Joint inversion for the spatial fault slip distribution of the 2015 Nepal Mw7.9 earthquake based on InSAR and GPS observations

An M(w)7. 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 degrees similar to 10 degrees,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. 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. (1) The coseismic deformation field derived from geodetic data shows that the Nepal M(w)7. 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(w)7. 2 aftershock on May 12, 2015 struck the slip deficit region left by the Nepal mainshock, which precisely filled the rupture gap. 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 similar to 60 km north of the MBT and at depth 8 similar to 9 km with an average dip angle of 9 degrees, 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 similar to 20 km. We envision that the Nepal event has ruptured a segment of the MBT, which could be the causative fault.