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Surprising tsunami triggers may lurk off California's coast, scientists say

Amina Khan, Los Angeles Times on

Published in Science & Technology News

LOS ANGELES — Although California’s most dangerous tsunamis come from thousands of miles away, scientists say they’ve pinpointed a wave trigger that’s much closer to home. Earthquakes along strike-slip faults can cause potentially dangerous waves in certain contexts, a new model shows — and such faults do exist right off parts of the Golden State’s shores.

If confirmed, the findings described in the Proceedings of the National Academy of Sciences could affect future local tsunami risk assessments for coastlines along California and beyond.

Tsunamis can be caused by a variety of events, including landslides, volcanic activity and most commonly, earthquakes.

But not every earthquake can trigger a rogue wave. Quakes along underwater thrust faults, in which one side gets pushed up higher than the other, are thought to be the main culprit, because the vertical motion can induce a wave in the water above.

Quakes along strike-slip faults like the San Andreas, in which two plates slide past one another, weren’t thought to cause tsunamis on their own because they cause largely horizontal motion.

That’s why the 2018 Sulawesi earthquake and tsunami in Indonesia raised geologists’ eyebrows.

 

On Sept. 28 of that year, a wave estimated to be 13 to 23 feet in height struck the provincial capital of Palu following a magnitude 7.5 earthquake that occurred along a strike-slip fault. Together, the two events killed thousands.

Scientists surprised at the power of the wave suggested that perhaps it caused landslides with vertical motion that was able to trigger the wave.

“We didn’t think so,” said Ares Rosakis, an engineer specializing in solid mechanics and the study’s senior author. The blame, he suspected, lay with the fault alone. “The unzipping of the bottom of the ocean in the Palu area ... would be enough to explain the creation of this tsunami.”

As Rosakis and his team — which included experts in every aspect of the process, including seismology and fluid dynamics — began probing this potential solution to the Palu mystery, they started seeing evidence that the earthquake rupture wasn’t your average unzipping of a fault. It fit the profile of a “supershear” event, in which the actual physical rupture moves faster than the seismic waves traveling through the material. That causes a triangle-shaped shock wave called a Mach Cone. (It’s very much akin to the sonic boom caused when an extremely fast plane moves faster than the speed of sound.)

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