No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Effects of topography and basins on seismic wave amplification: the Northern Chile coastal cliff and intramountainous basins
Abstract
During earthquakes, structural damage is often related to soil conditions. Following the 01 April 2014 Mw 8.1 Iquique earthquake in Northern Chile, damage to infrastructure was reported in the cities of Iquique and Alto Hospicio. In this study, we investigate the causes of site amplification in the region by numerically analyzing the effects of topography and basins on observed waveforms in the frequency range 0.1–3.5 Hz using the spectral element method. We show that topography produces changes in the amplitude of the seismic waves (amplification factors up to 2.2 in the frequency range 0.1–3.5 Hz) recorded by stations located in steep areas such as the ca. 1 km-high coastal scarp, a remarkable geomorphological feature that runs north–south, that is, parallel to the coast and the trench. The modeling also shows that secondary waves—probably related to reflections from the coastal scarp—propagate inland and offshore, augmenting the duration of the ground motion and the energy of the waveforms by up to a factor of three. Additionally, we find that, as expected, basins have a considerable effect on ground motion amplification at stations located within basins and in the surrounding areas. This can be attributed to the generation of multiple reflected waves in the basins, which increase both the amplitude and the duration of the ground motion, with an amplification factor of up to 3.9 for frequencies between 1.0 and 2.0 Hz. Comparisons between real and synthetic seismic waveforms accounting for the effects of topography and of basins show a good agreement in the frequency range between 0.1 and 0.5 Hz. However, for higher frequencies, the fit progressively deteriorates, especially for stations located in or near to areas of steep topography, basin areas, or sites with superficial soft sediments. The poor data misfit at high frequencies is most likely due to the effects of shallow, small-scale 3D velocity heterogeneity, which is not yet resolved in seismic images of our study region.
... This highlights the significance of model configuration for different ground motion characteristics. Compared to the LVL effect, the influence of the topography on ground motion is secondary, which coincides with the previous studies [23,41] . The amplification due to topography primarily exists at locations where there are variations in the terrain, notably evident at the edge of the western mountain range in Fig. 6d. ...
... This leads to a larger amplification factor almost everywhere on the outside slope of the surrounding mountain. This finding aligns with previous research, both analytical and numerical, indicating that the amplification caused by the mountain topography typically occurs on the mountain ridges or slopes [23,33,[41][42][43] . While the source is outside of the mountain region, the reflection of seismic wave amplitude is too subtle to discern. ...
Near-field ground motion amplification at sedimentary basins is widely observed and crucial to the earthquake hazard assessment. However, the effect of basin topography coupling with the low-velocity layer (LVL) in the ground motion amplification is yet to be fully understood. By constructing 3D basin models with surrounding mountain terrains and performing ground motion simulations, we compare the ground motion characteristics with different basin LVL depths and LVL velocities. The velocity contrast between LVL and bedrock controls the amplification magnitude. The maximum amplification area in the model changes from the central part to the periphery part of the basin with the velocity contrast decreasing and can be greatly influenced by the distance between the source and the basin. Amplification also spreads along the mountain edge circling the basin. Our work sheds light on the distribution of amplification within sedimentary basins surrounded by mountains, revealing that the velocity contrast between the LVL and bedrock plays a pivotal role in controlling the magnitude of amplification.
... The simulation methods incorporating geologic structures with a basin mainly vary between Finite Difference Method (FDM) (Frankel and Stephenson 2000;Graves et al. 2008;Olsen et al. 2006Olsen et al. , 2008Olsen, Pechmann, and Schuster 1995;Vidale and Helmberger 1988) and Finite Element Method (FEM) (Bakır, Özkan, and Cılız 2002;Bao 1998;Bielak, Ghattas, and Kim 2005;Ichimura, Hori, and Kuwamoto 2007). The Spectral Element Method (SEM) has also become globally popular in ground motion simulation recently for its flexibility in meshing, notable accuracy, and rapid convergence (De Martin et al. 2021;García-Pérez et al. 2021;Jayalakshmi et al. 2020Jayalakshmi et al. , 2021Komatitsch and Vilotte 1998;Komatitsch et al. 2004;Lee et al. 2008;Pilz et al. 2011;Raghukanth and Bhanu Teja 2012;Riaño, Lopez-Caballero, and Hollender 2024;Touhami et al. 2022). These studies account for the interaction between waves and the ground motion features over the deep basins. ...
... Therefore, it has been widely used in the simulation of seismic wave propagation at the global and local scales. [10][11][12][13][14] With the development of the physics-based ground motion simulations, the source-to-structure simulation, which is also called the "end-to-end simulation" has attracted increasing research attention in recent years. 10,[15][16][17][18] However, the seismic analysis for dams based on the source-to-structure simulation is still in its infancy. ...
The determination of spatially‐varying broadband ground motions is a prerequisite for the seismic analysis of dams. However, generating realistic ground motions at a specific dam site remains a challenge. Source‐to‐structure simulation, which considers source, propagation path, and site, is a promising way to reasonably synthesize site‐specific ground motions. This paper proposes a practical framework for the seismic analysis of dams based on a deterministic numerical source‐to‐structure simulation. The site‐specific broadband ground motions are generated by a physics‐based 3D numerical simulation using the spectral element method (SEM) in a coarse mesh, while the nonlinear dynamic response of the dam is simulated by the finite element method (FEM) in a fine mesh. The proposed framework bridges the gap between wavefield simulation and seismic analysis of dams. The seismic response analysis of Pacoima dam during the 1994 Northridge earthquake was conducted as a case study, making it the first demonstration of a complete source‐to‐structure simulation for realistic concrete dams using the purely numerical method. Results show that the calculated seismic response agrees with the observation, thus indicating the utility of the proposed framework for source‐to‐dam simulation.
... The increased intensity in Irkutsk can be explained, in particular, by the local site effects. Systematic local amplification within a relatively small area during strong earthquakes is largely believed to be related to the geological structure and the subsoil conditions (Celebi 1987;Bouckovalas and Kouretzis 2001;Sbarra et al. 2012;Burjánek et al. 2014;García-Pérez et al. 2021). The city of Irkutsk is located in the wide valley of the Angara River on its four terraces above the floodplain with Quaternary deposits, represented by alluvial, deluvial and eluvial complexes of more than 10 m thickness. ...
The article discusses the macroseismic data on the December 9, 2020, Mw 5.5 Kudara earthquake which occurred in the Selenga River delta (Middle Baikal, Eastern Siberia, Russia). This is the strongest seismic event in the study area over a 50-year period. The bulk of macroseismic data was obtained with internet questionnaire posted on the website of the Baikal Branch of the Geophysical Survey of the Russian Academy of Sciences. In total, 1433 responses were received from 120 places (villages, towns and cities) in Eastern Siberia, with most of them was obtained from large cities. In addition, a field survey was carried out in the near-field, which revealed slight non-structural damage to buildings in several settlements. The maximum intensity assessed VI–VII MSK-64 or VI EMS-98 was observed in the Kudara village. Taking into account the macroseismic data obtained, we have prepared a map of intensity data points. The largely observed shakings (intensity IV MSK-64/EMS-98) were noted at epicentral distances of over 500 km; intensity II MSK-64/EMS-98 was noted at a distance of up to 980 km. The Kudara earthquake allowed us to test the internet questionnaire in real conditions of strong seismic impact. The results obtained confirm the rather high efficiency of the internet questionnaire in the conditions of the Baikal region, however, some limitations associated with the responses spatial distribution are exist. A detailed study of the Kudara earthquake allowed us to gain new experience useful for the further development of macroseismology in Eastern Siberia.
The earthquakes in Pazarcık (M w 7.7) and Elbistan (M w 7.6), occurring along the East Anatolian Fault Zone (EAFZ) on February 6, 2023, caused significant damage and destruction to the built environment within the affected area. In this study, the preliminary site investigations were conducted in the Gölbaşı district, where the impacts of both earthquakes were severely felt, offering scientifically valuable information regarding the soil damage. Comprehensive liquefaction analyses were performed using the geotechnical laboratory test data on soil specimens collected from the Gölbaşı district. These analyses confirmed the liquefaction-induced ground failures observed immediately after the two earthquakes. Furthermore, microzonation data collected in the Gölbaşı district were consolidated, and seismic site response analyses were conducted. Simulations showed that local soils in the region could amplify seismic waves by a factor of two. Utilizing the calculated Peak Ground Acceleration (PGA) and amplification factors, GIS-based distribution maps of the entire area were developed. These maps serve as practical resources for practitioners and local planners, aiding in spatial settlement decisions and urban transformation planning. They contribute significantly to enhancing the understanding of earthquake hazards in the region.
The earthquakes in Pazarcık (Mw 7.7) and Elbistan (Mw 7.6), occurring along the East Anatolian Fault Zone (EAFZ) on February 6, 2023, caused significant damage and destruction to the built environment within the affected area. In this study, the preliminary site investigations were conducted in the Gölbaşı district, where the impacts of both earthquakes were severely felt, offering scientifically valuable information regarding the soil damage. Comprehensive liquefaction analyses were performed using the geotechnical laboratory test data on soil specimens collected from the Gölbaşı district. These analyses confirmed the liquefaction-induced ground failures observed immediately after the two earthquakes. Furthermore, microzonation data collected in the Gölbaşı district were consolidated, and seismic site response analyses were conducted. Simulations showed that local soils in the region could amplify seismic waves by a factor of two. Utilizing the calculated Peak Ground Acceleration (PGA) and amplification factors, GIS-based distribution maps of the entire area were developed. These maps serve as practical resources for practitioners and local planners, aiding in spatial settlement decisions and urban transformation planning. They contribute significantly to enhancing the understanding of earthquake hazards in the region.
It is estimated that 2 billion people will move to cities in the next 30 years, many of which possess high seismic risk, underscoring the importance of reliable hazard assessments. Current ground motion models for these assessments typically rely on an extensive catalogue of events to derive empirical ground motion prediction equations (GMPEs), which are often unavailable in developing countries. Considering the challenge, we choose an alternative method utilizing physics-based (PB) ground motion simulations and develop a simplified decomposition of ground motion estimation by considering regional attenuation (Δ) and local site amplification (A), thereby exploring how much of the observed variability can be explained solely by wave propagation effects. We deterministically evaluate these parameters in a virtual city named Tomorrowville, located in a 3D-layered crustal velocity model containing sedimentary basins, using randomly oriented extended sources. Using these physics-based empirical parameters (Δ and A), we evaluate the intensities, particularly peak ground acceleration (PGA), of hypothetical future earthquakes. The results suggest that the estimation of PGA using the deterministic Δ−A decomposition exhibits a robust spatial correlation with the PGA obtained from simulations within Tomorrowville. This method exposes an order-of-magnitude spatial variability in PGA within Tomorrowville, primarily associated with the near-surface geology and largely independent of the seismic source. In conclusion, advances in PB simulations and improved crustal structure determination offer the potential to overcome the limitations of earthquake data availability to some extent, enabling prompt evaluation of ground motion intensities.
Topography and soil different layer have significant effect on seismic response of structures. Most studies have focused on the amplification caused by a single topography; for example hill. This research, using Finite Difference Method numerical analyses, comprehensively studies the effects of adjacent hills, heterogeneity in the materials of the hills, and also hills different geometry shapes. In this paper, the amplification of homogeneous and non-homogenous hills with semicircular, triangular and trapezoid geometry shapes are investigated. The obtained results show that the geometry shape of adjacent hill has a significant effect on the base hill's amplification. Increasing the non-homogeneous adjacent hill's height, increases the base hill's amplification. The results demonstrate that increasing the height of the side hill up to 3 times the height of the studied base hill, increases the base hill's amplification up to 40%.
This paper evaluates the amplification factors at different locations of a tuff cliff subjected to seismic loading. We present a 1D and 2D modelling of a cliff located in the Sorrento Peninsula, a major Quaternary morpho-structural unit of the western flank of Southern Apennines. The geometry of our model is represented by a tuff cliff of 30 m height, with sand deposits at its base, covered by a 7-m-thick volcanoclastic layer. The geotechnical and geophysical parameters used for the analysis have been estimated via field measurements and laboratory tests. We selected seven scaled seismic events as input motions for the analysis, and the linear equivalent assumption was used in both 1D and 2D modelling. The general trend showed that in the cliff edge area, the amplification factor reaches a maximum value of 10 with a period of 0.11 s, while in other areas, its value decreases down to 2.90 seaside and 4.0 landside. The comparison of the 1D and 2D modelling highlights the importance of the topographic effects connected with a cliff. The average amplification factors (AF) for the 1D and the 2D models, have been computed within different time ranges, consistent with the ones adopted in the Italian microzonation studies (0.1–0.5 s, 0.4–0.8 s, 0.7–1.1 s).
Monitoring ground surface deformation is an essential component of volcano monitoring, along with other techniques, such as seismology, geochemistry and geology. This chapter presents the different sources at the origin of the deformation observed in a volcanic context, followed by a review of the various techniques used for measuring such deformation. It next reviews the conditions for deformation detection with these different techniques. Volcanoes are places of more or less developed hydrothermal activity, depending on sites. Pre‐eruptive deformations have spatial extensions and characteristic time scales which are different from a volcano to another and from an eruption to another. Detectability depends on the adequacy between the observed phenomena and the measurement technique. Finally, the chapter presents the most widespread deformation models for understanding measurements, as well as their contribution to monitoring.
Günümüzde doğal kaynaklı sismik yer araştırma yöntemleri kolay ölçü alımı ve veri-analizi nedeniyle, mikro bölgeleme çalışmalarında tercih edilen bir yöntemdir. Ankara Kentinin batısında yapılaşma için yoğun olarak tercih edilen Yenimahalle, Etimesgut ve Sincan ilçelerini kapsayan bölgede Ankara Çayı’nın yatağını oluşturan Kuvaterner yaşlı tortulun kalınlığını, makaslama dalgası hızının derinliğe bağlı değişimini ve rezonans frekansını belirlemek üzere 18 farklı noktada Genişletilmiş Uzamsal Öz İlişki yöntemi ile veri toplanmıştır. Elde edilen sonuçlar; Ankara Çayı tortullarının oluşturduğu Kuvaterner birimin sanılandan daha kalın ve değişken geometrili olduğuna, uluslararası yapı kodlarında ve yönetmeliklerde yer alan ve jeoteknik mühendisliğinde yaygın kullanılan Vs30 değerinin bu tür bölgeler için yeterli olamayacağına işaret etmektedir.
Observations of shear wave anisotropy are key for understanding the mineralogical structure and flow in the mantle. Several researchers have reported the presence of seismic anisotropy in the lowermost 150-250 km of the mantle (i.e., D
''
layer), based on differences in the arrival times of vertically (SV) and horizontally (SH) polarized shear waves. By computing waveforms at a period > 6 s for a wide range of 1-D and 3-D Earth structures, we illustrate that a time shift (i.e., apparent splitting) between SV and SH may appear in purely isotropic simulations. This may be misinterpreted as shear wave anisotropy. For near-surface earthquakes, apparent shear wave splitting can result from the interference of S with the surface reflection sS. For deep earthquakes, apparent splitting can be due to the S wave triplication in D
''
, reflections off discontinuities in the upper mantle, and 3-D heterogeneity. The wave effects due to anomalous isotropic structure may not be easily distinguished from purely anisotropic effects if the analysis does not involve full waveform simulations.
We conducted a regional seismic intensity survey in northern Chile following the 1 April 2014 M 8.2 Iquique earthquake to document the level of damage caused by the earthquake and tsunami. We observed very limited damage at the 204 sites surveyed. The modified Mercalli intensities (MMIs) reached a maximum value of VI adjacent to the earthquake rupture plane. The contours of our MMI map are similar in shape to the contour maps of peak ground acceleration (PGA) and peak ground velocity (PGV). A comparison of our seismic intensities with the recorded PGA and PGV values reveals that our MMI values are one to two units lower than predicted by groundmotion intensity conversion equations (GMICEs). We propose that this is due to the well-engineered buildings and moderate levels of high-frequency (0.5-5 Hz) ground shaking experienced during this earthquake. Whether existing GMICEs consistently overpredict seismic intensities for large subduction- zone earthquakes deserves further study.
We present new cosmogenic (¹⁰Be) exposure ages obtained on Pleistocene marine abrasion shore terraces of Northern Chile between 24°S and 32°S in order to evaluate the temporal and spatial variability of uplift rates along the coastal forearc. Both the dispersion of cosmogenic concentrations in samples from the same terrace and data obtained in vertical profiles show that onshore erosion rates, following emergence of paleoshorelines, approached 1 m/Myr. Therefore, minimum ages calculated without considering onshore erosion may be largely underestimated for Middle Pleistocene terraces. The elevation of the last interglacial (MIS-5) paleoshoreline is generally between 25 and 45 m amsl, suggesting that the entire coast of the study area has been uplifting during the Upper Pleistocene at rates approaching 0.3 mm/yr. Available ages for Middle Pleistocene terraces suggest similar uplift rates, except in the Altos de Talinay area where uplift may have been accelerated by the activity of the Puerto Aldea Fault. The maximum elevation of Pleistocene paleoshorelines is generally close to 250 m and there is no higher older Neogene marine sediment, which implies that uplift accelerated during the Pleistocene following a period of coastal stability or subsidence. We observe that the coastal morphology largely depends on the latitudinal climatic variability. North of 26.75°S, the coast is characterized by the presence of a high scarp associated with small and poorly preserved paleoshorelines at its foot. The existence of the coastal scarp in the northern part of the study area is permitted by the hyper-arid climate of the Atacama Desert. This particular morphology may explain why paleoshorelines evidencing coastal uplift are poorly preserved between 26.75°S and 24°S despite Upper Pleistocene uplift rates being comparable with those prevailing in the southern part of the study area.
We study the foreshocks and aftershocks of the 1 April 2014 Iquique earthquake of Mw 8.1. Most of these events were recorded by a large digital seismic network that included the Northern Chile permanent network and up to 26 temporary broadband digital stations. We relocated and computed moment tensors for 151 events of magnitude Mw ≥ 4.5. Most of the foreshocks and aftershocks of the Iquique earthquake are distributed to the southwest of the rupture zone. These events are located in a band of about 50 km from the trench, an area where few earthquakes occur elsewhere in Chile. Another important group of aftershocks is located above the plate interface, similar to those observed during the foreshock sequence. The depths of these events were constrained by regional moment tensor (RMT) solutions obtained using the records of the dense broad band network. The majority of the foreshocks and aftershocks were associated to the interplate contact, with dip and strike angles in good agreement with the characteristics of horst and graben structures (>2000 m offset) typical of the oceanic Nazca Plate at the trench and in the outer rise region. We propose that the spatial distribution of foreshocks and aftershocks, and its seismological characteristics were strongly controlled by the rheological and tectonics conditions of the extreme erosive margin of Northern Chile.
In this study, we present comparisons between the local soil conditions and the observed damage for the 1 April 2014 Iquique earthquake. Four cases in which site effects may have played a predominant role are analyzed: (1) the ZOFRI area, with damage in numerous structures; (2) the port of Iquique, in which one pier suffered large displacements; (3) the Dunas I building complex, where soil-structure interaction may have caused important structural damage; and (4) the city of Alto Hospicio, disturbed by the effects of saline soils. Geophysical characterization of the soils was in agreement with the observed damage in the first three cases, while in Alto Hospicio the earthquake damage cannot be directly related to geophysical characterization.
Strong ground motions recorded at 34 sites in the San Francisco Bay region from the Loma Prieta earthquake show marked variations in characteristics dependent on crustal structure and local geological conditions. Theoretical amplitude distributions and synthetic seismograms calculated for 10-layer models suggest that "bedrock' motions were elevated due in part to the wide-angle reflection of S energy from the base of a relatively thin (25 km) continental crust in the region. Characteristics of geologic and geotechnical units as currently mapped for the San Francisco Bay region show that average ratios of peak horizontal acceleration, velocity and displacement increase with decreasing mean shear-wave velocity. Ratios of peak acceleration for sites on "soil' are statistically larger than those for sites on "hard rock'. -from Authors
This paper focuses on a geophysical-based analysis of the soils in Arica and Iquique, both main cities in northern Chile. The large seismogenic coupling and the 136-year seismic gap predispose the north of the country to the highly likely seismic activity that could affect this area. This seismotectonic scenario highlights the importance of the assessment of earthquake hazards in urban domains. In this context, seismic microzoning emerges as a tool to identify areas that are more or less susceptible to ground motion amplification. In both cities, 148 sites were chosen to perform geophysical surveys based on surface wave methods, namely the spatial autocorrelation, frequency-wave number and H/V spectral ratio techniques. By inverting the measurements, local shear wave velocity profiles and predominant frequencies were derived. Additionally, seven boreholes were drilled to further complement the gathered data. With the available results, the cities were subdivided into different zones with similar properties in terms of the average shear wave velocity on the upper 30 m (V30s) and the predominant site’s frequency F0; these were parameters that were found consistent with the geology of each city. The collected information suggests zones that are more prone to ground motion amplification, namely: the northern side of Arica, the south-east area of Iquique, the northern limit of Iquique and the artificial landfill of the port of Iquique. The microzoning was compared to the ground motion records of the April 1, 2014, Iquique earthquake and showed to be consistent with the expected motion amplification in Iquique.
Measurements of ground motion generated by nuclear explosions in Nevada were made for 37 locations near San Francisco Bay, Calif. The results were compared with the San Francisco 1906 earthquake intensities and the strong-motion recordings of the San Francisco earthquake of Mar 22 1957. The recordings show marked amplitude variations which are related consistently to the geologic setting of the recording site. Consistent correlations of the results from the nuclear recordings with the 1906 earthquake intensities and the spectral amplification curves for the 1957 earthquake suggest that areas of high amplification determined from small ground motions may also be areas of high intensity in future earthquakes.
After some introductory remarks on the key aspects of seismic site effects evaluation and on new data of interest from recent earthquakes, the first part of the paper discusses and compares the performance of numerical 2D and 1D models, as well as of simple exact soulutions, in problems of local soil response and topographic amplification. The second part is devoted to the illustration of a very promising computational tool, i. e. the Fourier method for time-domain analyses of seismic wave propagation in heterogeneous elastic media.
A frequency-domain parametric study using generalized consistent transmitting boundaries has been performed to evaluate the significance of topo-graphic effects on the seismic response of steep slopes. The results show that the peak amplification of motion at the crest of a slope occurs at a normalized frequency 1t/2 = 0.2, where H is the slope height and 2 is the wavelength of the motion. The importance of the natural site frequency is illustrated by the analysis of a stepped layer over a half-space. It was found that the natural frequency of the region behind the crest can dominate the response, relative to the topographic effect, for the con-ditions studied. Moreover, the effect of topography can be handled separately from the amplification due to the natural frequency of the deposit behind the crest of the slope. This concept of separating the amplification caused by topography from that caused by the natural frequency is advantageous to the development of a simplified method to estimate topographic effects.
The seismic gap theory identifies regions of elevated hazard based on a lack of recent seismicity in comparison with other portions of a fault. It has successfully explained past earthquakes (see, for example, ref. 2) and is useful for qualitatively describing where large earthquakes might occur. A large earthquake had been expected in the subduction zone adjacent to northern Chile, which had not ruptured in a megathrust earthquake since a M ∼8.8 event in 1877. On 1 April 2014 a M 8.2 earthquake occurred within this seismic gap. Here we present an assessment of the seismotectonics of the March-April 2014 Iquique sequence, including analyses of earthquake relocations, moment tensors, finite fault models, moment deficit calculations and cumulative Coulomb stress transfer. This ensemble of information allows us to place the sequence within the context of regional seismicity and to identify areas of remaining and/or elevated hazard. Our results constrain the size and spatial extent of rupture, and indicate that this was not the earthquake that had been anticipated. Significant sections of the northern Chile subduction zone have not ruptured in almost 150 years, so it is likely that future megathrust earthquakes will occur to the south and potentially to the north of the 2014 Iquique sequence.
The subduction zone in northern Chile is a well-identified seismic gap that last ruptured in 1877. The moment magnitude (Mw) 8.1 Iquique earthquake of 1 April 2014 broke a highly coupled portion of this gap. To understand the seismicity preceding
this event, we studied the location and mechanisms of the foreshocks and computed Global Positioning System (GPS) time series
at stations located on shore. Seismicity off the coast of Iquique started to increase in January 2014. After 16 March, several
Mw > 6 events occurred near the low-coupled zone. These events migrated northward for ~50 kilometers until the 1 April earthquake
occurred. On 16 March, on-shore continuous GPS stations detected a westward motion that we model as a slow slip event situated
in the same area where the mainshock occurred.
This study examines the role of topographic effects on the prediction of earthquake ground motion. Ground motion prediction equations (GMPEs) are mathematical models that estimate the shaking level induced by an earthquake as a function of several parameters, such as magnitude, source-to-site distance, style of faulting and ground type. However, little importance is given to the effects of topography, which, as known, may play a significant role on the level, duration and frequency content of ground motion. Ridges and crests are often lost inside the large number of sites considered in the definition of a GMPE. Hence, it is presumable that current GMPEs are unable to accurately predict the shaking level at the top of a relief. The present work, which follows the article of Massa et al. about topographic effects, aims at overcoming this limitation by amending an existing GMPE with an additional term to account for the effects of surface topography at a specific site. First, experimental ground motion values and ground motions predicted by the attenuation model of Bindi et al. for five case studies are compared and contrasted in order to quantify their discrepancy and to identify anomalous behaviours of the sites investigated. Secondly, for the site of Narni (Central Italy), amplification factors derived from experimental measurements and numerical analyses are compared and contrasted, pointing out their impact on probabilistic seismic hazard analysis and design norms. In particular, with reference to the Italian building code, our results have highlighted the inadequacy of the national provisions concerning the definition of the seismic load at top of ridges and crests, evidencing a significant underestimation of ground motion around the site resonance frequency.
On April 1, 2014, a great (Mw 8.1) interplate thrust earthquake ruptured in the northern portion of the 1877 earthquake seismic gap in northern Chile. The sequence commenced on March 16, 2014 with a magnitude 6.7 thrust event, followed by thrust-faulting aftershocks that migrated northward ~40 km over two weeks to near the mainshock hypocenter. Guided by short-period teleseismic P wave back-projections and inversion of deep-water tsunami wave recordings, a finite-fault inversion of teleseismic P and SH waves using a geometry consistent with long-period seismic waves resolves a spatially compact large-slip (~2-6.7 m) zone located ~30 km down-dip and ~30 km along-strike south of the hypocenter; down-dip of the foreshock sequence. The mainshock seismic moment is 1.7 × 1021 Nm with a fault dip of 18°, radiated seismic energy of 4.5-8.4 × 1016 J, and static stress drop of ~2.5 MPa. Most of the 1877 gap remains unbroken and hazardous.
The paper presents an extensive review of topographic effects in seismology taking into account the knowledge of 40 yr of scientific literature. An overview of topographic effects based on experimental observations and numerical modelling is presented with the aim of highlighting meaning and causes of these phenomena as well as possible correlations between site response (fundamental frequency, amplification level) and geometrical (width and shape ratio of a relief) parameters. After a thorough summary of topographic effects, the paper focuses on five Italian sites whose seismic response is potentially affected by local morphology, as already evidenced by previous studies. In this study, seismic data recorded at these sites are analysed computing directional spectral ratios both in terms of horizontal to vertical spectral ratios (HVSRs) and, wherever possible, in terms of standard spectral ratios (SSRs). The analysis lead to the conclusion that wavefield tends to be polarized along a direction perpendicular to the main axis of a topographic irregularity, direction along which ground motion amplification is maximum. The final section of the article compares and contrasts different spectral ratio techniques in order to examine their effectiveness and reliability in detecting topographic effects. The examples discussed in the paper show that site responses based on HVSRs rather than SSR measurements could lead to misinterpretation of ground response results, both as concerns the definition of the site fundamental frequency and amplification level.
Results and findings of this work will be used as starting point to discuss the influence of topographic effects on ground motion prediction equations and regulations for design. These topics will be discussed in the companion article.
We propose a model for the effect of sedimentary basin depth on long- period response spectra. The model is based on the analysis of 3-D numerical simulations (finite element and finite difference) of long-period 2-10 s ground motions for a suite of sixty scenario earthquakes (Mw 6.3 to Mw 7.1) within the Los Angeles basin region. We find depth to the 1.5 km/s S-wave velocity isosurface to be a suitable predictor variable, and also present alternative versions of the model based on depths to the 1.0 and 2.5 km/s isosurfaces. The resulting mean basin-depth effect is period dependent, and both smoother (as a function of period and depth) and higher in amplitude than predictions from local 1-D models. The main requirement for the use of the results in construction of attenuation relationships is determining the extent to which the basin effect, as defined and quantified in this study, is already accounted for implicitly in existing attenuation relationships, through (1) departures of the average "rock" site from our idealized reference model, and (2) correlation of basin depth with other predictor variables (such as Vs30). DOI: 10.1193/1.2857545
A new 30-arc second resolution global topography/bathymetry grid (SRTM30_PLUS) has been developed from a wide variety of data sources. Land and ice topography comes from the SRTM30 and ICESat topography, respectively. Ocean bathymetry is based on a new satellite-gravity model where the gravity-to-topography ratio is calibrated using 298 million edited soundings. The main contribution of this study is the compilation and editing of the raw soundings, which come from NOAA, individual scientists, SIO, NGA, JAMSTEC, IFREMER, GEBCO, and NAVOCEANO. The gridded bathymetry is available for ftp download in the same format as the 33 tiles of SRTM30 topography. There are 33 matching tiles of source identification number to convey the provenance of every grid cell. The raw sounding data, converted to a simple common format, are also available for ftp download.
Local geological conditions, including both near-surface sedimentary layers and topographic features, are known to significantly influence ground motions caused by earthquakes. Microzonation maps use local geological conditions to characterize seismic hazard, but commonly incorporate the effect of only sedimentary layers. Microzonation does not take into account local topography, because significant topographic amplification is assumed to be rare. Here we show that, although the extent of structural damage in the 2010 Haiti earthquake was primarily due to poor construction, topographic amplification contributed significantly to damage in the district of Petionville, south of central Port-au-Prince. A large number of substantial, relatively well-built structures situated along a foothill ridge in this district sustained serious damage or collapse. Using recordings of aftershocks, we calculate the ground motion response at two seismic stations along the topographic ridge and at two stations in the adjacent valley. Ground motions on the ridge are amplified relative to both sites in the valley and a hard-rock reference site, and thus cannot be explained by sediment-induced amplification. Instead, the amplitude and predominant frequencies of ground motion indicate the amplification of seismic waves by a narrow, steep ridge. We suggest that microzonation maps can potentially be significantly improved by incorporation of topographic effects.
This work analyzes the morphological features of the western margin of the northern Chile Coastal Cordillera, character-ized by the presence of a major Coastal Cliff The main purpose of this research is to better understand the tectonic and climatic controls on the topography. The methodology used consists basically of a detailed study of the topography with quantitative geomorphic indexes such as incisión grade, hypsometric curves and integráis, thalweg profiles and sinuosity index. These are applied to digital elevation models and satellite images. The main results show that the topography of the western margin of the Coastal Cordillera, exhibits an increasing intensity of the erosión with latitude. This increment is non uniform. The tectonic processes that produced coastal subsidence/uplift and fault activity, and the erosión of the paleotopography of Coastal Cordillera due to processes related with the construction of the Coastal Cliff and increasing rainfall, modify the morphology of the coastal border of northern Chile. The existence and conservation of the Coastal Cliff results from the existance of non-equilibrium between the uplift and erosión rates. Both rates have one-two order magnitude difference, controlled by the extreme hyperarid climate of the Atacama Desert. These conditions prevailed at least for the last 2 m.yrs. The variations in the intensity of the erosión from north to south are interpreted as the result of the southward increase in precipitación by one order of magnitude.
The southern Andes between 33° and 45°S latitude are characterized by a series of complex basins that spanned the contemporaneous active continental margin, which itself was characterized by volcanic activity. The basins are filled with thick (up to 3000 m) accumulations of interbedded sedimentary and volcanic strata of late Oligocene-early Miocene age. We interpret that these basins developed during a phase of moderate extension within the plate margin system, triggered by an increased rate of convergence of the Farallon (Nazca) and South American plates between 28 and 26 Ma. This history is inconsistent with models of convergence that link high rates of convergence of a continental margin and an oceanic plate to strong compressional coupling. Although extensional basins of this age are only well-described in the southern Andes, the convergence history and volcanic chronology are similar farther north in the central Andes (18°-33°S), leading to the speculation that extension may have characterized the late Oligocene-early Miocene interval regionally. We hypothesize that this extension was a necessary condition to subsequent building of the modern Andes Mountains.
A high angle, west-vergent thrust system (WTS) located along the western border of the Altiplano in Northern Chile caused the westward translation of the metamorphic pre-Cambrian basement and Mesozoic rocks over late Tertiary deposits. This WTS, which was essential to the build-up of the western Altiplano, developed principally between 15 and 4.8 Ma. The chronological sequence of faults indicate a back-thrusting. The WTS forms an overstep thrust sequence with vertical throws increasing from West to East and exposes in this direction increasingly older units over younger ones.Taking the paleolake deposits of the Chucal Formation (25-19 Ma) and the ignimbrites of Oxaya Formation (19 Ma) as two regional reference levels, it is possible to calculate a 4,000 ± 200 m uplift associated to the WTS and a 392 ± 20 m/Ma uplift rate.The WTS on the west side of de Altiplano and the east vergent thrust and fold belt on the east border, indicate that this plateau is essentially a compressive asymmetric structure, formed by two thrust “belts” with opposing vergencies.
Amplification of seismic waves due to surface topography and subsurface soils has often been observed to cause intensive damage in past earthquakes. Due to its complexity, topographic amplification has not yet been considered in most seismic design codes. In this study, we simulate ground-motion amplification based on 3D Spectral Element Method, using Hong Kong Island as a local testbed site. The analyses revealed that topography amplification of ground motions is frequency-dependent. If the site is made of homogenous rock, the amplification factor is best correlated with the curvature smoothed over a characteristic length equal to half of the wavelength in rock. Amplification of high frequency wave is correlated with small-scale features, and amplification of long-period waves is correlated with large-scale features in horizontal dimension. The maximum topography amplification generally ranges from 1.6 to 2.0 on the protruded areas. When a low-velocity subsurface soil layer is considered, the topographic amplification pattern is significantly influenced by the thickness of the soil layer, as wavelength in soil is relatively short. The characteristic length reduces as soil thickness increases, and the amplification pattern becomes closely correlated to smaller-scale topographic features as well as slope angles. Results also show that the effect of material damping can be decoupled from the topographic effects and modeled using a theoretical attenuation factor. The study proposed parametric models to predict the topographic amplification considering subsurface soils, material damping and input wave frequencies, which gives accurate results with a standard deviation of residuals within 0.1-0.15.
Cambridge Core - Solid Earth Geophysics - Geophysics for the Mineral Exploration Geoscientist - by Michael Dentith
The comprehensive study of intramountain basins located in the Coastal Cordillera of the continental emergent Andean forearc in Northern Chile, enables the better understanding of the nature and evolution of the upper crustal deformation during the Neogene and Quaternary. A case study is the extensive extensional half-graben Alto Hospicio basin. The basin is cut by the Coastal Cliff, which exposes the deformed Neogene basin fill. Also exposed are several structural systems, some of which affect Quaternary surfaces. The results of the integrated geophysical surveys (Electromagnetic Transient and Gravity) allow us to fully constrain the geometry of the Alto Hospicio basin and the lithological relationship between the subsurface geological units. The structural geology analysis assesses the deformation regimes affecting the faults present in the basin and surrounding area. Altogether evidence a change in the deformation regime from an EW extensional deformation during the Miocene-Pliocene to a NS compression in the Quaternary as is presented in this study. We suggest this deformation change is related to a small change in the convergence vector orientation during the Pliocene.
Chile is frequently affected by large and potentially tsunamigenic and damaging earthquakes as a result of rapid convergence of the Nazca plate beneath the South America plate. TenM8 or larger earthquakes have occurred along the Chilean coast in the past century, the largest of which was the 1960 M 9.5 Valdivia earthquake. After the 2010 M 8.8 Maule earthquake, Chile began installing a modern real-time network of digital broadband/strong-motion and Global Positioning System (GPS) stations to better prepare for future and expected large damaging earthquakes. The network was designed to provide fast and accurate estimates of earthquake source parameters of potentially devastating earthquakes for emergency response applications, and was also capable of comprehensive characterization of Chilean seismicity necessary for long-term hazard assessment and mitigation activities. Beginning in 2013, the National Seismological Center (CSN) of the University of Chile was mandated by the Chilean government to implement a network of 65 permanent real-time broadband and strongmotion stations and 130 Global Navigation Satellite Systems (GNSS) monuments and receivers. These integrated sensor systems were designed to provide accurate automatic earthquake locations and magnitudes necessary for tsunami warning and impact assessment. In near-real-time applications, the GPS stations become critical for determination of fault finiteness of M ∼ 7 or larger earthquakes. Operations of these systems also provide new insights into long-term deformation and associated spatiotemporal variations in seismicity, which are necessary in long-term earthquake hazards assessment and mitigation. In addition to the real-time system described above, 297 strongmotion offline instruments complement the network for engineering purposes. Broadband data in real time are publicly available through the Incorporated Research Institutions for Seismology Data Management Center (IRIS-DMC) under networks C and C1. Strong-motion data for recorded accelerations larger than 2%g are available through the CSN webpage.
This study analyzed site effects including PGA and empirical transfer functions at 15 selected surface–downhole stations by using ground motion recordings of earthquakes with ML > 4 during 2012 and 2013 in Taiwan. In addition, we used all strong motion recordings of four large earthquakes (ML > 6) to generate intensity distribution maps and assess the differences in magnitude at the surface and downhole. The site amplification factors of the PGA were calculated using the ratio between the surface and downhole recordings. The mean PGA amplification factors ranged from 2 to an exaggerated value of 20 at different stations. In addition, the power law relationships between the PGAs at the surface and downhole were evaluated to understand how amplification varies as PGA increases. Strong ground motions with and without site effects throughout Taiwan could be observed by comparing intensity distribution maps generated using the surface and downhole accelerations from four large earthquakes with magnitudes > 6. Empirical transfer functions derived using the single-station and two-station methods at the same stations showed comparable dominant frequencies and amplification factors; however, the empirical transfer function derived using the two-station method showed clearer resonance peaks, not only at fundamental frequencies but also at higher mode resonance frequencies. The HHSR and the HVSR were highly similar, particularly at medium frequencies. This finding indicates that the HVSR can be used instead of the HHSR when only the surface recording is available. Moreover, the local magnitudes calculated using surface recordings were higher than those calculated using downhole recordings. The differences are attributed to the amplification caused by the sedimentary layers and resultant in 0.36, 0.46 and 0.49 on average for events with ML of > 6, 5–6, and 4–5. Furthermore, HHSRs at 5–10 Hz and 1.1–1.7 Hz were strongly correlated with PGA amplifications and ML differences, respectively.
Despite the ever increasing adoption of wave motion simulations for assessing seismic hazard, most assessment/simulations are still based on a flat surface earth model. The purpose of this paper is to quantify the effect of topographic irregularities on the ground motion and local site response by means of parametric investigations in the frequency-domain of typical two-dimensional features. To this end, we deploy best-practice tools for simulating seismic events in arbitrarily heterogeneous formations; these include: a forward wave simulator based on a hybrid formulation encompassing perfectly-matched-layers (PMLs); unstructured spectral elements for spatial discretization; and the Domain-Reduction-Method that permits placement of the seismic source within the computational domain, thus allowing consideration of realistic seismic scenarios. Of particular interest to this development is the study of the effects that various idealized topographic features have on the surface motion when compared against the response that is based on a flat-surface assumption. We report the results of parametric studies for various parameters, which show motion amplification that depends, as expected, on the relation between the topographic feature's characteristics and the dominant wavelength. More interestingly, we also report motion de-amplification patterns.
See my page for the full-text: https://sites.google.com/a/utexas.edu/babakpoursartip/curriculum-vitae
In this work, we study seismic wave amplification in alluvial basins having 3D standard geometries through the Fast Multipole Boundary Element Method in the frequency domain. We investigate how much 3D amplification differs from the 1D (horizontal layering) case. Considering incident fields of plane harmonic waves, we examine the relationships between the amplification level and the most relevant physical parameters of the problem (impedance contrast, 3D aspect ratio, vertical and oblique incidence of plane waves). The FMBEM results show that the most important parameters for wave amplification are the impedance contrast and the so-called equivalent shape ratio. Using these two parameters, we derive simple rules to compute the fundamental frequency for various 3D basin shapes and the corresponding 3D/1D amplification factor for 5% damping. Effects on amplification due to 3D basin asymmetry are also studied and incorporated in the derived rules.
This paper presents a set of deterministic 3-D ground motion simulations for the greater metropolitan area of Medellín in the Aburrá Valley, an earthquake-prone region of the Colombian Andes that exhibits moderate-to-strong topographic irregularities.We created the velocity model of the Aburrá Valley region (version 1) using the geological structures as a basis for determining the shear wave velocity. The irregular surficial topography is considered by means of a fictitious domain strategy. The simulations cover a 50 × 50 × 25 km3 volume, and four Mw = 5 rupture scenarios along a segment of the Romeral fault, a significant source of seismic activity in Colombia. In order to examine the sensitivity of ground motion to the irregular topography and the 3-D effects of the valley, each earthquake scenario was simulated with three different models: (i) realistic 3-D velocity structure plus realistic topography, (ii) realistic 3-D velocity structure without topography, and (iii) homogeneous half-space with realistic topography. Our results show how surface topography affects the ground response. In particular, our findings highlight the importance of the combined interaction between source-effects, source-directivity, focusing, soft-soil conditions, and 3-D topography.We provide quantitative evidence of this interaction and show that topographic amplification factors can be as high as 500 per cent at some locations. In other areas within the valley, the topographic effects result in relative reductions, but these lie in the 0-150 per cent range.
Three-dimensional seismic responses of two-dimensional topographies are studied by means of the indirect boundary element method (IBEM). The IBEM yields, in the presented form, very accurate results and has the advantage of low computation cost. In IBEM, diffracted waves are constructed in terms of single-layer boundary sources. The appropriate Green's functions used are those of a harmonic point force moving along the axis of the topography in a full space. Obtained results are compared against those published by other authors. -from Authors
The effect of topography on surface motion is investigated in the cases of incident SH, P and SV waves. Several types of topography ranging from a ridge to a valley are used. Different incidence angles are considered for a wavelength interval extending from 2h to 20h, where h is the vertical dimension of the anomaly. The computation is made by using a single frequency method developed by Aki and Larner and by synthesizing the results in order to get a solution in the time domain. A method of correction taking into account the residual stress is added. The surface displacement appears to be very much influenced by surface irregularities. In the case of a ridge, a zone of amplification takes place near the top, whereas, for a depression, a zone of attenuation occurs near the bottom. An application to the Pacoima Dam accelerograph site suggests that the high accelerations recorded there during the San Fernando earthquake could have been amplified between 30 and 50 per cent by the uneven topography.
Site-response experiments were performed 5 months after the MS = 7.8 central Chile earthquake of 3 March 1985 to identify amplification due to topography and geology.
Topographical amplification at Canal Beagle, a subdivision of Viña del Mar, was hypothesized immediately after the main event, when extensive damage was observed on the ridges of Canal Beagle. Using frequency-dependent spectral ratios of aftershock data obtained from a temporarily established dense array, it is shown that there is substantial amplification of motions at the ridges of Canal Beagle. The data set constitutes the first such set depicting topographical amplification at a heavily populated region and correlates well with the damage distribution observed during the main event.
Dense arrays established in Viña del Mar also yielded extensive data which are quantified to show that, in the range of frequencies of engineering interest, there was substantial amplification at different sites of different geological formations. To substantiate this, spectral ratios developed from the strong-motion records of the main event are used to show the extensive degree of amplification at an alluvial site as compared to a rock site. Similarly, spectral ratios developed from aftershocks recorded at comparable stations qualitatively confirm that the frequency ranges for which the amplification of motions occur are quite similar to those from strong-motion records. In case of weak motions, the denser arrays established temporarily as described herein can be used to identify the frequency ranges for which amplification occurs, to quantify the degree of frequency-dependent amplification and used in microzonation of closely spaced localities.
Simultaneous consideration of source, path, and site effects on ground motion during the Michoacan earthquake of 1985 allows us to draw coherent conclusions regarding the roles played for the disaster in Mexico City by the rupture process, the mode of propagation of the waves between the epicentral zone and Mexico City, and the local amplification. We examine two alternative source models associated with different crustal models to explain the characteristics of the vertical displacements recorded in Mexico City. Our preferred model attributes the cause of the enhanced 3 sec motion to the irregularity in the rupture propagation in addition to the effect of the local conditions in Mexico City. -from Authors
We built a 3D seismic model of the Po Plain and neighboring regions of northern Italy, covering altogether an area about 600 km by 300 km with an approximately 1 km spaced grid. We started by collecting an extensive and diverse set of geological and geophysical data, including seismic reflection and refraction profiles, borehole logs, and available geological information. Major geological boundaries and discontinuities have thus been identified and mapped into the model. We used kriging to interpolate the geographically sparse information into continuous surfaces delimiting geological bodies with laterally varying thickness. Seismic-wave properties have been assigned to each unit using a rule-based system and, VP, VS, and ρ derived from other studies. Sedimentary strata, although with varying levels of compaction and hence material properties, may locally reach a thickness of 15 km and give rise to significant effects in seismic-wave propagation.We have used our new model to compute the seismic response for two recent earthquakes, to test its performance. Results show that the 3D model reproduces the large amplitude and the long duration of shaking seen in the observed waveforms recorded on sediments, whereas paths outside the basin may be well fit by more homogeneous (1D) hard rock structure. We conclude that the new model is suited for simulation of wave propagation, mostly for T > 3 s, and may serve well as a constraint for earthquake location and further improvements via body- or surface-wave inversion. © 2015, Bulletin of the Seismological Society of America. All Rights Resereved.
In this paper, the influence of topography on ground-motion intensity parameter, response spectrum, peak ground acceleration(PGA) and the ratio of response spectrum were studied based on the measured data from large scale shaking table test and observation stations in XiShan park for Wenchuan earthquake. The results show that: in the EW direction, the height do not affect PGD, SM_A, VSI and HI, while V_RMS decreases slightly at the middle of the slope, the other intensity parameters increase with the increase of height. In the EW direction, the height has no influence on PGD, V_RMS, SMA, SED, A_RMS, VSI and HI, while other parameters increase as the height increase. In the UD direction, the height has no influence on SMV, PGD, V_RMS, SMV, ASI, VSI and HI, while A_RMS decreases at the middle of the slope, and the other intensity parameters increase as the height increase. Ignoring the local topographic effect, the amplitude of response spectrum increases with the increase of height at the part of short period(T <= 1s), the part of long period(T>1s) is not effected by height. The ground motion will be amplified by local canyon topography, and the influence of local topography is larger than height. The research carried out in paper will deepen the understanding of topographic effect.
The Salar de Atacama Basin holds important information regarding the tectonic activity, sedimentary environments and paleoclimate variations in northern Chile during Cretaceous times. About 4000 m of high-resolution stratigraphic columns of the Tonel, Purilactis and Barros Arana Formations reveal braided fluvial and alluvial facies, typical of arid to semi-arid environments, interrupted by scarce intervals with evaporitic, eolian and lacustrine sedimentation, displaying an overall coarsening-upward trend. Clast-count and point-count data evidence the progressive erosion from Mesozoic volcanic rocks to Paleozoic basement granitoids and deposits located around the Cordillera de Domeyko area, which is indicative of an unroofing process. The paleocurrent data show that the source area was located to the west. The U/Pb detrital zircon geochronological data give maximum depositional ages of 149 Ma for the base of the Tonel Formation (Agua Salada Member), and 107 Ma for its middle member (La Escalera Member); 79 Ma for the lower Purilactis Formation (Limón Verde Member), and 73 Ma for the Barros Arana Formation. The sources of these zircons were located mainly to the west, and comprised from the Coastal Cordillera to the Precordillera. The ages and pulses record the tectonic activity during the Peruvian Phase, which can be split into two large events; an early phase, around 107 Ma, showing uplift of the Coastal Cordillera area, and a late phase around 79 Ma indicating an eastward jump of the deformation front to the Cordillera de Domeyko area. The lack of internal deformation and the thicknesses measured suggest that deposition of the units occurred in the foredeep zone of an eastward-verging basin. This sedimentation would have ended with the K-T Phase, recognized in most of northern Chile.This article is protected by copyright. All rights reserved.
Le grand escarpement cotier du desert chilien qui est haut en moyenne de 700 m et se suit sur plus de 800 km constitue un trait geomorphologique majeur a l’echelle du Globe. Au Nord d’Iquique, il recule encore sous l’action des vagues mais, ailleurs, il prend l’aspect d’une falaise morte au pied de laquelle s’etend une plate-forme d’abrasion. L’abrupt derive probablement de failles verticales a fort rejet, apparues a la fin du Miocene, qui ont recule par erosion marine lors d’une importante transgression du Pliocene moyen a superieur. Il a peu evolue pendant le Quaternaire. Les pulsations transgressees de l’Ocean l’ont a peine retouche et la persistance de l’aridite, malgre l’occurrence de periodes a averses, lui a confere une remarquable immunite. Une tendance a la subsidence s’est manifestee.
An earthquake off Chile in 2014 occurred in a region where a great seismic event was expected. Two studies reveal that months of foreshocks and slow slip on the associated plate-boundary fault preceded the event. See Letters p.295 & p.299
The recent growth of seismic monitoring networks allows for systematic studies of local seismic effects at sites with pronounced topography. We applied a terrain classification method to identify such sites within Swiss and Japanese networks and compiled a data set of high-quality earthquake recordings. As a number of recent studies have found local effects to be directional at sites with strong topographic features, polarization analysis of particle motion was performed and azimuthally dependent resonant frequencies were estimated. The same procedure was also applied for available ambient vibration recordings. Moreover, average residuals with respect to ground motion prediction models for a reference bedrock were calculated to estimate the average amplification or deamplification for each station. On one hand, observed amplifications are found to be tightly linked with ground motion directionality as estimated by polarization analysis for both earthquake and ambient vibration recordings. On the other hand, we found no clear relation between local topographic features and observed amplification, so the local subsurface properties (i.e. shear wave velocity structure) seem to play the key role and not the geometry itself.
The Fourier amplitude ratio of the 19 September (M//s equals 8. 1) to the 21 September 1985 (M//s equals 7. 6), Michoacan earthquakes obtained from data in and near Mexico City increases from about 3. 5 at 0. 5 Hz to about 10 at 0. 2 Hz. This increase is neither seen in the coastal data nor in the teleseismic broadband GDSN P-wave spectra. We tentatively attribute the increase at frequencies of less than 0. 5 Hz to a special path and/or a depth of energy release effect for the 19 September earthquake. The damage to Mexico City is mostly related to 0. 5 Hz energy. The analysis of strong motion data recorded in Mexico City shows that the ground motions in the lake bed zone are amplified by 8 to 50 times with respect to a hill zone site in Ciudad Universitaria. The frequency at which the maximum relative amplification occurs varies from site to site and lies between 0. 2 and 0. 7 Hz.
Site response in Japan is characterized using thousands of surface and borehole recordings from events of moment magnitude M > 5.5 collected by the KiK-net network, including the 2011 M9.0 Tohoku earthquake. Site amplification is defined by the ratio of motions at the surface to those at depth (within the borehole), corrected for the depth effect due to destructive interference using a technique based on cross-spectral ratios between surface and down-hole motions. Site effects were particularly strong at high frequencies, despite the expectation that high-frequency response may be damped by nonlinear effects. In part, the large amplitudes at high frequencies are due to the prevalence of shallow soil conditions in Japan. We searched for typical symptoms for soil nonlinearity, such as a decrease in the predominant frequency and/or amplification, using spectral ratios of weak to strong ground motions. Localized nonlinearity occurred at some recording sites, but was not pervasive. We developed a general empirical model to express site amplification for the KiK-net sites as a function of common site variables, such as the average shear-wave velocity in the uppermost 30 m (VS30) and the horizontal-to-vertical (H/V) spectral ratio. We use the model to estimate site-corrected ground-motions for the Tohoku mainshock for a reference site condition; these motions are in reasonable agreement with the predictions of some of the published ground motion prediction equations for subduction zones.
It is possible to use the waveform data not only to derive the source mechanism of an earthquake but also to establish the hypocentral coordinates of the 'best point source' (the centroid of the stress glut density) at a given frequency. Thus two classical problems of seismology are combined into a single procedure. Given an estimate of the origin time, epicentral coordinates and depth, an initial moment tensor is derived using one of the variations of the method described in detail by Gilbert and Dziewonski (1975). This set of parameters represents the starting values for an iterative procedure in which perturbations to the elements of the moment tensor are found simultaneously with changes in the hypocentral parameters. In general, the method is stable, and convergence rapid. Although the approach is a general one, we present it here in the context of the analysis of long-period body wave data recorded by the instruments of the SRO and ASRO digital network. -Authors
Finite difference calculations for SH-motion, as well as field observations, show that topography can have significant effects on seismic waves when the incident wavelengths are comparable to the size of the topographic features and the topographic slopes are relatively steep. These effects are frequency-dependent and can range from amplification to deamplifieation at a given site. For the models considered in this paper, amplifications of 75 per cent as compared to motions produced in regions of no topography were found. A qualitative discussion of the strong-motion accelerogram recorded at Pacoima Dam from the San Fernando earthquake of February 9, 1971, suggests that the recorded accelerations could, at any period, be expected to differ by 25 to 50 per cent from those that would have been recorded if no topographic features were present. The complex terrain precludes a prediction of the actual magnitude or sign of the difference. The present results are probably most significant from the viewpoint of engineering seismology, for the conditions required for a significant topographic effect are most likely to occur in the period range of engineering interest.
It is well established that sedimentary basins can significantly amplify earthquake ground motion. However, the amplification at any given site can vary with earthquake location. To account for basin response in probabilistic seismic hazard analysis, therefore, we need to know the average amplification and intrinsic variability (standard deviation) at each site, given all earthquakes of concern in the region. Due to a dearth of empirical ground-motion observations, theoretical simulations constitute our best hope of addressing this issue. Here, 0-0.5 Hz finite-difference, finite-fault simulations are used to estimate the three-dimensional (3D) response of the Los Angeles basin to nine different earthquake scenarios. Amplification is quantified as the peak velocity obtained from the 3D simulation divided by that predicted using a regional one-dimensional (1D) crustal model. Average amplification factors are up to a factor of 4? with the values from individual scenarios typically differing by as much as a factor of 2.5. The average amplification correlates with basin depth, with values near unity at sites above sediments with thickness less than 2 km, and up to factors near 6 above the deepest ( approximate to 9 km) and steepest-dipping parts of the basin. There is also some indication that amplification factors are greater for events located farther from the basin edge. If the 3D amplification factors are divided by the 1D vertical SH-wave amplification below each site, they are lowered by up to a factor of 1.7. The duration of shaking in the 3D model is found to be longer, by up to more than 60 seconds, relative to the 1D basin response. The simulation of the 1994 Northridge earthquake reproduces recorded 0-0.5 Hz particle velocities relatively well, in particular at near-source stations. The synthetic and observed peak velocities agree within a factor of two and the log standard deviation of the residuals is 0.36. This is a reduction of 54% and 51% compared to the values obtained for the regional 1D model and a ID model defined by the velocity and density profile below a site in the middle of the basin (DOW), respectively. This result suggests that long-period ground-motion estimation can be improved considerably by including the 3D basin structure. However, there are uncertainties concerning accuracy of the basin model, model resolution, the omission of material with shear velocities lower than 1 km/s, and the fact that only nine scenarios have been considered. Therefore, the amplification factors reported here should be used with caution until they can be further tested against observations. However, the results do serve as a guide to what should be expected, particularly with respect to increased amplification factors at sites located above the deeper parts of the basin.
We used recordings of the M 6.8 Nisqually earthquake and its M L 3.4 aftershock to study site response and basin effects for 35 locations in Seattle, Washington. We determined site amplification from Fourier spectral ratios of the recorded horizontal ground motions, referenced to a soft-rock site. Soft-soil sites (generally National Earthquake Hazard Reduction Program [NEHRP] class E) on artificial fill and young alluvium have the largest 1-Hz amplifications (factors of 3–7) for both the mainshock and aftershock. These amplifications are correlated with areas of higher damage from the mainshock to major buildings and liquefaction. There are several indications of nonlinear response at the soft-soil sites for the mainshock ground motions, despite relatively modest peak accelerations in the S waves of 15%–22% g . First, the mainshock spectral ratios do not show amplification at 2–8 Hz as do the aftershock spectral ratios. Spectral peaks at frequencies below 2 Hz generally occur at lower frequencies for the mainshock spectral ratios than for the aftershock ratios. At one soft-soil site, there is a clear shift of the resonant frequency to a lower frequency for the mainshock compared with the aftershock. The frequency of this resonance increases in the coda of the mainshock record, indicating that the site response during the weaker motions of the coda is more linear than that of the initial S wave. Three of the soft-soil sites display cusped, one-sided mainshock accelerograms after the S wave. These soft-soil sites also show amplification at 10–20 Hz in the S wave, relative to the rock site, that is not observed for the aftershock. The cusped waveforms and 10–20-Hz amplification are symptomatic of nonlinear response at the soft-soil sites. These sites had nearby liquefaction. The largest amplifications for 0.5 Hz occur at soft-soil sites on the southern portion of the Seattle Basin. Stiff-soil sites (NEHRP classes D and C) on Pleistocence-age glacial deposits display similar spectral amplification for the mainshock and aftershock, indicating approximately linear response. The stiff-soil sites generally have moderate amplification (factors of 1.1–2.4) at 0.5 and 1 Hz. Amplifications at 1 and 5 Hz for all sites generally increase with decreasing shear-wave velocity measured in the top 30 m ( V s 30). However, larger amplifications at 0.5 and 1 Hz for sites with similar V s 30 values are observed for sites in the Seattle Basin, illustrating the amplification from the deeper (>30 m) sediments and the contribution from basin surface waves. Record sections for the mainshock and aftershock show that basin surface waves produce the peak velocities for many of the sites in the Seattle Basin and often dominate the amplitude at 1 Hz and lower frequencies.
We use the spectral-element method to simulate ground motion gener- ated by two recent and well-recorded small earthquakes in the Los Angeles basin. Simulations are performed using a new sedimentary basin model that is constrained by hundreds of petroleum-industry well logs and more than 20,000 km of seismic reflection profiles. The numerical simulations account for 3D variations of seismic- wave speeds and density, topography and bathymetry, and attenuation. Simulations for the 9 September 2001 Mw 4.2 Hollywood earthquake and the 3 September 2002 Mw 4.2 Yorba Linda earthquake demonstrate that the combination of a detailed sed- imentary basin model and an accurate numerical technique facilitates the simulation of ground motion at periods of 2 sec and longer inside the basin model and 6 sec and longer in the regional model. Peak ground displacement, velocity, and acceler- ation maps illustrate that significant amplification occurs in the basin.