The events in Sumatra in 2004 and Japan this spring (2011) have grabbed our attention, especially because evidence is accumulating that similarly devastating events could occur here in the Pacific NW. The basic mechanism at play is the plate collision between the Juan de Fuca (oceanic) plate with the North American (continental) plate along the Cascadia subduction zone. The downgoing plate scrapes against the base of the over-riding plate, and friction (and the strength of the opposing rocks) prevents the plates from sliding smoothly past one another. So, the rocks are deformed – that is, the western edge of the North American plate bends and is buckled up. We can see this happening by carefully measuring the rising elevation of the coast with GPS. At some point (and this is of GREAT interest!) the rocks break, generating an earthquake, the elastic bending of the plate edge is released, and the coastline falls abruptly. This creates a wave, called a tsunami (Japanese for great wave) that travels out from the place where the rocks break. The size of the earthquake and subsequent tsunami depend on the amount of motion across the break surface (called a “fault”) and the length of the fault along the subduction zone.

The last great earthquake and tsunami to hit the Pacific NW occurred January 21, 1700 at 9 pm local time. How do we know this? It’s a fascinating detective story, based on tree rings, Japanese records and geologic investigation of sediment deposits along the Oregon coast. Evidence for this event is found from northern California to northern Washington, so it was a fault that broke along the entire subduction zone. How often do these occur? When will the next one happen? These are truly important questions, and several investigators at Oregon State University and elsewhere are working hard to answer them with as high a degree of certainly as possible.

Dr. Chris Goldfinger of OSU has devised a novel way to uncover the ancient record of “mega” earthquakes in our regional, by coring into the marine sediments preserved along the continental margin. Earthquakes trigger under-water avalanches of shallow water sediments, that run down the continental slope into the abyssal plains. These deposits are called “turbidites” (after the turbid water caused by suspension of the fine grained sediment). These turbidites can be identified in the cores by characteristic layers of sandy to silty to muddy grain size. By counting these at several locations along the subduction zone, and knowing the ages of the turbidites (from 14C-dating), the average period between earthquakes can be determined. This turns out to be something like 500 years for a whole-region rupture, but more like 300 years for a rupture of the southern half. Thus, we’re overdue (from 1700) for the smaller event, and perhaps have some time before the next mega-earthquake. Goldfinger expresses the likelihood of an earthquake in terms of probability within a certain period (~37% chance of occurrence within the next 50 years).

Everyone should be aware of the evidence for recurrence of these major events, and the best estimates for the timing of the next one. Also, it’s important to know what to do when the next one strikes. Each coastal community has an evacuation plan, and signs that direct people to the closest safe elevated land. Tsunami waves can reach 20-30m high and can be focused by local shape of the ocean floor and estuaries to cover much higher elevations. As everyone who has seen videos of the Japanese tsunami of March 11, 2011 knows, these waves can easily move large buildings, ships, vehicles and cause enormous destruction and loss of life.