Cascadia is America's own tectonic version of Sumatra, where the magnitude 9.3 earthquake and tsunami of 2004 occurred. Stretching off the Pacific shore from northern California some 1300 kilometers to the tip of Vancouver Island, the Cascadia subduction zone appears capable of its own magnitude 9 earthquake. What do we know about its behavior and its history? What would that great Cascadia earthquake be like?
Subduction Zone Earthquakes, Cascadia and Elsewhere
Subduction zones are places where one lithospheric plate plunges beneath another (see "Subduction in a Nutshell"). They create three kinds of earthquakes: those within the upper plate, those within the lower plate, and those between the plates. The first two categories can include large, damaging quakes of magnitude (M) 7, comparable to the Northridge 1994 and Kobe 1995 events. They can damage whole cities and counties. But the third category is what concerns disaster officials. These great subduction events, M 8 and M 9, can release hundreds of times more energy and damage wide regions inhabited by millions of people. They are what everybody means by "the Big One."
Earthquakes get their energy from strain (distortion) built up in rocks from the stress forces along a fault (see "Earthquakes in a Nutshell"). Great subduction events are so large because the fault involved has a very large surface area on which rocks gather strain. Knowing this, we can easily find where the world's M 9 earthquakes happen by locating the longest subduction zones: southern Mexico and Central America, South America's Pacific coast, Iran and the Himalaya, western Indonesia, eastern Asia from New Guinea to Kamchatka, the Tonga Trench, the Aleutian Island chain and Alaska Peninsula, and Cascadia.
Magnitude-9 quakes differ from smaller ones in two distinct ways: they last longer and they have more low-frequency energy. They don't shake any harder, but the greater length of shaking causes more destruction. And the low frequencies are more effective at causing landslides, damaging large structures and exciting water bodies. Their power to move water accounts for the fearsome threat of tsunamis, both in the shaken region and on coastlines near and far (see more on tsunamis).
After the strain energy is released in great earthquakes, whole coastlines may subside as the crust relaxes. Offshore, the ocean floor may rise. Volcanoes may respond with their own activity. Low-lying lands may turn to mush from seismic liquefaction and widespread landslides may be triggered, sometimes creeping along for years afterward. These things may leave clues for future geologists.
Cascadia's Earthquake History
Studies of past subduction earthquakes are inexact things, based on finding their geologic signs: sudden changes of elevation that drown coastal forests, disturbances in ancient tree rings, buried beds of beach sand washed far inland and so on. Twenty-five years of research has determined that Big Ones affect Cascadia, or large parts of it, every few centuries. Times between events range from 200 to about 1000 years, and the average is around 500 years.
The most recent Big One is rather well dated, although no one in Cascadia at the time could write. It occurred around 9 p.m. on 26 January 1700. We know this because the tsunami it generated struck the shores of Japan the next day, where the authorities recorded the signs and damages. In Cascadia, tree rings, oral traditions of the local people and geologic evidence support this story.
The Coming Big One
We've seen enough recent M 9 earthquakes to have a good idea of what the next one will do to Cascadia: they struck inhabited regions in 1960 (Chile), 1964 (Alaska), 2004 (Sumatra) and 2010 (Chile again). The Cascadia Region Earthquake Workgroup (CREW) recently prepared a 24-page booklet, including photos from historic quakes, to bring the dreadful scenario to life:
- Strong shaking will last for 4 minutes, killing and injuring thousands.
- A tsunami up to 10 meters high will wash over the coast within minutes.
- Much of coastal Route 101 will be impassable due to wave and landslide damage.
- Parts of the coast will be cut off from inland cities when the roads are buried. Roads through the Cascades may likewise be blocked.
- For rescue, first aid, and immediate relief most places will be on their own.
- Utilities and transportation in the I-5/Highway 99 corridor will be disrupted for months.
- Cities may have "significant fatalities" as tall buildings collapse.
- Aftershocks will continue for years, some of them large earthquakes in themselves.
From Seattle on down, Cascadian governments are preparing for this event. (In this effort they have much to learn from Japan's Tokai Earthquake program.) The work ahead is enormous and will never be finished, but all of it will count: public education, setting up tsunami evacuation routes, strengthening buildings and building codes, conducting drills and more. The CREW pamphlet, Cascadia Subduction Zone Earthquakes: A magnitude 9.0 earthquake scenario, has more.