Topographic Amplification Effects Research Articles

Nonlinear seismic response analysis of slopes considering the coupled effect of slope geometry and soil stratigraphy

SUMMARY To investigate the effects of slope geometric parameters and soil stratigraphic properties on the topographic amplification of ground motions, a large number of 2-D horizontally layered slope models are constructed. First, the linear and nonlinear seismic responses of a slope model are compared, and the result shows that the nonlinear characteristics of soils should be considered when studying the amplifying effect of slope topography on ground motions. Then, the nonlinear seismic responses of these slope models are analysed from four aspects: the maximum shear strain in the slopes, the effects of geometry and stratigraphy on the seismic response, the distance between the maximum topographic amplification indicators and the slope crest, and the influence range of slope topography behind the slope crest. The results indicate that the amplifying effect of slope topography on ground motions increases with increasing slope height or decreasing average shear wave velocity of the overlying soil layers. Besides, the variation of the topographic amplification effect with slope gradient is significantly influenced by soil stratigraphic properties. The distance between the maximum topographic amplification indicators and the slope crest is mainly in the range of 0–60 m, and the influence range of slope topography behind the slope crest is mainly in the range of 0–150 m. Subsequently, approximate relations are derived based on regression analyses of simulation results, which can provide meaningful references for the seismic design and seismic retrofitting of engineering structures behind the slope crest. Finally, the effects of slope geometric parameters and soil stratigraphic properties on ground motion modifications are further evaluated according to the prediction curves provided by the approximate relations.

An Analytical Investigation of the Topographic Amplification Effects of Slopes under Seismic Loads

An Analytical Investigation of the Topographic Amplification Effects of Slopes under Seismic Loads

Comparison of empirically-based and physically-based analyses of coseismic landslides: A case study of the 2016 Kumamoto earthquake

This study compares the performance of empirically- and physically-based methods for predicting coseismic landslides around Aso caldera during the 2016 Mw 7.0 Kumamoto earthquake. The physically-based method couples a regional-scale wave propagation simulation with a site-scale Newmark-type sliding analysis, while the empirically-based method predicts sliding displacements based on seismic recordings and/or ground motion prediction equations. By using the well-documented landslide inventory, the predictive capacity of each method is quantitatively assessed.The case study demonstrates that the physically-based method has the advantage of simulating complicated near-fault and topographic effects. It captures around 44% of observed landslides at the caldera rim, which is significantly better than the 20% captured from the empirically-based prediction. Both methods have similar prediction performance, capturing around 56% actual landslides at the central cone region. The empirically-based method is simple to use and provides fast landslide prediction. It can be further improved by incorporating a simple prediction model for the topographic amplification effect, which increases the percentage of captured landslides by around 8% across the caldera rim.

Seismic response sensitivity of a V-shaped canyon-crossing bridge considering the near-source canyon topographic effects

Past seismic events have shown that the canyon-crossing bridges are more vulnerable due to the canyon topographic effects. The seismic responses of such bridges are affected by various influential factors, including source-to-canyon distance, depth-to-half-width ratio of the canyon, incident angle of the ground motion, and shear wave velocity. This study numerically examines the impacts of these factors on the seismic responses of a typical bridge across a V-shaped canyon, which are not adequately investigated in the existing literature. To represent the canyon topographic effects, the ground motions at each bridge pier location are synthesized by the scattering and diffraction models of seismic waves induced by the V-shaped canyon. The accuracy of the analytical model in describing the effect of the source location has been validated. A detailed parametric analysis is conducted to evaluate the importance of the main influential factors in the seismic design of the bridge. The results demonstrated that the incident angle and depth-to-half-width ratio result in significant effects on the bridge responses. When the incident angle is larger than 45°, the topographic amplification effect on the bridge responses should be considered in the seismic design of the bridge. The source location and soil property show certain influences on the seismic responses of the piers. The deep canyon can cause much higher bridge responses than the shallow canyon. When the depth-to-half-width ratio is larger than 0.4, the topographic irregularities can cause noticeable amplifications on the pier responses at the illuminated canyon side. After the earthquake events, the bearings experience noticeable residual deformations under the seismic waves with unequal amplitudes and phases induced by the local topography.

Seismic hazard prediction using multispectral amplification maps in a complex topographic area: A case study of Qiaozhuang town, Sichuan Province, Southwest China

Earthquakes can cause widely distributed slope failures and damage in mountainous areas. The accurate prediction of ground motions in mountainous areas is essential for managing the seismic risk of urban cities near mountains but is restricted primarily by complex seismic site amplification effects in areas of uneven terrain. This study selected Qiaozhuang town located in the Qingchuan—Pingwu fault zone, Southwest China, as a case study. A simulator for mapped seismic responses using a hybrid model (SiSeRHMap) was applied to compute the multispectral seismic topographic amplification maps at the three slope units surrounding Qiaozhuang town (Weigan hill, Mt. Dong, and Mt. Shizi). Post-earthquake damage survey maps, 1D seismic site response spectral ratios, and H/V spectral ratios of earthquake data were used to validate the computed seismic site amplification factors and resonance frequencies. The results suggest that strong topographic amplification effects usually occur at distinct slope locations, such as hilltops, convex slope positions, upslope, and narrow ridges. The computed topographic amplification factors in the study area reached up to 2.4 at upslope or hilltops, and the resonance frequencies were between 3 and 10 Hz. Topographic effects can be as important as stratigraphic effects when assessing seismic amplification effects in the study area. We conclude that both topographic and stratigraphic effects should be considered in the comprehensive seismic hazard assessment of the study area or other similar mountain towns.

An analytical solution of scattering of semi-circular hill on cylindrical SH waves

Irregular topography can induce scattering and diffraction of incident seismic waves and cause topographic amplification effects. In this paper, a theoretical model for the scattering and diffraction of cylindrical SH waves caused by a semi-circular hill is established for the first time to reveal the influence of source characteristics on the topographic amplification effects. The analytical solution of the theoretical model is derived via the wave function expansion method and image theory, and its accuracy is verified by checking the given boundary conditions. Base on the presented solution, the influences of source location, incident wave frequency, and hill radius on amplitude amplification are systematically investigated. Comparison of amplitude amplification around the hill caused by cylindrical waves and plane waves indicates that the effect of source distance on topographic amplification effects is not negligible unless the source distance exceeds 15 times the hill radius. It suggests that much attention should be paid to the effects of source location on topographic amplification effects, especially when the source is near the topography.

2-D Seismic Response Analysis of a Slope in the Tyrrhenian Area (Italy)

The Caronia area is located in the Tyrrhenian north-eastern sector of Sicily (Italy). Starting in 2010, attention focused on the study of landslides phenomena that occurred in this area, which caused significant economic damage to buildings and infrastructures and loss of productive activities. The site is characterized by geotechnical, geological and morphological heterogeneity, and for this reason the site is particularly prone to seismic topographic amplification effects. In this paper, the authors carried out numerical studies focused on the topographic seismic effect evaluation concerning the slope affected by the landslide phenomena. For this site, geotechnical characterization was available concerning both in-situ and laboratory tests; boreholes, piezometers, down-hole tests, multichannel analysis of surface waves tests, seismic tomographies and inclinometer measurements were carried out. Furthermore, 1-D and 2-D local seismic response analyses were carried out by using different synthetic seismograms related to the earthquake of Messina and Reggio Calabria on 28 December 1908. The results of the numerical analyses are presented in terms of response seismograms and response spectra at the surface.

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.

Evaluación de efectos de amplificación topográfica del suelo debidos a la respuesta sísmica en laderas de las urbanizaciones Shancayán y Nueva Esperanza de la ciudad de Huaraz en el año 2018

El objetivo general del estudio fue evaluar los efectos de amplificación topográfica del suelo debidos a la respuesta sísmica en laderas de las urbanizaciones Shancayán y Nueva Esperanza de la ciudad de Huaraz. La presente investigación es del tipo cuantitativo y aplicado, donde se ha realizado la evaluación de la respuesta sísmica del terreno en zonas de laderas que presentan condiciones desfavorables para la cimentación de edificaciones. Se ha evaluado la respuesta sísmica del suelo para un comportamiento lineal equivalente aplicado a un modelo geológico geotécnico; realizándose un análisis bidimensional, que modela el perfil estratigráfico del terreno utilizando el método de los elementos finitos. Los resultados de la amplificación topográfica sísmica nos indica que se presentan zonas con grado muy alto en pendientes mayores a 25° y zonas con grado alto en pendientes entre 15° a 25°. La conclusión del estudio indica que los efectos de la amplificación topográfica sísmica son considerables y aumentan a mayor pendiente del terreno, y tiene un efecto directo en el comportamiento sismorresistente de las edificaciones.

Probabilistic seismic landslide hazard assessment: a case study in Tianshui, Northwest China

Probabilistic analysis in the field of seismic landslide hazard assessment is often based on an estimate of uncertainties of geological, geotechnical, geomorphological and seismological parameters. However, real situations are very complex and thus uncertainties of some parameters such as water content conditions and critical displacement are difficult to describe with accurate mathematical models. In this study, we present a probabilistic methodology based on the probabilistic seismic hazard analysis method and the Newmark’s displacement model. The Tianshui seismic zone (105°00′–106°00′ E, 34°20′–34°40′ N) in the northeastern Tibetan Plateau were used as an example. Arias intensity with three standard probabilities of exceedance (63%, 10%, and 2% in 50 years) in accordance with building design provisions were used to compute Newmark displacements by incorporating the effects of topographic amplification. Probable scenarios of water content condition were considered and three water content conditions (dry, wet and saturated) were adopted to simulate the effect of pore-water on slope. The influence of 5 cm and 10 cm critical displacements were investigated in order to analyze the sensitivity of critical displacement to the probabilities of earthquake-induced landslide occurrence. The results show that water content in particular, have a great influence on the distribution of high seismic landslide hazard areas. Generally, the dry coverage analysis represents a lower bound for susceptibility and hazard assessment, and the saturated coverage analysis represents an upper bound to some extent. Moreover, high seismic landslide hazard areas are also influenced by the critical displacements. The slope failure probabilities during future earthquakes with critical displacements of 5 cm can increase by a factor of 1.2 to 2.3 as compared to that of 10 cm. It suggests that more efforts are required in order to obtain reasonable threshold values for slope failure. Considering the probable scenarios of water content condition which is varied with seasons, seismic landslide hazard assessments are carried out for frequent, occasional and rare earthquake occurrences in the Tianshui region, which can provide a valuable reference for landslide hazard management and infrastructure design in mountainous seismic zones.

Geotechnical advances in development of a seismic code for Hong Kong

Hong Kong is located in a low- to moderate-seismicity region with earthquake damage to buildings having been recorded in the past. On average, there are about two earth tremors felt every year. Nowadays, the city is a unique place in the world with a high-density population living in high-rise buildings and surrounded by steeply sloping terrain. The potential risk of earthquake-induced damage thus calls for the development of a seismic design code for buildings in Hong Kong. In recent years, several critical geotechnical studies have been completed focusing on enhancing earthquake resilience in Hong Kong. They have included: (a) updating of probabilistic seismic hazard assessment based on a recent earthquake catalogue; (b) examination of local site effects and topographic amplification effects based on a pilot-area-specific seismic microzonation study; (c) reviewing the earthquake-induced landslide susceptibility of the natural terrain; and (d) assessment of the seismic stability of man-made slopes and retaining walls. These seismic studies are pivotal to the development of the seismic design code which is being pursued in Hong Kong. This paper is an important milestone in providing a detailed account of these studies and their impact on the code development.

Low-frequency Topographic Amplification Effect of Lushan Earthquake Based on Three-dimensional Numerical Simulation

The study of the influence of terrain on ground motion is quantified as the amplification effect of terrain on ground motion. In this paper, we use the spectral element method to simulate the three-dimensional real terrain model, and select the aftershocks of the station ground motion observation records in the Lushan area as the source, and analyze the amplification effect of the three-dimensional topography on the ground motion of the Lushan earthquake in the low frequency range (1-5hz). In this paper, the Fourier model of the real terrain of Lushan and the half-space free model are used to calculate the Fourier amplitude spectrum. The results show that in the low frequency range, the ground fluctuations have a large variation, which will have complex effects on the propagation of seismic waves. Especially in the bulging mountain and depression terrain, the time course peak is large. In addition, this paper selects the mountain near the BXD station from the calculation area, and analyzes the terrain magnifying effect of the convex mountain. Under 2-3hz, the mountain slope near the source side has a magnification effect of 3-4 times.

A note on near-field site amplification effects of ground motion from a radially inhomogeneous valley

To improve the understanding of the near-field soil and topographic amplification effects, an analytical solution by the authors for the scattering of plane SH waves by a radially inhomogeneous semi-cylindrical valley is extended to the case of a line source of cylindrical SH waves. Upon confirmation of its accuracy with past exact solutions for a homogeneous and an inhomogeneous semi-cylindrical valley under far-field plane SH waves, the extended solution is used to calculate the ground motion amplification factors for both the homogeneous and inhomogeneous valleys subjected to near-field waves. A comprehensive parametric study is conducted with respect to the location of the wave source, the dimensionless frequency of the incident waves, and the inhomogeneity degree of the covering soil layer. It is found that more amplifications and reductions of ground motions will occur within a certain range in and around the valley as the sources are located further. Consistent with the far-field case, it is confirmed that an increase of the degree of inhomogeneity of the covering soil layer generally amplifies the ground motions significantly.

Site Effect Assessment of the Gros-Morne Hill Area in Port-au-Prince, Haiti, Part A: Geophysical-Seismological Survey Results

After the M = 7.0 Haiti earthquake in 2010, many teams completed seismic risk studies in Port-au-Prince to better understand why this not extraordinarily strong event had induced one of the most severe earthquake disasters in history (at least in the Western World). Most highlighted the low construction quality as the main cause for the disaster, but some also pointed to possible soil and topographic amplification effects, especially in the lower and central parts of Port-au-Prince (e.g., close to the harbor). However, very detailed local studies of such site effects have not been completed yet. A Belgian-Haitian collaboration project was established in order to develop a detailed local seismic hazard study for Gros-Morne hill located in the district of Pétion-Ville, southeast of Port-au-Prince. In order to have a better understanding of the amplification on the Gros-Morne hill, in the southeastern part of Port-au-Prince, site effects were investigated by using near surface geophysical methods. The horizontal to vertical spectral ratio technique was applied to ambient vibrations and earthquake data, and multichannel analysis of surface waves and P-wave refraction tomography calculation were applied to seismic data. Standard spectral ratios were computed for the S-wave windows of the earthquake data recorded by a small temporary seismic network. Electrical resistivity tomography profiles were also performed in order to image the structure of the subsurface and detect the presence of water, if any. The spectral ratio results generally show low to medium (1.5–6) resonance amplitudes at one or several different resonance frequencies (for the same site), between 0.5 and 25 Hz. At most of the investigated sites, the fundamental resonance frequency varies between 7 and 10 Hz. By using the multichannel surface wave analyses of the seismic data, we were able to determine shear wave velocities ranging between 200 and 850 m/s, up to a depth of about 15–20 m. From the refraction analysis, we were able to delineate P-waves velocities of 500 to 1500–2000 m/s at the studied sites. The outputs were locally compared with the resistivity data from the electrical profiles. Thus, the overall data indicate a moderate site effect at Gros-Morne hill, with a great variability in site amplification distribution. Initial estimates of local site effects were made on the basis of those outputs and the earthquake recordings. Our results are finally discussed with respect to outputs and interpretations that had been published earlier for the same site. Those results only partly confirm the strong seismic amplification effects highlighted by previous papers for this hill site, which had been explained by the effects of the local topographic and soil characteristics.

Numerical evaluation of topographic effects on seismic response of single-faced rock slopes

This paper investigates the seismic responses of homogenous single-faced rock slopes subjected to vertically propagating shear waves by numerical simulations in order to explore the topographic amplification of ground motion. The horizontal and vertical topographic amplification factors both on the free surface and in the slope are evaluated using parametric studies focusing on slope geometry, rock material, and input motion with the two-dimensional finite element code LS-DYNA. Comparison of the results obtained in this study with those of previous numerical analyses available in the literature and with the provisions of the existing seismic codes shows good agreement. Both qualitative and quantitative insights into the topographic amplification effects on the seismic responses of single-faced slopes are presented in this study. The results show that both slope geometry and rock material have great influences on the horizontal and vertical amplification factors. As for input motion, the magnitude and duration have negligible effects on the amplification factors when rock materials are homogeneous and elastic. However, the frequency extent of input motions has great impact on the amplification factors. It is also indicated that the modern seismic codes may underestimate the amplification effects of ground motion. Nevertheless, modification of the provisions of the codes may require more convincing evidence from reliable field experiments.

Ground Motion at a Semi-Cylindrical Valley Partially Filled with a Crescent-Shaped Soil Layer Under Incident Plane SH Waves

To elucidate the ground motion amplification due to soil and topographic effects, an analytical formulation based on wavefunction expansion is derived for the scattering of plane SH waves by a semi-cylindrical valley partially filled with a crescent-shaped soil layer. The site responses consisting of both soil and topographic effects from the partially filled alluvial valley and the pure topographic contribution from the homogeneous valley of the same geometry are calculated and compared. It is found that the soil amplification effects are usually larger than the topographic amplification effects within the alluvial valley, while the topographic effects dominate the amplification pattern of ground motions outside the alluvial valley. Generally, the maximum soil amplification generally far outweighs the maximum topographic amplification. The material parameters and filling degree of the soil layer are found to affect the magnitude and the pattern of ground motion amplitude on the valley surface depending on the irregular topography, the frequency content and obliquity of the wave incidence.

COMPARISON OF DISTRIBUTION OF LANDSLIDES TRIGGERED BY WENCHUAN, LUSHAN, AND LUDIAN EARTHQUAKES

All of the MS8.0 Wenchuan, MS7.0 Lushan, and MS6.5 Ludian earthquakes have triggered a large number of landslides. Due to differences in magnitude, geology, and topographical conditions, the distribution of seismic landslides vary widely. In this study, existing research results are combined to perform a contrastive analysis of seismic landslide distribution patterns based on three aspects:earthquake characteristics, geology, and topography. The following results are obtained. (1) The number and density of landslides due to the three earthquakes increase with the increase of peak ground acceleration(PGA) and magnitude. The number of landslides shows an increasing trend following an increase of intensity during the Wenchuan and Ludian earthquakes. However, the area affected by the relatively low-intensity Lushan earthquake also develops numerous landslides. (2) The maximum distance of influence of a geological fault on landslide distribution increases with the increase of magnitude. Within the range of 0.2 times of the maximum distance of influence, the Wenchuan earthquake has an 80% landslide distribution, whereas the other two earthquakes have only 30%. Moreover, the number of landslides decreases exponentially with distance of the Wenchuan earthquake from geological faults. (3) The distribution of seismic landslides is strongly influenced by topography. In the Ⅶ or higher intensity regions, the relief depth increases and earthquake-triggered landslides in high concentration areas have relatively higher relief. Furthermore, the slopes gradient with concentrated landslides can subsequently increase. Seismic landslides occur more frequently along mountain ridges and deep slopes in areas of higher relief, which may be related to the topographic amplification effect.

Time-Frequency Analysis of Seismic Wave Propagation across a Rock Mass Using the Discontinuous Deformation Analysis Method

To investigate the wave (including P-wave and S-wave) propagation across a rock mass, the seismic discontinuous deformation analysis (DDA) method is used, and a time-frequency analysis of the DDA simulation results is performed in this paper. First, compared with the theoretical solution, the correctness of the DDA to simulate the seismic wave propagation across both the interlayer and interface is validated. Second, the propagation effects of the seismic wave across the jointed rock mass are studied, showing that the joint stiffness, joint spacing, and joint number have different effects on the attenuation of seismic waves. Last, to analyze seismic wave propagation across a rock mass, the engineering site of the underground cavern complex of the Dagangshan hydropower station in southwest China is used as a typical case study. A number of valuable results are obtained by time-frequency analysis; an obvious topographic amplification effect and a joint abnormal area are found. From the analysis of the time-space effect, it may be concluded that the relative displacement of the rock mass is easily encountered under earthquake, which adversely affects the stability of the engineering site. The spectrum characteristics of the seismic dynamic responses at different locations are significantly different, and a weak area of the peak acceleration may be caused by the joint attenuation effect. Based on the joint time-frequency analysis, it is suggested that adjustments for both the peak acceleration and spectrum be made before taking the ground motion record as the incident wave.

Application and evaluation of a rapid response earthquake-triggered landslide model to the 25 April 2015 Mw 7.8 Gorkha earthquake, Nepal

The 25 April 2015 Mw 7.8 Gorkha earthquake produced strong ground motions across an approximately 250km by 100km swath in central Nepal. To assist disaster response activities, we modified an existing earthquake-triggered landslide model based on a Newmark sliding block analysis to estimate the extent and intensity of landsliding and landslide dam hazard. Landslide hazard maps were produced using Shuttle Radar Topography Mission (SRTM) digital topography, peak ground acceleration (PGA) information from the U.S. Geological Survey (USGS) ShakeMap program, and assumptions about the regional rock strength based on end-member values from previous studies. The instrumental record of seismicity in Nepal is poor, so PGA estimates were based on empirical Ground Motion Prediction Equations (GMPEs) constrained by teleseismic data and felt reports. We demonstrate a non-linear dependence of modeled landsliding on aggregate rock strength, where the number of landslides decreases exponentially with increasing rock strength. Model estimates are less sensitive to PGA at steep slopes (>60°) compared to moderate slopes (30–60°). We compare forward model results to an inventory of landslides triggered by the Gorkha earthquake. We show that moderate rock strength inputs over estimate landsliding in regions beyond the main slip patch, which may in part be related to poorly constrained PGA estimates for this event at far distances from the source area. Directly above the main slip patch, however, the moderate strength model accurately estimates the total number of landslides within the resolution of the model (landslides≥0.0162km2; observed n=2214, modeled n=2987), but the pattern of landsliding differs from observations. This discrepancy is likely due to the unaccounted for effects of variable material strength and local topographic amplification of strong ground motion, as well as other simplifying assumptions about source characteristics and their relationship to landsliding.

Simulation of Spatially Varying Ground Motions in V-shaped Symmetric Canyons

A method of simulating spatially varying ground motions in V-shaped symmetric canyon is presented. The topographic amplification effect is taken into account by considering a 2-dimension wave propagation model. Two approaches are developed to simulate spatially varying ground motions. First, the spatially varying ground motions are simulated by using the power spectral density and an empirical coherency loss function. Second, the spatially varying ground motions are simulated by using given ground motions and an empirical coherency loss function. These two proposed methods are validated by comparing statistical characteristics of the synthetic motions with target theoretical models.