Ground Motion Amplification Research Articles

Topographic effects on seismic responses of steep sloped superficially weathered rock: the case of ‘Rocher Noir’ at Boumerdes city in Algeria

Rocks often showed an alteration in their first meters of depth, by the presence of weathered portion through variable thickness. Concentration of building damage near the edge of hard or dense cliff has been observed during a number of earthquakes. The numerical analysis of topographic effects of rocky steep slopes knowingly neglects the stratigraphic effect. In previous studies, it was evoked the fracturing of the upper parts of the site of ‘Rocher Noir’ in Boumerdes City, and the obtained HVSR curves were flat or presenting a peak over 10 Hz, explained by the fact that the metamorphic outcropping is overlaid by sediments. In this paper, interaction between the effects of stratigraphic and topographic amplification of this weathered rocky site is investigated through 1D and 2D numerical models. The goal of these investigations is to provide a better understanding of the potential hazard that is represented by amplified ground motion due to the coupling effect (1D amplification and topographic amplification) and to quantify the weathering effects on topographic amplification. Seismic responses (amplification functions and response spectra) are calculated at representative points located in the upper surface of the rocky model, assumed either hard or weathered. The obtained results show that the presence of a weathered portion on the surface can modify the ground response in the sense of higher amplification, larger bandwidth (modifying of the mode vibration), multiple response spectra peaks and inducing a considerable vertical acceleration component. Ambient noise measurements were conducted, the HVSR results agree well with those obtained by modelling.

Modelling and assessment of earthquake ground response in areas characterised by a thick buried low-velocity layer

It is known that local geology can alter the ground motion characteristics at a particular site. Whereas some sites are characterised by outcropping low-velocity layers, others might have buried ones of considerable thickness which might still be a cause of ground motion amplification. The Maltese islands (Central Mediterranean) provide a good case study for profiles with a buried low-velocity layer. Parts of the islands are characterised by Upper Coralline Limestone covering a soft Blue Clay layer of considerable thickness (up to 75 m). This gives rise to a velocity inversion and makes the travel-time average shear-wave velocity (VS) in the upper 30 m not suitable for seismic microzonation purposes. This study presents a comprehensive site response analysis for such areas. Previously obtained shear-wave velocity (VS) profiles were used as an input to the equivalent linear programme SHAKE2000. Maps of amplification factors and 5% damped elastic response spectra confirm that the clay, even when buried under a hard outcropping layer, can still produce significant amplifications at frequencies which are of engineering interest when considering the recent urbanisation patterns. Moreover, the suitability of the VS30 and the travel-time average shear-wave velocity till bedrock parameters as proxies for site response was investigated, with the results highlighting the inadequacy of these parameters for sites characterised by buried low-velocity layers and the necessity for site-specific response studies.

Large shaking table tests of pile-supported structures in different ground conditions

This study investigates the dynamic response of structures and the effect of soil-structure interaction for flexible structures supported by pile foundations located in non-liquefiable and liquefiable sites as well as a rigid foundation. A series of large-scale shaking table experiments were performed, and accelerations and dynamic displacements of model superstructures were recorded during the shaking events. It was observed that the structure in non-liquefiable soil deposits had the highest accelerations due to the amplification of ground motion within the soil. On the other hand, the accelerations of the structure in liquefied site was small due to the increased dissipation of seismic energy. The structures founded on soil experienced residual (permanent) lateral displacements, with the permanent lateral displacement and tilting of the structure in the liquefied site being much larger. The structure on rigid foundation (i.e. foundation on rock or fixed base) experienced high accelerations and oscillatory displacement as the seismic energy could not be dissipated. Therefore, structures in liquefiable soil are prone to overturning damage due to failure of their foundations. Furthermore, the base shear force of the structure in the non-liquefied site was significantly larger than that in the liquefied site and fixed-base, indicating it would be more susceptible to shear failure. On the other hand, the fixed-base structure experienced relatively large acceleration and dynamic displacement responses, which indicates increased seismic demands on its structural members.

Evaluation of dynamic response of connected and non-connected piled raft systems using shaking table tests

Regarding the inadequacy of quantitative measurements of geotechnical aspects for assessing the behavior of the piled raft with different head connections during seismic events, A series of shaking table tests modeling, including un-piled-raft foundation, connected and non-connected piled-raft systems with 4 piles and dry sand were conducted. The efficiency of pile head separation and the material of the interposed layer between pile and cap for reducing the bending moment along with the piles length under different excitation scenarios were investigated by imposing base shaking levels with PGAs that ranged between 0.1 to 0.4 g, simulating weak to strong ground motions. Moreover, a comprehensive study is conducted on seismic ground motion amplification patterns induced by piled-raft systems in the soil body and caps as well as lateral raft movements.Based on the shaking table results, the non-connectivity between piles and caps decreased the maximum bending moment of piles by 20–60%, based on input motion amplitudes and cushion layer material. The dependency of both mechanism and amplitude of amplification on the level of base shaking and pile-raft connection details are also revealed.

Horizontal-to-Vertical Spectral Ratio (HVSR) IRIS Station Toolbox

AbstractThe horizontal-to-vertical spectral ratio (HVSR) for seismic ambient noise is a popular method that can be used to estimate the predominant frequency at a given site. In this article, we introduce the Incorporated Research Institutions for Seismology (IRIS) Data Management Center’s (DMC’s) openly available HVSR station toolbox. These tools offer a variety of ways to compute the spectral ratio by providing different averaging routines. The options range from the simple average of spectral ratios to the ratio of spectral averages. Computations take advantage of the available power spectral density estimates of ambient noise for the seismic stations, and they can be used to estimate the predominant frequency of the many three-component seismic stations available from the IRIS DMC. Furthermore, to facilitate the identification of the peaks in HVSR profiles for the assessment of the predominant frequency of station sites, the toolbox can also process the results of HVSR analysis to detect and rank HVSR peaks. To highlight the toolbox capabilities, three different examples of possible use of this toolbox for routine site-effect analysis are discussed: (1) site effects related to thawing in Arctic regions, (2) ground-motion amplification in urban area, and (3) estimation of station VS30.

Physical stratigraphy and geotechnical properties controlling the local seismic response in explosive volcanic settings: the Stracciacappa maar (central Italy)

Nowadays, policies addressed to prevention and mitigation of seismic risk need a consolidated methodology finalised to the assessment of local seismic response in explosive volcanic settings. The quantitative reconstruction of the subsoil model provides a key instrument to understand how the geometry and the internal architecture of outcropping and buried geological units have influence on the propagation of seismic waves. On this regard, we present a multidisciplinary approach in the test area of the Stracciacappa maar (Sabatini Volcanic District, central Italy), with the aim to reconstruct its physical stratigraphy and to discuss how subsoil heterogeneities control the 1D and 2D local seismic response in such a volcanic setting. We first introduce a new multidisciplinary dataset, including geological (fieldwork and log from a 45-m-thick continuous coring borehole), geophysical (electrical resistivity tomographies, single station noise measurements, and 2D passive seismic arrays), and geotechnical (simple shear tests performed on undisturbed samples) approaches. Then, we reconstruct the subsoil model for the Stracciacappa maar in terms of vertical setting and distribution of its mechanical lithotypes, which we investigate for 1D and 2D finite element site response analyses through the application of two different seismic scenarios: a volcanic event and a tectonic event. The numerical modelling documents a significant ground motion amplification (in the 1–1.5 Hz range) revealed for both seismic scenarios, with a maximum within the centre of the maar. The ground motion amplification is related to both 1D and 2D phenomena including lithological heterogeneity within the upper part of the maar section and interaction of direct S-waves with Rayleigh waves generated at edges of the most superficial lithotypes. Finally, we use these insights to associate the expected distribution of ground motion amplification with the physical stratigraphy of an explosive volcanic setting, with insights for seismic microzonation studies and local seismic response assessment in populated environments.

Seismic Response of a Mountain Ridge Prone to Landsliding

ABSTRACTDuring an earthquake, site effects can play an important role in triggering landslides. To document the seismic response of steep hillslopes, we deployed broadband seismometers across a mountain ridge in Taiwan, in an area with a high earthquake-induced landslide hazard. The ridge has a simple, representative shape, and landslides have previously occurred there. Our seismometer array has recorded continuously during more than 1 yr, with both ambient-noise and regional moderate earthquakes as sources. Processing horizontal and vertical signal components, we show that the ridge has a complex response, which we attribute to the combined effects of the subsurface geology and the topographic geometry. Amplification and directionality of ground motion are observed both high and low on the ridge, giving rise to localized, elevated, earthquake-induced landslide hazard. Our database contains earthquakes with mostly similar locations, making it difficult to determine the effect of earthquake back azimuth on the ridge response. A part of the ridge response, possibly due to topographic effects, seems to be explained by a model derived from a frequency scale curvature proxy at low frequency. If correct, this would be a promising first step toward improving local ground-motion estimation in mountain areas. However, the definition of appropriate scaling parameters of site effects based on geophysical measurements, for use in regional and global landslide hazard equations applicable to mountain areas with substantial regolith thickness, remains a significant challenge.

Non-stability of the noise HVSR at sites near or on topographic heights

SUMMARYThe horizontal to vertical spectral ratio (HVSR) of seismic noise is often used to investigate site effects, and it is usually assumed to be a stable feature of the site considered. Here we show that such an assumption is not always justified, and may lead to incorrect conclusions. The HVSR analysis was performed on ambient seismic noise recordings lasting from weeks to months at many sites in Calabria, Italy. Results show a variety of site effects, from the resonance of a shallow sedimentary layer to the polarized amplification of horizontal ground motion associated with topographic effects. We describe the results of seven sites whose HVSR is characterized by dual content: one that is persistent, and another appearing only occasionally. Two sites very near the coast of the Tyrrhenian sea and five sites in the Calabrian Arc mountains show the most remarkable results. The shape of the HVSR changes significantly at these sites when the amplitude of background noise increases in a broad frequency band during periods of bad weather. The occasional contribution to the HVSR consists of one or more peaks, depending on the site, that appear only when the amplitude of ambient noise is higher than usual. The seven sites where we observe the HVSR variability are all located in complex geological environments, on mountains, ridges or foothills. A variation of the HVSR correlated with the day–night cycle is also observed at some of these sites.

Evaluation of amplitude-scaled and spectrum-matched motions to estimate ground-motion amplification factors

Ground-motion amplification factors (GMAFs) are applied to characterize amplification of ground motions propagating from bedrock to ground surface, which are required to construct design response spectrum on ground surface in nuclear industry. To estimate GMAFs by ground response analysis (GRA), appropriate ground motions are required as input motions. This study investigates capabilities of amplitude-scaled and spectrum-matched ground motions to estimate mean GMAFs. It reports that amplitude-scaled motions lead to unbiased estimates of the mean GMAFs. Further investigation shows that, a suite of 5 spectrum-matched motions does not introduce significant bias of the mean GMAFs estimation if soil parameter variabilities are not considered in GRA; however, the bias will remarkably increase when soil parameter variabilities are considered. In addition, this study compares capabilities of spectrum-matched motions generated by different methods to estimate the mean GMAFs. Based on research findings in this study, issues concerning ground motions for GMAFs estimation are discussed.

A global hybridVS30map with a topographic slope–based default and regional map insets

Time-averaged shear wave velocity over the upper 30 m of the earth’s surface ( VS30) is a key parameter for estimating ground motion amplification as both a predictive and a diagnostic tool for earthquake hazards. The first-order approximation of VS30is commonly obtained through a topographic slope–based or terrain proxy due to the widely available nature of digital elevation models. However, better-constrained VS30maps have been developed in many regions. Such maps preferentially employ various combinations of VS30measurements, higher-resolution elevation models, lithologic, geologic, geomorphic, and other proxies and often utilize refined interpolation schemes. We develop a new hybrid global VS30map database that defaults to the global slope-based VS30map, but smoothly inserts regional VS30maps where available. In addition, we present comparisons of the default slope-based proxy maps against the new hybrid version in terms of VS30and amplification ratio maps, and uncertainties in assigned VS30values.

An inter-disciplinary and multi-scale approach to assess the spatial variability of ground motion for seismic microzonation: the case study of Cavezzo municipality in Northern Italy

Seismic microzonation represents a basic tool for prevention activity planning and land management. An extensive and detailed microzonation study was performed with reference to the territory of the Municipality of Cavezzo, damaged during the seismic sequence hitting Emilia-Romagna Region, Northern Italy, in 2012. In this paper, we discuss the work carried out to characterize the spatial variability of ground motion amplification due to local soil conditions in the municipality area.An inter- and multi-disciplinary approach is presented, involving geotechnical engineers, geophysicists, geologists and seismologists from different institutions, to thoroughly characterize the territory using complementary techniques with different level of resolution and confidence. A considerable amount of geomorphological, geological, hydrogeological, seismological, geotechnical and geophysical investigations was collected and processed for the purpose. A GIS-based (Geographic Information System) platform was initially setup to manage the gathered data, which now includes the results of about 1000 geotechnical and geophysical tests. Such an extended dataset was then used as a primary constraint for the creation of a comprehensive pseudo-3D geotechnical and seismo-stratigraphic model of the territory, consisting of a dense grid of one-dimensional vertical profiles to depict the variability of the soil properties over the area. The model was finally used as input for linear-equivalent ground response analysis. For the calculation of the amplification factors, special emphasis was given to the treatment and propagation of the uncertainties of the model parameters, whose different realizations have been accounted through a logic tree approach.

Parametric investigation on the effect of near-surface soil properties on the topographic amplification of ground motions

This study investigates the effect of near-surface soil properties on the topographic amplification of ground motions, based on the seismic response analysis of a 2D slope model with horizontally stratified soil medium. Finite difference simulations are performed to calculate the nonlinear elastoplastic response of the slope model, using strong motion records as excitation. Then a series of parametric analyses are carried out to quantitatively evaluate the influence of several factors, such as the shear-wave velocities of the superficial layers, thickness of the surface layer and soil material of the surface layer. The numerical results indicate that the decrease in the average shear-wave velocity of superficial layers would enhance the topographic effect in the area close to slope crest. In order to further assess the influence of different superficial soil profiles, seismic response analysis is conducted for a group of 2D slope models, which are built based on borehole data. The regression analysis of simulation results further confirms that the intensity of topographic effect gradually increases with decreasing VS30. Finally, the results of numerical analyses are compared with current seismic code provisions.

Influence of the variability of soil profile properties on weak and strong seismic response

Characterizing the potential effect of local site conditions on the amplification of ground motions is a critical aspect of seismic hazard and risk assessment. The aim of this study is to investigate the reliability and the limit of using the average shear wave velocity in the upper 30m of the soil profile vs,30, as single proxy, to characterize seismic site effects for weak and strong events.To this regard, a dataset of 300 one-dimensional soil profiles with a given vs,30 are generated through a Monte Carlo approach. Their seismic responses are computed for a set of 40 real accelerograms, with different seismic features. The vertical propagation from the bottom of the generated columns is modeled using a finite element spatial discretization, accounting for both linear and nonlinear soil behavior.The site dominant frequency f0 and the shear wave velocity gradient in the profile B30 are proposed as proxies to characterize seismic site effects and the variability of the response spectra for the numerical signals, at the free surface of the set of columns, is discussed. Correlations between site-specific amplification factors deduced using the numerical response spectra and the proposed site proxies are analyzed for different sub-ranges of periods. The obtained amplification factors are then compared to those proposed by different international and national design codes. The results, obtained under assumption of linear and nonlinear behavior of soil, emphasize the need to introduce complementary site parameters proxies, in addition to vs,30, to characterize the expected site effects in design response spectra.

Seismic emergency system evaluation: The role of seismic hazard and local effects

The emergency management system is a complex network consisting of structural components, such as buildings, infrastructure and emergency areas, and non-structural elements, such as procedures, emergency workers, information management, etc. This document describes the evaluation of only its structural components. The system was adopted at the territorial scale and is defined by a multi-municipality area extension. This paper presents a methodology for the probabilistic evaluation of operating the emergency structural system in the case of a seismic event occurrence and, in detail, it highlights the methodology for calculating the seismic hazard including seismic local effects (amplification and permanent co-seismic effects). Systemic seismic risk and associated impact were considered in a rigorous and unified way. Moreover, the importance of the interconnection and interdependence between elements at risk belonging to the same system and between different networks at urban or territorial scales is well known. The aim of the proposed methodology is to evaluate the operational losses of an emergency system considering the connections between the components of the system, contributing, in an original way, to the estimation of the seismic effects. The fundamental characteristics of this calculation are:•Estimating ground motion amplification at the territorial scale using a Vs30 from a probabilistic map obtained with the seismic microzonation national database;•Estimating shaking scenarios in terms of PGA and PGV depending on the Vs30;•Co-seismic permanent failure and deformations (landslide and liquefaction) evaluated with regression logistic methods.Finally, a case study is presented showing the application of the methodology on a real emergency system in an area of southern Italy.

Combining borehole log-stratigraphies and ambient vibration data to build a 3D Model of the Lower Var Valley, Nice (France)

In seismic hazard studies, site effects assessment is based on an accurate knowledge of mechanical properties and geometry of superficial geological formations. The Lower Var Valley (Nice, southeastern France) is a 2 km wide fluvial sedimentary basin where site effects have been observed on earthquake recordings. In addition, this area is a significant economically developing zone including the second largest airport of France. Therefore, it is highly important to define a 3D sub-surface model of the basin to understand the ground motion amplifications. Based on the compilation of several years of geotechnical and geophysical studies, the Quaternary alluvial deposits of the Lower Var Valley were investigated using 331 borehole log-stratigraphies and 439 ambient vibration measurements. These data were combined in a commercial software that models and allows to visualize geoscientific data and geological models in 3D. The borehole log-stratigraphies helped to define a representative lithostratigraphic soil profile and the geometry of the different sedimentary layers. Only few boreholes are reaching the engineering bedrock (Pliocene conglomerates characterized by a S-wave velocity larger than 800 m.s−1). From the ambient vibration data, the fundamental resonance frequency identified from H/V curves indicates the bedrock depth using the well-known f0 = Vs/4H relationship. These H/V curves were interpreted using geological knowledge of the studied zone.Our results show that the bedrock depth varies, in the center of the valley, between 50 m in the northern part of the studied zone and more than 100 m on the river estuary. First, the average S-wave velocity in the alluvial deposits is estimated according to combined analysis of the borehole log-stratigraphies and H/V calculations on ambient vibration measurements. The spatial variability of this velocity has been translated into a velocity zonation of the valley. Second, this zonation allows us to extrapolate the Quaternary-Pliocene limit in the whole valley. The 3D model of the Lower Var Valley will be useful in future numerical simulations of seismic ground motions in order to evaluate and mitigate seismic risk in the area.

Ground motion amplification models for Japan using machine learning techniques

Earthquake-induced ground motions can be altered by various factors that are associated with the characteristics of earthquake sources, paths, and sites. Conventionally, regression approaches have been used to develop empirical prediction models for ground motion amplifications. We developed models for ground motion amplifications based on three machine learning techniques (i.e., random forest, gradient boosting, and artificial neural network) using the database of the records at the KiK-net stations in Japan. The proposed machine learning based models outperforms the regression based model. The random forest based model provides the best estimation of amplification factors. Average shear wave velocity and the depth of the borehole are the two factors that influence the amplification model the most. Maps of the amplification factors for all KiK-net stations under moderate and large earthquake scenarios are provided. The three machine learning technique based models are also provided for the forward prediction of other earthquake scenarios.

Constraining Properties of Sedimentary Strata Using Receiver Functions: An Example from the Atlantic Coastal Plain of the Southeastern United States

ABSTRACTThickness and seismic velocities of sedimentary sequences strongly affect their response during earthquakes, which can prolong and amplify ground motions. We characterize shallow structure of Atlantic Coastal Plain (ACP) sediments using a passive-seismic approach based on high-frequency P-to-S receiver functions. We map the site-specific fundamental frequency for 64 USArray Transportable Array stations and confirm that the method yields results similar to those from traditional spectral ratio techniques, with fundamental frequencies between 0.1 and 1 Hz. In addition, using sediment S-wave reverberations and P-to-S phase arrival times measured directly from the receiver functions, we invert for average S- and P-wave velocity profiles of the ACP sedimentary strata. We find that VS increases with depth following a power-law relationship (VS∝z) whereas the increase of VP with depth is more difficult to constrain using converted wave methods; therefore, we choose to use the Brocher (2005) relationship to obtain VP through a VP/VS relationship. Finally, we use the variation of measured S-reverberation amplitudes with depth to validate these velocity profiles. These results have implications for seismic shaking across the ACP, which covers large portions of the eastern United States.

Assessment of seismic ground motion amplification and liquefaction at a volcanic area characterized by residual soils

A seismic hazard was assessed related to site effects at Abbadia San Salvatore, central Italy, on the Mt. Amiata slopes, an ancient volcanic area characterized by residual soils (thick layers of loose to dense sands originated from weathering of the trachydacitic lavas). The seismic ground amplification and soil liquefaction related to these layers were recognized as the major seismic hazards for the area. Geological, geophysical, and geotechnical surveys were carried out on the volcanic rocks. The Horizontal-to-Vertical Spectral Ratio (HVSR) analysis of 252 noise measurements and 29 shear-wave velocity models of the subsoil allowed a seismic microzonation of the studied area, distinguished by thick weathered volcanic sands and shear-wave impedance contrast with respect to the seismic bedrock (volcanic bedrock). The differentiation of classified zones allowed recognition of areas characterized by residual (almost undisturbed) soils from those with soils probably affected by flowing water. The analysis of hazards revealed that peak acceleration by seismic amplification of ground motion exceeded the value set by the national rules (0.175g) in a restricted area of the zone characterized by the most perturbed soils (Zone D); the potential occurrence of soil liquefaction was also greater in this zone. Finally, the study showed potential high hazards due to site effects of the volcanic mountainous area characterized by residual soils as opposed to an alluvial plain formed by volcanic debris where these effects have generally been more recognized.

Computation of Ground Motion Amplification Scenario in NCT Delhi for Earthquake Engineering Purposes and Seismic Microzonation

This paper presents a scenario for the spatial variation of the fundamental frequency of the sediment deposits above the basement and the corresponding amplification as well as the average spectral amplification in different frequency bandwidths for the National Capital Territory Delhi (the capital of India). The exposed central quartzite ridge and the Yamuna River channel are responsible for very large spatial variations of the fundamental frequency in the eastern part of the National Capital Territory Delhi. At 20% of the considered sites, a good match is obtained between the fundamental frequency computed numerically using available S-wave velocities to a certain depth and their extrapolation and that obtained experimentally. The computed fundamental and dominant frequencies reveal that both medium-rise (5–10 storey) and high-rise (> 10 storey) buildings in the western part and medium-rise buildings lying in the localities east of or very near to the Yamuna River may suffer heavy to very heavy damage due to the occurrence of the double resonance phenomenon. Furthermore, 1–2-storey buildings lying on the weathered exposed quartzite rock may also suffer heavy damage during local earthquakes because of the occurrence of double resonance. The possible reasons behind the lack of earthquake damage to the Qutab Minar, the tallest brick masonry minaret in the world, over the last 800 years may be the nonoccurrence of double resonance and almost no amplification in the low frequency range. There are two localities in the western part of the National Capital Territory Delhi, namely Kanganheri-Chhawla and Buradi, wherein all sorts of buildings are highly vulnerable to earthquake damage. For the closed Chhatarpur Basin and a semiclosed basin to its northeast, formed due to exposed quartzite rock, three-dimensional (3D) simulations are required to predict the characteristics of basin-generated surface waves and their focusing effects in the Chhatarpur Basin. The average spectral amplification map developed for the 0–10 Hz bandwidth depicts a range of 2.25–4.82 in the National Capital Territory Delhi and may be directly used to transfer the estimated seismic hazard at basement to the free surface.

Effects of Random Variations of Partially Saturated Soils Properties on Site Amplification of Seismic Ground Motions

Based on the principle of random vibration and the Tajimi-Kanai spectrum model, a seismic model with a given magnitude and epicenter distance is used to simulate the seismic motion. For the random variation of soil properties such as shear modulus, mass density, damping ratio and saturation of partially saturated soils, it is assumed that the input seismic wave is SH wave. The random dynamic response analysis of the site is conducted by using the linear wave propagation method and the Rosenblueth method, and the digital features of the amplified power spectral density function, the acceleration power spectrum density, the mean value and the standard deviation are obtained respectively. In a numerical example, a clay ground is considered. For the random variation of soil properties and the different of incident angles, the digital features of the amplified power spectral density function, the acceleration power spectral density, the mean value and the standard deviation are analyzed. The results show that the random variation of quasi saturated soils properties and the different of incident angles have great influence on the site amplification effect.