Understanding “slow-slip” earthquakes on the seafloor—seismic events that occur over a period of days or weeks—is giving researchers new insights into undersea earthquakes and the subsequent creation of tsunamis. Through an ocean discovery program supported by the National Science Foundation (NSF), scientists are studying the seafloor off the coast of Japan. The region could provide vital clues.
(From Phys.org) — Two tectonic plates, the Pacific Plate and the Eurasian Plate, meet there. In this ocean trench zone, the Pacific plate slides beneath the Eurasian plate. Such subduction zones are often associated with large earthquakes.
“This area is the shallowest part of the plate boundary system,” said Demian Saffer, a geoscientist at Penn State University. “If this region near the ocean trench slips in an earthquake, it has the potential to generate a large tsunami.”
Saffer and Eiichiro Araki, senior research scientist at the Japan Agency for Marine-Earth Science and Technology, published the results of their investigations of the plate boundary in this week’s issue of the journal Science.
The results are important for understanding tsunami risk, according to James Allan, program director in NSF’s Division of Ocean Sciences.
“Such tidal waves can affect the lives of hundreds of thousands of people and result in billions of dollars in damages, as happened in Southeast Asia in 2004,” Allan said. “This research underscores the importance of scientific drillship-based studies, and of collecting oceanographic and geologic data over long periods of time.”
The plate boundary earthquake zone off Japan’s coast forms part of the “ring of fire” that surrounds the Pacific Ocean. Once the end of a plate sliding—or subducting—beneath another reaches a certain depth, the material from the descending plate melts, forming volcanoes that often are located on land. Mount St. Helens in the U.S. is one of these volcanoes, as is Mount Fuji in Japan.
In 2009 and 2010, scientists with the IODP (Integrated Ocean Drilling Program, now the International Ocean Discovery Program) NanTroSEIZE (Nankai Trough Seismogenic Zone Experiment) project drilled two boreholes in the Nankai Trough southwest of Honshu, Japan. The holes were drilled from aboard a scientific drillship. In 2010, also from a scientific drillship, researchers installed monitoring instruments in the holes as part of a network that includes sensors on the seafloor. NSF supports the IODP.
The two boreholes are 6.6 miles apart, straddling the boundary of the last major earthquake in this area, which occurred in 1944 and measured magnitude 8.1. The resulting tsunami, which hit Tokyo, was 26 feet high.
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