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Multi-Scale Geophysical Characterization and Tsunami Modeling of Active Listric Normal Faults Offshore Grays Harbor, Wa

Description: 

Understanding the role of upper plate crustal faults in subduction zone strain accumulation and release is fundamental to improving earthquake and tsunami hazard characterization and risk mitigation. Here we take a closer look at a series of active listric normal faults located ~70 km offshore NW of Grays Harbor, Washington, to determine how these structures accommodate strain and how they may contribute to earthquake and tsunami hazards along the coast. We provide new constraints on the shallow structure and activity of these faults and use these observations to develop a suite of tsunami models that capture a range of possible rupture scenarios including those with and without megathrust rupture.

Analysis of multibeam bathymetry and high-resolution sparker and chirp seismic data collected between 2017 and 2023 document two areas of active faulting near the heads of Quinault and Grays canyon with individual fault trace lengths between 5 and 25 km. The faults are characterized by distinct seafloor scarps (heights ranging from 23-57 meters) cutting across the outer shelf and upper slope in water depths ranging from 160-600 meters. Interpretations of crustal-scale seismic data suggest these structures go listric at shallow depth (~3-4 km). We explore the tsunamigenic potential of these faults using estimates of fault rupture area derived from a combination of near-surface and crustal-scale seismic data. Preliminary GeoCLAW models of tsunami propagation and inundation resulting from listric normal fault sources predict maximum wave amplitudes along the coastline of less than one meter, with arrival times varying from 35 to 55 minutes. However, the modeled tsunami impacts from the listric fault source are overshadowed by those from megathrust sources. While these listric faults alone may not pose a significant tsunami hazard to coastal Washington, their presence may reflect repeated large-scale co-seismic seaward motion of the outer wedge, enabling large amounts of shallow slip and tsunamigenesis similar to that observed during the 2011 Tohoku earthquake in Japan.

Session: Six Decades of Tsunami Science: From the Source of the 1964 Tsunami to Modern Community Preparedness - I

Type: Oral

Date: 5/2/2024

Presentation Time: 02:30 PM (local time)

Presenting Author: Janet

Student Presenter: No

Invited Presentation: 

Authors