Probabilistic assessment of travel times in groundwater modeling

A Monte Carlo approach is described for the quantification of uncertainty on travel time estimates. A real (non synthetic) and exhaustive data set of natural genesis is used for reference. Using an approach based on binary indicators, constraint interval data are easily accommodated in the modeling process. It is shown how the incorporation of imprecise data can reduce drastically the uncertainty in the estimates. It is also shown that unrealistic results are obtained when a deterministic modeling is carried out using a kriging estimate of the transmissivity field. Problems related with using sequential indicator simulation for the generation of fields incorporating constraint interval data are discussed. The final results consists of 95% probability intervals of arrival times at selected control planes reflecting the original uncertainty on the transmissivity maps.     

Probabilistic assessment of travel times in groundwater modeling

A Monte Carlo approach is described for the quantification of uncertainty on travel time estimates. A real (non synthetic) and exhaustive data set of natural genesis is used for reference. Using an approach based on binary indicators, constraint interval data are easily accommodated in the modeling process. It is shown how the incorporation of imprecise data can reduce drastically the uncertainty in the estimates. It is also shown that unrealistic results are obtained when a deterministic modeling is carried out using a kriging estimate of the transmissivity field. Problems related with using sequential indicator simulation for the generation of fields incorporating constraint interval data are discussed. The final results consists of 95% probability intervals of arrival times at selected control planes reflecting the original uncertainty on the transmissivity maps.     

Probabilistic sinkhole modelling for hazard assessment

A method for quantitatively assessing sinkhole susceptibility (spatial probability) and hazard (spatio‐temporal probability) has been developed and independently tested in a 50 km² sector of the Ebro Valley evaporite karst. Three genetic types of sinkholes have been mapped in the floodplain and a terrace surface: 947 small cover‐collapse sinkholes (type 1, terrace), large collapse sinkholes (type 2, floodplain) and large subsidence depressions (type 3, floodplain). The type 1 sinkhole inventory includes two temporal populations: 447 sinkholes formed before 24 November 2005, and 500 between that date and 2 November 2006. Sinkhole susceptibility models have been elaborated analysing the statistical relationships between the sinkholes of the 2005 inventory and a set of potential conditioning factors. The independent evaluation (validation) of the susceptibility models by means of several strategies (random, sequentially excluded, and temporal) has allowed us to select the most significant variables for each sinkhole type and assess quantitatively the quality of models; which are reasonable for the three sinkhole types. Validation has also provided information on the contribution of specific variables and the effect of changing their accuracy to the prediction capability of models. Susceptibility models for type 3 sinkholes have been validated satisfactorily with the 2006 sinkhole inventory (temporal validation). The best susceptibility model has been transformed into a hazard map considering the frequency of sinkholes that occurred in each susceptibility class between 2005 and 2006, as well as their average size. The susceptibility and hazard models obtained could be used as an objective basis for the application of mitigation measures, either of preventive or corrective nature. Copyright © 2008 John Wiley & Sons, Ltd.     

Probabilistic seismic hazard assessment for Thailand

A set of probabilistic seismic hazard maps for Thailand has been derived using procedures developed for the latest US National Seismic Hazard Maps. In contrast to earlier hazard maps for this region, which are mostly computed using seismic source zone delineations, the presented maps are based on the combination of smoothed gridded seismicity, crustal-fault, and subduction source models. Thailand’s composite earthquake catalogue is revisited and expanded, covering a study area limited by 0°–30°N Latitude and 88°–110°E Longitude and the instrumental period from 1912 to 2007. The long-term slip rates and estimates of earthquake size from paleoseismological studies are incorporated through a crustal fault source model. Furthermore, the subduction source model is used to model the megathrust Sunda subduction zones, with variable characteristics along the strike of the faults. Epistemic uncertainty is taken into consideration by the logic tree framework incorporating basic quantities, such as different source modelling, maximum cut-off magnitudes and ground motion prediction equations. The ground motion hazard map is presented over a 10 km grid in terms of peak ground acceleration and spectral acceleration at 0.2, 1.0, and 2.0 undamped natural periods and a 5% critical damping ratio for 10 and 2% probabilities of exceedance in 50 years. The presented maps give expected ground motions that are based on more extensive data sources than applied in the development of previous maps. The main findings are that northern and western Thailand are subjected to the highest hazard. The largest contributors to short- and long-period ground motion hazard in the Bangkok region are from the nearby active faults and Sunda subduction zones, respectively.     

Probabilistic earthquake hazard assessment for Peninsular India

In this paper, a new probabilistic seismic hazard assessment (PSHA) is presented for Peninsular India. The PSHA has been performed using three different recurrence models: a classical seismic zonation model, a fault model, and a grid model. The development of a grid model based on a non-parameterized recurrence model using an adaptation of the Kernel-based method that has not been applied to this region before. The results obtained from the three models have been combined in a logic tree structure in order to investigate the impact of different weights of the models. Three suitable attenuation relations have been considered in terms of spectral acceleration for the stable continental crust as well as for the active crust within the Gujarat region. While Peninsular India has experienced large earthquakes, e.g., Latur and Jabalpur, it represents in general a stable continental region with little earthquake activity, as also confirmed in our hazard results. On the other hand, our study demonstrates that both the Gujarat and the Koyna regions are exposed to a high seismic hazard. The peak ground acceleration for 10 % exceedance in 50 years observed in Koyna is 0.4 g and in the Kutch region of Gujarat up to 0.3 g. With respect to spectral acceleration at 1 Hz, estimated ground motion amplitudes are higher in Gujarat than in the Koyna region due to the higher frequency of occurrence of larger earthquakes. We discuss the higher PGA levels for Koyna compared Gujarat and do not accept them uncritically.     

Probabilistic seismic hazard assessment for South-Eastern France

The accurate evaluation and appropriate treatment of uncertainties is of primary importance in modern probabilistic seismic hazard assessment (PSHA). One of the objectives of the SIGMA project was to establish a framework to improve knowledge and data on two target regions characterized by low-to-moderate seismic activity. In this paper, for South-Eastern France, we present the final PSHA performed within the SIGMA project. A new earthquake catalogue for France covering instrumental and historical periods was used for the calculation of the magnitude-frequency distributions. The hazard model incorporates area sources, smoothed seismicity and a 3D faults model. A set of recently developed ground motion prediction equations (GMPEs) from global and regional data, evaluated as adequately representing the ground motion characteristics in the region, was used to calculate the hazard. The magnitude-frequency distributions, maximum magnitude, faults slip rate and style-of-faulting are considered as additional source of epistemic uncertainties. The hazard results for generic rock condition (Vs30 = 800 m/s) are displayed for 20 sites in terms of uniform hazard spectra at two return periods (475 years and 10,000 years). The contributions of the epistemic uncertainties in the ground motion characterizations and in the seismic source characterization to the total hazard uncertainties are analyzed. Finally, we compare the results with existing models developed at national scale in the framework of the first generation of models supporting the Eurocode 8 enforcement, (MEDD 2002 and AFPS06) and at the European scale (within the SHARE project), highlighting significant discrepancies at short return periods.     

Probabilistic seismic hazard assessment of the Pyrenean region

A unified probabilistic seismic hazard assessment (PSHA) for the Pyrenean region has been performed by an international team composed of experts from Spain and France during the Interreg IIIA ISARD project. It is motivated by incoherencies between the seismic hazard zonations of the design codes of France and Spain and by the need for input data to be used to define earthquake scenarios. A great effort was invested in the homogenisation of the input data. All existing seismic data are collected in a database and lead to a unified catalogue using a local magnitude scale. PSHA has been performed using logic trees combined with Monte Carlo simulations to account for both epistemic and aleatory uncertainties. As an alternative to hazard calculation based on seismic sources zone models, a zoneless method is also used to produce a hazard map less dependant on zone boundaries. Two seismogenic source models were defined to take into account the different interpretations existing among specialists. A new regional ground-motion prediction equation based on regional data has been proposed. It was used in combination with published ground-motion prediction equations derived using European and Mediterranean data. The application of this methodology leads to the definition of seismic hazard maps for 475- and 1,975-year return periods for spectral accelerations at periods of 0 (corresponding to peak ground acceleration), 0.1, 0.3, 0.6, 1 and 2 s. Median and percentiles 15% and 85% acceleration contour lines are represented. Finally, the seismic catalogue is used to produce a map of the maximum acceleration expected for comparison with the probabilistic hazard maps. The hazard maps are produced using a grid of 0.1°. The results obtained may be useful for civil protection and risk prevention purposes in France, Spain and Andorra.     

Probabilistic assessment for seismic performance of port structures

Past experience has shown that ports are often susceptible to severe damage during earthquakes. From field damage data of 1995 Kobe earthquake, it is observed that the seismic behavior of port structures shows significant variability. In this study, a 2D numerical model, representing PC1 berth located in Port Island, Kobe and damaged in the 1995 Kobe earthquake, has been developed and used to simulate seismic behavior. It has been found that the uncertainties in the friction angle and the shear modulus of reclaimed soil contribute most to the variability of the residual horizontal displacement (RHD) response of the quay wall of port structures. To investigate the propagation of uncertainties of soil–structure system to the quay wall, a tornado diagram and a first-order second-moment analysis are used. Uncertainty of ground motions has also been investigated. Based on the results, design considerations have been provided.     

Probabilistic seismic hazard assessment of Eastern Marmara Region

The objective of this study is to evaluate the seismic hazard in Eastern Marmara Region using an improved probabilistic seismic hazard assessment methodology. Two significant improvements over the previous seismic hazard assessment practices are accomplished in this study: advanced seismic source characterization models in terms of source geometry and recurrence relationships are developed, and improved global ground motion models (NGA-W1 models) are employed to represent the ground motion variability. Planar fault segments are defined and a composite magnitude distribution model is used for all seismic sources in the region to properly represent the characteristic behavior of the North Anatolian Fault without the need for an additional background zone. Multi-segment ruptures are considered using the rupture model proposed by the Working Group on California Earthquake Probabilities (2003). Events in the earthquake catalogue are attributed to the fault zones and scenario weights are determined by releasing the accumulated seismic energy. The uniform hazard spectra at 10 % probability of exceedance in 50 years hazard level for different soil conditions (soil and rock) are revealed for specific locations in the region (Adapazar?, Düzce, Gemlik, Izmit, Iznik and Sapanca). Hazard maps of the region for rock site conditions at the selected hazard levels are provided to allow the readers perform site-specific hazard assessment and develop site-specific design spectrum for local site conditions.     

复杂构造地震波场分析(英文)

在我国的西部地区,地震波场十分复杂,信噪比低。本文采用波动方程正演模拟的方法研究复杂波场的形成原因。在模拟物性差异较大介质中的地震波场时,密度的影响不可忽略,因此,本文用含密度项的声波方程的交错网格有限差分法模拟地震波场并进行分析。设计了一个具有起伏地表、低速覆盖层和高速地层出露的复杂构造,从瞬时波场分布分析了形成复杂波场的原因。低速层对地震波场的影响明显,低速层中产生很强的槽波,低速层顶底形成的多次反射向地下传播又形成了复杂的反射波场。为了验证波场模拟结果的可靠性,对模拟波场用与正演模拟不同的算法进行叠前深度偏移,得到了与已知构造相同的偏移叠加剖面。     

Probabilistic seismic hazard assessment of southern part of Ghana

This paper presents a seismic hazard map for the southern part of Ghana prepared using the probabilistic approach, and seismic hazard assessment results for six cities. The seismic hazard map was prepared for 10% probability of exceedance for peak ground acceleration in 50 years. The input parameters used for the computations of hazard were obtained using data from a catalogue that was compiled and homogenised to moment magnitude (Mw). The catalogue covered a period of over a century (1615–2009). The hazard assessment is based on the Poisson model for earthquake occurrence, and hence, dependent events were identified and removed from the catalogue. The following attenuation relations were adopted and used in this study—Allen (for south and eastern Australia), Silva et al. (for Central and eastern North America), Campbell and Bozorgnia (for worldwide active-shallow-crust regions) and Chiou and Youngs (for worldwide active-shallow-crust regions). Logic-tree formalism was used to account for possible uncertainties associated with the attenuation relationships. OpenQuake software package was used for the hazard calculation. The highest level of seismic hazard is found in the Accra and Tema seismic zones, with estimated peak ground acceleration close to 0.2 g. The level of the seismic hazard in the southern part of Ghana diminishes with distance away from the Accra/Tema region to a value of 0.05 g at a distance of about 140 km.     

Probabilistic seismic demand model for pounding risk assessment

Earthquake‐induced pounding of adjacent structures can cause severe structural damage, and advanced probabilistic approaches are needed to obtain a reliable estimate of the risk of impact. This study aims to develop an efficient and accurate probabilistic seismic demand model (PSDM) for pounding risk assessment between adjacent buildings, which is suitable for use within modern performance‐based engineering frameworks. In developing a PSDM, different choices can be made regarding the intensity measures (IMs) to be used, the record selection, the analysis technique applied for estimating the system response at increasing IM levels, and the model to be employed for describing the response statistics given the IM. In the present paper, some of these choices are analyzed and evaluated first by performing an extensive parametric study for the adjacent buildings modeled as linear single‐degree‐of‐freedom systems, and successively by considering more complex nonlinear multi‐degree‐of‐freedom building models. An efficient and accurate PSDM is defined using advanced intensity measures and a bilinear regression model for the response samples obtained by cloud analysis. The results of the study demonstrate that the proposed PSDM allows accurate estimates of the risk of pounding to be obtained while limiting the number of simulations required. Copyright © 2016 John Wiley & Sons, Ltd.     

Probabilistic seismic response and reliability assessment of isolated bridges

Bridge seismic isolation strategy is based on the reduction of shear forces transmitted from the superstructure to the piers by two means: shifting natural period and earthquake input energy reduction by dissipation concentrated in protection devices. In this paper, a stochastic analysis of a simple isolated bridge model for different bridge and device parameters is conducted to assess the efficiency of this seismic protection strategy. To achieve this aim, a simple nonlinear softening constitutive law is adopted to model a wide range of isolation devices, characterized by only three essential mechanical parameters. As a consequence of the random nature of seismic motion, a probabilistic analysis is carried out and the time modulated Kanai-Tajimi stochastic process is adopted to represent the seismic action. The response covariance in the state space is obtained by solving the Lyapunov equation for a stochastic linearized system. After a sensitivity analysis, the failure probability referred to extreme displacement and the mean value of dissipated energy are assessed by using the introduced stochastic indices of seismic bridge protection efficiency. A parametric analysis for protective devices with different mechanical parameters is developed for a proper selection of parameters of isolation devices under different situations.     

Space-time ray method for seismic wave fields

Summary The space-time ray method can be applied to the evaluation and continuation (extrapolation) of the complete seismic wave field in laterally inhomogeneous media with curved interfaces. The wave field propagates along certain space-time curves, called space-time rays. Their space projections correspond to standard rays. Examples of possible applications of the space-time ray method, where the standard ray method fails, are as follows: a) The propagation of seismic waves in slightly dissipative media, b) The computation of seismic wave fields generated by seismic sources with direction-dependent source-time variations. c) Downward continuation of the seismic wave field (actual seismograms) measured at the Earth's surface.     

ULF wave occurrence statistics in a high-latitude HF Doppler sounder

Ultra low frequency (ULF) wave activity in the high-latitude ionosphere has been observed by a high frequency (HF) Doppler sounder located at Tromsø, Norway (69.71°N, 19.2°E geographic coordinates). A statistical study of the occurrence of these waves has been undertaken from data collected between 1979 and 1984. The diurnal, seasonal, solar cycle and geomagnetic activity variations in occurrence have been investigated. The findings demonstrate that the ability of the sounder to detect ULF wave signatures maximises at the equinoxes and that there is a peak in occurrence in the morning sector. The occurrence rate is fairly insensitive to changes associated with the solar cycle but increases with the level of geomagnetic activity. As a result, it has been possible to characterise the way in which prevailing ionospheric and magnetospheric conditions affect such observations of ULF waves.     

淀山湖风浪场的数值模拟

对淀山湖的风场和风浪场进行了观测,根据所观测的风场利用SWAN模型计算了湖区的风浪场.通过将计算的波高和观测值进行比较,说明计算结果的变化趋势和观测结果的变化趋势相吻合;观测结果的波动幅度较大,而计算结果比较平滑.总体说来,基于SWAN模型所计算的波高可信、可靠.在此基础上,利用SWAN模型较为系统地计算了不同水位和不...     

Simulation and analysis of point-source surface wave fields

The complexity of near surface intensifies the diversity of seismic wave fields, which makes study on near surface wavefields important in many aspects. The strong absorption of low velocity layer can affect the resolution of seismic data, and free boundary can cause surface wave. Considering the above problems, we focus on the Rayleigh wavefields simulation using finite-difference wave equation of higher-order staggered grids and PML boundary conditions. Free boundary, buried source and overlying low velocity layer are taken into consideration and point explosion source is adopted. Through some numerical simulation with different parameters, we quantitatively analyze relationship between wave intensity and source depth, as well as the energy variation with propagation and obtain some practical knowledge and conclusions.