顾及黏弹性松弛效应的2015年尼泊尔Mw7.9地震震后早期余滑模型

摘要

GPS data from continuously operating stations both in Nepal and in the southern QinghaiTibet plateau are processed and three-dimensional transient postseismic displacements are calculated during the first year after the earthquake.Significant postseismic displacements concentrate in northern Nepal,especially along the national boundaries between Nepal and China.The amplitudes and azimuths of the horizontal postseismic motions exhibit a pattern similar to that of the coseismic offsets.Almost all stations in Nepal record postseismic uplift.Kinematic finite fault afterslip models are constrained using GPS measurements.The preferred afterslip model shows that the afterslip occurred in the down-dip portion of the coseismic rupture with an average slip-depth of 20 km.The maximum depth of the raw afterslip model extends to 30 km.However,subtle afterslip is detected in the shallow portion of the fault.The moment release is estimated to be 8.8× 1019 Nm,equivalent to a magnitude of Mw7.3 and approximately 12％ of coseismic moment.Then,the residual afterslip model is inverted by using the residual data set between the raw GPS data and viscoelastic models,which is based on lateral rheological structure.The residual afterslip model shows narrower and shallower afterslip located in the downdip of the rupture with an average slip-depth of 16 km.The moment released by the pure afterslip model decreases to 5.7 × 1019 Nm,corresponding to Mw7.2.The afterslip-coseismic moment ratio drops to 8％.Our simple multiple mechanism model not only improves the data fit but also makes the residual afterslip distribution more physically reasonable.Our afterslip models show no slip within the shallow unbroken portion of the fault in the postseismic period,implying it is accumulating elastic strain and will be released through earthquake in the future.%处理尼泊尔境内及中国藏南的连续GPS数据,获得尼泊尔地震震后1 a的三维形变场.结果表明,震后形变主要发生在尼泊尔北部及中尼边境区域,水平形变以向南运动为主,垂直形变以隆升为主.采用有限断层模型反演的震后余滑分布范围十分广泛,主要集中在同震破裂下倾部分,向下延伸至30 km,平均余滑深度为20 km.震后1 a余滑释放的地震矩为8.8×1019 Nm,等价矩震级为Mw7.3,占同震释放总能量的12％.扣除黏弹性松弛效应之后,反演的余滑分布更加集中于同震破裂下倾区域,断层模型底部无余滑,平均余滑深度减小为16 km.经黏弹性松弛效应改正后的余滑模型释放的地震矩为5.7×1019 Nm,仅为同震释放能量的8％左右.该余滑模型不但提高了数据的拟合精度,且其分布特征更接近应力驱动的模型.由于震后余滑发生在深部而非浅部未破裂区域,说明断层浅部仍处在闭锁状态,其未来的地震危险性仍需密切关注.