One of the ocean's smallest creatures may give warnings of one of nature's largest destructive forces, a new study shows. Researchers have discovered that when Earth's tectonic plates are primed for tsunami-producing earthquakes, microscopic saltwater organisms called foraminifera replace their freshwater counterparts in coastal marshes. Monitoring the plate motion that causes this switch may provide a powerful new tool for forecasting tsunamis several years in advance.
A team led by geologists Andrea Hawkes of Dalhousie University in Nova Scotia and Jere Lipps of the University of California, Berkeley, discovered the tsunami-foraminifera connection while studying fossils of "forams" in coastal marshes in Alaska and Oregon. Layers of deep ocean sand also showed when enormous waves had struck the coast. Four times in the past 3,000 years, the researchers noticed, freshwater forams suddenly died out and were replaced by a saltwater species two to five years before a tsunami hit.
The great earthquakes that trigger tsunamis, such as the magnitude 9 tremblor that struck near Sumatra in December 2004, occur in subduction zones where one of Earth's tectonic plates is being forced beneath another plate. The two plates can become stuck as they move toward each other, and earthquakes occur when they free themselves and slip past one another. While the plates are stuck, the upper plate is bent downward as the lower plate tries to push beneath it, lowering the coastline slightly. This would allow salt water to infiltrate freshwater marshes that are near sea level, killing freshwater foraminifera and allowing saltwater species to thrive, says Lipps. If he's right, geoscientists could monitor tsunami-prone subduction zone areas by installing tiltmeters, instruments that measure small changes in slope. Such devices could give notice that a tsunami-producing earthquake is likely to occur in a few years, the researchers report in the current issue of GSA Bulletin.
"It's a very interesting idea that there would be these land level changes that would precede the earthquake," says Brian Atwater, a U.S. Geological Survey geologist based at the University of Washington, Seattle. But more evidence from additional locations around the globe is needed to determine if the phenomenon could be a reliable predictor, he says. The team is currently hunting for that evidence in mangroves and marshes in Mexico and New Zealand, and they hope to expand their search to Southeast Asia as well.