Source Modeling of the 2015 Mw 7.8 Nepal (Gorkha) Earthquake Sequence and Geodynamic Implications

Abstract:

We build upon the rapid earthquake response of the USGS NEIC to further refine source kinematics of the Gorkha, Nepal earthquake (4-25-15, M 7.8) and associated aftershocks. The earthquake was a low-angle thrust faulting event on the shallow décollement between the India and Eurasia plates, rupturing a section of the broader Himalaya Front that has been largely seismically quiescent since the adjacent segment to the south-east ruptured in the 1934 Nepal-Bihar M8.1 earthquake. We present a detailed multiple-event relocation analysis of earthquakes in the Himalaya region, using both teleseismic and regionally recorded data, over the half-century prior to, and approximately eight months following, the Gorkha earthquake. We also present moment tensor and finite fault analyses of the largest events, and integrate all of this information to place the Gorkha earthquake sequence into a seismotectonic context. Preliminary observations from this study include the following: 1) the Gorkha earthquake ruptured a ~120x80 km patch of the Main Himalayan Thrust (MHT) – the décollement defining the plate boundary at depth – over an area surrounding but predominantly north of the capital city of Kathmandu, 2) relocated aftershock hypocenters primarily halo the main slip patch and occur at or near the MHT with depths generally increasing to the north beneath the higher Himalaya along flatter section of the décollement, 3) the M7.3 aftershock occurred south-east of the mainshock slip patch, on a segment of the décollement that was positively stressed towards failure by the mainshock, 4) normal faulting earthquakes have occurred in the shallow crust above the MHT, and likely reflect readjustment of the stresses in the upper plate and, 5) we observe little evidence of seismicity on the near surface portion of the Main Frontal Thrust that lies south of Kathmandu. Results from this analysis provide constraints on where earthquake hazard remains high, and thus where future-damaging earthquakes may occur in this densely populated region.