in a practical setting are the same as those associated with conventional 1D site-response analysis. The results, which are affected by wave scattering attenuation, are compared to those from conventional laterally homogeneous 1D site-response analyses and 1D analyses with randomized velocity profiles extracted from heterogeneous 2D velocity model realizations. A sensitivity study, involving 5400 2D model realizations, investigates the influence of random field input parameters on wave scattering and site response. The computed ground surface acceleration waveforms and transfer functions show that this method is capable of scattering seismic waves. Multiple ground-motion intensity measures are analyzed to quantify this influence and distinguish between the effects of 1D vertical heterogeneities and averaging across many nodes and realizations, from the effects of wave scattering and 2D ground-motion phenomena. The redistribution of ground-motion energy across wider frequency bands and scattering attenuation of high-frequency waves in the 2D analyses resemble features observed in empirical transfer functions computed in other studies. While analyses with 1D randomized velocity profiles are able to replicate median results from 2D analyses for some low-frequency intensity measures (e.g. transfer functions at [Formula: see text] Hz, and spectral acceleration at the fundamental period), medians and standard deviations of high-frequency intensity measures (e.g. transfer function at [Formula: see text] Hz, [Formula: see text], and Arias intensity), which are influenced by wave scattering, are not appropriately captured. Given the equivalent input information requirements as conventional 1D analysis, and the availability of large computational resources, we advocate that the proposed 2D (and eventually 3D) approach is a fruitful path forward to improve the modeling of site-response physics and realize improved predictive capabilities.
Article
A Texas-specific [Formula: see text] map that uses geostatistical kriging integrated with a region-specific geologic proxy, field measurements of [Formula: see text], and P-wave seismogram estimates of [Formula: see text] is developed. The region-specific geologic proxy is used first to predict [Formula: see text] from the surface geologic conditions across the state, and then geostatistical kriging with an external drift is used to incorporate the local [Formula: see text] measurements/estimates into the map. Compared with the [Formula: see text] map of Texas developed from a topographic slope proxy, the Texas-specific [Formula: see text] map predicts larger [Formula: see text] values across much of Texas, except for the Gulf Coast region where the values are similar. The utilization of kriging brings the Texas-specific [Formula: see text] map into better agreement with the in situ measurements and estimates of [Formula: see text]. The sensitivity of predicted ground motions by ShakeMap to changes in [Formula: see text] values is evaluated with a scenario earthquake in the Dallas–Fort Worth area. The results suggest smaller predicted ground motions due to the generally larger values of [Formula: see text] in the Texas-specific [Formula: see text] map as compared to the [Formula: see text] from the topographic proxy.
Technical Report
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En el presente trabajo ponemos en evidencias fallas geológicas activas y cuaternarias, que se emplazan a lo largo de las pampas costeras, la Cordillera Occidental y el Altiplano. Para determinar esta relación de morfología y cinemática de fallas con la msagnitud del sismo, utilizamos un conjunto de metodologías, que incluyen las geología estructural, estratigrafía, geomorfología, sensores remotos y la paleosismología. Todas estas metodologías, contribuyeron a realizar análisis e identificar fallas geológicas activas a la región de Tacna. De acuerdo a la morfología y cinemática de las fallas Purgatorio-Mirave y Sama-Calientes, deducimos que estas fallas pueden generar sismos M>7. Además, en este trabajo, ponemos en evidencia por primera vez, una ruptura superficial de ~100 km asociada al Sistema de Falla Incapuquio. Entonces, con base a estudios neotectónicos y paleosismológicos, ponemos en evidencia de que ~100 km de escarpas de fallas de 2-4 m de altura fueron formadas por un terremoto de cinemática transpresional a lo largo de 100 km del segmento sur de la Falla de Incapuquio a principios del siglo XV (~AD 1400-1440). Las Las relaciones de escala empíricas sugieren que la magnitud de este terremoto habría sido de Mw 7,4-7,7 con al menos 2-3 m de deslizamiento neto. Eventos similares plantean un peligro significativo para las poblaciones asentadas en la región del antearco peruano. La fecha de este terremoto coincide con el colapso de la población de la civilización Chiribaya entre los años 1360 y 1400 d.C., presentamos evidencia de edificios dañados y desviados. Hacia los altos Andes, se ubican los volcanes activos de los Andes Centrales, adyacentes a ellos identificamos fallas de tipo normal, que generaron desplazamientos verticales de hasta ~20 m. Las fallas en el sector de Purupuruni, Paucarani, Vilacota, entre otros, son fallas que afectan morrenas glaciares y depósitos aluviales. De acuerdo a la longitud y desplazamiento vertical, sugerimos que estas fallas pueden generar sismos M>6.0. Los análisis neotectónicos y morfotectónicos sirven para caracterizar el potencial sismogénico de las estructuras tectónicas, lo que permite elaborar mapas de aceleración sísmica (peligro sísmico) deducido de la magnitud máxima posible que una falla activa podría generar. Asimismo, el movimiento del terreno por efectos del sismo genera procesos geológicos asociados, como por ejemplo movimientos en masa, procesos de licuación de suelos, asentamientos y grietas en el terreno, que afectan o podrían afectar ciudades, infraestructura y obras de gran envergadura. Con esta finalidad, generamos mapas de amenaza sísmica a partir de la interacción entre los mapas de susceptibilidad y el mapa de aceleración sísmica, que una falla activa podría generar. Se obtuvieron mapas de peligrosidad por movimientos en masa y los mapas de peligrosidad por procesos de licuefacción de suelos y/o asentamientos para las fallas más importantes de la región Tacna, mostrandosé que las fallas activas pueden generar aceleraciones locales altas y detonar procesos de movimentos en masa. Para poder identificar las zonas que se encuentran en riesgo o que serían afectadas por la reactivación de las fallas geológicas activas, elaboramos mapas de riesgo, treinta y uno (31) escenarios, para la región Tacna tomando como base los mapas de peligrosidad detonados por sismo, donde se superponen las capas de áreas restringidas y la capa de infraestructuras de la región (ciudades, población, hidroeléctricas, aeropuertos, represas, futuras obras de gran envergadura, etc.) De esta forma, obtenemos un mapa de riesgo donde se pueden identificar las ciudades u obras de gran envergadura que se verán afectadas por la reactivación de las fallas antes mencionadas. Estos mapas se consideran una herramienta importante para los Planes de Ordenamiento Territorial y son de gran ayuda para la toma de decisiones en planes de prevención en el marco de la gestión del riesgo de desastres. Se recomienda realizar trabajos paleosismológicos detallados en las fallas Purgatorio, Incapuquio, Sama-Calientes, Paucarani, Vilacota entre otros, con la finalidad de determinar la máxima magnitud posible y la recurrencia de estos eventos, debido a que adyacentes a estas zonas se ubican infraestructura privada y pública que podría ser afectada; en este sentido, esta información contribuirá a la Planificación del Territorio y crecimiento socio-económico del país.
Article
Most earthquake-related losses are enhanced by soil amplification phenomena, especially in areas where high vulnerability assets coexist. These mechanisms are frequently inferred from empirical formulas. These procedures are based on the average shear-wave velocity to 30 m depth (Vs30) as a proxy for amplification events. However, past analysis of spatial relationships between the Vs30 and nonlinear soil behaviour has frequently resulted in poor correlations, affecting all onward analysis. In this research, these relationships are further analysed using an Earthquake Loss Estimation Software. In this framework, the Algarve region and associated ground motion have been depicted based on a certified seismic catalogue. This region has a moderate seismic hazard but possesses a complex geology and dissimilar seismogenic sources. The source data for this research has been Vs30 data collected from geologic surveys made for the region in 2010 in the framework of the Study of Seismic Risk and Tsunamis in Algarve (ERSTA) coordinated by the Portuguese Civil Protection. Other data collected has been the Vs30 topography compiled by the USGS and geologic information available by the OneGeology collaborative project. The goal of this research is to identify inaccuracies that potentially occur in estimating the site effects by different approaches. In this framework three amplification factors based on different sources - Vs30 field-data, Vs30 using slope as proxy and Vs30 using geology as proxy – have been tested. The results have shown a good correlation between Vs30-field-data and geologic based Vs30 and a minor correlation between the former and slope based Vs30. However, regarding the seismic losses - building damage and human losses - the final results show similar values for all three amplification factors analysed.
Article
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A new model of the average shear-wave velocity in the uppermost 30 m was generated by extrapolation of discrete velocity using surface geology at several scales. Statistical methods have been applied to create a map that is no more complicated than is supported by the velocity data; several geologic units with similar responses are grouped together. The resulting map is simpler than previous ones and yet fits the observed velocity profiles better than earlier, more complicated maps. Analysis within a geographic information system will permit updates and modification of the map as new velocity data are added.
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In this study we used strong-motion data recorded from 1957 to 1995 to derive a mutually consistent set of near-source horizontal and vertical groundmotion (attenuation) relations for peak ground acceleration and 5%-damped pseudo-acceleration response spectra. The database consisted of up to 960 uncorrected accelerograms from 49 earthquakes and 443 processed accelerograms from 36 earthquakes of N-w 4.7-7.7. All of the events were from seismically and tectonically active, shallow crastal regions, located throughout the world. Some major findings of the study are (1) reverse- and thrust-faulting events have systematically higher amplitudes at. short periods, consistent with their higher dynamic stress drop; (2) very firm soil and soft rock sites have similar amplitudes, distinctively different from amplitudes on firm soil and firm rock sites; (3) the greatest differences in horizontal ground motion among the four site categories occur at long periods on firm rock sites, which have significantly lower amplitudes due to an absence of sediment amplification, and at short periods on firm soil sites, which: have relatively low amplitudes at large magnitudes and short distances due to nonlinear site effects; (4) vertical. ground motion exhibits similar behavior to horizontal motion for firm rock sites at long periods but has. relatively higher short-period amplitudes at short distances on firm soil sites due to a lack of nonlinear site effects, less anelastic attenuation, and phase conversions within the upper sediments. We used a relationship similar to that of Abrahamson and Silva (1997) to model hanging-wall effects but found these effects to be important only for the firmer site categories. The ground-motion relations do not include recordings from the 1999 M-w > 7 earthquakes in Taiwan and Turkey because there is, still no consensus among strong-motion seismologists as to why these events had such low ground motion. If these near-source amplitudes are later found to be atypical, their inclusion could lead to unconservative engineering estimates of ground motion. The study is intended to be a limited update of the ground-motion. relations previously developed. by us in 1994 and 1997, with the explicit purpose of providing engineers and seismologists with a mutually consistent set of near-source ground-motion relations to use in seismic hazard studies. The U.S. Geological Survey and the California Geological Survey have selected the updated relation as one of several that they are using in their 2002 revision of the U.S. and California seismic hazard maps. Being a limited update, the study does not explicitly address such topics as peak ground velocity, sediment depth, rupture directivity effects, or the use of the 30-m velocity or related National Earthquake Hazard Reduction Program site classes. These are topics of ongoing research and will be addressed in a future update.
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In October and November 2001 we performed the first urban shear-velocity transect, across 16 km of the Reno, Nevada area basin. The transect was completed by an average of 3-1/2 people in 9 days employing 45 Reftek RT-125 "Texan" portable recorders mated to 4.5 Hz vertical geophones. The records were analyzed using the refraction microtremor method of Louie (2001). Shear velocity, averaged over 30 meters depth (V 30) is a predictor of earthquake ground motion amplification and potential hazard in similar alluvium-filled basins. V 30 is the basis of site hazard classification under NEHRP-UBC provisions. A simple geologic map-based classification of nearly all of our transect line would be NEHRP-D. Our measurements of V 30 revealed that 82% of the transect is classified NEHRP-C. We also employed the results of a gravity basin-depth study to show that alluvium-depth modeling can be combined with transect V 30 measurements to effectively predict V 30 across the Reno basin. We concluded that unlike the Los Angeles basin, the Reno basin's greatest depths have stiffer, Tertiary sediments underlying the surface. Weaker soils appear to occur east of downtown Reno in the broad floodplain of the Truckee River.
Article
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This study determines site-response factors that can be applied as corrections to a rock-attenuation relationship for use in probabilistic seismic-hazard analysis. The site-response factors are amplitude and site-class dependent. These amplification factors are determined by averaging ratios between observed and predicted ground motions for peak ground acceleration (PGA) and for 5% damped response spectral acceleration at 0.3, 1.0, and 3.0 sec oscillator periods. The observations come from the strong-motion database of the Southern California Earthquake Center (SCEC), and the predictions are based on the Sadigh (1993) rock-attenuation relation. When separated and averaged according to surface geology, significantly different site-response factors are found for Quaternary and Mesozoic units, but a subclassification of Quaternary is generally not justified by the data. The low input motion amplification factors are consistent with those obtained from independent aftershock studies at the PGA and 0.3-second period. An observed trend of decreasing Quaternary site amplification with higher input motion is consistent with nonlinear soil behavior; however, the trend exists for Mesozoic sites as well, implying that this may be an artifact of the Sadigh relationship, There is a correlation between larger site-response factors and lower average shear-wave velocity in the upper 30 m for low predicted PGA input motions, with an increase in the correlation with increasing period. The 0.3-sec site response factors for Quaternary data in southern California determined in this study are consistent with 0.3-sec NEHRP site-response correction factors; however, at 1.0-sec period some inconsistencies remain. A trend is also seen with respect to sediment basin depth, where deeper sites have higher average site-response factors. These results constitute a customized attenuation relationship for southern California, The implication of these customized attenuation models with respect to probabilistic hazard analysis is examined in Field and Petersen (2000).
Article
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Las Vegas Valley has a rapidly growing population exceeding 1.5 mil-lion, subject to significant seismic risk. Surveys of shallow shear velocity performed in the Las Vegas urban area included a 13-km-long transect parallel to Las Vegas Blvd. (The Strip), and borehole and surface-wave measurements of 30 additional sites. The transect was completed quickly and economically using the refraction microtremor method, providing shear velocity versus depth profiles at 49 locations. The lowest velocities in the transect, NEHRP D class, are near intrabasin faults found near Interstate 15 and Lake Mead Blvd. Calcite cementation of alluvium (a.k.a. ca-liche) along the Las Vegas Strip elevates Vs30 values to 500–600 m/sec, NEHRP C class. Our transect measurements correlate poorly against geologic map units, which do not predict the conditions of any individual site with accuracy sufficient for en-gineering application. Some USDA soil map units do correlate, and Vs30 predictions based on measurements of soil units match transect measurements in the transect area. Extending soil-map predictions away from the area of dense measurement cov-erage generally failed to predict new measurements. Further, for several test sites the predictions were not conservative, in that the soil model predicted higher Vs30 than was later measured (predicting lesser potential ground motion). Subsurface infor-mation is needed to build a Vs30 model extending predictions throughout Las Vegas Valley. A detailed stratigraphic model built by correlating 1100 deep well logs in Las Vegas predicts Vs30 better than surface maps, but again only in parts of the Valley well-measured for velocity. The stratigraphic model yields good predictions of our transect Vs30 measurements. It is less accurate, although at least conservative, when extended to sites away from the transect.
Article
The authors have developed a method for assigning shear-wave velocities to the mapped geologic units in the San Francisco Bay region. In this chapter, the analysis is extended to include the Los Angeles region. Refs.
Article
One simple way of accounting for site conditions in calculating seismic hazards is to use the shear-wave velocity in the shallow subsurface to classify materials. The average shear-wave velocity to 30 m ( V 30s) has been used to develop site categories that can be used for modifying a calculated ground motion to account for site conditions. We have prepared a site-category map of California by first classifying the geologic units shown on 1:250,000 scale geologic maps. Our classification of geologic units is based on V 30s measured in 556 profiles and geological similarities between units for which we have V s data and the vast majority of units for which we have no data. We then digitized the geologic boundaries from those maps that separated units with different site classifications. V s data for California shows that several widespread geologic units have ranges of V 30s values that cross the boundaries between NEHRP-UBC site categories. The Franciscan Complex has V 30s values across UBC categories B and C with a mean value near the boundary between those two categories. Older alluvium and late Tertiary bedrock have V 30s values that range from about 300 to about 450 m/sec, across the boundary between categories C and D. To accommodate these units we have created intermediate categories, which we informally call BC and CD. Geologic units that have, or are interpreted to have, V 30s values near the boundary of the UBC categories are placed in these intermediate units. In testing our map against the available V 30s measurements, we have found that 74% of the measured V 30s values fall within the range assigned to the V s category where they fall on the map. This ratio is quite good considering the inherent problems in plotting site-specific data on a regional map and the variability of physical properties in geologic units. We have also calculated the mean and distribution of V 30s for each of our map units and prepared composite profiles, showing the variation of V s in the upper 100 m from the available V s data. These data show that the map categories that we have defined based on geologic units have different V s properties that can be taken into account in calculating seismic hazards.
Article
Attenuation relationships are presented for peak acceleration and response spectral accelerations from shallow crustal earthquakes. The relationships are based on strong motion data primarily from California earthquakes. Relationships are presented for strike-slip and reverse-faulting earthquakes, rock and deep firm soil deposits, earthquakes of moment magnitude M 4 to 8+, and distances up to 100 km.
Article
Site-amplification potential in a 140-km(2) area on the eastern shore of San Francisco Bay, California, was mapped with data from 210 seismic cone penetration test (SCPT) soundings. NEHRP V-S30 values were computed on a 50-m grid by both taking into account the thickness and using mean values of locally measured shear-wave velocities of shallow geologic units. The resulting map of NEHRP VS30 site classes differs from other published maps that (1) do not include unit thickness and (2) are based on regional compilations of velocity. Although much of the area in the new map is now classified as NEHRP Site Class D, the velocities of the geologic deposits within this area are either near the upper or lower V-S30 boundary of Class D. If maps of NEHRP site classes are to be based on geologic maps, velocity distributions of geologic units may need to be considered in the definition of V-S30 boundaries of NEHRP site classes.
Article
In this paper we summarize our recently-published work on estimating horizontal response spectra and peak acceleration for shallow earthquakes in western North America. Although none of the sets of coefficients given here for the equations are new, for the convenience of the reader and in keeping with the style of this special issue, we provide tables for estimating random horizontal-component peak acceleration and 5 percent damped pseudo-acceleration response spectra in terms of the natural, rather than common, logarithm of the ground-motion parameter. The equations give ground motion in terms of moment magnitude, distance, and site conditions for strike-slip, reverse-slip, or unspecified faulting mechanisms. Site conditions are represented by the shear velocity averaged over the upper 30 m, and recommended values of average shear velocity are given for typical rock and soil sites and for site categories used in the National Earthquake Hazards Reduction Program's recommended seismic code provisions. In addition, we stipulate more restrictive ranges of magnitude and distance for the use of our equations than in our previous publications. Finally, we provide tables of input parameters that include a few corrections to site classifications and earthquake magnitude (the corrections made a small enough difference in the ground-motion predictions that we chose not to change the coefficients of the prediction equations).
Article
In October and November 2001, we performed an urban shear-wave velocity transect across 16 km of the Reno, Nevada, area basin. Using the refraction microtremor method of Louie (2001) we determined shear-wave velocity versus depth profiles at 55 locations. Shear-wave velocity averaged to 30 m depth (Vs30) is one predictor of earthquake ground-motion amplification in similar alluvium-filled basins, and it is the basis of site hazard classification under National Earthquake Hazards Reduction Program-Uniform Building Code (nehrp-ubc) provisions. A geologic map-based nehrp classification along nearly all of our transect line would be nehrp-d, but our measurements of Vs30 revealed that 82% of the transect is classified nehrp-c. Relatively stiff Tertiary sediments underlie the surface of the Reno basin, and weaker soils occur east of downtown Reno in the floodplain of the Truckee River. Although 53 of our locations were on the geologically youngest and most active fluvial units, these sites showed Vs30 values ranging from 286 m/sec (nehrp-d) to 849 m/sec (nehrp-b). Mapped geologic and soil units are not accurate predictors of Vs30 measurements in this urban area. A test model based on gravity results showed Quaternary-alluvium depth can be combined with transect Vs30 measurements to predict Vs30 across the Reno basin.
Article
Site conditions can be classified by the average shear-wave velocity to 30 meters (Vs30) and used for estimating site effects in seismic hazard calculations. Large scale seismic hazard maps, which include site effects, may be produced, providing Vs30 can be well correlated with geologic units. Vs30 values for several geologic units can be easily classified into soil profile types of the UBC (ICBO 1997). Most geologic units have wide variations in Vs30 and some extensive geologic units, such as older alluvium, the Franciscan Complex or the Puente Formation cannot be easily classified.
Article
Using a database of 655 recordings from 58 earthquakes, empirical response spectral attenuation relations are derived for the average horizontal and vertical component for shallow earthquakes in active tectonic regions. A new feature in this model is the inclusion of a factor to distinguish between ground motions on the hanging wall and footwall of dipping faults. The site response is explicitly allowed to be non-linear with a dependence on the rock peak acceleration level.
Article
In July 2003 we evaluated a 60 km transect for shallow shear wave velocities along the San Gabriel River, in the San Gabriel Valley and Los Angeles Basin of southern California. The purpose of the experiment was to evaluate spatial variability of earthquake shaking hazard, and any velocity correlations to mapped geological and soil units. We recorded urban microtremor on 300-m linear arrays, analyzing Rayleigh-wave dispersion curves to forward-model 200 shallow shear-velocity profiles to depths exceeding 100 m. Each 300-m array had 15 seismometers at 20-m intervals. We used IRIS/PASSCAL "Texan" single-channel recorders with 4.5-Hz vertical geophones. Shear velocities, vertically averaged to 30-m depths (V30), provided NEHRP soil classifications of ground shaking potential. We also performed P-wave refraction analyses at several points along the transect. Arrays in a highway cut on the gneissic bedrock of the San Gabriel Mts. yielded V30 between 520 and 640 m/s, NEHRP hazard class C. Measurements approach NEHRP-B (>760 m/s) only at the locations of the coarsest surficial alluvium in Azusa, below the mouth of San Gabriel Canyon, and near Santa Fe Dam. The maximum V30 along the transect was 730 m/s. Our results suggest much of the line north of Whittier Narrows is NEHRP class C (350-760 m/s), or on the C/D boundary. We find NEHRP-D values for V30 (150-350 m/s) from Whittier Narrows south to Seal Beach. The lowest velocities from the transect, 230 m/s, are still well above the NEHRP-E range. The lowest velocities occur near the south end of the line at Alamitos Bay. At Seal Beach, in the littoral sands, velocities rise slightly toward the NEHRP-C/D boundary. We interpret a lesser shaking hazard in the northern San Gabriel Valley and at Alamitos Bay than do assessments based on existing geologic maps. In San Gabriel Mts. bedrock on the other hand, rated NEHRP-B/C on maps, we interpret a somewhat elevated hazard, finding a NEHRP-C velocity of 640 m/s at most. Variations in V30 we measured may well be explained by the presence of a steep hydraulic gradient and a large average clast size near the San Gabriel Mts., with the gradient decreasing and the sediments fining downstream to Alamitos Bay. Corroborating measurements near Alamitos Bay, Long Beach, and Azusa, at locations removed from the active river channel, proved to match results from the transect.
Article
This study assesses a 60 km NNE-SSW transect along the San Gabriel River for shallow shear velocities, in San Gabriel Valley and the Los Angeles Basin of southern California. We assessed a total of 214 sites, 199 along the transect at 300 m spacing, during a one-week field campaign with the refraction microtremor (ReMi) technique. The transect's maximum 30-meter shear velocity (Vs,) occurs in coarse alluvium of San Gabriel Valley where the San Gabriel River exits the San Gabriel Mts. ; at 730 m/s, upper NEHRP site class C. Much of the northeast section of the transect (in San Gabriel Valley) is also NEHRP class C, or near the CD class boundary. The section of the transect south from Whittier Narrows to Seal Beach shows NEHRP-D velocities in active alluvium. The transect's lowest Vs,, 230 m/s at the Alamitos Bay estuary, is also classed as NEHRP-D. An increase toward the NEHRP CD class boundary occurs at the shoreline beach outside Alamitos Bay, confirmed by additional measurements on Seal Beach. Our measured Vs 30 values generally show good correlation with published site-classification maps and existing borehole data sets. There is no evidence in our data for an increase in velocity predicted by Wills et al. (2000) at their "CD" to "BC" site classification boundary at the San Gabriel Mountains front, nor for any decrease at their "D" to "DE" class boundary at Alamitos Bay. Very large Vs 30 variations exist in soil and geologic units sampled by our survey. The Vs,variations we measured are smaller than Vs 30 variations of 30% or more we found between closely spaced (
Correlations between seismic wave velocities and physical properties of near-surface geologic materials in the southern San Francisco Bay region
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