Paper No. 281-12
Presentation Time: 9:00 AM-6:30 PM
COMPLICATED KINEMATICS IN THE SOUTHERN CASCADIA SUBDUCTION ZONE
MCPHERSON, Bob1, HEMPHILL-HALEY, Mark1, SMITH, Stewart2, WILLIAMS, Todd3 and PRYOR, Ian4, (1)Department of Geology, Humboldt State University, 1 Harpst Street, Arcata, CA 95521, (2)Earth and Space Sciences, University of Washington, Seattle, WA 98195, (3)Cascadia Geosciences, 561 School Road, McKinleyville, CA 95519, (4)Stillwater Sciences, 850 G Street, K, Arcata, CA 95521, rm4@humboldt.edu
Strain accumulation along the southern Cascadia subduction zone, within the overriding North American plate, is pulsed by frequent offshore earthquakes within the Gorda plate. These strike-slip earthquakes, responses to the Gorda plate being forced through a tectonic die between the Pacific and Juan de Fuca plates, result in triangular-shaped plate fragments moving beneath North America. The integrated effect of the movement of these fragments is to increase strain coseismically, in addition to interseismic strain, within the coupled zone from north of Humboldt Bay south to the Mendocino triple junction.
The style of strain accumulation in southern Cascadia differs from the rest of the subduction zone to the north, which is not complicated by the effects of offshore, oceanic plate deformation. As a result of this episodic strain character we propose that the southern Cascadia subduction zone is a distinct part of Cascadia. This southern portion has a pervasively sheared offshore deformation zone, a fold and thrust belt onshore, a profoundly different rate and style of seismicity, a different episodic tremor periodicity and slip character, a transitional geodetic signal that includes both the transform margin to the south and subduction margin to the north, and a different offshore turbidite record than the rest of Cascadia. We propose, based on these differences, it should be considered separately in seismic source and and hazard models.
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