Source model and ground shaking of the 2015 Gorkha, Nepal Mw7.8 earthquake

Abstract:

The 2015 Mw7.8 Gorkha, Nepal earthquake ruptured a previously locked portion of the Main Himalayan Thrust fault (MHT) that has not slipped in a large event since 1833 (Mw7.6). The earthquake was well recorded by geodetic (SAR, InSAR and GPS) and seismic instruments. In particular, high-rate (5Hz) GPS channels provide waveform records at local distances, with three stations located directly above the major asperities of the earthquake. Here we derive a kinematic rupture model of the earthquake by jointly inverting the seismic and geodetic data, using a Kostrov source time function with variable rise times. Our inversion result indicates that the earthquake had a weak initiation and ruptured unilaterally along strike towards the ESE, with an average rupture speed of 3.0km/s and total duration of ~60s. The preferred slip rate of the beginning portion of the rupture had a longer rise time compared with the strongest ruptures, which took place at ~22s and ~35s after the origin, located 30km to the northwest and 20km to the east of the Kathmandu valley, respectively. The horizontal vibration and amplification of ground shaking in the valley was well recorded by one of the GPS stations (NAST) and the accelerometric station (KANTP), with a dominant frequency of 0.25Hz. A simplified basin model with top shear wave speed of 250 m/s and geometry constrained by a previous micro-tremor study can largely explain the amplification and vibration, realized by 3D staggered-grid finite difference simulations. This study shows that ground shaking can be strongly affected by complexities of the rupture and velocity structure.