JOLT COULD TRIGGER COASTAL TSUNAMI
24 May 2007 - University of Southern California

With a strong enough jolt, a 7.6-magnitude earthquake, the seafloor under Catalina Island could be violently thrust upward, causing a tsunami along the Southern California coast, according to researchers at USC.

In a pair of journal articles recently published, researchers at the USC Viterbi School of Engineering described the tsunami hazard associated with offshore faults, including one that lies under Santa Catalina Island, just 25 miles off the Los Angeles coast.

“Catalina Island itself exists due to earthquake-related uplift on a geologic structure known as a restraining bend,” said Mark Legg, a geophysicist working with the USC researchers, in the August issue of Earthquake Spectra.

“Although faults offshore Los Angeles and Orange counties are mostly strike-slip, faults that move side to side, bends in the fault line produce areas where the ground is pushed up during major earthquakes,” Legg said. “One of these regions lies directly below Santa Catalina Island.”

“Strike-slip faults are not straight,” added Jose C. Borrero, assistant research professor in the USC Viterbi School, who worked with co-researcher Costas E. Synolakis, USC professor of civil and environmental engineering, on the study. “Bends in the fault trace produce regions where earthquake stresses cause the sea floor to pop up and generate a tsunami.”

When a large earthquake occurs at a restraining bend, like the bend under Catalina Island, the ground is pushed up and, in turn, pushes up the entire region that has created the island and its offshore flanks.

“Future earthquakes will push the region up further, possibly resulting in a tsunami,” Legg warned.

“Tsunami” is a Japanese word for waves caused by large motions of the sea floor, either through earthquakes, landslides or undersea volcanoes. They are generally associated with earthquakes that occur offshore and produce significant uplifting of the sea floor.

Legg was awarded a fellowship through the National Earthquake Hazards Reduction Program to conduct research at the USC Viterbi School, where he has combined his earthquake modeling with computer-simulation techniques developed at USC.

“We took a range of potential earthquakes and investigated the tsunami potential from each case," Borrero said. “We found there is significant amplification of tsunami energy into San Pedro Bay.”

The findings have important implications because San Pedro Bay’s south-facing shores are home to the largest container ports in the United States – the ports of Los Angeles and Long Beach.

Billions of dollars of materials pass through these ports every day. A large earthquake and tsunami could bring commerce to a halt, seriously affecting the economy of California and the rest of the nation.

“A magnitude 7.6 earthquake could cause seafloor uplift of six feet or more,” Borrero said. “That, in turn, would disturb the sea surface by the same amount, resulting in a tsunami.

The shallow San Pedro shelf offshore of Long Beach focuses the waves and amplifies them by one-and-a-half times, so the original six-foot wave would build to nine feet inside the harbor,” he said.

The researchers said waves of that size could smash small boats at their moorings, possibly flood low-lying areas in the ports and push huge oil tankers and cargo ships against piers, which may not withstand the force. The destruction could create oil spills and become a serious fire hazard.

In addition to the largest of the Catalina fault scenarios, the researchers looked at potential earthquakes on the Lasuen Knoll and San Mateo Thrust faults, which lie offshore of Orange and northern San Diego counties, and a large submarine landslide offshore of the Palos Verdes Peninsula.

“The results are similar, but show that the ports of Los Angeles and Long Beach are particularly vulnerable to locally generated tsunamis,” Borrero said.

But the analysis also offered some good news: The same features that focus and amplify tsunami waves also slow the waves’ arrival.

“Our models show that, depending on the source, there are anywhere from 15 to 20 minutes between the earthquake and the first significant waves in the ports,” Borrero said.

“This may give shippers enough time to evacuate dock workers and stop hazardous activities, such as cargo handling or offloading oil from tanker ships,” he added. “Every second would count.”

http://www.usc.edu

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