When a magnitude 7.2 earthquake struck off the California coast June 14, the greatest danger wasn’t swaying buildings or trembling ground. It was the possibility of a tsunami. Within minutes, the West Coast and Alaska Tsunami Warning Center sounded the alarm, alerting coastal areas from Alaska to Mexico about the potential for a monster wave.
Fortunately, a tsunami never materialized, but a simulation by University of California at Santa Cruz geophysicist Steven Ward makes clear that the West Coast has reason for concern. His recently unveiled computer model shows that a giant earthquake in that area could create up to 20-foot waves that would wash over the coastline. Arrival times vary from a few minutes near a northwest earthquake’s epicenter to about three hours in Southern California. “In deep water, they go about 500 miles an hour,” Ward says. “That’s pretty fast—as fast as a jet.”
Ward modeled the effects of a magnitude 9.2 earthquake in the Cascadia subduction zone—a 58,000-square-mile region in the Pacific from British Columbia to Northern California where the Juan de Fuca plate sinks beneath the North American plate. “At least five big tsunamis have hit in 2,000 years in Cascadia,” Ward says. “We know for sure that the last one happened in 1700—we even know the day of the week it happened because it was recorded in Japan.”
The Cascadia subduction zone is not the only source of West Coast earthquake tsunamis. In the past, plate motion along the Aleutian Islands in Alaska has also generated giant waves like the one that swept through Crescent City, California, in 1964 (inset), smashing homes and killing 11 people. Fortunately, June’s quake generated only a tiny swell because its motion was mostly horizontal, not vertical. The event did show, however, that much of the West Coast was not prepared. “A lot of communities in California didn’t get the warning, or even if they did, they didn’t know what to do with it,” Ward says. In the San Francisco area, word came an hour and a half late, he says. “If it had been serious, it would have been too late.”
How long would a tsunami from a magnitude 9.2 Cascadia-zone earthquake take to reach various points along the West Coast? And how high would the waves be?
Time (to the nearest 15 minutes)
Wave (run-up in feet)