Rupture Process of The MW7.9 2018 Alaska Earthquake Revealed by Flexible Finite-Fault Inversion of Teleseismic Data

The 2018 M_W7.9 Gulf of Alaska earthquake is one of the great examples exhibiting earthquake rupture complexity. In previous studies, the moment tensor solution showed a strike-slip faulting with a large non-double-couple component, and the teleseismic P waveform backprojection results indicated geometrically complex source evolution. In this study, we applied a flexible finite-fault inversion to teleseismic body waves of the 2018 Alaska earthquake. The applied method represents a slip direction with the five basis components of double-couple moment tensor, which makes it possible to estimate both spatiotemporal distribution of potency density and focal mechanisms on an assumed horizontal model plane and to mitigate the effect of modeling error originated from uncertainty of fault geometry. Our result suggests that the source process of this earthquake can be divided into three rupture episodes. The first rupture propagated mainly northward from the epicenter with a strike-slip mechanism until 8 s from the hypocentral time along a rapture path trending at NNW-SSE strike. Then, this northward rupture transfers to another segment in ~60 km northeastern region of the epicenter from 8 s to 16 s, which has not been resolved in the previous studies. This segment has the N-S trend, which shows different trend around the epicenter. After that, the rupture propagated to ENE and WSW directions from north part of the first rupture area with their segments located ~40 km northwest and ~60 km northeast of the epicenter, both have a common NE-SW trend. Hence by adopting the new framework of inversion, the extreme complexity of rupture evolution during the 2018 Alaska earthquake has been resolved even by the finite-fault inversion, by sufficiently reproducing the observed teleseismic waveforms.