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A geology based 3-D velocity model of the Los Angeles Basin sediments
Abstract
Seismic hazard studies of the Los Angeles basin area require a realistic seismic-velocity model. We use geologic information about depth to crystalline basement, depths to sedimentary horizons, uplift of sediments, and surface geology in a velocity-depth-age function to construct a three-dimensional velocity model. In earthquake location tests, the model predicts travel times satisfactorily, and in earthquake ground-motion simulations, the model correctly determines the timing and amplitude of late-arriving waves.
... To this end, the definition of the physical and geometrical attributes of the outcropping and subsurface geological units provides fundamental knowledge for several scientific topics and many engineering plans and operations. As an example, the integration of physical parameters of subsurface geological units into a well-defined 3D model can be applied to (i) to reduce the uncertainties for earthquake location, contributing to the calculation of more accurate focal mechanisms and performing wavepropagation and ground-motion simulations (e.g., Magistrale et al., 1996;Süss et al., 2001;Molinari et al., 2015;Livani et al., 90 2022), and (ii) understand, simulate, and predict the response of the geological body to subsurface natural and anthropic processes. The latter is the case of 3D geomechanical numerical models, which represent effective tools to evaluate and predict the possible effects -both at the surface and in the subsurface -of geofluid extraction and storage, to guarantee a safe management of such activities as well as to quantify and better understand the ongoing geological processes (e.g., tectonic deformation, natural subsidence, etc.; Teatini et al., 2006;Codegone et al., 2016;Benetatos et al., 2020). ...
... As an example, once populated with the values of seismic velocity, the 3D geological 585 model can find several applications in seismological studies. It can be used to improve the procedure and reduce the uncertainties during earthquake location, contribute to the calculation of more accurate focal mechanisms and perform wavepropagation and ground-motion simulations (e.g., Magistrale et al., 1996;Süss et al., 2001;Molinari et al., 2015;Livani et al., 2022). The 3D model also represents a starting model in perturbation studies, such as linearized inversions of travel times for crustal velocities (e.g., Magistrale and Day, 1999) or for studies related to the seismic waveforms for crustal structure and, 590 moreover, it can be used to derive densities and compare them to gravity observations (Roy and Clayton, 1999). ...
The Po Plain (Italy) is one of the most densely populated and productive regions of Europe, characterized by a flourishing economy (also linked to strategic subsurface resources) and several world cultural and natural heritage sites. The coupling of social-economic interests with geological hazards (i.e., seismic, subsidence and flooding hazards) in this area requires accurate knowledge of the subsurface geology, active geological processes, and impact of human activities on natural environments to mitigate the potential natural and anthropic risks. Most data unveiling the subsurface geology of this region were produced by the hydrocarbon exploration industry. Po Plain hosts indeed many hydrocarbon fields that were discovered since the early 1950s giving rise to the subsurface exploration through extensive seismic reflection surveys and drilling of numerous deep wells. In this work, geological-geophysical data from 160 deep wells drilled for hydrocarbon exploration/exploitation purposes in the Po Plain and in the facing northern Adriatic Sea have been collected and digitized along with several published geological cross-sections and maps. These data have been used to reconstruct the overall subsurface 3D architecture and to extract the physical properties of the subsurface geological units. The digitized data are suitable to be imported into geo-software environments so to derive the geophysical-mechanical properties of the geological units for a wealth of applied and scientific studies such as geomechanical, geophysical and seismological studies. The integrated dataset may represent a useful tool in defining strategies to ensure the safety of the urbanized areas and human activities and to reduce natural and anthropic risks that may affect this crucial region of Europe. Nowadays, such issues are particularly relevant for the underground industry development related to the increasing interest on possible CO2 and hydrogen underground storage, which can play a fundamental role in the energy transition process towards the decarbonisation goals.
... Extracting the average basin edge gradient from 11.25 to 13.25 km along profile A-A' in Figure 12 gives a dip angle of 72-73°. The SCEC CVMs have evolved from the original models of Magistrale et al. (1996Magistrale et al. ( , 2000. For the Los Angeles basin, an empirically determined velocity law for compacted sediments is used (Faust, 1951). ...
... Indeed, the northeastern components of the CSN operate within the surface expression of the lower Puente and Topanga units of the LA basin stratigraphic column, which were assembled early within the LA basin sequence and support a shallow sequence of basin rocks toward to the right of profile A-A' (Yerkes et al., 2005). In the Supporting Information S1, we further discuss these two main features in the context of fitting the rule-based CVM1 (Magistrale et al., 1996(Magistrale et al., , 2000 to profile A-A'. By perturbing the locations of the loosely constrained geological contacts that define the CVM1, we analyze the outcomes of our fully 3D inversion in terms of geological structure, and find that the steep basin sidewall is consistent with recently (≤4 Ma) active deformation. ...
The proliferation of dense arrays promises to improve our ability to image geological structures at the scales necessary for accurate assessment of seismic hazard. However, combining the resulting local high‐resolution tomography with existing regional models presents an ongoing challenge. We developed a framework based on the level‐set method that infers where local data provide meaningful constraints beyond those found in regional models ‐ for example the Community Velocity Models (CVMs) of southern California. This technique defines a volume within which updates are made to a reference CVM, with the boundary of the volume being part of the inversion rather than explicitly defined. By penalizing the complexity of the boundary, a minimal update that sufficiently explains the data is achieved. To test this framework, we use data from the Community Seismic Network, a dense permanent urban deployment. We inverted Love wave dispersion and amplification data, from the Mw 6.4 and 7.1 2019 Ridgecrest earthquakes. We invert for an update to CVM‐S4.26 using the Tikhonov Ensemble Sampling scheme, a highly efficient derivative‐free approximate Bayesian method. We find the data are best explained by a deepening of the Los Angeles Basin with its deepest part south of downtown Los Angeles, along with a steeper northeastern basin wall. This result offers new progress toward the parsimonious incorporation of detailed local basin models within regional reference models utilizing an objective framework and highlights the importance of accurate basin models when accounting for the amplification of surface waves in the high‐rise building response band.
... A USGS velocity model for the San Francisco bay area [17] and CUSVM for the central and eastern US [12] utilized polynomial parametric velocity functions. Exponential parametric functions have been used for the Canterbury region, New Zealand [18,19] and southern California [20]. However, with sufficient empirical constraints, no significant differences were observed between different forms of velocity functions [21]. ...
A new approach is presented to develop site signature consistent deep shear wave velocity profiles (V S profile) for the Mississippi embayment using generalized power-law functions. The approach utilizes 24 deep shear wave velocity profiles measured across the embayment to develop power-law shear wave velocity functions for the geologic units observed in the embayment. The velocity functions along with the layer interface boundaries are utilized to generate an initial V S profile, whose Vs is then adjusted to be consistent with the fundamental site frequency (i.e., the site signature). Using the developed approach, model V S profiles at 24 sites throughout the embayment are generated and compared to the corresponding measured Vs profiles. All modeled V S profiles matched the site fundamental frequency within one standard deviation. Percent differences calculated between the modeled V S profiles and measured V S profiles demonstrated good agreement with less than a 10% difference at most depth ranges. However, differences of up to 30% are observed for near surface layers. The V Savg of the modeled and measured V S profiles have a strong association with a Pearson correlation coefficient of 0.95, while the V S30 of the modeled and measured Vs profiles have a Pearson coefficient of 0.49, indicating a weaker association. As an independent verification, model V S profiles are generated at 11 independent sites and compared with the measured V S profiles at these sites. Percent differences calculated between the modeled and measured V S profiles at the independent sites are below 15% at most depth ranges, with a Pearson correlation coefficient calculated for V S150 of 0.75, indicating a strong association. With reasonable layer interface models, this new approach can be used to develop an updated 3D shear wave velocity model for the Mississippi embayment.
... Defining the geometries of the rock bodies and the spatial distribution of their mechanical properties allows running physically based numerical simulations (e.g., Mazzieri et al., 2013;Smerzini & Pitilakis, 2018;Giallini et al., 2020;Primofiore et al., 2020) and, consequently, investigating their role in the upward propagation of seismic waves, highlighting the possible occurrence of focussing, reflection, refraction, and/or amplification effects. In addition, geologybased 3D velocity models can be used to predict amplitude and frequency of the arriving seismic waves (e.g., Magistrale et al., 1996;Süss et al., 2001). However, the analysis of possible focussing, reflection, refraction, and/or amplification effects due to the structural-stratigraphic setting of the near-surface geological features (especially in the last several hundred metres) is often neglected in the evaluation of the local seismic response due to the lack of an accurate 3D velocity model of the near-surface geological features. ...
In this paper we present a new methodological approach which integrates geological and geophysical data into a 3D modelling process to be mainly employed in seismic hazard assessment studies of earthquake-prone areas around the world, as well as in applications for land use and urban planning. As a case study, the reconstruction of a geology-based 3D velocity model of the uppermost hundreds of metres of the Amatrice high-seismic-hazard area is described.
The model was constructed using geological (e.g., maps, cross-sections and core-wells) and geophysical (e.g., down-hole, MASW, refraction, and seismic noise measurements) data, which were georeferenced and uploaded into 3D geological modelling software, where faults, stratigraphic boundaries, and geophysical attributes were digitised, checked, hierarchised, and modelled. The performed 3D geological model was parametiresed with Vs and Vp velocities and, finally, the environmental noise (i.e., horizontal-to-vertical spectral ratio analysis, HVSR) recorded at some seismic stations was compared with the seismic responses modelled at some nearby control points.
In the study area, the proposed geology-based 3D velocity model represents both a new potential geophysical prediction tool for areas devoid of geophysical measurements (i.e. HVSR curves) and a potential input-model for future ground-motion and seismic-wave-propagation simulations aimed at a more precise local seismic response assessment and, consequently, at the development of more realistic seismic hazard scenarios.
The model here presented constitutes a first version of the 3D geological-geophysical model for the studied area, which will be improved with new data and more advanced algorithms available in the future.
... In addition, the following authors constructed 3D velocity models of sedimentary structures: Magistrale et al., (1996Magistrale et al., ( , 2000 and Süss and Shaw (2003) in the Los Angeles region, Kagawa et al. (2004) in Osaka, Pitarka et al. (2004) in the Gulf of Putjit, Manakou (2010) in the Miguton region of northern Greece and Irene Molinari et al. (2015) in the Po Plain, Italy. ...
As computing power and ground motion studies using numerical simulation methods have continuously improved, velocity model accuracies have become bottlenecks for simulation result accuracies. The characteristics of a hydrodynamic sedimentary environment in the Yuxi Basin are used as a classification standard, and a refined velocity model that depends on the lateral heterogeneity in sediments is constructed. The results of the lateral heterogeneity model are compared with those of the lateral uniformity model, and the distributions of the displacement peak ratio, focusing effect and edge effect change due to the refined basin structure. For the 4% of the surface area that is in the region with the largest amplification factor, the difference between the two models is greater than 20%, and this difference for nearly half of the surface area is greater than 5%.
... The technique (Herrmann, 2013) allows for the evaluation of partial derivatives of the Rayleigh wave group velocities with respect to the S-wave velocity and density for each layer. The model parameters are iteratively perturbed from the initial guess with a starting model taken from the Southern California Earthquake Center Community Velocity Model Version S-4.26 (Magistrale et al., 1996), which generally converges after a few iterations. The shear velocity profile is estimated for station pairs generated by every other station. ...
Plain Language Summary
We perform ambient noise tomography in the region surrounding the surface ruptures of the 2019 Ridgecrest M7.1 and M6.4 earthquakes. The imaging method uses locally sparse tomography (LST), a machine learning‐based method that directly learns the seismic travel time information from the data obtained from a coarse regional array and several dense arrays, with 342 seismic stations in total. The Rayleigh group speed obtained from LST outperforms the conventional regularized least‐squared inversion in travel time predictions and provides more details of the small‐scale geophysical structure. The 3D shear wave velocity model resulting from our imaging reveals a low velocity zone (LVZ) up to 5 km in width and ∼5 km deep surrounding the surface expressions of the 2019 Ridgecrest earthquakes. The average velocity inside the LVZ is 40% lower than that for the surrounding material. The relatively wide LVZ obtained from our imaging is strongly correlated with the distributed faulting from geological and geodetic observations, suggesting an origin as fault damage zones. We find correlation of other imaged LVZs in the model area with faults that have not experienced recent activity. Therefore, if these LVZs represent fault damage zones, they may have persisted for hundreds, maybe thousands of years.
... The second method is based on the geological properties of the region. Combining geological data obtained from boreholes like velocity profiles, rock densities, porosity and more with data on the horizontal and vertical geological structure of the region, like basement and mantle lithosphere depths, on every point in the earths sub-surface creates a more realistic model that mimics the sub-surface better (Magistrale et al., 1996;Lee and Bradley, 2015;Béthoux et al., 2016). Nevertheless, often the data that we use in order to construct the geological data is also based on inverting seismic data to get the wanted geological unit, for example the top of the crystalline basement or the seismic velocities of areas and layers we can't sample directly like buried sediments and deep mantle rocks, therefore some degree of uncertainty still remains in this method also. ...
Here we compile numerous regional seismic profiles, gravity profiles/campaigns obtained from both oil and gas industry explorations and researches conducted in the region to construct a new geological based 3-D velocity model for the Levant region. We then use the 3-D version of Nonlinloc to locate earthquakes at the periphery of Israel.
... We observe a greater depth to 3 km/s in the southeast portion of the LA and Ventura basins, and mid-range depths for the Central Valley and in the Salton Trough. This (Figure 3c, Figure S1a) agrees with previous studies (Berg et al., 2018;Fletcher & Erdem, 2017;Fliedner et al., 2000;Fuis et al., 2017;Han et al., 2016;Livers et al., 2012;Ma & Clayton, 2016;Magistrale et al., 1996). The Antelope Valley and Indian Wells Valley are shallower, fitting previous active-source studies (Lutter et al., 2004;Tape et al., 2010). ...
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.
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.
We have simulated 0-5 Hz deterministic wave propagation for a suite of 17 models of the 2014 Mw 5.1 La Habra, CA, earthquake with the Southern California Earthquake Center Community Velocity Model Version S4.26-M01 using a finite-fault source. Strong motion data at 259 sites within a 148 km by 140 km area are used to validate our simulations. Our simulations quantify the effects of statistical distributions of small-scale crustal heterogeneities (SSHs), frequency-dependent attenuation Q(f), surface topography, and near-surface low velocity material (via a 1D approximation) on the resulting ground motion synthetics. The shear wave quality factor QS(f) is parameterized as QS, 0 and QS, 0fγ for frequencies less than and higher than 1 Hz, respectively. We find the most favorable fit to data for models using ratios of QS, 0 to shear-wave velocity VS of 0.075-1.0 and γ values less than 0.6, with the best-fitting amplitude drop-off for the higher frequencies obtained for γ values of 0.2-0.4. Models including topography and a realistic near-surface weathering layer tend to increase peak velocities at mountain peaks and ridges, with a corresponding decrease behind the peaks and ridges in the direction of wave propagation. We find a clear negative correlation between the effects on peak ground velocity amplification and duration lengthening, suggesting that topography redistributes seismic energy from the large-amplitude first arrivals to the adjacent coda waves. A weathering layer with realistic near-surface low velocities is found to enhance the amplification at mountain peaks and ridges, and may partly explain the underprediction of the effects of topography on ground motions found in models. Our models including topography tend to improve the fit to data, as compared to models with a flat free surface, while our distributions of SSHs with constraints from borehole data fail to significantly improve the fit. Accuracy of the velocity model, particularly the near-surface low velocities, as well as the source description, controls the resolution with which the anelastic attenuation can be determined. Our results demonstrate that it is feasible to use fully deterministic physics-based simulations to estimate ground motions for seismic hazard analysis up to 5 Hz. Here, the effects of, and trade-offs with, near-surface low-velocity material, topography, SSHs and Q(f) become increasingly important as frequencies increase toward 5 Hz, and should be included in the calculations. Future improvement in community velocity models, wider access to computational resources, more efficient numerical codes and guidance from this study are bound to further constrain the ground motion models, leading to more accurate seismic hazard analysis.
The Los Angeles basin is one of the Neogene basins along the California continental margin that was formed by extension related to complex wrench-fault mechanisms. For the last 60 years, the biostratigraphic framework for correlation between the oil fields of the Los Angeles basin has been based on the benthic foraminiferal zones and divisions. With the utilization of other microfossil disciplines (e.g., siliceous and calcareous planktonic microfossils), a more refined biostratigraphic scheme has been developed. Correlation of plankton biostratigraphies with the radiometric time scale has in turn allowed correlation and calibration of benthic foraminiferal zonations. Three regional cross sections were constructed based on the thickness of the Neogene sedimentary package and based on the chronostratigraphic relationships between the benthic foraminiferal zones and the other fossil groups. -from Author
A tomographic inversion of the Pn arrivals in Southern California yields new information about wave velocities and topography on the Moho discontinuity. We produce maps of Pn velocity and Pn station delays. The Pn velocities do not show the dramatic correlation with surface faults that is found for the shallower Pg arrivals (Hearn and Clayton, 1986). This implies that the lower crust and mantle are largely decoupled from the upper crust. Undoubtedly, this is due to the different responses of the brittle upper crust and the ductile lower crust to tectonic and isostatic stresses. Detachment faults must play an important role in separating the crust.
In general, velocities on the American plate are higher than on the Pacific plate, but no distinct transition is observed. The Colorado River region has extremely thin crust due to basin-and-range type extension. The Transverse Ranges have a small root as seen in the station delays and which also results in slightly lower Pn velocities there. The Peninsula Ranges also have slow Pn velocities, but they do not have late station delays. Any root to the Peninsula Ranges must be very narrow. Isostatic balance must be maintained primarily through lateral density contrasts.
To prevent inadvertent triggering of earthquakes by fluid injection, the Geophysical Laboratory of the University of Southern California is engaging in a detailed study of microearthquake activities along a portion of the Newport-Inglewood fault where the accumulating tectonic stress is interacting with massive oil pumping and water flooding. Microearthquake monitoring began in Feb. 1971 and has since operated without interruption. A detailed crustal structure was obtained of the Los Angeles Basin from well-logging data and is use satisfactorily in accurate epicenter location. Many seismic events connected with the Newport-Inglewood fault system were accurately located to a third of a kilometer and fault-plane solutions were obtained for some. Hundreds of other events were recorded throughout S. California including the complete aftershock sequence of the San Fernando earthquake of Feb. 9, 1971. Also recorded were the Amchitka nuclear shot and several Nevada Test Site events. Waterflooding and oil production data have been compiled for several oil fields in the network operating area. Correlation between microearthquake events and waterflooding is suggested.
Velocity data are compiled from measurements of nearly 1,000,000 feet of section in 500 well surveys in the United States and Canada. Average velocities for shale and sandstone sections are arranged by depth and geologic age. Deviations from the mean values are attributed to lithologic variations. The variations of velocity with depth and time are studied independently in order to develop a quantitative relationship. It is concluded that the velocity for an average shale and sand section is given by the equation V = 125.3 ( ZT ) 1 6 , where V is velocity in feet per second, Z is depth in feet, and T is age in years. Velocities in limestone show less definite evidence of increase with age and depth.
A 3D P wave velocity model of the southern California crust is constructed by combining existing 1D models, each describing a region defined by surface geology. The model is calibrated with travel times from three explosions. The technique developed by Roecker (1981) is used to invert, starting with the forward model, about 21,000-P wave arrivals from earthquakes for hypocenters and block slownesses. The variance of these P wave travel time residuals decreases 47 percent during the inversion. Many of the blocks representing the upper crust and midcrust are well sampled and well resolved. The resulting model is useful both for locating earthquakes and for comparing the geologies of the different regions.
We have used a 3D finite-difference method to simulate ground motion from elastodynamic propagating ruptures with constant slip on faults in the metropolitan area of Los Angeles, California. Simulations are carried out for hypothetical M 6.75 earthquakes on the Palos Verdes and Elysian Park faults and, for comparison, an approximation to the 17 January 1994 M 6.7 Northridge earthquake. The dominant subsurface features of this area are the deep sedimentary Los Angeles basin and the smaller and shallower San Fernando basin. Simulated ground motions are restricted to the frequency range 0.0 to 0.4 Hz. Ground-motion time histories show that, in general, sites associated with the largest particle velocities and cumulative kinetic energies are located (1) in the epicentral area, (2) above the deepest parts of the basins, and (3) near the steepest edges of the Los Angeles basin. We find maximum particle velocities for the Palos Verdes, Elysian Park, and Northridge simulations of 0.44, 0.67, and 0.58 m/sec, respectively. In each case, both the directivity of the rupture and the lower impedance of the basins significantly amplify the ground motion. Although the gross radiation pattern from these ruptures is observable, the 3D basin structure distorts the wave field and becomes a source for edge-generated waves. Signal durations at some basin sites last beyond 90 sec due to Love waves and refracted 5 waves that propagate into the sediments from the basin edges. Compared with the Palos Verdes event, the durations are generally smaller for the Elysian Park earthquake due to a smaller amount of Love waves generated at the basin edges. A simple approximation to the Northridge earthquake reproduces the overall spatial pattern of the long-period particle velocities, successfully predicts the timing of late-arriving waves, and matches the peak velocities with discrepancies generally less than a factor of 2. However, for localized areas immediately north and south of the Santa Monica Mountains, the computed ground motion underpredicts the observed horizontal peak velocities but matches the vertical ones. The pattern of simulated total cumulative kinetic energies is similar to that for the damage intensities observed near the epicenter of the Northridge event. While the Northridge earthquake caused damage in the Los Angeles area, the 3D simulations show that earthquakes with the same magnitude on the Palos Verdes or Elysian Park faults produce more severe ground shaking in the Los Angeles basin.
Areal differences in damage caused by shaking from earthquakes commonly can be related to variations in near-surface geologic materials. For the Los Angeles region, a technique is presented for differentiating Quaternary sedimentary deposits that it is hoped will be transferrable to other areas. On the basis of a two-dimensional model of sedimentation pattern, the authors present a series of regional maps grouping the surficial deposits according to distinctive ranges in shear-wave velocity; these maps provide an approximate characterization of relative shaking response. Refs.