Probing the Southern Cascadia Plate Interface with a Dense Amphibious Seismic Array
Abstract
Convergence at the Cascadia subduction zone is primarily accumulated at seismogenic depths, and a significant portion is released down dip in episodic tremor and slip events (ETS); however, how strain is accommodated updip is unresolved. Regionally, the absence of offshore observations contributes to uncertainty in how the up-dip region behaves and its overall relationship to the seismic cycle. Additionally, the behavior of the zone between the nominally locked megathrust and the ETS region (~20-30 km depth) continues to be speculative in part due to the scarcity of plate interface seismicity. Here we use the 2014-2015 Cascadia Initiative amphibious seismic array to analyze seismicity in southernmost Cascadia. The dense ~10 km on and offshore station spacing of the array is favorable for determining accurate hypocentral locations. We build an exhaustive catalog of previously undetected events permitting increased scrutiny of the plate interface. All events were relocated within a local 3D velocity model and relative locations refined. The catalog reveals a lack of seismicity along the shallow plate interface, and the most likely explanation is that the interface is locally locked between 20 km depth to very close to the trench. We identified an anomalous cluster of seismicity located on or near the plate interface at the likely transition of downdip locking. A composite focal mechanism of these events is consistent with reverse faulting, and the orientation indicates that the events likely accommodate relative plate motion. Template matching of these events over the timespan 2006 - 2019 evinces that the cluster is only active from March 2014 through early 2015. The high waveform similarity, temporal clustering, and varied magnitude of events in this cluster are consistent with burst-type repeating earthquakes driven by slow slip. The timing of this unique cluster of seismicity is coincident with observed regional changes in plate coupling. Variations to the stress field generated by this coupling change may be responsible for triggering a local slow slip event too small to be detected geodetically.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMT055...07A
- Keywords:
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- 1242 Seismic cycle related deformations;
- GEODESY AND GRAVITY;
- 3613 Subduction zone processes;
- MINERALOGY AND PETROLOGY;
- 7240 Subduction zones;
- SEISMOLOGY;
- 8170 Subduction zone processes;
- TECTONOPHYSICS