Structural Heterogeneity in Source Zones of the 2018 Anchorage Intraslab Earthquake and the 1964 Alaska Megathrust Earthquake

Structural heterogeneities in subduction zones can affect slip behaviors of the megathrust faults and the generation of intraslab earthquakes. In this work we study the 3-D seismic structures (Vp, Vs, and Poisson's ratio) in and around the source zones of the 2018 Anchorage intraslab earthquake (Mw 7.1) and the 1964 Alaska megathrust earthquake (Mw 9.2). The Anchorage earthquake occurred in an anomalous zone within the subducting Yakutat/Pacific plate with a higher Poisson's ratio than the normal slab. Above the source zone, the overriding North American plate shows a low Vs and a high Poisson's ratio. These features indicate that strong dehydration occurs in the source zone and released fluids ascend into the overlying crust. Two areas with long-term slow slip events in the Upper and Lower Cook Inlet predominantly exhibit a high Poisson's ratio in the lowermost portion of the crust and the cold nose of the mantle wedge, whereas a low Poisson's ratio zone is revealed between them, suggesting that their segmentation is possibly related to localized slab-releasing fluids. In the Prince William Sound, the rupture of the 1964 Great Alaska earthquake initiated beneath a high-V and high Poisson's ratio zone of the overlying crust and the large slips occurred beneath a low-Vs and high Poisson's ratio zone, suggesting that lateral heterogeneities of the overriding plate may have played an important role in the nucleation and rupture processes of the Great Alaska earthquake.