Underwater Landslides Studied for Tsunami Risk

Coastal communities in California and elsewhere are at risk of being swept away by tsunamis generated by undersea landslides near shore, scientists say.

Until recently, experts believed the tidal waves were caused primarily by distant earthquakes or volcanoes below the ocean’s surface. That changed in 1998, when an underwater landslide generated a 50-foot wave that killed 2,200 people on the coast of Papua New Guinea.

And unlike tsunamis unleashed by distant quakes, those generated immediately offshore give only a few minutes’ warning before landfall.

“The biggest thing about local tsunamis is that they’re surprising,” said Philip Watts, president of Applied Fluids Engineering Inc. in Long Beach. “Coastal residents need to be educated about the hazards.”

Experts in the budding field of tsunamis and undersea landslides gathered at the fall meeting of the American Geophysical Union in December to discuss early attempts to locate hazardous areas and assess risk.

Detailed maps of the sea floor are still being collected and analyzed for evidence of past landslides and tsunamis. Researchers hope that, eventually, they will be able to predict the likelihood, location, size and motion of future landslides, and the size of waves given those factors.

Shortly after the Papua New Guinea disaster, researchers began looking more carefully at the sharp undersea cliffs and canyons off the coast of California, particularly at the southern end of the state.

Collapses at least hundreds of years old have been found stretching for miles in at least two areas-- near Santa Barbara and the Palos Verdes Peninsula in Los Angeles County.

Along the Santa Barbara Basin, a more than 80-square-mile section of sea canyon wall slid near Goleta. The area of failure is 9 miles long and 6.5 miles wide. The area fell more than 1,500 feet.

“We were surprised to see the extent and complexity of apparent mass sea floor wasting in the Santa Barbara Basin,” said Gary Greene of the Monterey Bay Aquarium Research Institute, which led the research.

Sketchy reports indicate a tsunami struck Santa Barbara in 1812, but it is not clear whether it was caused by the giant slide, Greene said. It’s also not clear how quickly the slide moved or whether it occurred all at once.

A much smaller slide took place nearby, and a nearly 2-mile crack extends away from that slide toward the bigger one. But researchers are not sure whether that means more sliding will occur in the future.

Other researchers discovered an undersea slide off the Palos Verdes Peninsula. It appears to be about the same size as the slide that occurred in Papua New Guinea, though no evidence of a tsunami has yet been found.

Similar to the Santa Barbara collapse, the slide’s speed of movement is not known. That could make a difference in determining whether it actually generated a tsunami or could create one in the future, 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 such massive landslides, other factors such as water movement and unstable rock can cause slides. In those cases, there might not be much warning to nearby communities, researchers said.

Experts acknowledge they are at an early stage in understanding the link between landslides and tsunamis. But that doesn’t mean the warnings should be dismissed, Locat said.

“The hazard is there. We aren’t able to quantify it at the moment,” he said. “The basic thing we did was to show there have been some landslides in the past. Why wouldn’t there be any in the future?”

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Applied Fluids Engineering Inc.: https://www.appliedfluids.com

International Tsunami Information Center: https://www.shoa.cl/oceano/itic/frontpage.html