Research shows that the subduction zone’s spreading faults increase the dangers of large earthquakes

Spreading faults in subduction zones increase the hazards of large earthquakes

Jessica DePaolis (second from left) and the team of researchers studied and compared sedimentary core samples on Montague Island, Alaska, and found evidence that four of the past eight earthquakes there involved secondary slip of the spreading fault attached to the subduction zone. caused an additional tsunami. Photo courtesy of Peter Haeussler. Credit: Peter Haeussler.

Research has provided new insight into the tectonic plate shifts that cause some of the largest earthquakes and tsunamis on Earth.

“This is the first study to use coastal geology to reconstruct the rupture history of the splay fault system,” said Jessica DePaolis, postdoctoral researcher in Virginia Tech’s Department of Geosciences. “These spreading faults are closer to the coast, so these tsunamis will hit the coastline faster than a tsunami generated solely by a subduction zone earthquake.”

Subduction zones around the world, areas where one tectonic plate slides beneath another, produce the largest earthquakes – those above magnitude 8.0 – creating tsunamis and changing ecosystems in their wake.

DePaolis, along with Tina Dura, assistant professor of natural hazards, and colleagues at the United States Geological Survey, found evidence that spreading faults, the faults in the Earth’s crust associated with the subduction zones, can shift during subduction zone earthquakes and contribute to local coastal destruction and ecological changes are occurring more regularly than previously realized.

Such a shift in the underwater spreading fault could trigger a tsunami that could reach the nearest shores in 30 minutes or less, DePaolis said.

Published in the Journal of Geophysical Research: Solid Earththe research should influence hazard awareness in subduction zones around the world. Spreading faults exist in subduction zones bordering Ecuador, Cascadia, Chile and Japan, suggesting that they may contribute to tsunami hazards in those locations as well.

When tectonic plates shift in a subduction zone, it happens miles below the ocean’s surface. Because spreading faults are associated with these zones, their location makes investigating them challenging.

Fortunately, secondary or surface effects of these shifts are geologically recorded on Montague Island in Alaska’s Prince William Sound, making it the only current landmass that sits on top of a spreading fault and exhibits such effects in the soil.

Spreading faults in subduction zones increase the hazards of large earthquakes

Magnified through a microscope, diatoms, a type of siliceous microalgae preserved in sediment, helped the researchers determine the salinity of the core samples. Photo courtesy of Jessica DePaolis. Credit: Jessica DePaolis.

Typically, the resulting lifting of land from the tectonic plate shifting beneath it, called uplift, due to subduction zone earthquakes can be as much as 1 to 3 meters. This applies to most onshore locations affected by the 1964 earthquake, which struck a magnitude of 9.2. However, on Montague Island, spreading faults caused a rise of 11 meters and caused the drainage of a coastal lagoon, effectively changing the ecosystem from a marine lagoon to a freshwater marsh.

“The island is kind of stuck in the middle of these spreading faults, so every time these spreading faults rupture, it’s actually recording the rise,” DePaolis said. “It has an exaggerated rise that is just not common in earthquakes that only occur in subduction zones.”

DePaolis and her team examined the effects of the dispersion fault breaks on Montague Island. By analyzing 42 sediment cores, they found stratigraphic evidence of the 1964 earthquake and a secondary shift caused by the spreading fault. They noticed that there was a clear sedimentary change from pre-earthquake lagoon silt to post-earthquake rooted soil.

“There are certainly islands that are uplifted by subduction zone earthquakes, but there aren’t necessarily faults going through them that are causing that exaggerated uplift, so it’s really a unique place,” said Dura, an affiliate faculty member of the Global Change Center and the Fralin Life Sciences Institute.

Researchers believe that a secondary shift in the spreading errors was possible. But that idea has only been theoretical until now, because this is the first known landmass where stratigraphic evidence has been recorded.

Team members also used diatoms, a type of siliceous microalgae preserved in the sediments and sensitive to changes in salinity, to reconstruct the paleoenvironmental changes that occurred after the 1964 earthquake. They found a clear shift from a highly saline marine lagoon environment beyond the reach of the tides, which indicated a coastal uplift.

By comparing findings from the core samples from the 1964 earthquake with samples deeper in the coastal stratigraphy, the research team discovered sedimentary and diatom evidence from three other cases where the spreading fault ruptured. This evidence correlated with four of the last eight documented subduction zone earthquakes in the region.

“There’s a tremendous amount of displacement on these fault lines that can produce very fast, localized, large tsunamis,” DePaolis said. “So you have a local tsunami that comes in very quickly and right after that you get the tsunami created by the subduction zone itself. Suddenly huge and destructive tsunamis come in in quick succession.”

More information:
Jessica M. DePaolis et al., Repeated coseismic uplift of coastal lagoons over the Patton Bay Splay Fault System, Montague Island, Alaska, USA, Journal of Geophysical Research: Solid Earth (2024). DOI: 10.1029/2023JB028552 , … 10.1029/2023JB028552

Provided by Virginia Tech

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