Uncalculated tsunami risks in major coastal areas

MADRID, 4 May. (EUROPA PRESS) –

Tsunami hazards have been overlooked in connection with underwater landslide faults close to shore, especially for cities adjacent to faults that cross bays inland.

Several areas around the world can fall into this category, including the area of San Francisco Bay, 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 of the California Institute of Technology, used the Blue Waters supercomputer at the National Center for Supercomputing Applications to model the tsunami hazards related to landslide faults around the world. The results are published in PNAS.

“Whenever we saw large earthquake-triggered tsunamis along slip faults, people assumed that maybe the earthquake had caused an underwater landslide, displacing the water that way“Rosakis said.

The researchers said that a slip fault exists when two blocks of rock in the fault line slide horizontally side by side. The San Andreas fault is an example of a slip fault.

In September 2018, a moderate magnitude 7.5 earthquake and unexpectedly powerful tsunami struck Palu, a city located on the inland side of Palu Bay on the Indonesian island of Sulawesi. The earthquake occurred along a northwest-southeast slip fault that runs through the city and sinks below the bay along the northwest coast of Palu.

“It looked like a bulldozer had entered and razed the city,” said co-author Costas Synolakis, a professor of civil engineering at the University of Southern California, who surveyed the area after the devastating event. “That is why it is so important that we try to understand what really happened.”

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There are studies that explore the connections between slip faults and tsunamis. However, they target specific fault systems or geographic locations, obscuring the complex details of the fault’s geometry and bathymetry, the study reports.

What is unique in our study is that instead of considering a specific location event, we focused on the fundamentals of a system of slip faults that interact within the confines of a narrow bay, “Elbanna said.” We opted to simulate a very basic flat fault that passes through a very simplified smooth-bottom bay, similar to a bathtub. Having this simplified baseline model allows us to generalize to anywhere on the planet that may be at risk. “

Intersonic earthquakes are fault ruptures that occur so rapidly that their movement exceeds the seismic shear waves they generate, like a sonic boom, but with the shock wave moving through the earth’s crust. The simulations found that intersonic earthquakes can provide sufficient energy and horizontal displacements. large enough to trigger large tsunami waves.

When such earthquakes occur within a narrow bay, the researchers reported three distinct phases that can lead to a tsunami: the initial movement of the fault and the shock wave that causes an almost instantaneous shaking of the coastal land; the displacement of water while the earthquake occurs; and the gravity-driven motion of the tsunami wave after the ground motion has subsided leading 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 earthquakes and subsequent water displacement that occur many miles offshore, an earthquake and tsunami that occur within the narrow confines of a bay will allow very little warning time for the shoreline“.

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Elbanna compares the effect of horizontal displacements of sliding faults to holding a cup of water in hand and shaking it horizontally.

“Splash motion is the result of horizontal shaking. When an earthquake occurs along a slip fault in a narrow bay, horizontal ground motion pushes and pulls the bay boundaries, which causes the displacement of the water in the vertical direction and the initiation of the tsunami “, He said.

“The physics-based model used in this study provides critical information on the hazard associated with landslide faults, in particular the need to account for such risk to mitigate future damage to other bays traversed by landslide faults,” he said undergraduate student Mohamed Abdelmeguid, who ran the simulations together with former graduate student Xiao Ma, currently a senior research scientist at Exxon Mobil.

The regions at risk identified by the team (Northern California, Turkey and Egypt) have experienced intersonic earthquakes in the past, and the researchers recommend reviewing the tsunami hazard classification of submarine slip faults, particularly those that pass through narrow bays.

“It may not look like the tsunami scene from Dwayne Johnson’s movie ‘San Andreas,’ but tsunami risk for Northern California and various locations around the world needs to be seriously reviewed“Elbanna said.

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