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Structural and Compositional Controls on Megathrust Slip Behavior Inferred From a 3D, Crustal-Scale, P-Wave Velocity Model of the Alaska Subduction Zone Spanning the Incoming and Overriding Plates

Description: 

Earthquake hazard assessment in subduction zones requires an understanding of how large-scale features of both the downgoing and overriding plates influence megathrust slip behavior. However, questions remain regarding the role of broad structural variability in modulating complex behaviors of megathrust faults. The Alaska subduction zone offshore the Alaskan Peninsula exhibits along-strike variations in geodetic plate coupling, seismicity, and earthquake rupture style, making it an excellent location for studying these structural influences. Here, we focus on the Semidi segment and SW Kodiak Asperity of the Alaska subduction zone. Plate coupling is typically high in our study area, with the largest degree of locking inferred near Kodiak Island which hosted part of the 1964 Mw 9.1 event. Coupling decreases to the west, with recent modeling showing a reduction in size of persistent asperities associated with past (1938-1964) and recent (2021) great earthquakes.

In this presentation, we will explore potential structural and compositional controls on spatial variations in megathrust slip behavior by analyzing a 3D P-wave velocity model within a 400- by 400-km area with resolution down to 25-30 km depth. The model was computed by traveltime tomography inversion of wide-angle first-arrivals interpreted from active-source ocean bottom seismometer data acquired as part of the Alaska Amphibious Community Seismic Experiment (AACSE). Results underscore the heterogeneity of the incoming plate marked by hotspot magmatism, fracture zones, and uneven sediment cover. Landward of the trench, variations in backstop geometry and slab velocities appear correlated with changes in sediment input and the possible downdip persistence of structural features of the incoming plate. In the overriding plate, velocity changes provide clearer evidence of differences in metamorphic grade between terranes and intrusions therein. This presentationwill focus on reconciling these observations with corresponding changes in megathrust plate coupling as well as with past and recent large earthquakes in the area.

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

Type: Oral

Date: 5/3/2024

Presentation Time: 11:00 AM (local time)

Presenting Author: Tanner

Student Presenter: Yes

Invited Presentation: 

Authors