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Multi-Resolution Imaging the Downdip Extent of the Subduction Megathrust

Description: 

Subduction zone megathrusts exhibit transitions from areas of large seismogenic moment release to small patches to segments undergoing slow slip. Imaging the megathrust region has the potential to assess the relative roles of rock rheology and pore pressure at different depths, and the length scales over which both vary. Because much of the megathrust remains resistant to active-source imaging, earthquake-based methods provide the primary method of sampling deeper portions. We have taken advantage of a number of recent broadband deployments along the Alaska subduction segments, the forearc is relatively accessible. Array-based receiver function studies map out a regional low-velocity channel with thickness a few km or less, which appears to define the megathrust. It extends from the Canadian border west through the rupture area of the Mw9.3 1964 earthquake with little variation in properties along strike or downdip. New methodologies show that the layer is characterized by Poisson ratios similar to those of common rock lithologies without evidence of substantial overpressure; past estimates of high Vp/Vs likely result from signal bias. At higher frequencies (1-10 Hz), large-amplitude P-to-S waves are observed for nearby earthquakes. They systematically arise from the top of the same low-velocity zone, but show much greater heterogeneity in amplitude over length scales of tens of km. This scale is reminiscent of scales associated with rupture processes, indicating an important role for small-scale material heterogeneity along the fault surface. Overall, these observations highlight the importance of rock material properties and their heterogeneity in controlling slip behavior.

Session: Subduction Zone Structure From Trench to Arc

Type: Oral

Date: 4/19/2023

Presentation Time: 10:45 AM (local time)

Presenting Author: Geoffrey A. Abers

Student Presenter: No

Invited Presentation: 

Authors