Study: Calif.Threatened by Tsunamis
S A N F R A N C I S C O, Dec. 19, 2000 -- Tsunamis generated by underwater landslidespose a serious threat to coastal communities in California andelsewhere, say researchers who are trying to determine wheresubmarine slips are most likely to occur.
Until recently, experts believed tsunamis were caused by distantundersea earthquakes or volcanoes. That changed in 1998, when aquake-triggered underwater landslide generated a 50-foot wave thatkilled 2,200 people on the coast of Papua New Guinea.
Unlike tsunamis unleashed by distant quakes, locally generatedtsunamis give only a few minutes warning before landfall.
“The biggest thing about local tsunamis is that they’resurprising,” said Philip Watts, president of Applied FluidsEngineering Inc. in Long Beach. “Coastal residents need to beeducated about the hazards.”
Sea Floor Maps
On Monday, experts in the budding field of tsunamis and undersealandslides gathered at the fall meeting of the American GeophysicalUnion to discuss early attempts to locate hazardous areas andassess risk, particularly in Southern California.
Detailed maps of the sea floor are still being collected andanalyzed for evidence of past landslides and tsunamis. Researchershope to eventually predict the likelihood, location, size andmotion of future landslides, and the size of waves given thosefactors.
Shortly after the Papua New Guinea disaster, researchers beganlooking more carefully at the sharp undersea cliffs and canyons offthe coast of California, particularly at the southern end of thestate.
Collapses stretching for miles were found in at least two areas,near Santa Barbara and the Palos Verdes Peninsula in Los AngelesCounty.
Along the Santa Barbara Basin, a more than 80-square-milesection of sea canyon wall slid near Goleta. The area of failure is9 miles long, 6.5 miles wide. The area fell more than 1,500 feet.
“We were surprised to see the extent and complexity of apparentmass sea floor wasting in the Santa Barbara Basin,” said GaryGreene of the Monterey Bay Aquarium Research Institute, which ledthe research.
Sketchy reports indicate a tsunami struck Santa Barbara in 1812,but it is not clear whether it was caused by the giant slide, hesaid. It’s also not clear how quickly the slide moved or whether itoccurred all at once.
Another, much smaller slide took place not far from the largeone, and a nearly 2-mile crack extends away from that slide towardthe bigger one. But researchers are not sure whether that meansmore sliding will occur in the future.
Earthquakes Usually to Blame
Other researchers discovered an undersea slide off the PalosVerdes Peninsula in Los Angeles County. It appears to be about thesame size as the slide that occurred in Papua New Guinea, though noevidence of a tsunami has yet been found.
Similar to the Santa Barbara collapse, the slide’s speed ofmovement is not known. That could make a difference in determiningwhether it actually generated a tsunami or could create one in thefuture, said Jacques Locat of Quebec’s Laval University.
“If a slide moves slowly, you wouldn’t notice much,” he said.
Though earthquakes are the most likely culprit behind suchmassive landslide, other factors such as water movement andunstable rock can cause slides. In those cases, there might not bemuch warning to nearby communities, researchers said.
Experts admit they are at an early stage in understanding thelink between landslides and tsunamis. But that doesn’t mean thewarnings should be dismissed, Locat said.
“The hazard is there. We aren’t able to quantify it at themoment,” he said. “The basic thing we did was to show there havebeen some landslides in the past. Why wouldn’t there be any in thefuture?”