An Earthquake Nest in Cascadia
Date: 4/24/2019
Time: 09:15 AM
Room: Puget Sound
A cluster of temporally persistent seismicity (ML < 3.2) at > 60 kilometres depth below the Georgia Strait in southern British Columbia is investigated using data for the period 1985 to 2018. This isolated concentration of intermediate-depth intraslab earthquakes appears to be unique in Cascadia and meets the criteria of an earthquake nest. 129 relocated hypocenters elucidate two northwestward dipping structures likely within the subducting Juan de Fuca mantle within a ~30x10x14 km 3 volume. Focal mechanisms for 15 well recorded events represent a mixture of strike-slip and reverse faulting, and a stress regime approximating down-dip tension and plate-normal compression, consistent with previous stress inversions of regional intraplate seismicity. Dehydration embrittlement of antigorite is suspected as the primary agent of seismogenesis and is supported on several grounds. Converted seismic phases inferred to originate at the Juan de Fuca slab interface are observed at several seismic receivers and are consistent with a local slab depth of ~45 km, significantly shallower than recent JdF plate models. The source region displays an azimuthally variable VP/VS ratio that is best explained by a highly anisotropic source volume resulting from preferred orientation of minerals and/or organized fluid filled porosity. Finally, the nest is located within the extrapolated bounds of a propagator wake within the Juan de Fuca plate.
Presenting Author: Reid Merrill
Authors
Reid Merrill rmerrill@eoas.ubc.ca University of British Columbia, Vancouver, British Columbia, Canada Presenting Author
Corresponding Author
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Michael G Bostock bostock@eos.ubc.ca University of British Columbia, Vancouver, British Columbia, Canada |
An Earthquake Nest in Cascadia
Category
Characterizing Faults, Folds, Earthquakes and Related Hazards in the Pacific Northwest