INTRAPLATE EARTHQUAKE POTENTIAL IN THE CASCADIA SUBDUCTION ZONE BENEATH WESTERN OREGON

An examination of the relatively brief 160-year historical record indicates that no large intraplate earthquakes (M > 6) have occurred within the subducting Juan de Fuca plate of the central Cascadia subduction zone from south of the Puget Sound to the Oregon-California border. The lone exception may be a M 7.3 earthquake near Brookings, Oregon, that shook much of the coastal Pacific Northwest in 1873. The more complete instrumental record since 1960, indicates that very few intraplate earthquakes, as small as M 3, have occurred within the central Cascadia subduction zone. In contrast, the Puget Sound has been repeatedly shaken by large intermediate-depth (> 40 km) intraplate earthquakes including events in 1939 (M 5¾), 1946 (M 6.3), 1949 (M 7.1), 1965 (M 6.5), and 2001 (M 6.8). Also, at the southern end of the Cascadia subduction zone, the Gorda block has also been the source of several M ³ 7.0 earthquakes in historical times although all of these events have been offshore and hence at relatively shallow depth (< 20 km). A critical issue for western Oregon and southwestern Washington is what is the potential for such large intraplate earthquakes. In particular, the future occurrence of a large event beneath the urban corridor of Vancouver, Portland, Salem, and Eugene would be damaging. An analysis of the limited information on the thermal and physical properties, geometry, and seismicity of the central Cascadia subduction zone and comparisons with other young, warm subduction zones worldwide suggests that the absence of intermediate-depth earthquakes is not unusual. The warm temperatures within the Juan de Fuca plate and possibly lower level of tensional stresses due to the comparatively gently-dipping plate probably results in long recurrence intervals (hundreds of years) for such earthquakes. In contrast, the arch in the Juan de Fuca plate and possible existence of a tear fault probably acts to concentrate stresses in the region resulting in large events. The Gorda block, which was probably the site of the 1873 earthquake, exhibits a significantly different deformational history possibly due to different physical properties and a stress regime heavily influenced by the Mendocino Triple Junction. Interestingly, no large intermediate depth earthquakes have been observed within the Gorda block in historical times.