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Variations in the Alaska-Aleutian Subduction Megathrust Properties Along Strike Using Several Seismic Imaging Techniques

Description: 

The Alaska-Aleutian subduction zone is one of the most seismically active plate boundaries in the world. Many of the subduction zone characteristics are linked to the shear zone at the top of the subducting plate. However, our interpretation of this region is limited by the discrepancy observed between seismic imaging techniques. We investigate this discrepancy and changes in the Alaska subduction zone properties along strike using a combination of seismic imaging techniques. We analyze data from 34 onshore and offshore broadband seismic stations from the Alaska forearc, recorded by the Alaska Amphibious Community Seismic Experiment (AACSE; 2018-2019) and surrounding permanent stations. At each station, we identify high-frequency P to S (PS; 1-10 Hz) mode conversions from local in-slab earthquakes at the subducting plate interface. We also generate receiver functions (RFs) from teleseismic earthquakes (0.03 – 1.5 Hz) to identify the PS arrivals associated with the subducting plate. The PS arrival time is dictated by the depth of structures to which the local and RF PS arrivals are sensitive. Using 3D ray tracing, we find local PS arrivals are sensitive to structures 10 km shallower than RF PS arrivals. The depth of structures seen by local PS corresponds to the top of the high reflectivity shear zone in co-located active source profiles, while the RFs illuminate the oceanic Moho. These results and full waveform modeling indicate that local PS arrivals are sensitive to finer-scale structures within the shear zone, while RFs respond to longer-wavelength structures. Therefore, the discrepancy observed between seismic imaging techniques is explained by frequency-dependent sensitivity to structure. Local Ps arrivals are observed across the Kenai, Semidi, and Shumagin segments but are absent around Kodiak. The lack of local PS conversions below Kodiak could indicate the shear zone and upper plate have similar seismic properties, or that there is no shear zone present. Along-strike variability in PS arrivals provides valuable insight into changes in shear zone properties.

Session: Structure and Behavior of the Alaska-Aleutian Subduction Zone - II

Type: Oral

Date: 5/3/2024

Presentation Time: 11:30 AM (local time)

Presenting Author: Geoffrey

Student Presenter: Yes

Invited Presentation: 

Authors