Ground motion characterization and seismic hazard assessment in Spain: context,problems and recent developments

This article points out some particular features conditioning seismic hazard assessments (SHA) in Spain, a region with low–moderate seismicity. Although sized earthquakes occurred in the past, as evidenced by historical documents and neotectonic studies, no large events occurred during the last decades. The absence of strong motion records corresponding to earthquakes with magnitude larger than 5.5 is an important obstacle for the development of ground motion models constrained by local data, with the consequent difficulty in SHA studies. In this paper, some recent developments aiming at providing solutions to these difficulties are presented. Specifically, a strong motion databank containing a massive collection of accelerograms and response spectra from different configurations source-path-site corresponding to earthquakes all over the world is introduced, together with software utilities for its management. A first application of this databank is the development of specific ground motion models for Spain and for the Mediterranean region that predict peak ground accelerations as a function of several definitions of magnitude, distance and soil class. The predictive power of these ground motion models is tested by contrasting their estimates with recently recorded ground motions. The comparison between our ground-motion models with others proposed in the literature for other areas reveals a regular overestimation of the expected ground motions at Spanish sites by the non-local models. Consequently, SHA studies based in external models may overestimate the predicted hazard at the Iberian sites. In the last part of the paper a method for checking whether the response spectra proposed in the Spanish Building Code (NCSE-02) are consistent with actual accelerometric data from recent low magnitude earthquakes is applied. The spectral shapes of the Spanish Building Code NCSE-02 are compared with the response spectral shapes deduced from the available accelerograms by normalising the response spectra with the recorded PGA. It is appreciated that the NCSE-02 spectral shapes are exceeded by a large number of actual spectral shapes for short periods (around 0.2 s), a result to be taken into account in further revisions of the NCSE-02 code. The issues tackled in this work constitute not only an improvement for ground-motion characterisation in Spain, but also provide guidelines of general interest for potential applications in other regions with similar seismicity.     

Corresponding Relation Between the Space-time Evolution of Seismic Apparent Strain and the Region of Strong Earthquakes in Yunnan

On the basis of our predecessors' research,we study the distribution and the space-time evolution characteristics of the seismic apparent strain field in Yunnan since the 1970's using the seismic data of Yunnan and its surrounding areas.The result shows that there is a rather strong corresponding relationship between the anomaly region of seismic apparent strain and strong earthquakes.In the nine earthquakes studied,anomaly areas of seismic apparent strain had appeared before eight earthquakes,including five occurring in the anomaly region and three on the edge.Finally,the investigative result is demonstrated primarily.     

强地震前乾陵台地倾斜短临暂态异常图像特征研究

系统整理分析了1983—2008年乾陵台地倾斜仪记录到的暂态异常图像,发现其与周围4000km以内6．5级以上强震有一定的对应关系。异常图像以渐变、突变阶跃、畸变及脉动为主要特征,异常出现时间为震前0-40天,主要集中在震前4天。     

龙岩地震台GM4型磁通门磁力仪典型干扰分析

GM4型磁通门磁力仪是"十五"项目中地磁学科主要仪器。龙岩台GM4型磁通门磁力仪主要受到来自铁磁性物质和电磁类型的干扰。由已知干扰类型,对记录曲线典型畸变形态进行对比、分析、总结,可以更好分辨干扰,进行恰当、合理、准确地预处理,保证数据的连续、可靠、精确。     

中国大陆及邻区大尺度浅源强震空间分布的不均匀性和板块耦合的力学机制

对近代中国大陆及其邻区板内浅源强震空间分布的大尺度不均匀性进行研究，得出：（１）该区存在４个主要的高地震活动区，即华北地震区（３０°～４２°Ｎ）、东南沿海地震区（１９°～２５°Ｎ）、南北地震区（缅甸—中国—蒙古）和中亚地震区（帕米尔—天山—贝加尔湖）；（２）这４个地震区与该区周围板块（欧亚板块与北美—太平洋—菲律宾海—印度板块）之间边界上地震耦合强度大的段落大致垂直；（３）作用于板块边界不同段落上的应力水平差异，可能是中国大陆及其邻区板内近代大尺度强震活动空间分布不均匀性的控制因素     

Probabilistic aftershock hazard assessment II: application of strong ground motion simulations

Probabilistic aftershock hazard assessment (PAHA) has been introduced by Wiemer (Geophys Res Lett 27:3405–3408, 2000). The method, in its original form, utilizes attenuation relations in evaluating peak ground velocity (PGV) exceedence probability. We substitute the attenuation relations together with their uncertainties by strong ground motion simulations for a set of scenarios. The main advantage of such an approach is that the simulations account for specific details of the aftershock source effects (faulting style, slip distribution, position of the nucleation point, etc.). Mean PGVs and their standard deviations are retrieved from the simulation results obtained by the new hybrid k-squared source model, and they are used for the PAHA analysis at a station under study. The model chosen for the testing purposes is inspired by the Izmit A25 aftershock (M _w  = 5.8) that occurred 26 days after the mainshock. The PAHA maps are compared with (1) those obtained by the use of attenuation relations and (2) the peak values of ten selected strong-motion recordings written by the aftershock at epicentral distances <50 km. We conclude that, although the overall hazard decay with increasing fault distance is similar, the PAHA maps obtained by the use of simulations exhibit remanent radiation pattern effect and prolongation in the strike direction due to the directivity effect pronounced for some of the scenarios. As regard the comparison with real data, we conclude that the PAHA maps agree with observed peak values due to appropriate attenuation model adopted in the analysis.     

Truncation of the distribution of ground-motion residuals

Recent studies to assess very long-term seismic hazard in the USA and in Europe have highlighted the importance of the upper tail of the ground-motion distribution at the very low annual frequencies of exceedance required by these projects. In particular, the use of an unbounded lognormal distribution to represent the aleatory variability of ground motions leads to very high and potentially unphysical estimates of the expected level of shaking. Current practice in seismic hazard analysis consists of truncating the ground-motion distribution at a fixed number (ε _max) of standard deviations (σ). However, there is a general lack of consensus regarding the truncation level to adopt. This paper investigates whether a physical basis for choosing ε _max can be found, by examining records with large positive residuals from the dataset used to derive one of the ground-motion models of the Next Generation Attenuation (NGA) project. In particular, interpretations of the selected records in terms of causative physical mechanisms are reviewed. This leads to the conclusion that even in well-documented cases, it is not possible to establish a robust correlation between specific physical mechanisms and large values of the residuals, and thus obtain direct physical constraints on ε _max. Alternative approaches based on absolute levels of ground motion and numerical simulations are discussed. However, the choice of ε _max is likely to remain a matter of judgment for the foreseeable future, in view of the large epistemic uncertainties associated with these alternatives. Additional issues arise from the coupling between ε _max and σ, which causes the truncation level in terms of absolute ground motion to be dependent on the predictive equation used. Furthermore, the absolute truncation level implied by ε _max will also be affected if σ is reduced significantly. These factors contribute to rendering a truncation scheme based on a single ε _max value impractical.     

中国大陆强震活动时空特征分析

本文在以往研究的基础上分析中国大陆强震活动的时空特征, 以中国大陆1901年以来7级及其以上强震年频次为样本, 采用最优分割将中国大陆地震活动状态分为三种时段: 少发段、 平均段和频发段。 中国大陆强震时间活动呈现非稳态现象, 强震频发段间存在短期的地震平静, 由强震平均段到频发段过渡期间同样存在强震少发段, 而少发段也有7级以上强震发生。 以此为基础结合中国大陆活动地块边界带, 研究不同强震丛发时段前地震活动的空间分布特点以及强震丛发时段的主体活动地区。 研究结果表明, 1901年以来中国大陆强震活动总体来讲, 1955年前后表现出不同的空间特征: 1955年之前强震活动主体区域为中国西部及周边大三角地区的三条边界; 1955年之后强震活动主体区域为华北地区、 巴颜喀拉地块区及南北地震带南段地区。 不同相邻强震丛发时段的主体活动地区有一定的交替性, 少发时段或增多时段的强震往往是上一个丛发时段的主体区内强震活动的延续, 或者发生在下一个强震主体活动区内或周边地区。     

用准周期（PP）方法预测地震序列中早期强余震

研究了地震序列的准周期性,提出了预测早期强余震发生时间的准周期方法及其定量预报指标,同时进行了回溯性预报检验,Ｒ值评分为０．４５。表明该方法具有一定的预报效能。     

伽师强震群震源特征及震源机制力学成因分析

应用改进的主地震相对定位法对17年伽师强震群3级以上地震进行了精确定位,根据较强地震的空间分布和震源机制解推断出伽师震群的发震构造为北北西向的雁行断裂.根据伽师震群地震的2177个P波初动方向记录,计算了伽师震群的平均震源机制解.基于Silver的震源模型,由震源谱推断了伽师震群主要地震的破裂方向,破裂尺度及应力降.文中最后用右阶雁行断裂的数值模型计算了伽师震群的发震构造所产生的扰动应力场的空间分布图像,用其解释了序列地震震源机制的多样性和低应力降现象,并认为特定的雁行发震构造与强震的多发性有关.