Inversion Of Fault Slip From The 2020 M7.8 Alaska Earthquake And Its Triggering Effects

On July 22,2020,a M7.8 earthquake occurred in Alaska,USA.The earthquake occurred in the Shumagin gap on the subduction zone of North American-Pacific plate.There has been no major earthquake in the Shumagin gap in the past hundred years,and its seismic risk has always been concerned.Existing studies have shown that the plate boundary of the gap is in a low coupling state.The occurrence of this large thrust earthquake has aroused the concern of scientific researchers about the seismic risk of Shumagin gap.At the same time,it also provides a rare opportunity to deeply understand the characteristics of seismicity in the weak coupling area of such subduction zone.After the M 7.8 earthquake,the aftershock sequence further ruptured along the westward Shumagin gap,and the M 7.6 strike-slip earthquake occurred 80km southwest after three mouths.It is worth studying about triggering an unexpected earthquake in an uncoupled subduction zone.This paper will comprehensively use GPS and seismic wave data to study the deformation characteristics,dynamic mechanism and triggering effects of the M 7.8 earthquake.The research work mainly includes the following parts:(1)Construction of earth medium model and fault model and selection of inversion method;inversion of fault kinematic parameters of the M7.8 earthquake based on GPS and seismic wave data,study of seismic slip distribution and rupture process.(2)By comparing the inversion results of different data,study the constraint effect of various data in joint inversion;comparing the results of different strikes and dips,discuss the sensitivity of geometric parameters in inversion.(3)Analyze the GPS pre-seismic and coseismic deformation observation data,and study the deformation characteristics of the thrust earthquake and the M 7.8 earthquake.(4)Calculate the change of Coulomb stress caused by the M 7.8 earthquake.Analyze the triggering effect on aftershocks and the M 7.6 strike-slip earthquake and explore the seismic hazard of the Shumagin gap.The main research results of the paper show that:(1)The joint inversion scalar seismic moment is about 7.69×10²⁰N·m,and the moment magnitude is M_w 7.86.The main slip area is located in the deep part of the fault west of the epicenter,the depth is concentrated at 23-40km,and the maximum slip is 2.22m.The rupture of this earthquake was divided into three stages,lasting for 90 s.The rupture started at the epicenter at a depth of 28km,and the second stage started at the 20^th second.At this time,the rupture speed increased significantly,and the deep rupture to the west,with the highest peak at 2.7km/s in the 35^th second.The third stage began at the 60^th second,and the rupture extended from the southwest to the surface at a speed of nearly 1km/s along the fault.(2)The near-field data has good constraint on the rupture directionality,but it has poor constraint on the moment magnitude and is more sensitive to the change of fault parameters.The far-field data is the opposite.The strike deviation has little influence on the slip distribution,and the dip deviation has a great influence on the fault edge and the small rupture area,and both of them have an influence on the moment magnitude.(3)Larger horizontal and vertical displacements occurred in the hanging wall of the fault.The vertical displacement increased significantly in the overhang area,and there was a significant negative change away from the rupture area,reflecting the subsidence of the surface in this section under the action of elastic rebound.(4)The change of Coulomb stress has a good corresponding relationship with the occurrence of aftershocks.Nearly 73%of the aftershocks fall in the positive change area,and about 68%of the aftershocks fall in the area with the change threshold greater than 0.01MPa.The friction coefficient has little influence on the spatial distribution of Coulomb stress change,but has a certain influence on the magnitude of the change.The north-south fault zone near the Shumagin gap may be the main cause of the M 7.6 strike-slip earthquake.The fault had sufficient right-side shear stress accumulation and triggered the M 7.6 strike-slip earthquakes under the stress loading generated by the M 7.8 earthquake.Under the action of the M 7.8earthquake and aftershocks,the stress in the shallow area of the Shumagin gap was greatly increased,and this event increased the risk of shallow earthquakes in this area in the future.