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Distribution and Focal Mechanisms of Incoming Plate Earthquakes Along the Alaska Subduction Zone

Description: 

We analyze the variation in earthquake locations and focal mechanisms along the Alaska subduction zone using ocean-bottom seismic data from the Alaska Amphibious Community Seismic Experiment (AACSE), EarthScope TA, and other land-based seismic networks. Earthquake locations are determined with a nonlinear relocation program (Lomax et al., 2000) incorporating a three-dimensional velocity model based on ambient noise Rayleigh wave tomography. Focal mechanisms are determined using a waveform inversion (Herrmann, 2013) for the largest events. We manually pick P and S first motions to determine the focal mechanisms for the smaller events (Snoke, 2003). Seismicity is higher in the Shumagin Gap, with extensional faulting as deep as 30 km below the seafloor, and predominantly normal faulting. These earthquakes are mostly confined within 50 km of the trench, matching the region of fault scarps observed in bathymetric profiles. A swarm of over 50 earthquakes occurred along one of those faults over several days in April 2019 with magnitudes ranging from 2 to 3.25. This swarm could be caused by fluid circulation along a plate-bending fault extending into the mantle, promoting mantle serpentinization and the subduction of water at the Shumagin Gap. Downdip of the trench at the Shumagin Gap, some normal faulting earthquakes are deeper than the plate interface, indicating that plate-bending faults are reactivated after subduction. The Semidi Segment to the east has less seismicity and more diverse focal mechanisms. There are strike-slip earthquakes with north-south oriented left-lateral fault planes in addition to normal faulting events. This change in the dominant faulting mechanism coincides with other changes across the Alaska subduction zone including incoming plate curvature, fabric orientation, and sediment thickness. These incoming plate variations are also reflected in the changes in megathrust coupling, which is higher in the Semidi Segment than the Shumagin Gap.

Session: Crustal Deformation and Seismic Hazard in Western Canada, Cascadia and Alaska

Type: Oral

Date: 4/20/2023

Presentation Time: 08:45 AM (local time)

Presenting Author: Patrick Matulka

Student Presenter: Yes

Invited Presentation: 

Authors