Subduction zone fault spacing complex hazards

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Jessica DePaolis (second from left) and a team of researchers studied and compared sedimentary core samples on Montagu Island, Alaska, and found evidence that four of the past eight earthquakes there involved secondary slip from a dilatational fault associated with the subduction zone, which triggered an additional tsunami. Photo courtesy of Peter Haussler.

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Credit: Photo by Peter Haussler.

Pioneering research has provided new insight into the shifts of tectonic plates 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 an extensional fault system,” said Jessica DePaolis, a postdoctoral fellow in the Department of Geosciences at Virginia Tech. “These divergent faults are closer to the coast, so these tsunami waves will be faster to hit the coast than a tsunami generated solely by a subduction zone earthquake.”

Subduction zones around the world, areas where one tectonic plate moves under another, create the largest earthquakes—those with a magnitude greater than 8.0—triggering tsunamis and changing ecosystems in their wake. DePaolis, along with Tina Dora, assistant professor of natural hazards, and colleagues from the United States Geological Survey, found evidence that divergent faults, which are crustal faults associated with subduction zones, may shift during subduction zone earthquakes and contribute to local coastal destruction and change. environment more regularly than previously expected.

Such a shift in a fault extending underwater could create a tsunami that could reach the nearest shores in 30 minutes or less, DePaulis said.

The study was published in the journal Geophysical Research: Solid Earth, and should influence awareness of risks in subduction zones around the world. Scattered faults are found in subduction zones bordering Ecuador, Cascadia, Chile, and Japan, suggesting that they may contribute to tsunami hazards at those locations as well.

When tectonic plates move at a subduction zone, it happens miles below the ocean's surface. Because divergent faults are connected to these areas, their location makes searching for them difficult.

Fortunately, the secondary or superficial effects of these transformations have been recorded geologically at Montagu Island in Alaska's Prince William Sound, making it the only current land mass above an extensional fault to show such effects in its soils.

Normally, the uplift caused by tectonic plates moving underneath, called uplift, from subduction zone earthquakes can reach 1 to 3 metres. This is true of most land-based sites affected by the 1964 earthquake, which hit 9.2 on the Richter scale. However, on Montagu Island, spaced faults created 11 meters of uplift and began draining a coastal lagoon, effectively changing its ecosystem from a marine lagoon to a freshwater marsh.

“The island is kind of stuck in the middle of these faults, so any time these faults rupture, they actually record uplift,” DePaulis said. “It has this exaggerated height, which is not common in earthquakes that only occur in a subduction zone.”

DePaolis and her team examined the effects of an extensional fault rupture on Montagu Island. By analyzing 42 sediment samples, they found stratigraphic evidence of the 1964 earthquake and secondary metamorphism caused by the extensional fault. They noticed a clear change in the sediments from lake silt before the earthquake to rooted soil after the earthquake.

“There are certainly islands that are uplifted by subduction zone earthquakes, but they don't necessarily have faults running through them that cause this exaggerated uplift, so it's a really unique place,” said Dora, an affiliate faculty member at the Center for Global Change and the Fralin Life Sciences Institute. .

The researchers believe that a secondary shift from divergence errors was possible. But this idea has been only theoretical so far because this is the first known landmass to record stratigraphic evidence.

The team members also used diatoms, a type of siliceous microalgae preserved within sediments that are sensitive to changes in salinity, to reconstruct ancient environmental changes that occurred following the 1964 earthquake. They found a clear shift from a hypersaline marine lake environment beyond the reach of the tides, suggesting To the height of the coast.

By comparing core sample results from the 1964 earthquake with samples deeper in coastal strata, the research team discovered sedimentary and diatomaceous evidence for three other cases in which an extensional fault ruptured. This evidence is associated with four of the last eight documented subduction zone earthquakes in the region.

“There is a tremendous amount of displacement on these faults that can create really fast, localized, large tsunamis,” DePaulis said. “So, the local tsunami comes very quickly, and right after that, we will have the tsunami that was created by the subduction zone itself. Suddenly, you have these huge, destructive tsunamis coming quickly one after the other.”

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Journal of Solid Earth Geophysical Research

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