Tsunami on the West Coast

The two main tectonic regions of the west coast of North America are the transverse fault zone area of California, and the subduction zone of the Pacific Northwest (Oregon, Washington, and British Columbia). The two zones have very different seismic and tsunami hazards.

Pacific Northwest

The Cascadia subduction zone is where the Pacific tectonic plate dives under the North American Plate. Similar subduction zones have been in the news quite a bit recently: a shallow, relatively small-magnitude earthquakes in New Zealand, and today’s catastrophic megaquake in Japan. These events could easily happen anywhere along the Cascadia zone.

Today’s event is a mirror image of the 1700 Cascadia earthquake. On the evening of January 26th, 1700, a magnitude 9 earthquake off the coast of Vancouver Island created a tsunami that immediately flooded the local area, and eventually reached Japan (whose records are why we can pin down the date so precisely).

Southern BC tsunami hazard map

Southern BC tsunami hazard map.

The Cascadia subduction zone is tectonically active, with a major earthquake expected every 300 to 700 years. Predictions are constantly changing, but the most typical probabilities for Vancouver are 11% odds within 50 years of a major earthquake (probable impact on buildings). The impact of a M6 or M7 earthquake could easily be like either of the Canterbury or Christchurch earthquakes depending on the location of the epicenter. A smaller shallow earthquake probably won’t trigger a tsunami (except maybe very small local events from landslides into the water), but a major M8 or M9 subduction quake would generate a tsunami across the Pacific.

From the lurid red on the hazard map, the entire west coast is exposed to tsunami, while anything tucked in behind it is relatively protected. This is because the island effectively stops waves: while Victoria was under tsunami advisory this morning, Vancouver wasn’t.

California

The San Andreas Fault is a transverse fault: the Pacific plate rub past the North American Plate. This results in frequent, shallow, relatively low-magnitude (M5, M6, maybe M7) earthquakes. The Bay Area has a substantial risk of a major earthquake in the coming decades, but the only tsunami risk is if the earthquakes cause landslides into the water, producing small local tsunami (Models of a 10cm tsunami from the 1906 earthquake).

11 March 2011 tsunami generated by the Japanese earthquake as it nears Emeryville, California. Photography credit to Steven Winter.

The primary risk of tsunami for California is from tsunami generated elsewhere on the Pacific Rim. For the most part, this means tsunami generated by major subduction earthquakes in Japan (like today), but events generated closer to home in the northern subduction zone (like the 1964 Alaska, and the 1700 Cascadia earthquakes) can also impact California. Tsunami look like a rapidly rising tide; the tsunami as it arrived in the Bay Area today could easily be mistaken for a tidal bore.

Prediction & Warnings

Although height is tricky, it is relatively straightforward to predict tsunami arrival times from a given source area. Tsunami generated in Japan or Chile take approximately 9-12 hours to reach the west coast. Tsunami generated in along the Cascadia subduction zone will have an immediate local impact, and reach California within a few hours.

Use the USGS California inundation hazard map (searchable by address) to determine your personal hazard.

Related Reading:

USGS Could it happen here? of tsunami hazard for the United States.
Canadian hazards map with notable historic events.
Animations of historic and forecasted tsunami along the west coast.
CREW Working group reports on shallow, deep, and subduction earthquakes in the Cascadia zone.
USGS technical report on tsunami probabilities for the Cascadia subduction zone.
Working group report on last-mile tsunami warning notification for the Pacific Northwest.
Earth Observatory supersite for the Pacific Northwest.

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