Recent Advances In Understanding Earthquake Hazards and Risk In the Pacific Northwest

The instrumental record of seismicity in the Pacific Northwest is too brief to provide a full understanding of earthquake hazards in the region. In the past 130 years, ten moderate to large earthquakes have struck southwest British Columbia and adjacent Washington State. Some of these earthquakes occurred on faults cutting North American crust; others had had much deeper sources, in the Juan de Fuca plate, which is subducting beneath North America at the Cascadia subduction zone. Geological and geophysical evidence demonstrates that a third, much larger (M8-9) type of earthquake can occur at the boundary between the Juan de Fuca and North America plates. These subduction earthquakes are much rarer than large crustal and intraslab events but, by virtue of their size, affect a much larger area. Geological and geophysical studies during the past decade have greatly improved our understanding of seismic hazards and risk in the region. Improvements in high-precision GPS monitoring of surface deformation, coupled with improved heat-flow modeling have better defined the locked and transitional portions of the Cascadia megathrust fault. In particular, surface deformation and tremors during recurrent “silent slip events” have provided a better delineation of both the down-dip limit of the megathrust locked zone and the landward extent of the transition zone, both of which are critical for estimating the magnitude and likely effects of subduction earthquake on cities in the Pacific Northwest. Identification and characterization of active, east-striking, oblique-slip faults that cut the crust of North America have significantly altered perceptions of seismic risk in the Portland and Seattle-Tacoma areas. Earthquakes on these faults, some of which may exceed M7, result from northerly compression induced by clockwise rotation of North American crust in western Washington.