If coastal subsidence is common before subduction zone quakes, areas such as those ringing the Pacific Rim could be on the lookout for subsidence as a warning of possible future megathrust quakes like the Dec. 26 9.0 Sumatra-Andaman Islands earthquake, the researchers say.
"In areas along subduction zones, like the Northwest coast of the United States, we should look to see where the land has subsided and put instruments there to monitor it," said Jere Lipps, a UC Berkeley professor of integrative biology and study coauthor. "If the land continues to subside instead of relaxing back to normal, it could indicate a big earthquake and a tsunami might occur some time in the next few years."
Subduction zones are areas where one of the Earth's tectonic plates slips under another, raising mountain ranges along the margin sprinkled with volcanoes. Quakes occur when the edge of the overlying plate sticks to the subducting plate, causing a slight dip nearest the zone and a slight bulge farther away. Eventually, the stuck edge lets go in a massive quake, after which the margin relaxes to pre-quake levels. Because plates thrust over one another in these zones, the quakes are referred to as megathrust earthquakes. They often generate tsunamis.
"What we think happens is, two to five years before a quake, the plate boundary starts to drag down ever so slightly before it fractures," said David B. Scott, the Killam Professor of Earth Sciences at Dalhousie University in Halifax." Before that, you've got subsidence that is almost undetectable to the people living there. But you could
Contact: Robert Sanders
University of California - Berkeley