Analysis of landslide damage caused by the 2008 Wenchuan earthquake using strong motion data: A case study in the Beichuan county town

The county town of Beichuan county, China, experienced catastrophic destruction due to landslides induce by the 2008 Wenchuan earthquake. In consideration of the special location of the county town, this paper selected the landslides induced in the town as representative of large-scale near-rupture landslides, and quantitatively analyzed why the landslide damage was so destructive in the town by using strong motion data obtained from the Wenchuan earthquake in the Longmenshan area. Three methods were employed to estimate the landslide damage using strong motion data. (1) Peak ground accelerations (PGAs) on the hanging wall were used to evaluate the PGAs on the landslide sites in the town. The evaluated average PGAs were all greater than 1 g, indicating that the ground motion intensity was very strong during the earthquake. (2) Acceleration time histories, from another station with similar geological conditions to the town, were used to evaluate the critical acceleration changing range, and the estimated values showed the geological conditions were very susceptible to earthquakes. (3) Acceleration time histories, from two stations on the hanging and foot walls of the rupture, and near the town, were used to calculate the Newmark displacements, and all the evaluated displacements indicated that landslides were very likely. The results show that the slopes, susceptible to earthquakes in the Beichuan county town, were easily triggered under such strong ground-motion intensity and developed into large-scale catastrophic events.     

考虑动态临界加速度的地震边坡永久位移研究

Newmark永久位移是评价边坡在地震时稳定性的一个重要指标,近年来广泛应用于地震边坡危险性评价中。传统Newmark永久位移法在计算临界加速度时假定其为常数,未考虑滑动面上抗剪强度参数的变化,过低估计了边坡的永久位移。为了解决这一问题,本文从岩土结构理论获得思路,详细分析滑块底面抗剪强度参数在地震中的变化过程,以边坡震动过程中黏聚力逐步丧失为基本思路,在黏聚力符合一定概率分布的基础上,提出了一种利用蒙特卡罗法模拟其动态减小过程从而实现临界加速度动态变化的计算方法。经过算例计算,黏聚力和临界加速度体现了地震过程中边坡滑块黏聚力和临界加速度的动态变化,位移大小符合地震边坡实际位移的常规数值。本文提出的蒙特卡罗法实现动态黏聚力和动态临界加速度的计算过程与地震时程相对应,不仅在一定程度上解决了抗剪强度参数的动态变化问题,还解决了传统Newmark位移计算中永久位移比实际位移偏小的问题。     

STUDY ON THE DISTRIBUTION PATTERN OF EARTHQUAKE- TRIGGERED LANDSLIDES BASED ON SEISMIC LANDSLIDE SUSCEPTIBILITY ANALYSIS: A CASE STUDY OF LANDSLIDES TRIGGERED BY THE MS6.5 LUDIAN EARTHQUAKE IN 2014

On August 3, 2014, an M_W6.5 earthquake occurred in Ludian County, Yunnan Province, which triggered significant landslides and caused serious ground damages and casualties. Compared with the existing events of earthquake-triggered landslides, the spatial distribution of co-seismic landslides during the Ludian earthquake showed a special pattern. The relationship between the co-seismic landslides and the epicenter or the known faults is not obvious, and the maximum landslide density doesn't appear in the area near the epicenter. Peak ground acceleration (PGA), which usually is used to judge the limit boundary of co-seismic landslide distribution, cannot explain this distribution pattern. Instead of correlating geological and topographic factors with the co-seismic landslide distribution pattern, this study focuses on analyzing the influence of seismic landslide susceptibility on the co-seismic distribution. Seismic landslide susceptibility comes from a calculation of critical acceleration values using a simplified Newmark block model analysis and represents slope stability under seismic loading. Both DEM (SRTM 90m)and geological map (1 ︰ 200^￣￣000)are used as inputs to calculate critical acceleration values. Results show that the most susceptible slopes with the smallest critical accelerations are generally concentrated along the banks of rivers. The stable slopes, which have the larger critical accelerations and are comparably stable, are in the places adjacent to the epicenter. Comparison of the distribution of slope stability and the real landslides triggered by the 2014 M_W6.1 Ludian earthquake shows a good spatial correlation, meaning seismic landslide susceptibility controls the co-seismic landslide distributions to a certain degree. Moreover, our study provides a plausible explanation on the special distribution pattern of Ludian earthquake triggered landslides. Also the paper discusses the advantages of using the seismic landslide susceptibility as a basic map, which will offer an additional tool that can be used to assist in post-disaster response activities as well as seismic landslides hazards zonation.     

Methodology for seismic and post-seismic stability assessment of natural clay slopes based on a viscoplastic behaviour model in simplified dynamic analysis

A co-seismic viscoplastic sliding model, composed of two consequential behaviour phases, was realised in order to assess the co-seismic and post-seismic stability of natural slopes. The model takes into consideration the development and distribution of available strengths in pre-seismic conditions, as well as the viscoplastic behaviours manifested during monotonic and impulsive fast shearing tests on different clayey soils. In relation to the strength increase produced by the shear displacement rate, phase I is present during sliding on pre-existing failure surfaces at the residual state and/or in weak bands at the fully softened state. In this latter case, this is limited to small displacements. Conversely, phase II is characterised by strength decrease and occurs if and when the inertial dynamic load mobilises the “impulsive critical shear strength”, which is greater than the shear strength available in the pre-seismic static field. This implies the development of a first failure or a new failure surface with high shear displacements along these surfaces. The simplified dynamic analysis for infinite slopes, integrated by the behaviour model introduced in this paper, highlights a less conservative nature in comparison to that of the classic Newmark approach with one single exception. This occurs on attainment of the “static break point” where the co-seismic displacements obtained are comparable or even greater than those attainable from the classic Newmark approach. Furthermore, in relation to the co-seismic development of shear strength, it is possible to estimate in the short term as well as in the long term the post-seismic instability after the main shock.     

Centrifuge modeling of a dry sandy slope response to earthquake loading

This paper presents results of a series of centrifuge models of dry, sandy slopes excited by earthquakes and cyclic waves under 50g centrifugal acceleration to investigate the dynamic performance of slopes. Test results of four model slopes with different profiles stimulated by the adjusted El Centro earthquakes with various peak accelerations reveal the response amplification mechanism of the slope. By calculating the response spectra of recorded acceleration time histories, it was observed that the different frequency contents of the input event were amplified to different degrees. The model slope showed a completely different response under the cyclic wave with a constant frequency and amplitude in that the spectral amplification factor curves had no prominent peak values. These findings suggest that dynamic centrifuge tests excited with a real ground motion are able to better reflect the response characteristics of a slope rather than the tests with cyclic loading.     

Earthquake-reactivated landslide scenarios in Southern Italy based on spectral-matching input analysis

The Tyrrhenian portion of the Calabria region (southern Italy) is particularly prone to landslides as a consequence of intense morphodynamic processes. These processes affect the slopes that are composed of highly jointed metamorphic rock masses. Moreover, the frequent intense rainfalls and the up to Mw 7.0 regional earthquakes represent the main landslide triggering factors. An area of approximately $45\,\hbox {km}^{2}$ was selected as a test site in the context of a regional project aimed at reconstructing possible earthquake-reactivated landslide scenarios (i.e., referred to already existing landslide masses). An inventory map led to the identification of 175 landslides, including rock slides, earth slides and rock falls. Ground-motion scenarios based on a spectral-matching method were derived to evaluate the expected earthquake-induced displacements of the existing landslides. Naturally recorded acceleration time histories were selected from international ground-motion databases based on a similarity index and considered representative of the seismological features of the considered seismic sources (i.e., epicentral distance, magnitude, focal mechanism). Spectral attenuation was considered, according to well-established attenuation laws, to define the expected response spectrum at the outcropping bedrock corresponding to each existing landslide. Subsequently, the selected natural records were modified to guarantee spectral matching with the attenuated response spectra at each landslide site. The derived time histories were used to compute co-seismic displacements via the classic Newmark’s sliding-block method. Different scenarios of co-seismic landslide displacements or collapse were generated for different pore-water pressure hypotheses. The strongest $\hbox {Mw}>6$ seismic scenario (Messina Straits seismogenic source) indicated an exceedance probability of earthquake-induced co-seismic landslide collapse varying from 20 to 55 % with the increasing severity of the pore-water pressures. This probability corresponds to a percentage of co-seismic landslide displacements up to 40 % of the total inventoried landslides. The exceedance probability indicated that co-seismic landslide collapse drops below 20 % for $\hbox {Mw}<6$ seismic scenarios. In contrast, if a uniform probability is assumed for the seismic action occurrence, i.e., return periods of 475 and 2,475 years, the total percentage of landslide co-seismic displacements could be as high as 70 and 90 %, respectively, for the considered pore-water pressures.     

A methodology to select seismic coefficients based on upper bound “Newmark” displacements using earthquake records from Turkey

Strong motion records taken during earthquakes in Turkey are used to calculate Newmark displacements in slopes. These displacements are then utilized in developing a novel displacement-based methodology to select the seismic coefficient which is used to calculate pseudostatic safety factor. In the first step of the study, calculated Newmark displacements are evaluated in three different categories which are as follows: using all data, using data for different earthquake magnitude (M) ranges with and without distance constraint and using data for different peak acceleration (a_max) ranges. For all categories, different equations are obtained to assign slope displacements as a function of the ratio of yield acceleration to peak acceleration. The results show that categorization of data is an important issue, because the displacements are earthquake magnitude and peak acceleration dependent. In the second step, equations obtained for different peak acceleration ranges are used to propose charts linking upper bound slope displacements (D), seismic coefficients (k_h) and pseudostatic safety factors (PSF), which are three important parameters of a pseudostatic approach. This enables the k_h values be chosen based on the allowable displacements, instead of the current applications based on judgement and expertise. The results show that k_h values for any allowable displacement should be based on anticipated a_max values, while use of high PSF values results in lower displacements. Extensive comparison with solutions from the literature is also made. The methodology is best suited for earthquake triggered shallow landslides in natural slopes, consisting of materials which do not lose strength during dynamic loading.     

基于修正岩土体强度参数的简化纽马克位移法地震滑坡危险性快速评估技术

地震滑坡危险性评估可为震后应急响应等提供科学的决策依据。纽马克位移法可不依赖同震滑坡编目快速评估同震滑坡危险性。工程岩体物理力学参数是该方法的核心参数之一,但其赋值过于单一,难以反映复杂地质背景下岩体强度的空间差异性。针对上述问题,本文在分析地震滑坡影响因子的基础上,选择距断层距离、高程和距水系距离作为影响岩体强度的评价指标并建立岩体强度评价模型,获得区域岩体强度修正系数,进而修正传统方法的临界加速度。结合震后的即时地震动峰值加速度,采用简化纽马克位移法计算边坡累积位移,开展地震滑坡危险性快速评估,并以汶川M_W7.9地震的地震滑坡危险性评估为例验证本文方法。结果表明,相对于传统方法,本文方法划分的地震滑坡危险区与同震滑坡分布更加一致。     

Ground motion selection for seismic slope displacement analysis using a generalized intensity measure distribution method

Selecting ground motions based on the generalized intensity measure distribution (GIMD) approach has many appealing features, but it has not been fully verified in engineering practice. In this paper, several suites of ground motions, which have almost identical distributions of spectral acceleration (SA) ordinates but different distributions of non‐SA intensity measures, are selected using the GIMD‐based approach for a given earthquake scenario. The selected ground motion suites are used to compute the sliding displacements of various slopes. Comparisons of the resulting displacements demonstrate that selecting ground motions with biased distribution of some intensity measures (ie, Arias intensity) may yield systematic biases (up to 60% for some slope types). Therefore, compared to the ground motions selected based only on the distribution of SA ordinates, the ground motion suite selected by the GIMD‐based approach can better represent the various characteristics of earthquake loadings, resulting in generally unbiased estimation in specific engineering applications.     

地震作用下黄土边坡动力响应的时频特征分析

地震作用下黄土边坡的动力响应特征与变形失稳机制是具有重要理论与实践意义的课题,但从动力响应频谱特性方面开展的研究还相对较少。以大型振动台模型试验获得的黄土边坡地震动峰值加速度数据为基础,通过分析其变化规律,着重从频谱特性的角度分析,讨论黄土边坡的动力失稳机制。进一步通过对坡面不同高程测点、边坡内部垂直方向以及水平方向上测点的加速度时程进行绝对加速度反应谱分析,从频谱变化角度提出黄土边坡的动力失稳机制。研究表明,黄土边坡在地震动作用下的响应过程可以分为三个阶段：弹性阶段、塑性阶段与破坏阶段;黄土边坡进入破坏阶段时均会伴随反应谱峰值的增幅或者主周期的变化,在弹性阶段反应谱加速度峰值增幅与输入地震动幅值增幅一致,进入塑性阶段后反应谱峰值增幅比输入地震动幅值增幅小;研究提出将反应谱首峰的凸显情况作为坡体破坏程度的判断依据之一。     

Real-time mapping of earthquake-induced landslides

Rigid sliding block analysis is a common analytical procedure used to predict the potential for earthquake-induced landslides for natural slopes. Currently, predictive models provide the expected level of displacement as a function of the characteristics of the slope (e.g., geometry, strength, yield acceleration) and the characteristics of earthquake shaking (e.g., peak ground acceleration, peak ground velocity). These predictive models are used for developing seismic landslide hazard maps which identify zones with risk of earthquake-induced landslides. Alternatively, these models can be combined with Shakemaps to generate “near-real-time” Slidemaps which could be used, among others, as a tool in disaster management. Shakemaps (a publicly available free service of the United States Geological Survey, USGS) provide near-real-time ground motion conditions during the time of an earthquake event. The ground motion parameters provided by a Shakemap are very useful for the development of Slidemaps. By providing ground motion parameters from an actual earthquake event, Shakemaps also serve as a tool to decouple the uncertainty of the ground motion in sliding displacements prediction. Campania region in Italy is studied for assessing the applicability of using Shakemaps for regional landslide-risk assessment. This region is selected based on the availability of soil shear strength parameters and the proximity to the 1980 Irpina (M _w  = 6.9) Earthquake.     

Predictive model of Arias intensity and Newmark displacement for regional scale evaluation of earthquake-induced landslide hazard in Greece

Defining the possible scenario of earthquake-induced landslides, Arias intensity is frequently used as a shaking parameter, being considered the most suitable for characterising earthquake impact, while Newmark׳s sliding-block model is widely used to predict the performance of natural slopes during earthquake shaking. In the present study we aim at providing tools for the assessment of the hazard related to earthquake-induced landslides at regional scale, by means of new empirical equations for the prediction of Arias intensity along with an empirical estimator of coseismic landslide displacements based on Newmark׳s model. The regression data, consisting of 205 strong motion recordings relative to 98 earthquakes, were subdivided into a training dataset, used to calculate equation parameters, and a validation dataset, used to compare the prediction performance among different possible functional forms and with equations derived from previous studies carried out for other regions using global and/or regional datasets. Equations predicting Arias intensities expected in Greece at known distances from seismic sources of defined magnitude proved to provide more accurate estimates if site condition and focal mechanism influence can be taken into account. Concerning the empirical estimator of Newmark displacements, we conducted rigorous Newmark analysis on 267 one-component records yielding a dataset containing 507 Newmark displacements, with the aim of developing a regression equation that is more suitable and effective for the seismotectonic environment of Greece and could be used for regional-scale seismic landslide hazard mapping. The regression analysis showed a noticeable higher goodness of fit of the proposed relations compared to formulas derived from worldwide data, suggesting a significant improvement of the empirical relation effectiveness from the use of a regionally-specific strong-motion dataset.     

SLIDING SYSTEM PREDICTING LARGE PERMANENT CO-SEISMIC MOVEMENTS OF SLOPES

The sliding-block model is used widely for the prediction of permanent co-seismic displacements of slopes and earth structures. This model assumes motion in an inclined plane but does not consider the decrease in inclination of the sliding soil mass as a result of its downward motion, which is the usual state in the field. For the investigation of the above effect, this paper considers the dynamic motion of a variant sliding system; a perfectly flexible chain sliding along planes with gradually gentler inclinations. The governing equation of motion of the new model is approximately derived and solved for a number of earthquakes. The cases of ground slips where the permanent displacement of the new model differs considerably from that of the sliding-block model are detected and an improved method predicting permanent seismic deformations is proposed.     

黄土斜坡地震动时程分析

应用地震危险性分析理论和地震动人工合成技术，给出Newmark法中所需的地震动时程，解决了斜坡稳定性分析Newmark法中难以选取合适地震动时程的难题。通过对黄土斜坡实例计算，给出了坡体中地震动峰值加速度与深度的关系；在计算坡体位移时，提出了等效峰值加速度的概念；对比了使用地面地震动时程和使用坡体内等效地震动时程的计算结果。     

Earthquake induced displacements of slopes

The estimation of permanent slope displacements induced by earthquakes is examined. Predictive formulae and graphs for co-seismic and post-seismic permanent displacements for translational movements are presented which allow the assessment of the vulnerability of natural and man-made slopes subjected to earthquakes.     

Empirical models for predicting lateral spreading considering the effect of regional seismicity

A revised empirical model has been developed for predicting liquefaction-induced lateral spreading displacement (LD) as a function of both response spectral acceleration derived from strong-motion atte...     

考虑动态临界加速度的地震边坡永久位移预测模型研究

在区域边坡地震危险性评价中主要采用永久位移预测模型进行地震边坡永久位移计算.永久位移预测模型以Newmark滑块理论为基础,通过大量实测地震时程记录统计拟合得出.针对Newmark理论中滑动面抗剪强度参数保持不变和已有位移预测模型的计算位移小于实测位移的问题,利用动态临界加速度理论,分别构建含有峰值加速度和阿里亚斯强度...     

APPLICATION OF TWO SIMPLIFIED NEWMARK MODELS TO THE ASSESSMENT OF LANDSLIDES TRIGGERED BY THE 2008 WENCHUAN EARTHQUAKE

At present, with the wide application of the Newmark method, various Newmark empirical formulas with different ground motion parameters have been fitted by many researchers based on global strong-motion records. However, the existing study about the Wenchuan earthquake does not quantitatively evaluate the applicability of different Newmark models based on the actual landslides distribution. The aim of this paper is to present a comparison between observed landslides from the 2008 Wenchuan earthquake and predicted landslides using Newmark displacement method based on different ground motion parameters. The factor-of-safety map and critical acceleration(a_c)map in the study area are obtained by using the terrain data and geological data. The distribution of Arias intensity(I_a)and PGA in the study area is obtained by using the attenuation formulas of Arias intensity(I_a)and PGA, which is regressed by Wenchuan ground motion records. Based on the distribution of Arias intensity(I_a)and PGA parameters, we obtained the predicted locations of landslide using Newmark regression equations which are generated using global strong-motion records. The results shows that the assessment results can better reflect the macroscopic distribution characteristics of co-seismic landslides, most predicted landslide cells are distributed on the two sides of the Beichuan-Yingxiu Fault, especially the Pengguan complex rock mass in the hanging wall. The abilities to predict landslide occurrence of the two Newmark simplified models are different. On the whole, the evaluated result of simplified model based on parameter I_a is better than that based on PGA parameter. The GFC values obtained by the Newmark model of I_a and PGA parameters are 65.7% and 34.9%respectively. The evaluated result based on I_a can better reflect the macro distribution of coseismic landslides. The Ls_Pred value based on the Newmark model of parameter I_a is 26.5%, and the Ls_Pred value based on the Newmark model of PGA parameter is 10.3%. However the total area of predicted landslides accounts for 2.4% of the study area, which indicates that the predicted landslide cells are greater than the observed landslide cells. This reminds us that depending on the current input of shear strength and ground-motion parameters, we can only conduct landslide hazard assessment in macro areas, the ability to predict landslide can be improved using more accurate topographic data and input parameters.     

the faulting characteristics of 2008 wenchuan ms8.0 earthquake and its relation with strong ground motion

The 2008 M_S8 Wenchuan earthquake occurs on a high angle listric thrust fault. It is the first time that the near and far field strong ground motion was observed for such special type thrust earthquake. This paper jointly interprets the distribution of peak acceleration of ground motion data with seismogenic structure and slip propagating process to investigate how high angle listric thrust fault controls the pattern of strong ground motion. We found that the distribution of peak acceleration of strong ground motion during the Wenchuan earthquake has four distinctive features: 1)The peak acceleration of ground motion inside the Longmenshan fault zone is large, that is, nearly twice as strong as that outside the fault zone; 2)This earthquake produces significant vertical ground motion, prevailing against horizontal components in the near field; 3)The far field records show that the peak acceleration is generally higher and attenuates slower versus station-fault distance in the hanging wall. It is doubtful that the attenuation of horizontal components also has the hanging wall effect since no evidence yet proving that the unexpected high value at long distance need be omitted; 4)As to the attenuation in directions parallel to the source fault(Yingxiu-Beichuan Fault), the far field records also exhibit azimuthal heterogeneity that the peak acceleration of horizontal components decreases slower in the north-northeastern direction in which the co-seismic slip propagates than that in the backward way. However, the attenuation of vertical component displays very weak heterogeneity of this kind. Synthetically considered with shallow dislocation, high dip angle, and prevailing vertical deformation during co-seismic process of the Wenchuan earthquake, our near and far field ground motion records reflect the truth that the magnitude of ground motion is principally determined by slip type of earthquake and actual distance between the slipping source patches and stations. As a further interpretation, the uniqueness of high angle listric thrust results in that the ground motion effects of the Wenchuan earthquake are similar to that due to a common thrust earthquake in some components while differ in the others.     

Newmark design spectra considering earthquake magnitudes and site categories

Newmark design spectra have been implemented in many building codes, especially in building codes for critical structures. Previous studies show that Newmark design spectra exhibit lower amplitudes at high frequencies and larger amplitudes at low frequencies in comparison with spectra developed by statistical methods. To resolve this problem, this study considers three suites of ground motions recorded at three types of sites. Using these ground motions, influences of the shear-wave velocity, earthquake magnitudes, source-to-site distances on the ratios of ground motion parameters are studied, and spectrum amplification factors are statistically calculated. Spectral bounds for combinations of three site categories and two cases of earthquake magnitudes are estimated. Site design spectrum coefficients for the three site categories considering earthquake magnitudes are established. The problems of Newmark design spectra could be resolved by using the site design spectrum coefficients to modify the spectral values of Newmark design spectra in the acceleration sensitive, velocity sensitive, and displacement sensitive regions.