基于广义反演技术的汶川地震地震动场地效应研究

It is well known that the earthquake damage and ground motion are affacted by the local site condition strongly. The evidence almost can be found in every post-earthquake investigation of great earthquake, such as the 1985 Mexico Earthquake, 1976 Tangshan Earthquake and 2008 Wenchuan Earthquake. The concept of site classification has been gradually incorporated in seismic codes in many countries. It can be said that the site effect has being a hot research topic. There are some studies related to the site effect in Wenchuan Earthquake that have been done. However, the given research results are almost qualitative, which either analyzing the ground motion characteristics associated with the different site condition, or explaining the abnormal seismic intensity due to site amplification, etc. The quantitative research has never been seen in published literatures yet. Therefore, it is an essential work that this project will evaluate the site effect quantitatively by using the generalized inversion technique. The first step in this project is to collect the strong motion recordings in Wenchuan Earthquake from the China National Strong Motion Observation Network System (NSMONS). The recordings from aftershocks will be made up a dataset and took the data processing. Secondly, the source, path and site effect of the processed recordings will be separated by the generalized inversion method. The Bootstrap technique will be used for verifying the stability of the inversion procedure and the Standard Spectral Ration (SSR) method will be used for examining its accuracy. Then the site predominant frequency and site amplification response of every strong motion station will be obtained. Based on the borehole data, the relationship between the average site amplification for each frequency band and the Vs20 (the average uppermost-20m shear wave velocity) will be regressed. The map of the site amplification factor around Wenchuan Area will be complied by using Krikin interpolation technique. Finally, the recordings from main shock will be added into the previous dataset, and the generalized inversion will be operated again. Compared with the results from two inversion operation, the soil nonlinearity will be identified and the nonlinear level will be analyzed. We will give the threshold of ground motion parameters (e.g. PGA or PGV) in Wenchuan Earthquake associated with the occurrence of soil nonlinearity. The key deliverables will be the versatile evaluation method of site effect based on the generalized inversion technique and the map of the site amplification factor around Wenchuan Area, which could be tailed for the specific application, such as the ground motion simulation, ShakeMap produce and earthquake intensity rapid releasing, and so on.

地震动场地效应一直以来是地震工程领域的一个研究热点。本项目将利用国家强震动台网上百个固定及流动台站在汶川地震主震及余震中获取的大量高质量加速度记录，通过广义反演技术计算这些台站的场地反应，运用Bootstrap技术验证反演过程的稳定性以及运用标准谱比法验证反演结果的可靠性；对反演结果计算各台站场地卓越频率及场地放大因子，结合台站钻孔波速资料，拟合不同频率段的场地放大因子与20m上覆土层平均剪切波速(Vs20)的关系，通过克里金插值法计算汶川地震近场区域不同频率的场地放大因子分布图；进行二次广义反演获得台站在主震中的场地反应，识别汶川地震地震动土层非线性效应，分析发生非线性反应的程度以及研究汶川地震中发生土层非线性反应的地震动PGA、PGV阈值。最终完成对汶川地震近场地震动的场地特征分析。本研究思路和方法以及相应成果可以直接运用于地震动模拟、震动图快速生成、地震烈度速报等研究工作中。

场地效应一直以来是地震工程领域研究的热点。2008年汶川地震中，我国强震动观测网络(NSMONS)在主震和余震中分别获得了大量高质量的自由场加速度记录。这些记录为研究汶川地震场地效应提供了良好契机。本项目以此为基础数据，从场地反应广义反演和场地反应非线性识别两方面开展研究，主要工作及成果如下：.(1) 选取了汶川地震余震中43个强震动台站在132个地震中获取的928组记录，采用广义反演法（GIT法）分离了其震源、路径、场地三项因素；针对台站062WUD，采用标准谱比法（SSR法）估计其场地反应并与GIT法的结果进行比较，显示两者较为一致，验证了广义反演结果的可靠性；在对数刻度下回归了1.0-5.0Hz、5.0-10.0Hz及1.0-10.0Hz频率段内的平均场地放大因子与上地表土层20m深度的等效剪切波速Vs20值的线性关系，结果显示两者具有较好的相关性；.(2) 在运用广义反演法对汶川地震场地非线性反应进行识别与分析过程中，定义了一种新的表示场地至地震源（site-to-source）距离的形式——凹凸体距，并验证了其相比目前常用的破裂距、断层距和震源距更能科学近似地表示大震中场地至地震源的距离，解决了广义反演方法不适用于大地震的局限性；.(3) 采用GIT法和HVSR法分别对强震动台站在汶川地震主震中的场地地震反应进行非线性识别与分析，结果表明当地震动PGA大于300cm/s2或PGV大于20cm/s时，场地地震反应出现明显的非线性现象，对应台站051AXT、051GYS、051GYZ、051JYC、051JYD、051JYH、051JZW、051MXN、051SFB和062WUD在汶川地震主震中产生了显著的场地非线性反应；.(4) 在对汶川地震场地地震反应进行非线性识别过程中，提出了一种新的定义场地反应非线性程度的参数——ADNL，计算了各台站的ADNL值以及其他类似参数fNL、DNL和PNL，并与地震动参数PGA进行经验回归分析，结果显示各参数与PGA都存在强烈相关性。.本项目有助于拓宽强震动记录在我国科学研究和工程实践领域的应用范围，推进强震观测事业的发展。