Past earthquakes in the Cascadia region

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Earthquakes in British Columbia
Each year more than 1000 earthquakes are recorded and located in western Canada. The Pacific Coast is the most earthquake-prone region of Canada. In the offshore region to the west of Vancouver Island, more than 100 earthquakes of magnitude 5 or greater (large enough to cause damage had they been closer to land) have occurred during the past 70 years.  

The west coast of Canada is one of the few areas in the world where all three types of plate movements take place (convergent, divergent, and transform), resulting in significant earthquake activity. Earthquakes in this region occur along the faults in the offshore region (e.g., the M=8.1 Queen Charlotte Island earthquake of 1949); within the subducting ocean plate (e.g., a magnitude 6.5 earthquake beneath downtown Seattle in 1965); and within the continental crust (e.g., a magnitude 7.3 earthquake on central Vancouver Island in 1946). Moving inland from the coast (and the active plate boundaries), the frequency and size of the earthquakes decreases. Saskatchewan and Manitoba are the least earthquake-prone areas in Canada.

For more information about earthquakes in British Columbia -

Earthquakes in Washington and Oregon 

Pre-instrumental Earthquakes
One of the largest earthquakes to occur in Washington before modern monitoring instrumentation existed, is thought to be the 1872 earthquake, magnitude 7.0, that hit near Lake Chelan, Washington. This earthquake was felt from British Columbia, Canada, to Oregon and from the Pacific Ocean to Montana. Another historical earthquake that caused damage in Washington state is the 1936 Milton-Freewater earthquake. Two of the largest earthquakes in Oregon occurred in 1910 and 1993. The 1910 earthquake was the largest historical shock within the state’s boundaries at a magnitude of 6.8, but it occurred too far offshore to cause damage, whereas the damaging 1993 earthquake was the largest historical earthquake beneath the land area of Oregon, with a magnitude of 5.9.

Instrumentally Characterized Earthquakes

Portland
Portland is part of a poorly understood region. Several distinct faults with uncertain rates of motion have been mapped. Six earthquakes of Richter magnitude (Ml) 5 or greater have occurred within the greater Portland area in historical times, including the damaging 1962 Ml5.5 Portland and 1993 Ml 5.6 Scott Mills earthquakes. There are at least three crustal faults beneath the Portland metropolitan area that could also generate more damaging crustal earthquakes of Ml6.5 or larger, which may occur in the Portland region on average about every 1,000 years.

Puget Sound
Most small and moderate sized earthquakes in the Washington and Oregon occur in the Puget Sound region.  This region of high population density is occasionally home of shallow crustal earthquakes inside the North American plate, which accommodate the north-south directed stresses within the Puget Sound area. The most notable earthquake is the 900 AD M~7.2 earthquake on the Seattle fault, which would be disastrous were it to recur today. The most recent moderate M5.3 earthquake hit Duvall in 1996, and a similar event struck nearly the same spot in 1932. From detailed geological mapping, several active fault strands criss-cross the region. Aside from the Seattle Fault, the Olympia, Tacoma, and South Whidbey Island fault zones are considered likely locations for future earthquakes. 

Information about what is known about crustal faults within the crust of Puget Sound can be found in the Atlas of Active Shallow Tectonic Deformation in the Puget Lowland, Washington.

An example of a scenario, describing the setting and consequences of a hypothetical earthquake in Puget Sound, is contained in the Fact Sheet "A Magnitude 7.1 Earthquake in the Tacoma Fault Zone—A Plausible Scenario for the Southern Puget Sound Region, Washington".

The Puget Sound region also is home to most of Washington and Oregon’s intraplate earthquakes (those occurring within the subducting Juan de Fuca plate).  In chronologic order these include the 1939 M6.2 Bremerton, 1946 M5.8 Olympia, 1949 M7.1 Olympia, 1965 M6.5 SeaTac, 1999 M5.8 Satsop, 2001 M6.8 Nisqually, and 2009 M4.5 Kingston earthquakes. 

Eastern Washington and Oregon
The Basin-and-Range geologic province is centered on Nevada, stretches northwest into eastern Oregon, west and south into California, and east into Utah. The crust in the Basin and Range province is stretching (extending) in an east-west direction, leading to primarily normal faulting. Earthquakes of this type tend to be in the upper crust, and strike infrequently. Three events, M6.0, M5.9, and M5.1 near Klamath Falls in 1993 are the only events larger than M5 since the region began seismic monitoring in the 1970s.

The Yakima fold and thrust belt region in eastern Washington is under north-south compression, and is marked by east-west ridges underlain by folds and faults of thrusting mechanisms. The compression is slow, only a few mm per year, but large earthquakes are possible. In fact, the most widely felt earthquake in the Pacific Northwest struck in this region in 1872.

Cascades
Seismicity in the Cascade Mountains of Washington away from volcanic centers takes place mostly on shallow crustal faults under the same general north-south compression responsible for earthquakes throughout Washington and western Oregon. They usually have strike-slip mechanisms with some component of thrust or reverse faulting. In Oregon seismicity in the Cascades is more transitional between the normal faulting of the extensional basin and range to the east and the north-south compression seen in western Oregon and most of Washington. The transition between these two environments is somewhere between Mount St. Helens and Mount Hood.  The largest recent earthquakes include the 2006 M4.5 Cowlitz Chimneys and 1981 M5.5 Elk Lake earthquakes.

For more information about earthquakes in Washington and Oregon -

  • Washington-specific general info about earthquakes, maps, publications from the Washington State Department of Natural Resources: http://www.dnr.wa.gov/ResearchScience/GeologyEarthSciences/Pages/Home.aspx
  • Washington-specific general info about earthquakes, maps, preparedness, and response from the Washington Military Department Emergency Management Division: http://www.emd.wa.gov/hazards/haz_earthquakes.shtml
  • Oregon-specific general info about earthquakes, maps, publications from the Oregon Department of Geology and Mineral Industries: http://www.oregongeology.org/sub/earthquakes/EQs.htm
  • Oregon-specific general info about earthquakes, maps, preparedness, and response from the Oregon Emergency Management Department: http://www.oregon.gov/OMD/OEM/plans_train/earthquake.shtml
  • Pacific Northwest Seismic Network real-time earthquake monitoring information, seismic data: http://www.pnsn.org/ 
  • Pacific Northwest Geodetic Array real-time deformation monitoring information, GPS and other slow deformation measurements: http://www.geodesy.cwu.edu/ 

Earthquakes in Northern California
Since 1900, nearly 40 M6 or larger earthquakes have occurred in northern California and in the adjacent offshore areas.  The southern boundary of the Cascadia subduction zone terminates in northern California at Cape Mendocino, which is where three plates (Gorda, Pacific, and North American) meet at a ‘triple junction’. To the north of the triple junction, the southernmost section of the Cascadia subduction zone is comprised of the Gorda plate descending beneath the North American plate, much like the Juan de Fuca plate in central and northern Cascadia.  

The southern boundary of the Gorda plate, where it meets the Pacific plate, is formed by the Mendocino Fault.  Here these two plates move parallel to one another, so that the fault motion is mostly sideways rather than vertical or dipping as on the interface between two converging plates. This type of boundary is called ‘strike-slip’ or ‘transform’.   More than two-thirds of California’s large historic earthquakes have been located offshore on faults within the Gorda plate and along its bounding faults. Fortunately many of these earthquakes have been too far offshore to cause damage, although 13 were close enough to the coast to knock down chimneys and damage buildings.   Only some of these offshore earthquakes are capable of generating tsunamis, because the seafloor must be displaced vertically to create a tsunami.  Thus, earthquakes that break the seafloor along the interface between the subducting Gorda and overlying North American plate are most likely to cause tsunamis, but tsunamis are less likely due to transform Gorda-Pacific plate earthquakes because the motion is mostly parallel to the Earth’s surface, and without much vertical component.

For more information about earthquakes in northern California -

  • Pacific Northwest Geodetic Array real-time deformation monitoring information, GPS and other slow deformation measurements: http://www.geodesy.cwu.edu/
  • California Emergency Management Agency information about earthquake preparedness, response, recovery, and mitigation: http://www.oes.ca.gov/