Geologist: Devastating Cascadia earthquakes are coming; Here's the NW WA shake primer

Earthquakes are common in Western Washington, but we don't know when they will happen. Most earthquakes are small and of little consequence, but there is the potential for large, damaging earthquakes. Understanding where earthquakes occur, why they occur, and their likelihood of occurrence is helpful in preparing for and planning for eventual shaking.

Western Washington earthquakes occur in three distinct regions:

The locked portion of the deep Cascadia subduction zone within the slab below shallow crustal faults

The Cascadia Subduction Zone (CSZ) is about 620 miles long and extends from northern Vancouver Island to northern California. The CSZ is where the Juan de Fuca plate, which is moving to the northeast, is subducted under the North American plate, which is moving west.

For the first 30 to 40 miles of plate interaction, the plates are locked together and not moving. Pressure builds up in the closed portion and the plates will eventually slide, causing an earthquake. The plates have slid, causing large earthquakes at least 19 times over the past 10,000 years. The CSZ's most recent earthquake occurred on January 26, 1700, when it ruptured along its entire 620-mile length.

There is a moderate possibility of an earthquake measuring 9 on the Richter scale in the CSZ within the next 50 years. The M9 earthquake will be more than 1,200 times larger than the 2001 Nisqually earthquake and will cause a catastrophic aftershock that could last five to six minutes.

Bellingham: 6 feet of tsunami

In Bellingham, strong to very strong ground shaking at Modified Mercalli Intensity (MMI) is expected. Stone buildings and chimneys are likely to be damaged with possible liquefaction and lateral spread of loose saturated soil in the old town, port and other areas of loose fill near the beach. The M9 earthquake will generate a tsunami of 60 to more than 80 feet on the ocean coast, and more than 6 feet in Bellingham Bay in the Nooksack River Delta, with the tsunami extending to the outskirts of Ferndale.

Deep intraplate earthquakes have been responsible for some of the largest historical earthquakes in Washington, including the 1949 Olympia, 1965 Seattle, and 2001 Nisqually earthquakes. The foci of intraplate earthquakes define a relatively narrow region underlying Puget Sound. Intraplate earthquakes occur due to the bending of the subducting plate as it sinks into the mantle. They occur at depths of 18 to 42 miles, have no surface rupture and do not cause tsunamis.

The 2001 Nisqually earthquake was the fourth most damaging earthquake in U.S. history with damages amounting to more than $2 billion. There is a slab embedded under all of Western Washington, and there is a high probability of an earthquake occurring within the plates within the next 50 years.

Four active crustal faults at Whatcom

There are many active crustal rifts in the Puget Sound Salish Sea area. The faults are due to pressure from Western Washington on British Columbia's Coast Range, causing the Earth's crust to fracture. There are four active faults in Whatcom County: Boulder Creek, Drayton Harbor, Birch Bay, and Sandy Point. The Boulder Creek fault is located in the North Fork Nooksack River valley near Kendall and Maple Falls. It has slipped three times in the past 8,000 years, causing earthquakes estimated at M6.3 to M6.8.

The most recent earthquake on the Boulder Creek Fault occurred approximately 900 to 1,000 years ago and caused several large seismic landslides in western Whatcom County. Modeling conducted by WDNR showed that a M6.8 earthquake would cause intense MMI ground shaking near Maple Falls with strong ground shaking throughout western Whatcom and Skagit counties.

The Drayton Harbour, Birch Bay, and Sandy Point faults have all slipped over the past 1,000 years, and are all capable of causing an M6 to M6.5 earthquake, and because all three of these faults trend offshore, they can also cause tsunamis. There is a small possibility of a fault-caused earthquake in Whatcom County within the next 50 years.

A large and devastating earthquake may occur. In the event of a major earthquake, we must descend, cover and hold steady. There are many good sources of information on how to prepare, survive, and recover from an earthquake. The Whatcom County Hazard Mitigation Guide is a good local resource.

References:

Atwater, B. et al., 2005, The Orphan Tsunami of 1700 – Japanese Evidence for an Original North American Earthquake, USGS Professional Paper 1707 Sherrod, P. L., et al., 2013, Holocene tectonics and fault reactivation in the northern Cascade foothills, Washington, Atmosphere, Issue 9, p. 827-852. Kelsey, H.M., et al., 2012, Holocene rifting in the Bellingham foreland basin: deformation of the upper plate at the northern end of the Cascadia subduction zone, Journal of Geophysical Research: Solid Earth, Issue 117.

Cliff Netter is a licensed geologist and engineering geologist in Washington State. He retired after working as a principal geologist for a large international geotechnical and environmental engineering company. Although his practice was global, he was based in the Seattle area, maintaining his roots and interest in local geology and tectonics. He holds a bachelor's degree in geology from WWU and a master's degree. in Geology from the University of Calgary. He currently lives in Bellingham.

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