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Using Small Earthquakes and the Amphibious Cascadia Initiative Dataset to Probe Cascadia Subduction Zone Heterogeneities

Session: Earthquake Science, Hazards and Policy in Cascadia I

Type: Oral

Date: 4/20/2021

Presentation Time: 02:45 PM Pacific

Description: 

The Cascadia Subduction Zone (CSZ) has a rich paleoseismic history of large to great earthquake ruptures, the most recent being a M9 in 1700. Historically, the margin is notable for being seismically quiet and lacking even small- to moderate-magnitude (< M4) earthquakes. Given the potential of a future megathrust rupture, mapping high coseismic slip patches/strong asperities on the plate interface is imperative in order to best estimate expected ground motions and tsunami inundation potential for the high population centers along the margin. We improve upon existing land-seismometer derived catalogs for the CSZ by using the amphibious Cascadia Initiative (CI; 2011-2015) seismic array and robust subspace detection. We identify small offshore and coastal earthquakes within the seismogenic zone and use them to map along-strike variations in frictional conditions on the plate interface.

We have detected and located more than 2000 earthquakes over most of the CI deployment (July 2011-August 2014). 1594 of these earthquakes were previously undetected, and a few thousand detections remain to be located. Small earthquake locations generally agree with previously identified offshore paleo-rupture segment extents, and with the M9 1700 rupture model. Along the northern end of the margin, small earthquakes indicate that the identified 1700 asperities may be smaller than modeled or smooth over smaller, closely located asperities. Possible rupture barriers exist at the tip of the Olympic Peninsula and at the OR-CA border, which correlate with paleo-rupture segments, but are absent from the 1700 model and geodetic locking model. Both the geodetic locking model and the small earthquakes presented here point to weak coupling offshore central OR, which is inconsistent with the 1700 rupture model. The small earthquakes presented here demonstrate broad agreement with previous models of locking and asperities along the margin, but refine these with illumination of small-scale locking variations.

Presenting Author: Emily A. Morton

Student Presenter: No

Authors