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Surprising causes of the tsunami could lurk off the coast of California



Although the most dangerous tsunami in California comes thousands of miles away, scientists say they have identified a wave trigger that is much closer to home. Shock fault earthquakes can cause potentially dangerous waves in certain contexts, a new model shows – and such faults exist in the immediate vicinity of the shores of Golden State.

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

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

But not every earthquake can cause a fraudulent wave. Earthquakes on underwater faults, in which one side is pushed higher than the other, are thought to be the main culprit, as vertical movement can cause a wave in the water from above.

Earthquakes on impact-slip faults such as San Andreas, in which two slabs slide one after the other, were not thought to cause a tsunami in themselves, as they caused a largely horizontal movement.

That is why the earthquake and tsunami in Sulawesi in 201

8 in Indonesia raised the eyebrows of geologists.

On September 28 of the same year, a wave estimated at 13 to 23 feet hit the capital of Palu province after a magnitude 7.5 earthquake. Together, the two events killed thousands.

Scientists, surprised by the strength of the wave, suggest that this may have caused landslides with vertical motion, which is able to trigger the wave.

“We didn’t think so,” said Ares Rosakis, an engineer specializing in solid mechanics and one of the study’s greatest authors. The guilt, he suspected, lay only with guilt. “Unbuttoning the ocean floor in the Palu area … would be enough to explain the creation of this tsunami.”

As Rosakis and his team – involving experts in every aspect of the process, including seismology and fluid dynamics – began exploring this potential solution to the Palu mystery, they began to see evidence that the earthquake rupture was not your average unpacking error. . It fits into the profile of a “shear” event, in which the actual physical destruction moves faster than the seismic waves traveling through the material. This causes a triangle-shaped shock wave called a Mach cone. (This is very similar to the sound boom caused when an extremely fast plane is moving faster than the speed of sound.)

They presented this information in a powerful computer model that took into account seismic waves, the destruction of the earthquake, the shape of the bottom of Palu Bay and the dynamics of the fluids of the wave itself. The result: A landslide by mistake in a landslide like the one that devastated Palu could indeed cause a tsunami.

“It’s a fascinating study from a physics perspective,” said Eric Geist, a geophysicist at the U.S. Geological Survey based in Moffett, California. “It’s kind of like a new way to look at generating a tsunami.”

Sophisticated models like the one that combines earthquake and tsunami dynamics 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 compares it to the violent staining you can see after hitting a tall, skinny cup, compared to the gentler movement seen in a wider vessel.

Rosakis and colleagues identified areas along the Northern California coast that matched the profile, including San Francisco Bay and Tomales County. The San Andreas Fault is located right in the sea in both areas and both are narrow enough to show some very violent swings.

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

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

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

The study helps explain how tremors during impact slip the mistakes could generate a tsunami and why the risk they pose may have been underestimated, he said. But he added: “I don’t think it will change the way we do things in terms of tsunami forecasting or hazard assessment.”

This is because tsunami hazard maps are usually made by assessing the effects of the largest and most potentially harmful tsunamis.

For California, these would be great waves caused by, say, a magnitude 9 earthquake across the Pacific Ocean or a similar major earthquake near the Cascade subduction zone north of Cape Mendocino, said Rick Wilson, a senior engineering geologist with the California Geological Survey. which updates tsunami danger maps in the state. It is unlikely that any additional tsunami risk, if confirmed, would fail in these worst-case scenarios.

“We will continue to review articles like this to see if we really need to update something, but based on our experience and all the great sources we have included in the maps, we feel quite comfortable that existing maps are safe to use. and good to use, ”Wilson said.

The results of the study could affect the probability estimates of hazards, which take into account not only the worst case, but also the probability of earthquakes of varying magnitude, Arkas said.

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

“I guess this document will make a number of tsunami models take a closer look” in the role of earthquakes during impact faults, Dengler said. “We also need to take a closer look at the conditions that allow supercutting to occur. If this idea is more closely monitored, it will be included in the next generation of tsunami maps. “

Although it is important to understand the potential risk of a tsunami from these local sources, it is unlikely to affect the way the tsunami forecast is conducted, as ground destruction information is not available in real time, Arkas said.

Even if it was, he added, “many of the faults in this type of mechanism can be found in close proximity to the shore, and although they are dangerous locally, there will be no time for local warning due to the proximity of the source.”

Dengler said that in these situations, the earthquake itself is a warning.

“As far as public safety is concerned, when you feel an earthquake near the coast, especially one that shakes for a long time, accept that a tsunami could be generated and the coastal area evacuated,” she said. “A festival was taking place in Palu during the earthquake, which brought many people from the interior to the bay. Many of these people may not have realized that the trembling was their warning to get ashore. “




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