by Staff Writers
Paris (AFP) Dec 05, 2012
Seismologists on Wednesday unveiled a map highlighting zones most prone to unleashing the world's biggest earthquakes, including the 9.0 monster that struck northeastern Japan in March 2011.
These mega-quakes spring mostly from regions where fractures on the sea floor intersect with subduction zones, or zones where one slab of Earth's crust is diving slowly under another, they said.
The evidence is statistical, coming from data from 1,500 earthquakes.
Over the past century, half of the 50 largest earthquakes with a magnitude of 8.4 or more have occurred at these dangerous intersections, the scientists said.
The rate rises to 87 percent for the 15 biggest quakes above 8.6 magnitude, including behemoths that have shaken Chile and Alaska.
Subduction zones are well known for engendering very powerful quakes.
Some regions are notorious, such as the Sunda Trench off Indonesia, which spawned the 9.1-magnitude Indian Ocean quake and tsunami of December 2004.
Others, though, are not picked up as potential threats, as they only generate powerful earthquakes every few centuries or even millennia.
Such long periods are called "supercycles" and because they last happened so long ago, they are not documented in human history.
The worrisome intersections are points where fractures, caused by the movement of ridges on the ocean floor, cross with a subduction zone at a boundary on Earth's tectonic plates.
Why these meeting points become hotspots is unclear.
One theory is that the complex fractures act as a brake, preventing the slow subduction movement. As a result, the stress accumulates over very long periods until it is released catastrophically.
"Even though we don't fully understand the physics of long earthquake cycles, any improvements that can be made using statistical data analysis should be considered, as they can help reduce earthquake damage and loss of life," said seismologist Dietmar Mueller at the University of Sydney in Australia.
The new map is published in the journal Solid Earth by the European Geosciences Union (EGU).
Earth regions 'primed' for earthquakes
Geophysicists from Stanford University are presenting findings from studies of the regions at a meeting of the American Geophysical Union in San Francisco this week, a university release reported.
The Himalayan range was formed and remains active due to the collision of the Indian and Asian continental plates, creating a fault knows as the Main Himalayan Thrust where 20 seismometers have been gathering data for two years.
Stanford geophysics doctoral student Warren Caldwell noted the MHT has historically been responsible for a magnitude 8 to 9 earthquake every several hundred years.
"What we're observing doesn't bear on where we are in the earthquake cycle, but it has implications in predicting earthquake magnitude," Caldwell said.
Recent detections of magma and water around the MHT indicate which segments of the thrust will rupture during an earthquake, said Caldwell's adviser, geophysics Professor Simon Klemperer.
"The findings are important for creating risk assessments and disaster plans for the heavily populated cities in the region," he said.
Studies were also presented at the AGU meeting focusing on the Pacific Northwest's Cascadia subduction zone, which stretches from northern California to Vancouver Island but has not experienced a major seismic event since it ruptured in 1700.
Stanford postdoctoral scholar Annemarie Baltay said measurements of small seismic tremors in the region can be utilized to determine how ground motion from larger events might behave.
"We can't predict when an earthquake will occur, but we can try to be very prepared for them," Baltay said. "Looking at these episodic tremor events can help us constrain what the ground motion might be like in a certain place during an earthquake."
The 1700 earthquake, estimated at between 8.7-9.2 magnitude, shook the region and created a tsunami that reached Japan.
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