DISCUSSION ON ATTENUATION CHARACTERISTICS, SITE RESPONSE AND MAGNITUDE DETERMINATION IN SICHUAN

doi:10.3969/j.issn.0253-4967.2016.04.016

Abstract

Abstract:

The attenuation characteristics and site response are calculated respectively for each individual tectonic unit in Sichuan (Sichuan Basin,west Sichuan plateau and Panzhihua-Xichang area),using digital waveform data recorded by regional seismic networks and relevant seismic phase data collected from China Seismograph Network.The frequency dependent Q(f) is obtained by the iterative grid-search technique described by Atkinson and Mereu based on trilinear geometrical spreading model.The source spectra are determined by the model of Brune and the site responses of seismic stations are derived by Moya's method using genetic algorithms.Comparison to conventional M_L estimates shows that the network local magnitude bias is quite significant at low and intermediate magnitudes.The bias at the j^th station for the i^th event is defined as ΔM_ij=M_ij-M_i, where ΔM_ij is the station magnitude and M_i the network-average value.For comparison,we mapped the spatial distribution of biases by digital seismograms recorded from 10535 earthquakes of magnitude 2.5≤M_L≤4.9 that occurred in Sichuan from January 1,2009 to June 30,2015.Based on the above data,the attenuation characteristics,site response and their effects on magnitude determination in Sichuan are analyzed.Our results demonstrate that the associated model for regional quality factor for frequencies can be expressed as Q₁(f)=450.6f^{0.513 4} for Sichuan Basin,Q₂(f)=136.6f^{0.581 3} for west Sichuan Plateau and Q₃(f)=101.9f^{0.666 3} for Panzhihua-Xichang area.Site response results indicate that different stations show different amplifications.Maps of biases appear to be different,but with similar dominant spatial distribution.For stations in Sichuan Basin,their greater magnitudes are functions of low attenuation in structure and amplification effects of both seismic stations and basin effects.For stations in west Sichuan Plateau,the possible causes of these lower magnitudes are severe dependence upon source region due to extreme lateral variations in either structure or path effect attenuation.For stations in Panzhihua-Xichang area,broken medium caused by strong tectonic activity or large earthquakes and heat flow up-welling along active faults may be the main reasons of low magnitude values when earthquakes occur in western Sichuan and eastern Tibetan region.And the greater magnitudes for earthquakes along the Longmen Mountains appear to be well correlated with edge effect of sedimentary basin on strong ground motion.In our study,stations magnitude biases appear to be extremely correlated with tectonic structures and different regions when seismic rays passing through,magnitudes are affected significantly by lateral variations in attenuation characters rather than site responses.

Key words: Q value, site response, geometrical spreading function, magnitude deviation

摘要：

利用四川区域地震台网的数字波形资料以及中国地震台网中心的有关震相数据，选取互相衔接的3段地震波几何衰减模型，利用基于遗传算法的Atkinson和Mereu方法，分别反演四川盆地、川西高原及攀枝花-西昌地区的介质品质因子。采用Brune的ω平方模型约束震源位移谱，使用Moya方法联合多台多地震数据求得各地震台站的场地响应。根据波形数据和观测报告，计算出2009年1月1日至2015年6月30日10 535次2.5≤M_L≤4.9近震单台震级测定值与台网平均震级的差，绘制各次地震的单台震级偏差空间分布图像。结果显示：四川盆地介质衰减模型呈现高Q₀和低衰减系数γ的特点，攀西地区表现出低Q₀和较高衰减系数γ；岩石台基对地震波信号在不同频率点的放大作用不相同；单台震级偏差优势分布基本一致，在构造稳定的四川盆地，介质品质因子高（低衰减），地震波在该区域能有效传播，盆地表层松软沉积层对入射波有放大作用，造成该区域单台震级高异常；川西大部分地震台站台基增益较大，但单台震级明显偏低，推测与松潘-甘孜块体地壳深度软流圈的物质流动及该区域低Q₀异常（高衰减）有关；攀西地区断陷拉张、流体侵入及沉积填充等损耗地震波能量，常使该地区台站测定的川西藏东地震震级偏小，但同样台站测定的龙门山断裂带上中强地震震级常较台网平均震级更大，可能是受盆地边缘强烈不对称高幅值带的影响。从研究结果看，四川地区介质衰减对地震射线振幅的影响较地震台站的台基增益大，因此校正四川区域地震台网单台震级时，应结合介质衰减及台站场地响应的空间特性进行考虑。

关键词: Q值, 场地响应, 几何衰减函数, 震级偏差

CLC Number:

P315.3⁺2

WU Wei-wei, SU Jin-rong, WEI Ya-ling, WU Peng, LI Jun, SUN Wei. DISCUSSION ON ATTENUATION CHARACTERISTICS, SITE RESPONSE AND MAGNITUDE DETERMINATION IN SICHUAN[J]. SEISMOLOGY AND GEOLOGY, 2016, 38(4): 1005-1018.

吴微微, 苏金蓉, 魏娅玲, 吴朋, 李俊, 孙玮. 四川地区介质衰减、场地响应与震级测定的讨论[J]. 地震地质, 2016, 38(4): 1005-1018.

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