The risk of a massive tsunami striking the West Coast may be greater than previously thought, according to an analysis of sedimentation along the Gulf of Alaska coast.

In 1964, a magnitude 9.2 earthquake off the coast of Alaska generated a wall of water more than 40 feet high that hit Alaska, British Columbia, Oregon and parts of California, killing 130 people and causing hundreds of millions of dollars in damage. Now, geologists say an even bigger tsunami could someday be in store for the West Coast.

The 1964 earthquake occurred in the Aleutian subduction zone, where the Pacific plate is being pushed beneath the North American continent. When scientists studied evidence of older quakes in the Aleutian zone and the subduction zone immediately to the east, they found the two have ruptured simultaneously in the past. Because the size of an earthquake depends in part on the length of the fault that breaks, together these fault zones are capable of a more massive jolt.

"People knew that there's a section of fault that extends to the east that generates earthquakes," said geologist Ronald Bruhn of the University of Utah, who co-authored the paper published January in Quaternary Science Reviews. "But they hadn't made the connection or had the dating information to link it to the big Aleutian subduction zone."

Just east of the Aleutian zone, the Pacific plate is being pushed beneath a relatively small chunk of continent called the Yakutat microplate. When Bruhn and his colleagues compared evidence of past earthquakes preserved in the sediments of the two regions using radiocarbon dating, they discovered that at least twice in the past 1,500 years, both faults appear to have ruptured at once.

Tsunamis form when a major earthquake causes a sudden rise or fall in the sea floor which rapidly displaces a large amount of water. A simultaneous rupture of the faults could lead to a far larger earthquake — and more devastating tsunami — than the one in 1964. Because the Yakutat microplate lies beneath a very shallow section of the Pacific Ocean, an earthquake in this region would create a particularly massive wave.

"Try putting your hand at the bottom of a deep bathtub," Bruhn said. "Then all of a sudden raise it up with your palm facing up — that disturbance will create a bit of turbulence. But if you place your hand in shallow water, and raise it up to do the same thing, you'll get a considerably larger surface disturbance."

In addition, the transition from shallow water in the Yakutat region to the much deeper ocean covering the Aleutian zone could create the perfect environment for underwater landslides, which would then generate even more big waves. Without creating numerical models, geologists can't estimate exactly how big a multi-segment tsunami might be, but they think the damage could be devastating.

"California, and particularly San Francisco, is most vulnerable from tsunamis originating from earthquakes in the Aleutians, among all possible tsunamis emanating from the different subduction zones around the Pacific," said geologist Costas Synolakis, director of the Tsunami Research Center at the University of Southern California, who was not involved in the research.

A multi-segment earthquake like the one described in the paper could generate foot-high waves off the San Francisco coast, Synolakis said, and small waves on the open ocean become huge waves as they get close to shore. For example, the massive 2004 Sumatra tsunami, which killed more than 300,000 people, had a height of only about two feet on the open ocean. "A 30-centimeter (11.8-inch) tsunami a few miles offshore the Golden Gate is something to worry about," Synolakis said. "What's new here is the evidence that Sumatra-style events are not only possible but likely."

See Also:

Image**: Tsunami damage to Kodiak, Alaska following 1964 Good Friday Earthquake, NOAA.

Follow us on Twitter @wiredscience, and on Facebook.