四川地区场地放大系数特征分析及在强地震动模拟中的应用——以2022年芦山M_(S)6.1地震为例

Analysis of site amplification coefficient characteristics of Sichuan and its application in strong ground-motion simulation:A case study of 2022 Lushan MS6.1 earthquake

基于四川地区62个强震动台站的场地资料,依据中国抗震规范及美国下一代衰减的Geomatrix(GMX)场地分类方法将台站场地进行了划分,并以台站的场地放大系数和高频衰减参数(κ0)为基础数据,研究了不同场地类型的场地放大系数平均特征.结果显示,在中国Ⅱ类场地中,受狭窄山间谷底地形效应影响,满足GMX的C类场地在大于10 Hz的高频段有明显放大;受深厚软弱沉积层影响,满足GMX的D类场地在小于5 Hz的低频段放大效应显著.考虑震源滑动模型、山区和盆地品质因子差异、局部场地效应,采用区域内余震平均应力降,通过随机有限断层法模拟了2022年芦山M_(S)6.1地震中9个断层距(Rrup)<100 km的强震动台站的加速度时程、傅里叶振幅谱(FAS)和反应谱(PSA).结果显示,模拟结果与观测加速度时程的S波部分符合良好.在8 s以下周期范围内模拟与观测FAS和PSA形状和幅值基本一致.较好地模拟结果支持此次地震为2013年芦山M_(S)7.0芦山地震的一次余震.另外,模拟结果体现了此次地震在Rrup<35 km范围内有明显的方向性效应,与观测数据相符.最后,考虑各类场地的场地放大系数和κ0的平均特性,比较了此次地震中Rrup<150 km的167个盆地和130个山区烈度台观测和模拟峰值加速度(PGA)和峰值速度(PGV),以及周期为0.5 s、1.0 s、5.0 s和8.0 s的PSA幅值随断层距的衰减特性.结果显示,山区和盆地的地震动参数随距离的衰减特性较为相似,山区和盆地的PGA、PGV强度相当.但是,在周期大于1.0 s的长周期段,盆地的反应谱幅值整体上大于山区.

Following the site classification methods proposed the Chinese seismic code(Code16)and the Next Generation Attenuation(GMX),the site conditions of 62 strong motion observation stations located in Sichuan,China were classified based on their site conditions.Then,the site amplification coefficients and high-frequency attenuation parameters(κ0)of these selected stations were used to investigate the local site effect characteristics of each site classes in average sense.The result shows that,in the range of Code16 site classⅡ,at frequencies above 10 Hz,the amplification of GMX site class C is significantly enhanced by the topographical effect due to narrow mountainous valley site condition;whereas at frequencies below 5 Hz,the amplification of GMX site class D is noticeably enhanced by the deep soft sediments.Considering the source slip model,the differences of quality factor between mountainous and basin aeras,the local site effects,and the average stress drop of regional aftershocks,the acceleration time series,Fourier Amplitude spectrum(FAS)and 5%-damped pseudo-spectral acceleration(PSA)of 9 stations located within 100 km rupture distance(Rrup)range with respect to the 2022 Lushan M_(S)6.1 earthquake were simulated using the stochastic finite-fault simulation method.The result shows that,the simulated time series fit well with the S-wave portion of observation.Moreover,at period shorter than 8 s,the shapes of simulated FAS and PSA matched those of observations.The satisfactory simulation result supported that the current earthquake is most likely to be an aftershock of the 2013 Lushan M_(S)7.0 earthquake.Moreover,the simulation result captured the obvious directivity effect observed when Rrup<35 km.Finally,taking the average site amplifications andκ0 values of each site classes into account,the attenuation characteristics with respect to Rrup of simulated and observed peak ground acceleration(PGA),peak ground velocity(PGV),and spectral acceleration at periods of 0.5 s,1.0 s,5.0 s,and 8.0 for 167 basin and 130 mountain stations with Rrup<150 km were compared.The result indicates that,the attenuation rate of ground motion parameters and the levels of PGA and PGV of basin and mountain stations are similar.However,at period longer than 1.0 s,the spectral accelerations of basin are systematically larger than those of mountain area.