COLLABORATIVE RESEARCH: Seismic and Aseismic Slip Interactions on a Subduction Megathrust, Guerrero, Mexico

墨西哥格雷罗州的一次瞬变事件沿着走向从东向西传播，在巨型逆冲的锁定过渡附近，它在1998年初的几个月内影响了格雷罗州的整个地震空区。运动GPS数据表明，1998年格雷罗地震可能与1995年科帕拉7.3级地震后的余滑有关。科帕拉地震位于格雷罗缺口以东约120公里处，但在地震破裂后的头六个月里，余滑似乎沿着走向向西传播了长达100公里。奇怪的是，在无地震滑动事件之前、期间和之后的几个月里，格雷罗州巨型逆冲带上的背景地震活动异常平静，但在瞬变结束几个月后恢复到正常或高于正常水平。这些观察提出了几个基本问题。首先，格雷罗的无震滑移事件真的是在科帕拉地震后作为余滑开始的吗？或者，1995-96年在阿卡普尔科以东和1998年在阿卡普尔科以西发生的无地震滑动仅仅是巧合吗？其次，这些无震滑动事件所产生的应力变化如何影响大断裂上的地震活动？本研究项目利用1994.2-1997.3年间收集的JERS-1干涉合成孔径雷达数据来检验这些问题，以估计科帕拉事件的同震滑动和阿卡普尔科以东地区的震后余滑。此外，来自格雷罗加速度计网络的强地面运动地震数据和来自墨西哥区域网络的宽带数据正被用来估计以前从未检查过的Mw7级地震的震源参数。正在使用现代网络差分技术重新定位这些事件，并对震源机制、时刻释放、应力降和其他震源参数进行了表征。研究小组正在将大地测量源和地震源与过去十年来俯冲巨型逆冲的滑移历史以及库仑破坏准则随时间的相关变化结合起来。研究人员还使用巨型逆冲上的非线性动力摩擦模型，评估了科帕拉地震余滑和格雷罗滑移瞬变之间的时空联系，以及无震滑移和地震活动之间的关系。

A transient event in Guerrero, Mexico, propagated from east to west along-strike, near the locking transition on the megathrust, and it affected the entire length of the Guerrero seismic gap over a period of months during early 1998. Campaign GPS data suggest the possibility that the 1998 Guerrero event is causally related to afterslip following the 1995 Mw=7.3 Copala earthquake. The Copala earthquake was located about 120 km east of the Guerrero gap, but afterslip appears to have propagated westward along-strike as much as 100 km during the first six months following the seismic rupture. Curiously, background seismic activity on the Guerrero megathrust unusually quiescent during the several months before, during and after the aseismic slip event, but returns to normal or above-normal levels several months after the transient ended. These observations raise several fundamental questions. First, did the Guerrero aseismic slip event really begin as afterslip following the Copala earthquake? Or is the occurrence of aseismic slip east of Acapulco in 1995-96 and west of Acapulco in 1998 merely coincidental? Secondly, how did the stress changes generated by these aseismic slip events influence seismic activity on the megathrust?This research project is examining these questions using JERS-1 InSAR data collected during the years 1994.2 to 1997.3 to estimate coseismic slip of the Copala event and postseismic afterslip in the region east of Acapulco. Also, strong ground motion seismic data from the Guerrero accelerograph network and broadband data from the Mexican regional network are being used to estimate earthquake source parameters from events with magnitude Mw7 that had never been examined previously. The events are being relocated using modern network differencing techniques, and the focal mechanisms, moment release, stress drops and other source parameters are also being characterized. The research team is combining the geodetic and seismic source to the slip history on the subduction megathrust during the past decade, and the associated change in Coulomb failure criteria as a function of time. The researchers are also assessing the space-time connection between Copala earthquake afterslip and the Guerrero slip transient, as well as the relationship between aseismic slip and seismic activity, using a model of nonlinear dynamic friction on the megathrust.