Focal mechanism solutions for shallow earthquakes throughout the Cascadia plate system indicate that the primary regional stress is northerly compression, even though the Juan de Fuca plate generally is thought to be subducting N50¿E. This compressional stress is pervasive throughout the Gorda-Juan de Fuca-Explorer plate system and much of the adjoining section of North American plate. Modeling, using a discrete element code, shows that this stress primarily is due to the Pacific plate being driven into the Gorda block and Juan de Fuca plate (at the Mendocino and Blanco fracture zones). This plate collision causes compression of the offshore plate system northward through the 45 ¿W trending Vancouver Island and leads to transfer of northerly compression into the overriding plate. The northerly compression in the Cascadia plate system has caused large, crustal earthquakes in the regions of Vancouver and northern California and may be capable of causing large earthquakes also in and offshore of Washington and Oregon.

Several independent lines of evidence confirm that the Cascadia subduction interface is coupled strongly and that subduction primarily must be accompanied by significant earthquakes, rather than occurring aseismically. For numerous earthquakes within the subducted plate, focal mechanisms indicate extensional stresses that trend downdip. These earthquakes reflect plate extension this is due to sinking of the more deeply subducted plate and to plate motion being resisted at the shallow subduction interface. The slab pull force of the sinking plate also can lead to earthquakes at the shallow subduction interface and the potential exists for M 7.5--8.0 subduction earthquakes to occur at segments of the subduction boundary at Oregon. The in-plate driving forces of the offshore Cascadia plate system do not appear to contribute significantly to regional stresses. Stresses and earthquakes at the Cascadia province primarily result from the action of the Pacific plate on the Gorda-Juan de Fuca-Explorer plate system, a transfer of this action into the overriding North American plate, and the independent sinking and slab pull force of the subducted oceanic lithosphere. ¿ American Geophysical Union 1989