Where Do Earthquakes Come From
Most earthquakes in our region result directly or indirectly from the convergence of three tectonic plates: the continental North America plate with two oceanic plates, the Juan de Fuca and Gorda plates. The oceanic crust is denser than the continental crust, so the Juan de Fuca and Gorda plates sink, or subduct, beneath the North America plate. Stress builds up on the interface between the plates and on faults within the plates. Faults are surfaces within the Earth that occasionally break, releasing stored-up stresses as earthquakes.
Three different types of earthquakes have periodically shaken Cascadia. All types of earthquakes can trigger landslides and liquefaction (when soil liquefies during shaking). Each type, however, starts at a different depth underground and has different characteristics.
Shallow or crustal earthquakes
Most earthquakes are a result of fault movement in the crust, a relatively thin layer on the Earth’s surface. In Cascadia, most earthquakes are shallow quakes that occur within the crust of the North America plate to a depth of about 20 miles (35 km). Shallow earthquakes cause destruction near the epicenter, where the shaking is strongest. Strong shaking may last 20 to 60 seconds. Aftershocks are common and may cause further damage. Small shallow earthquakes occur every day in Cascadia; damaging quakes occur every few decades.
Scotts Mills, Oregon (1993): The M5.6 “Spring Break Quake” near Scotts Mills in Marion County (about 32 miles south of Portland) on March 25, 1993 was a shallow or crustal earthquake. Ground shaking was felt throughout the Willamette Valley, from Roseburg, Oregon to Seattle. The quake caused $25 to 30 million in damages, including to Molalla High School and the State Capitol in Salem. No serious injuries were reported.
More information: CREW has produced a 30-page report that describes shallow earthquakes and their impact on Cascadia. Download PDF.
Deep or intraplate earthquakes
Deep earthquakes, also called intraplate quakes, take place within the oceanic Juan de Fuca plate (beneath Oregon and Washington) and the Gorda plate (beneath northwestern California) as they subduct beneath the North America plate. Deep earthquakes occur at depths of 30 to 50 miles (80 to 45 km) beneath much of Oregon and Washington, and at depths of about 25 miles (40 km) beneath northwestern California.
Because the faults that break during the earthquake are so deep, the seismic wave energy they radiate spreads over a much larger area than in a shallow quake. Therefore, the area directly above the epicenter feels less severe shaking than from a shallow earthquake of the same magnitude, but the shaking affects a larger area. Few, if any, aftershocks occur.
Damaging deep earthquakes occur every ten to 30 years in the Puget Sound area and less frequently elsewhere in Cascadia. The most recent deep events struck Cascadia in 1949, 1965 and 2001.
Nisqually, Washington (2001): The M6.8 earthquake that struck the Puget Sound area on Feb. 28, 2001 was a deep earthquake. It was centered about 30 miles (50 km) beneath the Earth’s surface and was felt from British Columbia to Utah, although property damage was largely confined to western Washington. Building walls crumbled, bridge supports cracked, and costs ultimately reached $4 billion. More than 400 were injured.
More information: CREW has produced a 28-page report that summarizes the threat from deep earthquakes and includes pre-quake efforts that limited damage during the Nisqually event. Download PDF.
Cascadia subduction zone earthquakes
When experts talk about the potential “Big One,” they mean a Cascadia subduction zone earthquake. When a magnitude 8 to 9 subduction earthquake occurs, it will cause many deaths and injuries and widespread property damage unless we prepare for it.
The Cascadia subduction zone lies offshore from northern California to southwestern British Columbia, where two tectonic plates -- the North America plate and the Juan de Fuca plate -- come together to form an 800-mile long earthquake fault. Earthquakes generated here trigger far more widespread effects than other types of quakes in the region. The ground could shake for four minutes -- or longer in some places. Aftershocks up to M7 could occur, causing even greater damage. In contrast to more common shallow and deep earthquakes, a subduction zone quake will generate a destructive tsunami, a series of waves up to 30 feet (10 m) high that will hit the Cascadia coast and travel across the Pacific Ocean toward Alaska, Hawaii and Asia.
Scientists believe the most recent subduction zone earthquake, a M9 event, occurred in January 1700. The best available evidence indicates that subduction zone quakes occur, on average, every 500 to 600 years, but the intervals between events have been as short as short as 100 to 300 years. The bottom line: All Cascadia residents should prepare to experience a subduction zone quake in their lifetimes.
Alaska (1964): The M 9.2 “Good Friday” earthquake that struck the Prince William Sound area of Alaska on March 27, 1964 was a subduction zone earthquake. Strong shaking lasted four to six minutes, depending on location. In one example of the widespread property damage caused by this quake, about 30 blocks of dwellings and commercial buildings were damaged or destroyed in downtown Anchorage. A devastating tsunami followed the earthquake, causing far more loss of life than the quake itself. Twelve people died from the quake; 119 people died from the tsunami.
More information: CREW has produced a 24-page report that presents what may occur if a M9 subduction zone earthquake strikes Cascadia. Download PDF.