Mapping a seismic barrier across the Shumagin Gap from satellite goce derivatives direct and inverse modeling

Abstract

The earthquake potential of the segment known as the Shumagin gap, located along the southwestern Alaska convergent margin, has been debated for over 40 years. This portion of the megathrust has not experienced a historically great earthquake with a magnitude (Mw) greater than Mw = 8.0, or at least none has been recorded in the instrumental era, exhibiting a moderate to low slip deficit. On 22 July 2020, an Mw = 7.8 thrust-fault earthquake ruptured a deeper portion of the megathrust along the eastern edge of the Shumagin Gap. Aftershocks following this event, including an Mw = 7.6 strike-slip earthquake on 19 October 2020, delineated an approximate north-south fault zone. Later, on 29 July 2021, an Mw = 8.2 thrust-fault earthquake ruptured the Semidi segment to the East. In this work, we examined the coseismic behavior of the Shumagin gap and adjacent Semidi segment along the Alaska margin from direct and inverse models obtained from satellite-derived gravity data. The distribution of the vertical gravity gradient shows a saddle point topography along the Shumagin Gap where the aftershocks of the July 2020 Mw = 7.8 earthquake concentrated in a nest. The gravity disturbance and the inverse model of mass anomalies also show an along-strike segmentation. Anomalous mass inferred along the Shumagin Gap, is consistent with the seismicity, focal mechanisms, and recently published works, suggesting that this segment hosts a seismic barrier (along-strike) that limits earthquakes with magnitudes Mw > 8.0. On the other hand, an across-strike (along-dip) segmentation is inferred from Tzz, which is consistent with vertical motion models. Comparison of the interplate coupling and b-values distribution to the vertical gravity gradient, allowed mapping main asperities in the region suggesting that the area to the west of the Shumagin gap could host a great megathrust earthquake in the future.