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

墨西哥格雷罗的一个瞬变事件沿走向从东向西传播，靠近大型逆冲断层的锁定过渡，它在1998年初的几个月内影响了格雷罗地震空区的整个长度。活动GPS数据表明，1998年的格雷罗事件可能与1995年Mw=7.3科帕拉地震后的余震有关。科帕拉地震位于格雷罗缺口以东约120公里处，但在地震破裂后的前六个月内，后滑似乎沿走向向西传播了100公里。奇怪的是，格雷罗巨型逆冲断层上的背景地震活动在地震滑动事件之前、期间和之后的几个月里异常平静，但在瞬变结束几个月后又恢复到正常或高于正常水平。这些意见提出了几个基本问题。首先，格雷罗地震滑动事件真的是科帕拉地震后的余震开始的吗？或者，1995-96年阿卡普尔科以东和1998年阿卡普尔科以西发生的地震只是巧合？第二，这些地震滑动事件产生的应力变化如何影响巨型逆冲断层上的地震活动？本研究项目正在利用1994年2月至1997年3月期间收集的JERS-1干涉合成孔径雷达数据对这些问题进行研究，以估计阿卡普尔科以东地区科帕拉事件的同震滑动和震后余震滑动。此外，来自格雷罗加速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在被用来估计以前从未检查过的Mw 7级事件的震源参数。正在利用现代网络差分技术重新定位这些事件，并确定震源机制、力矩释放、应力降和其他震源参数的特征。研究小组正在将大地测量和地震源与过去十年中俯冲大推力断层的滑动历史相结合，以及库仑破坏标准随时间的相关变化。研究人员还评估了科帕拉地震后滑动和格雷罗滑动瞬变之间的时空联系，以及地震滑动和地震活动之间的关系，使用大型逆冲断层上的非线性动态摩擦模型。

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.