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Nature of Intermediate-Depth Earthquakes in the Eastern Aleutian Subduction Zone

Session: Amphibious Seismic Studies of Plate Boundary Structure and Processes [Poster]

Type: Poster

Date: 4/20/2021

Presentation Time: 04:15 PM Pacific

Description: 

It is challenging to determine with confidence what geological units of subducting oceanic plates host intermediate-depth earthquakes, specifically those in the upper-plane of the double seismic zone, because beyond ~40 km depth earthquake hypocenters are the primary constraint on the location of the plate interface. The 2011 ALEUT seismic reflection survey in the eastern Aleutian subduction zone, Alaska, imaged clear plate-interface reflections to depths of ~60km, providing a rare opportunity to study intermediate-depth earthquake hypocenters with respect to robust, independent constraints on the interface. This survey encompasses the Shumagin gap, the Semidi segment and the SW portion of the Kodiak asperity. We use grid-search and double-difference methods to relocate earthquake catalogs from the Alaska Earthquake Center (AEC) and the Alaska Amphibious Community Seismic Experiment (AACSE) using a local 3D velocity model derived from the ALEUT reflection experiment, ensuring consistency between hypocenter and reflection depths. There are relatively few earthquakes recorded in the AEC catalog between the trench and where the plate interface reaches depths of ~25 km, and initial earthquake catalogs from the AACSE suggest that this is not merely because of poor station coverage offshore. Our initial results indicate that, where the plate interface is deeper than 50 km, there is a distinct gap between the plate interface and the top of the upper-plane earthquakes. It is not yet clear whether these deeper earthquakes are entirely constrained to the oceanic mantle, or whether some occur in the lower crust. We additionally investigate along-strike trends in upper-plane seismicity and consider their relations to varying properties of the incoming oceanic plate.

Presenting Author: Alexandre P. Plourde

Student Presenter: No

Authors