2015年尼泊尔地震破裂过程的统一模型

摘要

模拟2015年尼泊尔地震(主震Mw7.8及最大余震Mw7.3)GPS/InSAR同震位移、远震体波、高频GPS位移波形和强震加速度记录,构建统一震源模型.统一模型分布特征主要由InSAR观测决定,地震矩释放过程则与P波模型相似,静态与高频GPS观测增加了对破裂时空特征的约束强度;各种比对表明,该模型对各基于单一类型反演模型具有很好的兼容性,棋盘测试展现其具有更优空间分辨率,最小可恢复20 km×20 km尺度的空间特征,压缩了非同震信号或误差导致的零散瑕疵,主、余震破裂具有更好的空间对应关系.主震展布范围为140 km×80 km;4 m以上破裂集中在加德满都以北30 km、深度15 km的狭长区域内,最大滑动量为7.4m;破裂持续总时长为60 s,破裂速度为3.3 km· s-1,子断层上升时间在10s内.Mw7.3余震破裂区域位于主震东侧边缘,滑动量围绕震中扩散,扩展范围为30 km×20 km,最大滑动量约为4.4m,总破裂持续时间为35 s.本次地震中静态和高频的GPS观测亦具备独立约束主震破裂扩展过程的能力.%On 25 April 2015,the Gorkha earthquake struck the central Nepal,for which diverse data sets including the surface displacements field derived by both InSAR and static GPS,and especially strong ground motions recorded by high-rate GPS are available to depict its rupture process,providing an unprecedented opportunity to assess their contributions to the inversion of the source parameters of the large megathrust earthquake.We investigate the space-time history of fault slip during the Gorkha earthquake mainshock (Mw =7.8) and its largest aftershock (Mw =7.3,happened 12 days later) using separate and joint inversions of high-rate GPS,static GPS,InSAR data,teleseismic waveform and strong-motion data to pursue a self-consistent and compatible rupture model.After obtaining the preferred rupture velocity and subfault rise time by using the tradeoff line between the cross correlation coefficients of observed and synthetic waveforms of HRGPS versus rupture velocity and subfault rise time,we performed separate inversions of the individual datasets.Separately inverted models present different slip patterns due to the intrinsic resolution of different datasets.Finally,two joint inversions of the near-field datasets and all datasets have been carried out.The joint model from near-field datasets improves the resolution of GPS model,but is not better than InSAR model.However,the scattered slip patches due to the noncoseismic deformation or observation errors have been depressed in the joint models.The optimal joint model of all datasets,supplemented by far-field observation,can be regarded as a unified model for preserving the common features of all separate inversions and yielding good combined resolution of slip.The Checkerboard tests also show that the unified model has the best spatial resolution for almost recovering the 20 km × 20 km slip patch and is more stable to rupture velocity variance.The slip pattern is mainly constrained by InSAR and the temporal process agrees well with teleseismic P waves model.The preferred unified model reveals the slip zone of mainshock extending about 140 km along strike and about 80 km down dip.The large slip patch (slip ＞4 m) of an elongated shape is located 15 km deep and 30 km north of Kathmandu,with a maximum slip of 7.4 m.The slip process lasted 60 s with a rupture velocity of about 3.3 km· s 1,and with the rise time of 10 s for each subfault.The slip of Mw7.3 aftershock lies to the east of the boundary of mainshock slip zone within an area about 30 km along strike and about 20 km down dip,features a compact pattern with a maximum slip of 4.4 m and total lasing time of 35 s.