A new study found evidence that uplift in a subduction zone in Alaska and the Aleutian Islands caused additional tsunami activity in half of the last eight earthquakes. New information about the shifting tectonic plates that cause the world’s largest earthquakes and tsunamis has been revealed through groundbreaking research.

“This is the first study to use coastal geology to reconstruct the rupture history of a fault system,” said Jessica DePaolis, a doctoral student in Virginia Tech’s Department of Earth Sciences. “These faults are closer to the coast, so these tsunamis will hit the coast faster than a tsunami caused by just a subduction zone earthquake.”

Skew defects and their effects

Subduction zones around the world, where one tectonic plate slides under another, create the largest earthquakes, greater than magnitude 8.0, that trigger tsunamis and subsequently alter ecosystems. DePaolis, along with assistant professor of natural hazards Tina Dura and colleagues at the U.S. Geological Survey (USGS), found evidence that crustal faults associated with subduction zones can slip during subduction zone earthquakes and contribute to local coastal destruction, and that environmental changes are more widespread. than previously thought.

Such an underwater fault shift could create a tsunami that could reach nearby shores in 30 minutes or less, DePaolis said.

A study published today (May 20) Journal of Geophysical Research: Solid Earth should influence hazard awareness in subduction zones worldwide. Faults exist in subduction zones bordering Ecuador, Cascadia, Chile, and Japan, indicating that they may also contribute to tsunami hazards in these regions.

Problems in fault investigation

When tectonic plates move in a subduction zone, this occurs many kilometers below the ocean surface. Since faults are associated with these regions, their locations make studies difficult. Fortunately, the effects of these secondary or surface-level landslides have been geologically recorded on Montague Island in Prince William Sound, Alaska; This makes it the only landmass today to lie on a fault that exhibits such effects on its soil.

As a general rule, the elevation of land by the shifting of the underlying tectonic plate is called uplift; Subduction zone earthquakes can range from 1 to 3 metres. This is true for most onshore locations affected by the 1964 earthquake measuring 9.2 on the Richter scale. But on Montague Island, faults created an 11-foot uplift and began drying out the coastal lagoon, effectively transforming its ecosystem from a marine lagoon to a freshwater marsh.

Unique geological studies of Montague Island

“The island is kind of stuck in the middle of these faults, so every time these faults break, uplift actually occurs,” DePaolis said. “It has an exaggerated uplift that is not common for subduction-only earthquakes.”

DePaolis and his team investigated the effects of oblique fault ruptures on Montague Island. By analyzing 42 sediment cores, they found stratigraphic evidence of the 1964 earthquake and secondary faulting. They observed a distinct change in the sediment layer, from pre-earthquake lagoon silt to post-earthquake rooted soil.

“Of course there are islands that rise up during subduction zone earthquakes, but they don’t necessarily have faults running through them that cause extreme uplift, so this is a really unique place,” said Dura, a faculty member affiliated with the Center for Global Change. and Fralin Life Sciences Institute.

Researchers believe a secondary slide from the faults is possible. But this idea has remained only theoretical so far because it is the first known landmass to record stratigraphic evidence.

Advanced methods and results

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

By comparing core results from the 1964 earthquake with deeper samples in coastal stratigraphy, the research team found sedimentary and diatom evidence of three other examples of fault rupture. This evidence has been linked to four of the last eight documented subduction zone earthquakes in the region.

“There’s a tremendous amount of displacement on these faults that can create really fast, localized, large tsunamis,” DePaolis said. “So a local tsunami is approaching very quickly and will be immediately followed by a tsunami created by the subduction zone. Suddenly, you’re faced with huge, devastating tsunamis that seem to come in rapid succession.”