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Tsunami by the Bay: A Worst-Case Scenario

Half Moon Bay hammered, Sausalito saturated; a primer on effects of hypothetical wave

Last week, a tsunami about 10 meters high wiped out whole towns on Japan's coastline. Moving at the speed of a jet plane across the ocean, it caused an estimated $40 million in damage across the California coast.

In the Bay Area, it's hard not to wonder — what if the Next Big One is just a precursor to the real threat, a wall of water?

Rick Wilson of the California Geological Survey has worked with city planners around the Bay Area to be ready for a large tidal wave. According to Wilson, the worst-case scenario involves a 9.0 or larger quake coming from the Aleutian Islands, west of Alaska. That region has subduction zones, in which tectonic plates move under one another, much like those around Japan.

The wave sparked by this hypothetical quake would then hurtle toward the Californian coast. In about four and half hours, Pacifica would see 20-foot-tall waves that would surge onto land. San Francisco, at the mouth of the bay, would see waves around 12-15 feet above sea level, Richmond and Alameda would get 8- to 10-foot waves, Sausalito 8-foot waves.

Wilson said that Half Moon Bay would be particularly hammered: waves there could reach 30 feet above sea level. 

(Click here to see animated simulations of tsunamis striking the Bay Area.)

The Bay Area doesn't have the worst odds, tsunami-wise, but it doesn't have the best, either. Tsunamis are a regular feature of Bay Area seismic activity — according to Steven Ward, a geophysicist at the University of California, Santa Cruz, the 1989 Loma Prieta quake triggered a 25-centimeter tsunami — but huge waves are rare. Paul Segall, a professor in the Department of Geophysics of Stanford University who studies earthquakes, said that there had not been a tsunami on the level of Japan’s in the Bay Area in recorded history.

An earthquake struck off the coast of Santa Barbara in 1812, triggering a tsunami that reportedly damaged buildings and swept a boat half a mile inland. The Alaskan earthquake of 1964 caused a tsunami that killed 11 people in Crescent City. And in 1700, an earthquake near Northern California triggered a tsunami that struck the shore of Japan.

One of the reasons for the region's historically small tsunamis is that local tectonic plates tend to move horizontally rather than vertically — strike-slip, in earthquake parlance, as opposed to thrust — displacing less water than the seismic action that hit the Sumatra region in 2004, Chile and Indonesia more recently and Japan last week. 

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