OBTAINING PROBABILISTIC ESTIMATES OF DISPLACEMENT ON A LANDSLIDE DURING FUTURE EARTHQUAKES

The assessment of earthquake triggered landslide hazard may be undertaken using both deterministic and probabilistic techniques. Probabilistic methods have been developed because much of the data can be considered as random variables where parameters such as the angle of internal friction and moisture content do not have a single fixed value but may assume any number of values across a range. This random variability can be modelled by a probability density function (PDF) which describes the relative likeli-hood that a random variable will assume a particular value. Instead of using just the average or expected value of an input parameter, the complete range of possible values can be used to estimate a range of possible outcomes. Thus the probability of a slope being unstable can be obtained rather than a single indicator of stability. Such proba-bilistic analyses allow for the incorporation of the likely variability of each parameter and therefore allow a more intimate assessment of slope stability to be derived. Utilising empirical relationships for calculating earthquake ground motions and associated slope displacement, an investigation was undertaken to identify the contribution that modern simulation techniques could make to the assessment of earthquake-triggered landslides. To achieve this, geotechnical and earthquake data obtained from a deep-seated landslide triggered during the M _w 7.0 Loma Prieta earthquake was used. By incorporating the variability of the geotechnical parameters and the uncertainty in earthquake location the model derived the probabilities associated with increasing amounts of slope displacement during future probable earthquakes. Analysis was undertaken for four of the principal fault segments in the San Francisco Bay area. These estimates were then combined with the occurrence probabilities of the earthquakes to provide temporal estimates of dis-placement for a 30 year period. Results indicated that a M _w 7.0 earthquake located on the Peninsula Segment of the San Andreas fault was most hazardous with a 11% chance of minor slope displacement (≥0.10 m) and a 6% chance of moderate slope displacement (≥0.30 m) within the next 30 years.     

SEISMIC HAZARD ASSESSMENT IN FLORENCE CITY ITALY

A seismic hazard analysis of Florence city was performed in the frame of a project concerning the dynamic behaviour of cable-stayed bridges. Both a probabilistic approach and a methodology based on the use of a local macroseismic catalogue were applied. A local catalogue was expressly compiled for this purpose, to collect the macroseismic intensities actually observed at the site as a result of past earthquakes. This sort of catalogue is an independent tool to verify the assumptions of the probabilistic approach (seismic zoning, earthquake recurrence relation, attenuation model), though it can supply results in terms of macroseismic intensity only and reflects the effective seismic history at the site, without taking into account any variability. The Cornell' methodology was used to assess probabilistic hazard in terms of macroseismic intensity, peak ground acceleration, peak ground velocity, and pseudovelocity uniform response spectra. The local catalogue points out level VII of the Mercalli-Cancani-Sieberg scale (MCS) as the maximum intensity historically observed in Florence. The probabilistic approach leads to the consideration of intensity VIII MCS as the maximum credible for the city. The probabilistic analysis in terms of ground motion was performed using attenuation relations estimated for alluvium sites, since the geology of Florence area is represented by fluvial and lacustrine deposits of various thickness. Peak ground acceleration values with 90% non exceedence probability in 50 and 500 years are respectively 145 and 219 cm/s's for a shallow alluvium site, and 95 and 157 cm/s's for a deep alluvium site; the corresponding peak ground velocity values for sites located on alluvium are 6.41 and 11.76 cm/s. Uniform response spectra are provided for shallow and deep alluvium sites, according to frequency-dependent attenuation relations estimated from strong Italian earthquakes.     

The coastal landslides of Shetland

Little is known of hard-rock coastal landsliding in Scotland. We identify 128 individual coastal landslides or landslide complexes >50 m wide along the coasts of Shetland. Most are apparently translational slides characterized by headscarps, displaced blocks and/or debris runout, but 13 deep-seated failures with tension cracks up to 200 m inland from cliff crests were also identified. Thirty-one sites exhibit evidence of at least localized recent activity. Landslide distribution is primarily determined by the distribution of coastal cliffs >30 m high, and they are preferentially developed on metasedimentary rocks. Analysis of 16 landslides on Fetlar (NE Shetland) indicates that most are translational dip-slip failures; 3 represent deep-seated failures and several exhibit active frontal erosion attributable to basal sapping by storm waves. As these landslides terminate in shallow water, failure was probably initiated when rising sea level resulted in footslope erosion and upslope propagation of instability, causing downslope displacement of landslide blocks on upper slopes. ¹⁰Be exposure dating of two headscarps yielded ages of 4.8?±?0.2 ka and 4.4?±?0.2 ka, consistent with the onset of footslope erosion as sea level rose. Our results suggest that landslides have played a hitherto undocumented but important role in retreat of cliffed coastlines in Scotland.     

Seismic Hazard Analysis Using Discrete Faults in Northwestern Pakistan: Part II – Results of Seismic Hazard Analysis

This article is the second of two companion articles that evaluate seismic hazard in northwestern (NW) Pakistan. Using the properties and characteristics of discrete faults in NW Pakistan described in the first article, probabilistic and deterministic seismic hazard analyses for 11 major cities in NW Pakistan were conducted. The results from both probabilistic and deterministic seismic hazard analyses exhibit good agreement. Median deterministic spectra compare favorably with uniform hazard spectra (UHS) for 475- or 975-year return periods, while the 84^th-percentile deterministic spectra compare favorably with the UHS for a 2475-year return period. Peak ground accelerations (PGAs) for 2475-year return periods exceed 1.0 g for the cities of Kaghan and Muzaffarabad, which are surrounded by major faults. The PGAs for a 475-year return period for these cities are approximately 0.6g — 3 to 4 times greater than estimates by previous studies using diffuse areal source zones. The PGAs for some cities located farther from faults (including Astor, Malakand, Mangla, Peshawar, and Talagang) are similar to those predicted using diffuse areal source zones. Seismic hazard maps for PGA and spectral accelerations at periods of 0.2 s and 1.0 s corresponding to three return period (2475, 975, and 475 years) were produced. Based on deaggregation results, a discussion of the conditional mean spectra for engineering applications is presented.     

Seismic Rotational Displacements of Gravity Quay Walls Considering Excess Pore Pressure in Backfill Soils

The effect of excess pore pressure developed in backfill soil during earthquake is an important consideration in rotational displacement prediction of gravity quay walls. Based on Newmark’s sliding block concept and stress-based excess pore pressure model, a new method is proposed to predict the critical rotational acceleration and angular acceleration time histories considering the development process of excess pore pressure in earthquake events. Then, the rotational displacement of gravity quay walls is predicted according to the calculated angular acceleration time histories. By using the proposed method, the effects of various parameters involved in the calculation have been studied by carrying out a parameter study. Analysis results reveal that the influence of excess pore pressure on the rotational displacement of gravity quay walls with saturated backfill soil is significant, so, can not be ignored; and rotational displacement is sensitive to the magnitude of earthquake, horizontal and vertical seismic accelerations of ground motion, wall and soil friction angle, and soil relative density. When the rotation and sliding of wall occur simultaneously, rotation and sliding will be inhibited by each other.     

Considering Soil-Nonlinearity Effect on Seismic Performance Evaluation of Structures based on Pushover Analysis

According to the most of current seismic codes, nonlinear soil behavior is commonly ignored in seismic evaluation procedure of the structures. To contribute on this matter, a pushover analysis method incorporating the probabilistic seismic hazard analysis (PSHA) is proposed to evaluate the effect of nonlinear soil response on seismic performance of a structure. The PSHA outcomes considering soil nonlinearity effect is involved in the analysis procedures by modifying the site-specific response spectrum. Results showed that incorporation of nonlinear soil behavior leads to an increase in displacement demand of structures which should accurately be considered in seismic design/assessment procedure. Results of implemented procedure are confirmed with the estimated displacement demand including soil-structure interaction (SSI).     

Critical Issues on Probabilistic Earthquake Loss Assessment

Probabilistic loss modeling can be used to develop risk reduction measures, such as the identification of regions more prone to human and economic losses, or to develop financial mechanisms to transfer the earthquake risk from local governments to the private sector. This study addresses several critical issues in probabilistic loss modeling, and provides recommendations depending on the intended final use of the risk results. Modeling issues related to convergence in probabilistic event-based analysis; consideration of epistemic uncertainties within a logic tree; generation of different types of loss exceedance curves; and derivation of risk maps are thoroughly investigated. The Metropolitan Area of Lisbon is used to explore these issues, and it is demonstrated that different assumptions in the loss modeling process can lead to considerably different risk results. Furthermore, the findings and recommendations of this study are also relevant for institutions that promote the assessment of earthquake hazard and risk, such as the Global Earthquake Model Foundation.     

The economic development of West Africa

Almost all documented landslides in Scotland belong to one of four categories: non‐rotational rock slope failures (major rockfalls, translational slides, topples or sags, often in some combination); rotational rock slope failures; debris flows; and translational slides in drift or regolith. Non‐rotational rock slope failures are particularly common on Highland metamorphic rocks, especially schists, and on igneous scarps in the Hebrides and Midland Valley; rotational rock slope failures almost invariably occur in weak sedimentary rocks overlain by resistant igneous rocks; and debris flows are most common on sandy drift or regolith. Former glacial activity has also been of importance in determining landslide distribution. The great majority of rock slope failures are ancient features, but debris flows and small translational slides in drift (both triggered mainly by intensive rainstorms) are much more frequent at present and cause much greater damage, particularly to communications.     

Stochastic Modeling of Earthquake Scenarios in Greater Tehran

Tehran, the capital of Iran, with millions of inhabitants, has been affected several times by historical and recent earthquakes that confirm the importance of seismic hazard assessment for the area. The main objective of this article is to present a probabilistic procedure to construct time series compatible with the source-path and site reflecting the influence of different magnitude events at different distances that may occur during a specified time period. A Monte Carlo approach is used to generate numerous synthetic catalogs for the evaluation of the probabilistic seismic hazard in greater Tehran over hard rock site for a return period of 475 years. The disaggregation of the seismic hazard is carried out to identify hazard-dominating events and to associate them with one or more specific faults, rather than a given distance. The stochastic finite-fault technique based on region specific seismic parameters is used to generate time series of earthquake scenario.     

SEISMIC HAZARD ASSESSMENT OF METROPOLITAN TEHRAN,IRAN

This paper presents a probabilistic seismic hazard assessment of Tehran, the capital of Iran. Two maps have been prepared to indicate the earthquake hazard of Tehran and its vicinity in the form of iso-acceleration contour lines. They display the probabilistic estimate of Peak Ground Acceleration (PGA) over bedrock for the return periods of 475 and 950 years. Tehran is a densely populated metropolitan in which more than 10 million people live. Many destructive earthquakes happened in Iran in the last centuries. It comes from historical references that at least 6 times, Tehran has been destroyed by catastrophic earthquakes. The oldest one happened in the 4th century BC. A collected catalogue, containing both historical and instrumental events and covering the period from the 4th century BC to 1999 is then used. Seismic sources are modelled and recurrence relationship is established. For this purpose the method proposed by Kijko [2000] was employed considering uncertainty in magnitude and incomplete earthquake catalogue. The calculations were performed using the logic tree method and three weighted attenuation relationships; Ramazi [1999], 0.4, Ambraseys and Bommer [1991], 0.35, and Sarma and Srbulov [1996], 0.25. Seismic hazard assessment is then carried out for 12×11 grid points using SEISRISK III. Finally, two seismic hazard maps of the studied area based on Peak Ground Acceleration (PGA) over bedrock for 10% probability of exceedance in two life cycles of 50 and 100 years are presented. The results showed that the PGA ranges from 0.27(g) to 0.46(g) for a return period of 475 years and from 0.33(g) to 0.55(g) for a return period of 950 years. Since population is very dense in Tehran and vulnerability of buildings is high, the risk of future earthquakes will be very significant.     

SEISMIC WAVE ATTENUATION FOR VRANCEA EVENTS REVISITED

New aspects of the frequency-dependent attenuation of the seismic waves travelling from Vrancea subcrustal sources toward NW (Transylvanian Basin) and SE (Romanian Plain) are evidenced by the recent experimental data made available by the CALIXTO'99 tomography experiment. The observations validate the previous theoretical computations performed for the assessment, by means of a deterministic approach, of the seismic hazard in Romania. They reveal an essential aspect of the seismic ground motion attenuation that has important implications on the probabilistic assessment of seismic hazard from Vrancea intermediate-depth earthquakes. The attenuation toward NW is shown to be a much stronger frequency-dependent effect than the attenuation toward SE and the seismic hazard computed by the deterministic approach fits satisfactorily well the observed ground motion distribution in the low-frequency band (<1Hz). The apparent contradiction with the historically-based intensity maps arises mainly from a systematic difference in the eigenperiods (type and size) of the buildings in the intra- and extra-Carpathians regions, thus the existing macroseismic data, based on buildings of small dimensions, i.e. with high eigenfrequency (5–10 Hz), can hardly be representative of the real hazard for new and large dimension, tall buildings, with eigenfrequency above 1 Hz.     

Seismic Reliability of Code-Conforming Italian Buildings

ABSTRACT

This paper presents and discusses some research results related to the seismic failure risk of standard, residential and industrial, buildings designed for damage, and life-safety according to the Italian seismic code, which is somewhat similar to Eurocode 8. The five considered structural typologies are as follows: masonry, cast-in-place reinforced concrete, precast reinforced concrete, steel, and base-isolated buildings. The archetype structures have been designed according to standard practice at three sites, representative of the seismic hazard across the country. Seismic risk is defined here as the annual rate of earthquakes able to cause structural failure in terms of usability-preventing damage and global collapse. For each structure, the failure rates have been evaluated in the framework of performance-based earthquake engineering, that is, via integration of site’s probabilistic hazard and structural fragility. The former has been computed consistently with the official hazard model for Italy that is also used to define design actions in the code. The latter has been addressed via nonlinear dynamic analysis of three-dimensional numerical structural models. Results indicate that, generally, design procedures are such that seismic structural reliability tends to decrease with increasing seismic hazard of the building site, despite the homogeneous return period of exceedance of the design seismic ground-motion.     

高精度变形监测预警系统在明显陵遗址保护中的应用研究

为在遗址完整性保护与滑坡灾害预警方面制定切实有效的解决方案,使用基于高精度全球卫星定位系统GPS与远程遥测终端RTU的高精度变形监测预警系统,并结合无线通讯方式实现对遗址保护区陵墓边坡的安全监测。系统完成墓冢内部环境监测与滑坡方位的监测定位。并通过通用无线分组业务GPRS方式将监测参数与警情信号实时传输至监控中心,在管理软件平台上进行实时显示并提示安保人员及时出警。该系统可针对不同规模野外文物保护区和古墓群实现有效的滑坡监测与预警防范,具有广泛的应用推广价值。     

EARTHQUAKE DAMAGE SCENARIOS AND SEISMIC HAZARD OF MESSINA,NORTH-EASTERN SICILY (ITALY) AS INFERRED FROM HISTORICAL DATA

A study aimed at evaluating earthquake damage scenarios and seismic hazard of Messina using historical data, is presented. The analysis of coeval reports allowed us to reconstruct the seismic history of the city and to obtain a homogeneous earthquake site catalogue based on intensity assessed by the European Macroseismic Scale 1998. In the last 1200 years Messina was destroyed once (1908, intensity X-XI EMS) and suffered effects estimated between intensities VII and IX EMS many times (e.g. 853, 1169, 1494, 1509, 1599, 1693, 1783, 1894, 1909). Destruction or severe damage which affected the city are mainly related to earthquakes occurring in the Messina Straits and Southern Calabria, while slighter, moderate effects are usually due to shocks taking place in the seismogenic sources of SE Sicily, Gulf of Patti and Northern Calabria. The damage scenarios of the most relevant events, delineated using coeval urban plans of the city, showed that damage distribution is strongly conditioned by the different soil response. A probabilistic seismic hazard assessment was obtained by using site observed intensities: The expected intensity in a time span of 50 years (i.e. maximum intensity characterised by at least 10% exceedance probability in 50 years) is DC EMS; the expected intensity in a time span of 300 years (10% exceedance probability in 300 years) is X EMS.     

SENSITIVITY OF PARAMETERS FOR PROBABILISTIC SEISMIC HAZARD ANALYSIS USING A LOGIC TREE APPROACH

The sensitivity of different parameters used in probabilistic seismic hazard calculation is investigated by different logic tree runs with alternative magnitude sets, source zone models and attenuation relations, and with different sets of values for the seismicity parameters and the <r-value. Also the influence from the different parameters on the hazard uncertainty, represented by fractiles, is investigated. The calculations are made for peak ground acceleration at a site near Aachen in the Lower Rhine Embayment. The model where the site is located in a larger source zone gives lower hazard values. This is typical for the case where the seismicity near the site is high relative to its surrounding. The hazard curves for the different attenuation functions are similar, an effect of the similarity of the functions themselves. However, a large sensitivity of this parameter is indicated for small mean return periods. An increased α-value implies a moderate increase of the hazard at long mean return periods. The hazard is increasing for decreased focal depth, decreased β-value and increased maximum expected magnitude, respectively. However, the effects are noteworthy only at low hazard levels for variations in the focal depth and to some extent in the maximum expected magnitude. Finally, decreasing the minimum magnitude thought to be of engineering relevance causes a drastic increase of the hazard at small mean return periods.     

Predicting Probabilistic-Based Strong Ground Motion Time Series for Citadel of Arg-E-Bam (South-East of Iran)

The main objective of this article is to present a probabilistic-based strong motion compatible with the source-path and site soil condition given the probability of exceedence for citadel of Arg-e-Bam site bed rock (South-East of Iran). A Fourier amplitude spectral attenuation relation for bed rock beneath the site is proposed which permits the estimation of time-histories through a probabilistic seismic hazard analysis procedure. Due to lack of data, the two well-known simulation techniques, point source and finite fault models have been used for generating hundreds of strong motion as input data. Tens of model parameter values such as stress-drop nucleation points were used, in each specified magnitude-distance, to reduce the uncertainty effects inherently existing in seismological/geological parameters. The proposed attenuation relation is validated by comparing the estimated strong motion, in the form of Fourier amplitude spectral, using the proposed attenuation relation with those of recorded ground motion data at three stations far away from the assumed source so that the results would not be influenced by the near source problems such as directivity and fling step. The results of proposed technique is assessed by comparing the estimated response spectra, with 10% probability of exceedence and 5% damping ratio, with those of traditional uniform hazard spectra. The proposed technique is supposed to be used in retrofitting procedure of international historical adobe structures in Arg-e-Bam site, which have been damaged during the destructive Bam earthquake 2003, Iran     

Data Generation Processes for Spatial Series: The Example of Ephemeral Channel Form

Recently developed techniques for time series analysis are applied to channel width and slope series collected in small ephemeral channels in Southern Italy. Tests for the difference between trend and difference stationarity, although developed in the field of econometrics, are found to be more suitable for data generation processes in geomorphology, where physical causes of deterministic trends are evident. These tests are important because the modeling of causal flows and development of dynamic predictive models depends on the type of data generation process that is identified. The ephemeral channel data exhibit trend stationarity and the results of spectral analysis and vector autoregression analysis of detrended channel width and slope data are described.     

南京长江大桥桥头堡抗震加固受力性能研究

为研究南京长江大桥桥头堡填充墙加固对结构抗震性能的影响,提出考虑较高填充墙开裂的双斜撑模型,用于罕遇地震工况下桥头堡的抗震性能计算,同时提出考虑填充墙加固后刚度增强效应的建模方法。利用SAP2000建立了加固前后用于多遇地震工况、设防地震工况及罕遇地震工况的分析模型,并选取了桥头堡在1974年经历的实际地震激励时程及El-Centro地震激励时程作为激励进行了加固前后结构的抗震性能分析比较。研究发现,填充墙加固后桥头堡的抗震性能有了明显提升,其位移响应峰值约下降8%~23%,层间位移角响应峰值约下降12%~22%,而且桥头堡结构在设防地震情况下2个方向的层间位移角均满足了规范要求。另外,在罕遇地震工况下,未加固的填充墙开裂会使结构的扭转刚度下降,而填充墙加固可有效提升结构的扭转刚度,降低桥头堡在地震时发生扭转振动的概率。这2种地震荷载激励的分析结果差异约在3%~21%不等,且响应峰值出现的位置也有一些不同,故对桥头堡进行抗震时程分析时建议选取多条地震波输入综合分析。研究成果可为同类型的钢筋混凝土建筑遗产的抗震加固提供借鉴和参考。     

Response of Alum Rock springs to the October 30, 2007 Alum Rock earthquake and implications for the origin of increased discharge after earthquakes

The origins of increased stream flow and spring discharge following earthquakes have been the subject of controversy, in large part because there are many models to explain observations and few measurements suitable for distinguishing between hypotheses. On October 30, 2007 a magnitude 5.5 earthquake occurred near the Alum Rock springs, California, USA. Within a day we documented a several‐fold increase in discharge. Over the following year, we have monitored a gradual return towards pre‐earthquake properties, but for the largest springs there appears to be a permanent increase in discharge. The Alum Rock springs discharge waters that are a mixture between modern (shallow) meteoric water and old (deep) connate waters expelled by regional transpression. After the earthquake, there was a small and temporary decrease in the fraction of connate water in the largest springs. Accompanying this geochemical change was a small (1–2°C) temperature decrease. Combined with the rapid response, this implies that the increased discharge has a shallow origin. Increased discharge at these springs occurs both for earthquakes that cause static volumetric expansion and for those that cause contraction, supporting models in which dynamic strains are responsible for the subsurface changes that cause flow to increase. We make a quantitative comparison between the observed changes and model predictions for three types of models: (i) a permanent increase in permeability; (ii) an increase in permeability followed by a gradual decrease to its pre‐earthquake value; and (iii) an increase of hydraulic head in the groundwater system discharging at the springs. We show that models in which the permeability of the fracture system feeding the springs increases after the earthquake are in general consistent with the changes in discharge. The postseismic decrease in discharge could either reflect the groundwater system adjusting to the new, higher permeability or a gradual return of permeability to pre‐earthquake values; the available data do not allow us to distinguish between these two scenarios. However, the response of these springs to another earthquake will provide critical constraints on the changes that occur in the subsurface and should permit a test of all three types of models.     

SEISMIC HAZARD ASSESSMENT OF UNITED ARAB EMIRATES AND ITS SURROUNDINGS

This paper presents the seismic hazard assessment and seismic zoning of the United Arab Emirates (UAE) and its surroundings based on the probabilistic approach. The area that has been studied lies between 50°E-60°E and 20°N-30°N and spans several Gulf countries. First, the tectonics of the area and its surroundings is reviewed. An updated catalogue, containing both historical and instrumental events is used. Seismic source regions are modelled and relationships between earthquake magnitude and earthquake frequency is established. A modified attenuation relation for Zagros region is adopted. Seismic hazard assessment is then carried out for 20 km interval grid points. Seismic hazard maps of the studied area based on probable Peak Ground Acceleration (PGA) for 10% probability of exceedance for time-spans of 50, 100 and 200 years are shown. A seismic zone map is also shown for a 475-year return period. Although the results of the seismic hazard assessment indicated that UAE has moderate to low seismic hazard levels, nevertheless high seismic activities in the northern part of UAE warrant attention. The northern Emirates region is the most seismically active part of UAE. The PGA on bedrock in this region ranges between 0.22 g for a return period of 475 years to 0.38 g for a return period of 1900 years. This magnitude of PGA, together with amplification from local site effect, can cause structural damage to key structures and lifeline systems.