Two Rupture Stages of the 2020 Mw 7.6 Strike-Slip Earthquake in the Shumagin Gap, Alaska

The October 19, 2020 Mw 7.6 earthquake is an intraplate strike-slip event within the Shumagin Gap, and its aftershocks showed a north-south rupture extent for the mainshock. Here we relocated the earthquake sequence and investigated the source process of the mainshock by back-projection and waveform inversion. Our results show that the rupture of the mainshock can be divided into two segments. In segment 1, the earthquake started to rupture along a steep fault plane with a strike of 20° and a dip angle of 80°. which is very similar to the preferred fault plane with strike of 15° and dip angle of 81° derived from P wave first motions. In stage 2, the rupture propagated southeast along a gradual fault plane with strike of 345° and dip angle of 55°, similar to the preferred fault plane in the W-phase inversion (USGS). Those rupture patterns are further validated by the back-projection results using data recorded at Global and Chinese stations. using the above observations to set fault parameters, we performed tele-seismic waveform inversion to inverse the slips in the fault planes. The slip occurs on the of south fault is dominated by strike-slip, and the maximum slip of is 3.5 m. The total source duration is ~28 s, which is similar to what we get from back-projections. Comparisons of the earthquake sequence and the source process of the mainshock for the 1999 Mw 7.0 and 2020 strike-slip events indicate the intraplate earthquakes in subducting slabs may be related to the pre-existing plate fabric from magnetic anomaly.