Study finds evidence that spreading faults in the subduction zone increase the risk of large earthquakes

Research has revealed new insights into the shifting tectonic plates that cause some of Earth’s most powerful earthquakes and tsunamis.

“This is the first study to use coastal geology to reconstruct the rupture history of the fault system,” said Jessica DePaolis, a postdoctoral researcher in the Department of Geosciences at Virginia Tech. “These faults are closer to the coast, so these tsunamis will reach the coast faster than a tsunami that only comes from a subduction zone earthquake.”

Subduction zones around the world, areas where one tectonic plate shifts beneath another, cause the largest earthquakes—those over magnitude 8.0—that trigger tsunamis and subsequently alter ecosystems.

DePaolis, along with Tina Dura, assistant professor of natural hazards, and colleagues from the United States Geological Survey, found evidence that splay faults, the crustal faults associated with subduction zones, can shift during earthquakes in subduction zones and contribute to the destruction of local coastlines more often than ecological changes accepted so far.

Such a displacement of the fault underwater could trigger a tsunami that could reach the nearest coast in 30 minutes or less, DePaolis said.

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

When tectonic plates shift in a subduction zone, they happen miles below the ocean’s surface. Because splay faults are associated with these zones, their location makes their exploration challenging.

Fortunately, secondary or near-surface effects of these shifts have been geologically recorded on Montague Island in Prince William Sound, Alaska, making it currently the only landmass lying on a fault with such effects in the ground.

Typically, the resulting lifting of the land from the displaced tectonic plate beneath it, called uplift, can be as much as 1 to 3 metres during subduction zone earthquakes. This is true for most onshore sites affected by the 1964 earthquake, which measured 9.2 on the Richter scale. However, on Montague Island, splay faulting caused uplift of 11 metres and initiated the drainage of a coastal lagoon, transforming the ecosystem from a marine lagoon into a freshwater marsh.

“The island is kind of stuck in the middle of these spreading faults, and every time these spreading faults rupture, it actually records the uplift,” DePaolis said. “There’s this exaggerated uplift that’s just not common in earthquakes that only occur in the subduction zone.”

DePaolis and her team studied the effects of splay fractures on Montague Island. By analyzing 42 sediment cores, they found stratigraphic evidence of the 1964 earthquake and secondary displacement caused by the Splay fault. They found that there was a significant sediment change from lagoon mud before the earthquake to rooted soil after the earthquake.

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

Researchers have believed that secondary displacement from the spreading faults was possible. But this idea has so far only been theoretical, as this is the first known landmass to show stratigraphic evidence.

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

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

“There are enormous displacements occurring on these faults that can result in very rapid, localized, large tsunamis,” DePaolis said. “So you have this local tsunami coming very quickly, and right behind that comes the tsunami caused by the subduction zone itself. Suddenly, these massive and destructive tsunamis are coming our way in quick succession.