When you think of earthquakes in the U.S., you usually think of Southern California and the San Andreas fault. In 1906, such an earthquake destroyed San Francisco. The next “big one” could certainly hit along the San Andreas, but there is another, lesser known possibility: off the coast of Oregon, Washington and British Columbia along the Cascadia subduction zone, the site of many historic earthquakes.
Natural Resources Canada sets the scene:
“At 9PM on January 26, 1700 one of the world’s largest earthquakes occurred along the west coast of North America. The undersea Cascadia thrust fault ruptured along a 1000 km length, from mid Vancouver Island to northern California in a great earthquake, producing tremendous shaking and a huge tsunami that swept across the Pacific. The Cascadia fault is the boundary between two of the Earth’s tectonic plates: the smaller offshore Juan de Fuca plate that is sliding under the much larger North American plate.” That earthquake is estimated to have been magnitude 9.0. You can read accounts of it here and a paper recounting American Indian stories about an M7.3 earthquake along the related Seattle fault that happened about A.D. 900.
Rob Witter, coastal geologist with the Oregon Department of Geology and Mineral Industries, says “there’s a 10 to 14 percent chance a powerful earthquake and tsunami will strike the Oregon coast in the next 50 years.” That prediction is based on several independent studies. (See here and here.)
According to the U.S. Geological Survey, “Analysis of geologic deposits indicates that a number of earthquakes, possibly of magnitude 8-9, have occurred in the past,” along the Cascadia subduction zone. See here for USGS map and cross sections.
According to Natural Resources Canada, evidence for multiple large earthquakes in the region include:
Buried tidal marsh or coastal forest soils point to sudden land subsidence of about 1 meter occurring at the same time from Vancouver Island to Northern California.
Changes in tree ring growth from coastal old-growth also suggest a sudden, widespread subsidence and drowning of roots.
Sand layers on top of the buried coastal marshes, driven in from offshore bars by the wave of the large tsunami that rushed into the subsided coastal region.
Silt turbidite (landslide) layers on the deep sea floor far off the coast from underwater landslides, likely caused by strong seismic shaking.
Tsunami evidence from:
local sources – marine organisms swept into and preserved in the bottom muds of coastal lakes that are separated from the ocean by land elevations of some 5 m high distant sources – large tsunami in Japan with no local Japanese earthquake.
We really don’t know where or when the “big one” will hit, but geologic evidence shows that the northwest coast of the U.S. has all the makings.
See also: The Measure of an Earthquake.