Complicated Kinematics In The Southern Cascadia Subduction Zone

In the southern Cascadia subduction zone, strain accumulation in the overriding North American plate is pulsed by frequent offshore earthquakes. These offshore strike-slip earthquakes, responses of the Gorda plate being forced through a tectonic die, result in triangular-shaped fragments of this plate moving under the North American plate. The integrated effect of the movement of fragments of the Gorda plate as a result of these numerous offshore quakes is to increase strain coseismically 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 subduction zone's loading to the north, which is uncomplicated by the effects of offshore earthquakes. We propose that the southern Cascadia subduction zone is a distinct segment, pervasively sheared offshore, with a fold and thrust belt onshore, and should be considered independently in hazard models from the rest of the Cascadia subduction zone. Our findings suggest that the Cascadia subduction zone has profound "edge effects" near its southern boundary.