Although California’s most dangerous tsunamis come from thousands of miles away, scientists say they have identified a wave trigger much closer to home. Earthquakes along slip-resistant faults can cause potentially dangerous waves in some settings, a new model shows – and such faults exist just off the shores of the Golden State.
If confirmed, the findings described in Proceedings of the National Academy of Sciences could affect future local tsunami risk assessments for coasts along California and beyond.
Tsunamis can be caused by a variety of events, including landslides, volcanic activity, and most commonly earthquakes.
But not all earthquakes can trigger a rogue wave. Earthquakes along submarine thrust faults, in which one side is pushed higher than the other, are considered the main culprit, as vertical movement can induce a wave in the water above.
Quakes along non-slip faults like the San Andreas, in which two plates slide over each other, are not believed to cause tsunamis on their own as they cause largely horizontal movement.
This is why the Sulawesi earthquake and tsunami in Indonesia in 2018 raised the eyebrows of geologists.
On September 28 of that year, a wave estimated to be 13 to 23 feet in height struck the provincial capital of Palu following a 7.5 magnitude earthquake that occurred along a dropout fault. Together, the two events killed thousands of people.
Scientists surprised by the power of the wave suggested that it may have caused landslides with vertical movement that was able to trigger the wave.
“We didn’t think so,” said Ares Rosakis, an engineer specializing in solid mechanics and lead author of the study. The blame, he suspected, was only the fault. “The decompression of the ocean floor in the Palu region … 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 probe this potential solution to the Palu mystery, they began to see evidence that the rupture of the earthquake was not your average decompression of a fault. It fits the profile of a “supershear” event, in which the actual physical fracture moves faster than the seismic waves passing through the material. This causes a triangle-shaped shock wave called the Mach Cone. (This is very similar to the sound boom caused when an extremely fast airplane is moving faster than the speed of sound.)
They fed this information into a powerful computer model that took into account seismic waves, the earthquake breaking, the shape of the bottom of Palu Bay, and the fluid dynamics of the wave itself. The result: an earthquake on an anti-slip fault like the one that devastated Palu could indeed trigger a tsunami.
“This is a fascinating study in terms of physics,” said Eric Geist, a geophysicist with the US Geological Survey based at Moffett Field in California. “It’s sort of a new way of looking at the tsunami generation.”
Complex models like this, which combine the dynamics of the earthquake and tsunami, require powerful computers but are likely to become the norm, he added.
Part of the reason for the Sulawesi tsunami was the very narrow shape of Palu Bay. Rosakis likened it to the violent sway you might see after jostling a tall, skinny glass, compared to the smoother motion you would see in a larger vessel.
Rosakis and his colleagues highlighted areas along the northern California coast that fit the profile, including San Francisco Bay and Tomales Bay in Marin County. The San Andreas Fault sits just offshore of these two areas, and both are narrow enough to potentially see some very violent sloshing.
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,” said Rosakis, “so a lot more work needs to be done to be incorporated into 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 faults can generate tsunamis, and why the risk they pose may have been underestimated, he said. But he added: “I don’t think it’s going to change the way we do things in terms of forecasting tsunamis or assessing hazards.”
Indeed, tsunami risk maps are generally established by estimating the effects of the most significant and potentially damaging tsunamis.
For California, these would be large waves caused, for example, by a magnitude 9 earthquake thousands of miles across the Pacific Ocean, or an equally large earthquake off the subduction zone of Cascadia north of Cape Mendocino, said Rick Wilson, a senior engineering geologist. with the California Geological Survey, which updated the state’s tsunami hazard maps. It is unlikely that an additional tsunami risk, if confirmed, will make a dent in these worst-case scenarios.
“We will continue to review articles like this to see if we really need to update anything, but based on our experience and all the great sources that we have incorporated into the maps, we are fairly convinced that the existing cards are safe. to use and good to use, ”Wilson said.
The results of the study may have an effect on probabilistic risk assessments, which consider not only the worst-case scenario, but also the likelihood of earthquakes of various magnitudes occurring, Arcas said.
Ultimately, it will take time for more scientists to independently test whether they can replicate the results, said Lori Dengler, a geophysicist and professor emeritus at Humboldt State University.
“I suspect this paper will cause a number of tsunami modellers to take a closer look” at the role of earthquakes along strike-slip faults, Dengler said. “There is also a need to take a closer look at the conditions that allow the supershear to occur. If this idea stands up to closer scrutiny, it will be included in the next generation of tsunami maps. “
While it is important to understand the potential risk of a tsunami from these local sources, it is unlikely that it will affect the way tsunami forecasting is conducted as information on ground failure is not available on time. real, Arcas said.
Even if that were the case, he added, “many of the faults of this type of mechanism can be found near the coast, and although they could be dangerous locally, there would be no time for a local warning due to proximity to the source. ”
Dengler said that in these situations, the earthquake itself is the warning.
“Regarding public safety – any time you feel an earthquake near the coast, especially one where the tremors last a long time, assume that a tsunami could occur and evacuate the coastal area,” he said. she declared. “In Palu there was a festival going on around the time of the earthquake which brought many people from inland to the bay. Many of these people may not have recognized that the tremors were their warning to move away from the coast.