2018年12月1日美国阿拉斯加M_W7.0地震震源参数及破裂过程

THE SOURCE PARAMETERS AND RUPTURE PROCESS OF THE M_W7.0 EARTHQUAKE IN ALASKA,USA ON DECEMBER 1,2018

文中对2018年12月1日发生在美国阿拉斯加州的MW7.0地震开展了震源参数以及破裂过程的反演研究,并综合研究结果探讨了此次地震发生的动力学背景。震源机制反演结果表明,此次地震为拉张型正断地震,矩心相对于初始震中位置向NE偏移约10km。破裂过程反演的结果显示此次地震的滑动量分布比较集中,主要发生在长30km、宽20km的区域内,最大滑移量达3.6m。此外,破裂并非简单地以震源为中心对称分布。此次地震的破裂方向和余震分布均呈NE向延伸的趋势,发震断层的西南段则出现地震空区,由此可初步判断该地震是一次发生在太平洋板块与北美板块俯冲碰撞带后缘的弧后拉张环境中的典型正断型地震事件。由于太平洋俯冲板块在向N俯冲的过程中受高温高压作用影响,造成太平洋板片的俯冲角度变陡、向后弯曲变形,由此在碰撞带的后缘形成拉张环境,造成此次阿拉斯加MW7.0地震的发生。

A magnitude MW7.0 earthquake struck north of Anchorage,Alaska,USA on 1 December 2018.This earthquake occurred in the Alaska-Aleutian subduction zone,on a fault within the subducting Pacific slab rather than on the shallower boundary between the Pacific and North American plates.In order to better understand the earthquake source characteristics and slip distribution of source rupture process as well as to explore the effect of tectonic environment on dynamic triggering of earthquake,the faulting geometry,slip distribution,seismic moment,source time function are estimated from broadband waveforms downloaded from IRIS Data Management Center.We use the regional broadband waveforms to infer the source parameters with ISOLA package and the teleseismic body wave recorded by stations of the Global Seismic Network is employed to conduct slip distribution inversion with iterative deconvolution method.The focal mechanism solution indicates that the Alaska earthquake occurred as the result of tensile-type normal faulting,the estimated centroid depth from waveform inversion shows that the earthquake occurred at the depth of 56.5 km,and the centroid location is10 km far away in northeast direction relative to the location of initial epicenter.We use the aftershock distribution to constrain the fault-plane strike of a normal fault to set up the finite fault model,the finite fault inversion shows that the earthquake slip distribution is concentrated mainly on a rectangular area with 30 km×20 km,and the maximum slip is up to 3.6 m.In addition,the slip distribution shows an asymmetrical distribution and the range of possible rupture direction,the direction of rupture extends to the northeast direction,which is same as that of aftershock distribution for a period of ten days after the mainshock.It is interesting to note that a seismic gap appears in the southwest of the seismogenic fault,we initially determined that the earthquake was a typical normal fault-type earthquake that occurred in the back-arc extensional environment of the subduction collision zone between the Pacific plate and the North American plate,this earthquake was not related to tectonic movement of faults near the Earth’s surface.Due to the influence of high temperature and pressure during the subduction of the Pacific plate toward to the north,the subduction angle of the Pacific plate becomes steep,causing consequently the backward bending deformation,thus forming to a tensile environment at the trailing edge of the collision zone and generating the MW7.0 earthquake in Alaska.