Energy-based average stress drop and its uncertainty during the 2015 M w 7.8 Nepal earthquake constrained by geodetic data and its implications to earthquake dynamics

Abstract

The slip and static stress drop of the 2015 Mw 7.8 Gorkha, Nepal earthquake has been studied using its excellent near-source static observations and a newly developed finite fault inversion algorithm with an average stress drop constraint. A series of nonlinear inversions with different target stress drops are conducted to search for the solution that not only most accurately fits the geodetic data, but also has an energy-based stress drop, ΔτE, matching the target value. Our result reveals that the misfit to the geodetic data gradually decreases when the ΔτE of the inverted slip model increases from 2 to 7MPa; but becomes nearly constant when ΔτE further increases. Hence, only the lower bound of ΔτE, that is, ΔτEmin(∼7MPa), of the Gorkha earthquake can be constrained with near-fault geodetic measurements, consistent with our previous study using just far-field seismic data. The upper bound of ΔτE can be constrained with an extra constraint on the maximum local stress drop. An artificial upper bound can also be reached if using a large subfault size to represent the source. The lower bound of ΔτE leads to the lower bound of the apparent available energy and the upper bound of the radiation efficiency (ηR, 0.09-0.15), though the latter is also sensitive to the determination of the radiated seismic energy. We find that the lower ηR and the reported high rupture velocity (80-93 per cent shear wave speed) can be reconciled by considering the aspect ratio of the dominant slip patch. We recommend using ΔτEmin to replace various slip smoothing constraints to stabilize the finite fault inversion because of its clear physical meaning.