3-D seismic exploration data and the study of shallow marine geology

In 2005, Geology Today carried an article (v.22, n.5) describing how 3D exploration seismic data are being used to map sub-surface structure and stratigraphy, in fine detail, over hundreds and even thousands of square kilometres of the sea bed. The article in question was concerned primarily with the various ways in which 3D seismic data are applied to the study of stratigraphy and basin analysis. However, the potential uses of 3D exploration data for investigating other kinds of geological phenomena are many and varied, and significant advances may be made in our understanding of the geological past using such data. The present article describes how 3D seismic data can be used to investigate aspects of the upper Tertiary and Quaternary geology of marine areas, drawing on examples from the Central North Sea and West Shetland Slope. These originally came to light in the course of conducting geo-hazard surveys on behalf of the hydrocarbons industry.     

A check-up on seismic hazard assessment: Tirana case study

Based on the data of consequences of the 9 January 1988 earthquake that hit Tirana city, a comparative study is made of these consequences with the seismic hazard assessment by microzoning studies finished on the eve of this earthquake. It is shown that the methodologies used to assess the seismic hazard by engineering geology, geophysical, instrumental and analytical methods, follows the same trends as the distribution of the consequences of this earthquake.Paper presented at the 21st General Assembly of the European Seismological Commission, Sofia, 1988.     

银川盆地构造发展——深地震反射剖面揭示浅部地质与深部构造的联系

横跨银川盆地北西西向的深地震反射剖面,清晰揭示了银川盆地边界断裂以及整个地壳的结构构造特征,这对研究具活动大陆裂谷性质的银川盆地浅-深构造关系具有重大的意义。贺兰山东麓山前断裂、黄河断裂作为银川盆地的西、东边界断裂,前者为一条缓倾斜、延伸至上、下地壳边界的犁式断裂,而后者则为一条切穿地壳并延伸进入上地幔的深大断裂。根据深地震反射剖面揭示的地壳结构特征,银川盆地浅部结构并非前人认为的"堑中堑"结构,而是表现为由一系列东倾犁式正断层控制的新生代断陷。略微下凹的Moho面几何形态以及厚2~3.2 km的层状强反射带为下地壳最显著的反射特征。Moho面深度与强反射带厚度变化趋势与银川盆地沉积厚度变化趋势几乎一致。本文认为,强反射带的成因可能是由源自地幔的基性岩浆以岩席状的形式底侵进入地壳底部造成的,而这部分形成强反射带的物质可能补偿了因银川盆地断陷而造成的地壳减薄,最终导致银川盆地之下Moho面并未像之前所认为的那样隆起。     

A new evaluation of seismic hazard for the northwestern part of Saudi Arabia

Natural Hazards - A new evaluation of seismic hazard in the northwestern part of Saudi Arabia area has been carried out, for the minimization and mitigation of earthquake losses. The study area...     

汶川和芦山地震前后浅部应力对深部应力的响应及地震危险性分析

活动断层上盘浅部的高应力异常通常被认为是强震前兆之一,而低应力一般认为不会发生地震是安全的。然而多个震例表明,低应力状态下的地震危险性具有多解性。为进一步探讨强震前的应力异常特征,本研究利用2008年汶川M_S8.0级地震和2013年芦山M_S7.0级地震前的地应力测量结果和地震活动性参数b值,在考虑深部应力对浅部应力影响强度的基础上,建立了浅部应力对深部应力的响应模式,并对强震发生前不同的应力状态与地震危险性的关系进行了讨论。结果表明：活动断层上盘低b值对应浅部高地应力为潜在地震危险区,而对应低地应力则为地震危险性不确定区,需要借助更多的手段进一步确定;2008年汶川M_S8.0级地震前的低应力状态可能反映了区域应力存在较强的调整。本研究对运用断层浅部应力状态评估断层地震危险性具有一定的启示意义。     

Sensitivity analysis of seismic hazard for the northwestern portion of the state of Gujarat, India

We test the sensitivity of seismic hazard to three fault source models for the northwestern portion of Gujarat, India. The models incorporate different characteristic earthquake magnitudes on three faults with individual recurrence intervals of either 800 or 1600 years. These recurrence intervals imply that large earthquakes occur on one of these faults every 266–533 years, similar to the rate of historic large earthquakes in this region during the past two centuries and for earthquakes in intraplate environments like the New Madrid region in the central United States. If one assumes a recurrence interval of 800 years for large earthquakes on each of three local faults, the peak ground accelerations (PGA; horizontal) and 1-Hz spectral acceleration ground motions (5% damping) are greater than 1 g over a broad region for a 2% probability of exceedance in 50 years' hazard level. These probabilistic PGAs at this hazard level are similar to median deterministic ground motions. The PGAs for 10% in 50 years' hazard level are considerably lower, generally ranging between 0.2 g and 0.7 g across northwestern Gujarat. Ground motions calculated from our models that consider fault interevent times of 800 years are considerably higher than other published models even though they imply similar recurrence intervals. These higher ground motions are mainly caused by the application of intraplate attenuation relations, which account for less severe attenuation of seismic waves when compared to the crustal interplate relations used in these previous studies. For sites in Bhuj and Ahmedabad, magnitude (M) 7 3/4 earthquakes contribute most to the PGA and the 0.2- and 1-s spectral acceleration ground motion maps at the two considered hazard levels.     

Comparative study of three probabilistic methods for seismic hazard analysis: case studies of Sochi and Kamchatka

Natural Hazards - This study examines the effect of the procedures used in three different probabilistic seismic hazard analysis (PSHA) methods for estimating the rates of exceedance of ground...     

Classifying zones of suitability for manual drilling using textural and hydraulic parameters of shallow aquifers: a case study in northwestern Senegal

A method is proposed that uses analysis of borehole stratigraphic logs for the characterization of shallow aquifers and for the assessment of areas suitable for manual drilling. The model is based on available borehole-log parameters: depth to hard rock, depth to water, thickness of laterite and hydraulic transmissivity of the shallow aquifer. The model is applied to a study area in northwestern Senegal. A dataset of boreholes logs has been processed using a software package (TANGAFRIC) developed during the research. After a manual procedure to assign a standard category describing the lithological characteristics, the next step is the automated extraction of different textural parameters and the estimation of hydraulic conductivity using reference values available in the literature. The hydraulic conductivity values estimated from stratigraphic data have been partially validated, by comparing them with measured values from a series of pumping tests carried out in large-diameter wells. The results show that this method is able to produce a reliable interpretation of the shallow hydrogeological context using information generally available in the region. The research contributes to improving the identification of areas where conditions are suitable for manual drilling. This is achieved by applying the described method, based on a structured and semi-quantitative approach, to classify the zones of suitability for given manual drilling techniques using data available in most African countries. Ultimately, this work will support proposed international programs aimed at promoting low-cost water supply in Africa and enhancing access to safe drinking water for the population.     

Influence of slow active faults in probabilistic seismic hazard assessment: the northwestern margin of the València trough

Areas of low strain rate are typically characterized by low to moderate seismicity. The earthquake catalogs for these regions do not usually include large earthquakes because of their long recurrence periods. In cases where the recurrence period of large earthquakes is much longer than the catalog time span, probabilistic seismic hazard is underestimated. The information provided by geological and paleo-seismological studies can potentially improve seismic hazard estimation through renewal models, which assume characteristic earthquakes. In this work, we compare the differences produced when active faults in the northwestern margin of the València trough are introduced in hazard analysis. The differences between the models demonstrate that the introduction of faults in zones characterized by low seismic activity can give rise to significant changes in the hazard values and location. The earthquake and fault seismic parameters (recurrence interval, segmentation or fault length that controls the maximum magnitude earthquake and time elapsed since the last event or T_e) were studied to ascertain their effect on the final hazard results. The most critical parameter is the recurrence interval, where shorter recurrences produce higher hazard values. The next most important parameter is the fault segmentation. Higher hazard values are obtained when the fault has segments capable of producing big earthquakes. Finally, the least critical parameter is the time elapsed since the last event (T_e), when longer T_e produces higher hazard values.     

基于斜坡单元的滑坡风险识别

识别斜坡地质灾害风险已成为西南山区地质灾害防治的重要基础工作,通过GIS软件划分出孕灾斜坡单元,选取坡度、坡高、覆盖层厚度、滑体土类型、人类工程活动、日最大降雨量等6项指标作为评价因子,利用层次分析法开展山区大比例尺地质灾害危险性评价,初步评价出研究区危险性斜坡。以万山区1:1万地质灾害风险调查获取的斜坡剖面模型为基础,通过取样、试验,综合选取岩土体物理力学参数,利用有限元软件对暴雨状态下的高危险斜坡进行数值模拟,通过强度折减计算出安全系数均小于1.05,并存在不同程度的塑性贯通区。其分析结果与层次分析法评价结果一致,可作为西南山区浅层土质滑坡隐患识别的定量化评价方法。     

Hydrodynamic response of subduction zones to seismic activity: A case study for the Costa Rica margin

Thermal anomalies in tectonically active areas are often attributed to sub-seafloor fluid circulation and faulting mechanisms, particularly in subduction zones where the largest thrust earthquakes occur. Postseismic fluid flow is enabled by the poroelastic response of the fault system to the earthquake's strain field, as well as by the rupturing of permeability barriers in the vicinity of the fault zone. We investigated the relative importance of these mechanisms on postseismic pore-pressure diffusion and advective heat transport in the subduction zone setting. A two-dimensional numerical fluid flow and heat transport model was developed for the Costa Rica subduction zone offshore of the Nicoya Peninsula. The flow and transport model was coupled with an earthquake strain model to quantify the effects of coseismic strain and permeability enhancement on fluid pressures and temperatures within the Costa Rica margin. Coseismic changes in pore pressure and postseismic pore-pressure diffusion were found to be sensitive to the compressibility of the porous medium, and patterns of pore-pressure recovery were more complex than that predicted by theoretical faulting models. Coseismic contraction and extension of the crust produced high fluid pressures close to the fault, while the inflow of fluid from depth increased fluid pressures several years following the simulated fault slip. Crustal deformation alone was not observed to perturb the temperature field. Laterally extensive permeability increases of two orders of magnitude along the décollement were required to produce small changes in heat flow. Local permeability changes in the upper slope region of least five orders of magnitude were necessary to noticeably affect heat flow. The results of the numerical simulations may help to refine conceptual faulting models and provide guidance for locating long-term hydrologic monitoring sites at Costa Rica and other subduction zones.     

浅层高分辨率反射地震试验研究：以雄安新区第四系为例

雄安新区城市规划建设对地下空间的高精度勘探提出了高要求,作为具有高分辨率的地面物探方法,反射波地震勘探技术具有良好应用前景。本文探究了厚覆盖区条件下的高精度有效反射地震探测技术,并利用反射纵波技术在雄安新区进行提高浅层分辨率的地震勘探方法试验。根据实际探测目标,通过对比分析不同检波器、震源类型、道距大小及接收排列长度等不同采集因素试验效果,探究已有条件下适宜的震源类型和最佳采集参数。结合实际资料处理和分层解释,试验研究表明,采用高频检波器、选择可控震源激发效果最佳,针对试验目标,结合小道距、适当排列接收等措施,可有效提高厚覆盖区50~300 m深度内反射纵波地震勘探的纵向分辨率,试验获得了丰富的有效波组信息,为内部地层精细划分和解释提供了依据。该方法可为类似环境下城市地下空间的精细探测提供有益参考借鉴。     

Probabilistic seismic hazard analysis of interconnected infrastructure: a case of Iranian high-pressure gas supply system

Seismic hazard maps are widely used for engineering design, land-use planning, and disaster mitigation. The development of the new seismic hazard map of Iran with regard to the specification of Iranian high-pressure gas network is based on probabilistic seismic hazard analysis using the historical and new earthquakes data, geology, tectonics, fault activity, and seismic zone models in Iran. The map displays the probabilistic estimates of peak ground acceleration for the return period of 2,475 year (2 % probability in 50 years). The results presented in this study will provide the basis for the preparation of risk map, the estimation of insurance premiums, finding best paths for future pipelines, planning, and relocating lifeline facilities especially for interconnected infrastructures.     

A Moore's cellular automaton model to get probabilistic seismic hazard maps for different magnitude releases: A case study for Greece

Cellular automata are simple mathematical idealizations of natural systems and they supply useful models for many investigations in natural science. Examples include sandpile models, forest fire models, and slider block models used in seismology. In the present paper, they have been used for establishing temporal relations between the energy releases of the seismic events that occurred in neighboring parts of the crust. The catalogue is divided into time intervals, and the region is divided into cells which are declared active or inactive by means of a threshold energy release criterion. Thus, a pattern of active and inactive cells which evolves over time is determined. A stochastic cellular automaton is constructed starting with these patterns, in order to simulate their spatio-temporal evolution, by supposing a Moore's neighborhood interaction between the cells. The best model is chosen by maximizing the mutual information between the past and the future states. Finally, a Probabilistic Seismic Hazard Map is given for the different energy releases considered. The method has been applied to the Greece catalogue from 1900 to 1999. The Probabilistic Seismic Hazard Maps for energies corresponding to m = 4 and m = 5 are close to the real seismicity after the data in that area, and they correspond to a background seismicity in the whole area. This background seismicity seems to cover the whole area in periods of around 25–50 years. The optimum cell size is in agreement with other studies; for m > 6 the optimum area increases according to the threshold of clear spatial resolution, and the active cells are not so clustered. The results are coherent with other hazard studies in the zone and with the seismicity recorded after the data set, as well as provide an interaction model which points out the large scale nature of the earthquake occurrence.     

Integrating long-term seismic risk changes into improving emergency response and land-use planning: a case study for the Hsinchu City, Taiwan

An increasing number of rapidly growing urban areas in Asia are becoming more vulnerable to seismic hazards in their development process. However, local authorities rarely integrate seismic risk into the procedure of emergency and land-use planning. This article explores the question of whether seismic risks for urban areas are increasing or diminishing over time, while trends such as population growth and land development in hazard-prone areas increase the potential for loss in disasters. The net effects of such urbanization factors are examined through the use of simulation models that estimate building inventory and seismic loss changes. Seismic losses are modeled for a comparative analysis under the same hypothetical earthquake events hitting at different points in a city area’s long-term development. A case study of seismic risk assessments is illustrated by the Hsinchu City, Taiwan. Results of a prospective analysis indicate that, for the same seismic events, overall risk is expected to increase due to a forecast 2.9 % growth in building inventory. This increment in loss is largely attributed to a large amount of initial buildings predicted to be developed into commercial and industrial uses. However, the spatial pattern of risk would change slightly; particularly, the southeastern, eastern, and some older core areas would be the most vulnerable and risky both at current and future time periods. The approach here enables city planners to incorporate seismic risk analysis into predisaster emergency and land-use planning to encourage risk-reduction strategies.     

Numerical evaluation of seismic response of shallow foundation on loose silt and silty sand

This study includes the results of a set of numerical simulations carried out for sands containing plastic/non-plastic fines, and silts with relative densities of approximately 30?40% under different surcharges on the shallow foundation using FLAC 2D. Each model was subjected to three ground motion events, obtained by scaling the amplitude of the El Centro (1940), Kobe (1995) and Kocaeli (1999) Q12earthquakes. Dynamic behaviour of loose deposits underlying shallow foundations is evaluated through fully coupled nonlinear effective stress dynamic analyses. Effects of nonlinear soil structure interaction (SSI) were also considered by using interface elements. This parametric study evaluates the effects of soil type, structure weight, liquefiable soil layer thickness, event parameters (e.g., moment magnitude of earthquake (M _w ), peak ground acceleration PGA, PGV/PGA ratio and the duration of strong motion (D _5?95) and their interactions on the seismic responses. Investigation on the effects of these parameters and their complex interactions can be a valuable tool to gain new insights for improved seismic design and construction.     

Qualitative hazard and risk assessment of landslides: a practical framework for a case study in China

Natural Hazards - Landslides are presented in various types; some of which are unique or completely different from those in other countries due to geological conditions in China. Baoji City in...