NEWLY DETECTED CASCADIA SUBDUCTION ZONE EARTHQUAKES OFFSHORE CENTRAL OREGON USING THE CASCADIA INITIATIVE AMPHIBIOUS DATASE

Understanding the spatial extent and properties of interplate seismicity in the Cascadia Subduction Zone (CSZ) will illuminate important insights into potential for future great earthquakes. Use of existing seismic data does not provide high-confidence results for the CSZ because of the lack of historically recorded great earthquakes, as well as the paucity of small to moderate sized earthquakes in the historic catalogs, which were compiled based on detections from land-based instruments positioned far from the geodetically determined, offshore, locked seismogenic zone. Catalog magnitudes of completeness thus systematically under-represent small earthquakes. This issue is addressed through the 2011-2015 Cascadia Initiative (CI) amphibious seismic array deployment, which combined coastal land seismometers with more than 60 ocean-bottom seismometers (OBS) situated directly above the presumed seismogenic zone. We search these seismic data for small, previously undetected interplate earthquakes to help identify locations of seismic patches on the megathrust.

We apply automated subspace detection, which leverages high signal similarity at a recording station among earthquakes having similar mechanisms and source locations. Our subspace is built from events identified using existing catalogs, that appear to have occurred on the plate interface. Waveforms for these events are extracted from CI OBS and land seismic data and scanned against continuous data to identify additional events whose waveforms surpass a selected similarity threshold. We focus on two repeating earthquake clusters off the Oregon coast, one of which is associated with a subducted seamount. Our analysis yields >60 new earthquakes (Md 1.5-2.7). The seamount area events locate within the overriding and subducting plates, downdip of the seamount, rather than coincident with the seamount on the interface, suggesting that they define a fracture/damage zone around the seamount. Both clusters occurred at the boundaries of two modeled high slip patches of the M9 1700 event; the non-seamount cluster may be part of a persistent seismic patch, while the seamount subducts aseismically, as a boundary between the two patches.