Previously unrecognized tsunami risk identified in coastal cities – sciencedaily

A new study has found that overlooked tsunami hazards were linked to underwater slip-resistance faults near the shore, especially for coastal towns adjacent to faults that cut through inland bays. Several regions of the world can fall into this category, including the San Francisco Bay Area, Izmit Bay in Turkey, and the Gulf of Al-Aqaba in Egypt.

The study led by Ahmed Elbanna, professor of civil and environmental engineering at the University of Illinois Urbana-Champaign and professor Ares Rosakis at the California Institute of Technology used the Blue Waters supercomputer from the National Center for Supercomputing Applications to model the risks. of tsunami linked to strike-slip faults around the globe. The results are published in the Proceedings of the National Academy of Sciences.

“Whenever we have seen large tsunamis triggered by earthquakes along non-slip faults, people have speculated that the earthquake may have caused an underwater landslide, displacing water from that. way, ”Rosakis said.

The researchers said there is an anti-slip fault when two blocks of rock on the fault line slide horizontally over each other. The San Andreas fault is an example of a slip fault.

In September 2018, a moderate 7.5 magnitude earthquake and unexpectedly powerful tsunami swept through Palu, a town inside Palu Bay on the Indonesian island of Sulawesi. The earthquake occurred along a loose north-west-south-east trending fault that crosses the city and plunges under the bay along the north-west shore of Palu.

“It looked like a bulldozer had entered and leveled the city,” said co-author Costas Synolakis, president of Athens College and professor of civil engineering at the University of Southern California, who inspected the area. region following the devastating event. “This is why it is so important that we try to understand what really happened.”

Studies exploring the links between anti-slip faults and tsunamis exist. However, they focus on specific fault systems or geographic locations, obscuring the intricate details of fault geometry and bathymetry, the study reports.

“What is unique about our study is that instead of considering a location-specific event, we focused on the fundamentals of a non-slip fault system interacting within the confines of a narrow bay.” , Elbanna said. “We chose to simulate a very basic flat fault crossing a bay with a very simplified smooth bottom, similar to a bathtub. Having this basic, simplified model allows us to generalize to any place on the planet that may be at risk.

Intersonic earthquakes are fault ruptures that occur so quickly that their motion exceeds the seismic shear waves they generate – like a sonic boom, but with the shock wave moving through the earth’s crust. Simulations have shown that intersonic earthquakes can provide enough energy and horizontal displacements large enough to trigger large tsunami waves.

When such earthquakes occur in a narrow bay, researchers have reported three distinct phases that can lead to a tsunami: the initial movement of the fault and the shock wave causing almost instantaneous shaking of coastal lands; displacement of water during the earthquake; and the gravitational movement of the tsunami wave after the ground movement has subsided which carries the wave to shore.

“Each of these phases will have a different effect depending on the unique geography of the surrounding land and the bathymetry of the bay,” Elbanna said. “And, unlike the earthquakes and subsequent water displacements that occur several miles offshore, an earthquake and tsunami that occurs within the narrow confines of a bay will leave very little time to go. ‘alert for the coast. “

Elbanna likens the effect of horizontally moving non-slip faults to holding a cup of water in your hand and shaking it horizontally.

“The swaying motion is the result of the horizontal shaking. When an earthquake occurs along a strike-slip fault in a narrow bay, the horizontal motion of the ground pushes and pulls the boundaries of the bay, which causes the water to move vertically and initiate the tsunami, ”he said.

“The physics-based model used in this study provides critical insight into the danger associated with slip faults, in particular, the need to account for this risk to mitigate future damage to other bays crossed by slip faults,” said said an Illinois graduate. student Mohamed Abdelmeguid, who conducted the simulations with former graduate student Xiao Ma, currently a senior researcher at Exxon Mobil.

The risk regions identified by the team – Northern California, Turkey and Egypt – have experienced intersonic earthquakes in the past, and the researchers recommend reviewing the tsunami risk rating of the strike-slip faults below. -marines, especially those crossing narrow bays.

“It may not sound like the tsunami scene in Dwayne Johnson’s film ‘San Andreas’, but the tsunami risk for Northern California and several places around the world needs to be seriously reviewed,” Elbanna said.

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