Earthquakes in the Cascadia Region

The Cascadia Region, or the Pacific Northwest, as it is also called, which encompasses British Columbia, Washington, Oregon and Northern California, is a very seismically active area of the world. Below is an exerpt from an FAQ which answers many questions about the causes, magnitudes and frequency of earthquakes in this area. It was prepared by the Pacific Northwest Seismograph Network (PNSN), which operates seismograph stations and locates earthquakes in Washington.

Q: Why does the Pacific Northwest have earthquakes?

A: We are located at a convergent continental boundary, where two tectonic plates are colliding. This boundary is called the Cascadia Subduction Zone. It lies offshore and runs from British Columbia to northern California. The two plates are converging at a rate of about M 3-4 cm/year (1-2 inches/year), and the northeast-moving Juan de Fuca Plate is pushing into North America, causing stress to accumulate. Earthquakes are caused by the abrupt release of this slowly accumulated stress. More information and plate-tectonic cartoon

Q: Where are the major faults in the Pacific Northwest?

A: There are many faults in the Pacific Northwest that can produce damaging earthquakes, including hard-to-identify faults that exist entirely underground and have not been identified at the earth's surface. At the same time, some mapped faults have been located that have not generated earthquakes in recent geologic time. New faults continue to be discovered as more field observations and earthquake data are collected.

There are three different sources for damaging earthquakes in the Pacific Northwest. The first of these is the "Cascadia Subduction Zone", a 1000 km long thrust fault which is the convergent boundary between the Juan de Fuca and North American plates and is the most extensive fault in the Pacific Northwest area. It surfaces about 50 miles offshore along the coasts of British Columbia, Washington, Oregon and northern California. No earthquakes have occurred on this source zone during the last 200 years, but in 1700 this zone produced an earthquake estimated at magnitude 9.0.

The second source for damaging earthquakes is the Benioff Zone. This zone results from pressure and temperature conditions in the subducting plate as it reaches the upper mantle beneath Puget Sound. The Benioff Zone produces deep earthquakes beneath the Puget lowland with magnitudes as large as 7.5. Damaging earthquakes of this type occur every few decades. The 2001 Nisqually Earthquake is the most recent example.

The third source consists of shallow crustal earthquakes (depths of 0 to 20 km) on faults within the North American continental plate. Past earthquakes have revealed some shallow fault structures, including the Western Rainier Seismic Zone and the Mt. St. Helens Seismic Zone. Geologic evidence indicates that a large crustal fault, the Seattle Fault, runs east-west through Seattle from Issaquah to Bremerton. This fault generated an earthquake of estimated magnitude 7.3 approximately 1100 years ago. Other crustal faults capable of producing damaging earthquakes are located in almost every area of Oregon, Washington, and British Columbia.

Q: How often do earthquakes occur in the Pacific Northwest?

A: Typically, each year the PNSN locates over 1000 earthquakes with magnitude 1.0 or greater in Washington and Oregon. Of these, approximately two dozen are large enough to be felt. These felt events are a reminder that the Pacific Northwest is an earthquake-prone region. As residents of the Pacific Northwest, we should be prepared for the consequences of larger earthquakes that could result in damage to the transportation systems and lifelines. There have been about 25 damaging earthquakes in Washington and Oregon since 1872. During the 20th century about 17 people lost their lives due to earthquakes in the Pacific Northwest.