四川近震震级确定及地震波衰减特征的区域性差异

摘要

We select concentrations of earthquakes of varying times,spaces and magnitudes,and calculate station magnitude bias and network-average values,based on digital waveform data recorded by regional seismic networks and relevant seismic phase data as collected by the China Earthquake Networks Center.We perform.quantitative statistical analysis of the magnitude deviation of each station after eliminating effects due to source,path and direction.Considering different tectonic structures in Sichuan Basin,the western Sichuan plateau and the Panzhihua-Xichang area,the problem of local magnitude bias is discussed from the point of view of regional seismic wave attenuation characteristics,in order to render the magnitude correction value of station more reasonable.In our study,for stations in Sichuan basin and the south section of Longmenshan fault,greater magnitudes are functions of low attenuation in structure and amplification of both basin effects and seismic stations;these require negative correction.For stations in the northern and middle sections of Longmenshan fault,their lower magnitudes require positive correction.For stations in western Sichuan plateau,the possible causes of the lower magnitudes are severe dependence upon source region due to low Q value,path effect attenuation,and small magnitude estimated by weak S-wave.For stations in the Panzhihua-Xichang area,magnitude deviation varies with the location of epicenter;the greater magnitudevaluesoccurwhen earthquakes occur in Sichuan basin appear to be well correlated with edge effect of sedimentary basin on strong ground motion;these require negative correction.Furthermore,the ray path passes through the Sichuan plateau and Panzhihua-Xichang area,so that the two high attenuation areas may be the main cause of low magnitude values for plateau earthquakes,thus requiring positive correction.%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,选取大量时间、地点相对集中的地震来测定每个台站的震级与台网平均震级的偏差,通过震级残差统计分析方法,对各子台震级偏差进行定量统计分析,避开震源、路径、方位等耦合因素对震级测定的影响.结合四川盆地、川西高原及攀枝花-西昌地区不同的地质构造,从地震波衰减特征区域性差异的角度探讨近震震级(2.5≤ML≤4.9)的估算问题,使计算的单台震级校正值更合理.研究表明,位于四川盆地腹地及龙门山断裂带南段的地震台站,受松软沉积层对入射波放大作用和地震台站场地响应的共同影响,震级普遍偏大,作负校正;龙门山断裂带北中段台站估算盆地地震震级时震级偏小,作正校正;川西地区介质品质因子低,地震射线途径该区域时能量衰减大,加上“弱S波形”估算的近震震级偏小,作正校正;攀西地区单台震级偏差随震中位置不同而有变化,估算盆地地震震级时,受盆地边缘强烈不对称高幅值带的影响,单台震级常偏大,作负校正;估算高原地震震级时,射线路径穿越了川西、攀西两个高衰减区域,单台震级偏小,作正校正.