Geometric Controls on Pulse‐Like Rupture in a Dynamic Model of the 2015 Gorkha Earthquake

The 15 April 2015 M-w 7.8 Nepal Gorkha earthquake occurred on a shallowly dipping portion of the Main Himalayan Thrust (MHT). Notable features of the event include (1) the dominance of a slip pulse of about 6-s duration that unlocked the lower edge of the MHT and (2) the near-horizontal fault geometry, which, combined with proximity of the free surface, allows surface-reflected phases to break the across-fault symmetries of the seismic wavefield. Our dynamic rupture simulations in an elastoplastic medium yield earthquake parameters comparable to those deduced from kinematic inversions, including seismic moment and rupture velocity. The simulations reproduce pulse-like behavior predicting pulse widths in agreement with those kinematic studies and supporting an interpretation in which the pulse-like time dependence of slip is principally controlled by rupture geometry. This inference is strongly supported by comparison of synthetic ground velocity with the near-field high-rate GPS recording at station KKN4, which shows close agreement in pulse width, amplitude, and pulse shape. That comparison also constrains the updip extent of rupture and disfavors significant coseismic slip on the shallow ramp segment. Over most of the rupture length, the simulated rupture propagates at a near-constant maximum velocity (similar to 90% of the S wave speed) that is controlled by the antiplane geometry and off-fault plastic yielding. Simulations also reveal the role of reflected seismic waves from the free surface, which may have contributed similar to 30% elongation of the slip pulse, and show the potential for significant free-surface interaction effects in shallow events of similar geometry.