Teleseismic Source Modelling of Strong-to-major Himalayan Earthquakes

Abstract:

We study the source mechanism and finite fault slip of recent four strong-to-major earthquakes in the Himalaya. These are the 2005 Kashmir (M_w 7.6), 2011 Sikkim (M_w 6.9), 2015 Gorkha (M_w 7.8) and its largest aftershock Dolakha (M_w 7.3) earthquakes. These Himalayan regions are segmented with various geodetic slip rates, and to investigate source characteristics from western, central, and eastern parts is important. The Sikkim earthquake ruptured the underthrusting Indian plate beneath the Himalaya, while the Kashmir and Nepal earthquakes occurred on or above the Main Himalayan Thrust (MHT), an interface between the downgoing Indian plate and the overriding Himalayan wedge. The Nepal events ruptured the downdip locked segment on the MHT, while the Kashmir event produced a surface rupture. We use teleseismic P-waveform inversion to model the moment magnitude, focal mechanism, slip distribution and maximum slip for all four events. The earthquake rupture is modeled as multiple-shock sequences on a finite fault plane. The slip distribution is modeled on a rectangular fault surface, parameterized as regular grids. The Gorkha mainshock ruptured a ~150 km long thrust fault dipping to the NE, with two major subevents at ~15 s and ~27.5 s around hypocentral depth of ~14 km. The larger sub-event has a maximum slip of 7.4 m and is located ~90 km eastward of the epicenter, close to Kathmandu. The Dolakha aftershock is located adjacent to the mainshock rupture termination to the east, and is also modeled as a thrust earthquake (strike ~325°, dip ~12°, depth ~15 km) with maximum slip of 4.6 m. Our teleseismic source inversion revealed a small slip gap between the Gorkha mainshock and the Dolakha aftershock, indicating the remaining stress around the significant slip. The Kashmir earthquake occurred on a NE dipping thrust fault (strike ~327°, dip ~31°, depth ~12 km) and had a bilateral rupture, with maximum slip of 8.9 m near the hypocenter. This event consisted of two major sub-events at ~12.5 s and ~25 s, where the fault plane strike changed from ~313° to ~348°, respectively. Finally, the Sikkim earthquake has a strike-slip mechanism on a near vertical fault plane striking ~315° and dipping ~86° with maximum slip around 0.8 m.