Aftershock Distributions, Moment Tensors and Spatio-temporal Evolution of the Stress Field Associated with Two M7.1 Alaskan Intraslab Earthquakes

Abstract:

The January 24 2016 Iniskin earthquake struck around 120 km below central southern Alaska. Finite fault solutions indicate that the main rupture propagated up-dip and was followed by more than 140 aftershocks with Mw > 3. Aftershock relocations using hypoDD localize half of the aftershocks along a well-defined lineation. The intersection of the mainshock rupture plane with the Jadamec & Billen (2010) slab model matches the orientation of the lineation to within 6 degrees. This geometrical correspondence suggests that the mainshock terminated its rupture at a stratigraphic level within the subducting plate such as the slab Moho. We apply the relative moment tensor inversion developed by Plourde & Bostock (2019) and recovered 170 focal mechanisms for events with Mw > 3 and which occurred between January 2014 and November 2018. Seventy percent of moment tensor solutions form four main clusters, all of which share a nodal plane similar to the mainshock rupture plane. Stress inversions indicate that the compressional and neural stress components have similar eigenvalues for a few days immediately after the mainshock. On a longer time scale, the M7.1 earthquake caused a small change on the stress regime of the region that returned to a quasi-equilibrium in 2017.

The November 30 2018 Anchorage earthquake was followed by more than 400 events of Mw > 3 hundreds of aftershocks, ranging in depth from 20 and 60 km, forming a diffuse cloud about the mainshock. Their distribution does not allow the determination of a single mainshock rupture plane with certainty. As for the Iniskin earthquake, we are computing moment tensors for local events including aftershocks to elucidate the structure(s) responsible for the event and constrain the spatio-temporal evolution of the ambient stress regime before and after the mainshock.