基于InSAR和GPS观测数据的尼泊尔地震发震断层特征参数联合反演研究

利用日本ALOS-2和欧空局Sentinel-1A卫星获得的尼泊尔地震同震形变场,结合GPS同震位移数据,联合反演了断层滑动分布特征和空间展布.结果表明:尼泊尔地震的同震形变场主要集中在150km×100km的范围内,且分为南北两个相邻的形变中心,南形变中心的视线向抬升量约为1.2m,北形变中心的视线向沉降量约为0.8m,均位于发震断层上盘.位于形变抬升区的KKN4和NAST两个GPS站,抬升量和南向运动量均达到了m级,而远离震区的其他GPS台水平和垂直观测量均在1cm以内.联合反演得到的断层位错分布主要集中在沿走向150km,沿倾向70km的范围内,最大滑动量为5.59m,平均滑动量为0.94m.断层面倾角在浅部约为7°,随着深度增加,倾角逐渐变大,到垂直深度20km时倾角接近12°;5月12日MW7.2级余震位于主震破裂区的"凹"型滑动缺损区域;主震破裂区的上边界与MBT空间位置十分吻合,主震破裂区主要集中的MBT以北50~60km处,垂直深度为8~9km,倾角为9°,继续向北时主震破裂面以10°~12°的倾角向深延伸,在18~20km可能与MHT交汇.因此,初步判定MBT为此次地震的发震断层.

Joint inversion for the spatial fault slip distribution of the 2015 Nepal MW7.9 earthquake based on InSAR and GPS observations

An M_W7.9 earthquake, located at the front of the India-Eurasia collision belt, struck Nepal on April 25th, 2015. Focal mechanisms from different organizations show a dip angle of 7°~10°, which indicate a typical Himalayan-type low-angle thrusting earthquake. Almost no surface ruptures were found after the earthquake, making it difficult to interpret the spatial characteristic of the coseismic slip distribution, and most importantly, the possible causative fault buried underneath the sub-surface. We seek to answer these questions by joint inversion of InSAR coseismic deformation observed by ALOS-2 and Sentinel-1A SAR satellites, as well as some GPS measurements.#br#The method of joint inversion was used to analyze the spatial characteristic of the coseismic slip distribution and to infer the possible causative source fault of the 2015 Nepal earthquake. We collected the Sentinel-1A data immediately after the mainshock and processed them using the GAMMA software. Besides, the relevant ALOS-2 and GPS data were also collected. The quadtree sampling method was then used to resample the InSAR deformation results. In order to obtain a robust result and to reduce the uncertainties of the inversion, initial parameters for the fault were assigned according to focal mechanisms from Global CMT, USGS and GFZ.#br#(1) The coseismic deformation field derived from geodetic data shows that the Nepal M_W7.9 event is mainly distributed within a 150 km long and 100 km wide range, with two peak deformation centers aligned close to each other in north-south direction reaching about 1.2 m and about 0.8 m, respectively, and both of which are located on the hanging wall side, demonstrating a low-angle thrust. The InSAR results confirm that the surface trace of the causative source fault coincides with the MBT. Two GPS stations, namely KKN4 and NAST, about 80 km away from the epicenter on the hanging wall side, have peak displacements over 1 m. However, GPS stations at the far field decay very rapidly, which only have about 1 cm of coseismic displacements in both horizontal and vertical directions.(2) Based on the sensitivity iterative fitting method, a satisfactory fit to the GPS offsets and the InSAR displacements were achieved. Residuals for the ALOS-2 are between -10 cm and 10 cm, while residuals for the Sentinel-1A data are within -15 cm and 15 cm. Discrepancies arise from different coherences for C and L bands. Offsets for the GPS sites near the epicenter region are fitted well.However, sites away from the epicenter region have relatively poorer fitting especially in the vertical direction, which may be associated with the poor accuracy in GPS vertical measurements.(3) The inverted slip dislocations on the fault are mainly distributed in 150 km along strike and 70 km along down-dip direction. The maximum slip inverted reaches 5.59 m and the average is 0.94 m. The inverted dip angle of the fault model is 7°at shallow depth and 12° at deeper depth of 20 km. Results also show that with depth increasing, the dips of the causative fault increase as well, indicating reverse-listric shape of the fault. Slip more than 4 m is mainly concentrated between depths of 8 km and 10 km. Aftershocks are mainly distributed around the main rupture zone. The M_W7.2 aftershock on May 12, 2015 struck the slip deficit region left by the Nepal mainshock, which precisely filled the rupture gap.#br#The coseismic deformation field was derived for the Nepal earthquake and based on which the fault spatial slip distribution and the spatial characteristic of the coseismic slip distribution were inverted. The Nepal event ruptured a segment 50~60 km north of the MBT and at depth 8~9 km with an average dip angle of 9°, which, if projected onto the surface, is well aligned with the previously mapped fault MBT. And further north the Nepal rupture segment merges with MHT at depth of 18~20 km.We envision that the Nepal event has ruptured a segment of the MBT, which could be the causative fault.