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Surprising tsunami triggers may be lurking off the coast of California, researchers say

POINT REYES NATIONAL SEASHORE, CA - JULY 20, 2016: Clear water and deep blue skies at Tomales Bay on July 20, 2016 in Point Reyes National Seashore, California.  (Gina Ferazzi / Los Angeles Times)

The clear waters of Marin County Tomales Bay, whose shape and proximity to the San Andreas Fault could make it more vulnerable to tsunamis than scientists have previously realized. (Gina Ferazzi / Los Angeles Times)

Although California’s most dangerous tsunamis are thousands of miles away, researchers say they have found a wave trigger that is much closer to home. Earthquakes along strike failures can cause potentially dangerous waves in certain contexts, a new model shows – and such failures are found just outside parts of the Golden State’s shores.

If confirmed, the results described in the Proceedings of the National Academy of Sciences may 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 often earthquakes.

But not every earthquake can trigger a ripple wave. Earthquakes along underwater pressure faults, where one side is pushed higher up than the other, are thought to be the biggest culprit because the vertical movement can cause a wave in the water above.

Earthquakes along strike-slip faults such as San Andreas, where two plates slide past each other, were not considered to cause tsunamis alone because they cause largely horizontal motion.

Therefore, the earthquake and tsunami in Indonesia in Sulawesi in 2018 raised the eyebrows of geologists.

On September 28 of the same year, a wave estimated to be 13 to 23 feet hit the provincial capital Palu after a 7.5 magnitude earthquake occurred along a strike fault. Together, the two events killed thousands.

Scientists surprised by the force of the wave suggested that it might have caused landslides with vertical motion that were able to trigger the wave.

“We did not think so,” said Ares Rosakis, an engineer specializing in solid mechanics and one of the study’s senior authors. The blame, he suspected, lay alone with the fault. “The unpacking of the seabed in the Palu area … would be enough to explain the creation of this tsunami.”

As Rosakis and his team – which included experts in all aspects of the process, including seismology and fluid dynamics – began to investigate this potential solution to the Palu mystery, they began to see evidence that the earthquake was not your average unpacking of a fault. It fits the profile of a “supershear” event where the actual physical fracture moves faster than the seismic waves moving through the material. It causes a triangular shock wave called Mach Cone. (It’s very similar to the sound barrier caused when an extremely fast plane moves faster than the speed of sound.)

They introduced this information into a powerful computer model that took into account the seismic waves, the earthquake rupture, the shape of the bottom of Palu Bay, and the fluid dynamics of the wave itself. The result: An earthquake with a strike error like the one that destroyed Palu could actually trigger a tsunami.

“This is a fascinating study in physics,” said Eric Geist, a research geophysicist with the U.S. Geological Survey based in California’s Moffett Field. “It’s a kind of new way of looking at tsunami generation.”

Complex models like this, which combine the dynamics of both earthquakes and tsunamis, require powerful computers, but are likely to become the norm, he added.

Part of the cause of the Sulawesi tsunami was the very narrow shape of Palu Bay. Rosakis compared it to the violent sloshing you can see after throwing a tall, thin glass along with the gentler movement that could be seen in a wider vessel.

Rosakis and his colleagues pointed to areas along the Northern California coast that fit the profile, including San Francisco Bay and Tomales Bay in Marin County. The San Andreas bug is just outside both of these areas, and both are narrow enough to potentially see something very violent shoot.

Other bays around the world also fit the profile, including Izmit Bay in Turkey and the bay formed by the Gulf of Aqaba in Egypt.

“This is the first time that this has been recognized,” Rosakis said, “so really a lot more work needs to be done to be placed inside the hazard assessment maps. At this point, it is completely absent – these areas are declared safe. ”

Diego Arcas, director of the NOAA Center for Tsunami Research, called the authors’ conclusion “interesting.”

The study helps explain how earthquakes along strike failures can generate tsunamis and why the risk they pose may have been underestimated, he said. But he added: “I do not think it will change the way we do things in terms of tsunami forecasts or hazard assessment.”

This is because tsunami hazard maps are generally produced by estimating the effects of the largest and most potentially harmful tsunamis.

For California, it would be large waves caused by an earthquake measuring 9,000 miles across the Pacific Ocean or a similar large earthquake off the Cascadia subduction zone north of Cape Mendocino, said Rick Wilson, a senior engineering geologist. with the California Geological Survey, which has updated the state’s tsunami hazard map. It is unlikely that additional tsunami risks, if confirmed, will make a dent in these worst-case scenarios.

“We will continue to review papers like this to see if we really need to update anything, but based on our experience and all the major sources that we have incorporated into the maps, we feel pretty confident that the existing cards are safe to use and good to use, ”said Wilson.

The study results could have an effect on likely hazard assessments, which not only consider the worst case scenario, but also the likelihood of earthquakes of various magnitudes occurring, Arcas said.

Ultimately, it will take time for more researchers to independently test whether they can reproduce the results, said Lori Dengler, a geophysicist and professor emeritus at Humboldt State University.

“My guess is that this paper will get a number of tsunami modelers to take a closer look at the role of the earthquake along strike failures,” Dengler said. “The conditions that allow supershear to occur must also be further investigated. If this idea holds up to more careful control, it will be included in the next generation of tsunami maps.”

While it is important to understand the potential tsunami risk from these local sources, it is unlikely to affect how tsunami forecasts are carried out because information on landslides is not available in real time, Arcas said.

Although it was, he added, “many of the faults with this type of mechanism can be found near the coast, and although they can be dangerous locally, there would be no time for a local warning due to the proximity of the source.”

Dengler said the quake in itself is the warning in these situations.

“As far as public safety is concerned – assume that when you feel an earthquake near the coast, especially one where the tremor lasts a long time, a tsunami can occur and evacuate the coastal area,” she said. “In Palu, a festival took place at the time of the earthquake that brought many people from inland areas to the bay. Many of these people may not have recognized that the quake was their warning to get off shore.”

This story originally appeared in the Los Angeles Times.

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