SAN FRANCISCO--Coastal residents in Southern California face a risk of giant waves from massive undersea landslides, scientists have learned. New research suggests that surges of water tens of meters high could wash inland if offshore slopes collapse near Santa Barbara and Los Angeles. However, geologists can't yet say how often such events occur or whether other slopes pose similar threats, according to reports here on 19 December at a meeting of the American Geophysical Union (AGU).
Marine geologists knew that big earthquakes under the seafloor trigger tsunamis that can cross entire ocean basins. But in July 1998, a horrific tsunami in Papua New Guinea called attention to the havoc that landslides can wreak at close range. An earthquake dislodged a 4-cubic-kilometer chunk from an offshore slope, spawning a 15-meter tsunami that engulfed coastal villages. More than 2200 people died. "That was a watershed," says tsunami modeler Costas Synolakis of the University of Southern California in Los Angeles. "People realized this was a hazard we had totally overlooked."
Such waves may have washed over Southern California, too. High-resolution sonar mapping and remotely operated submersibles reveal the true extent of a past slope collapse offshore from Goleta, California, near Santa Barbara. The landslide is 10.5 kilometers wide and 14.5 kilometers long, containing about half the volume of material that broke loose in Papua New Guinea. Parts of the slide may have triggered a reported tsunami in the Santa Barbara Channel in 1812, says marine geologist H. Gary Greene of the Monterey Bay Aquarium Research Institute in Moss Landing, California. However, his team doesn't yet know when the slope fell apart.
A similar slide off the Palos Verdes peninsula near Long Beach probably struck within the last 1000 years, says geological engineer Jacques Locat of Laval University in Sainte-Foy, Quebec. His survey indicates that the offshore slope there is normally stable, so it must have taken a large quake to set it in motion. Such a landslide today could unleash a tsunami 10 to 45 meters high, he says. However, his models depend on unknown factors, he notes, such as how fast the slide occurs and whether it happens all at once.
Those uncertainties and a lack of detailed undersea mapping in the rest of the western United States make an assessment of tsunami hazard impossible for now, says geologist Robert Bohannon of the U.S. Geological Survey in Menlo Park, California. "We don't know why some areas of the steep slope all along the coast have failed and others have not," Bohannon says. Seepage of fluids through seafloor vents near faults may weaken the slopes, he and Greene both observe.