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Weak Geological Barrier may Cause Severe Earthquakes in US

by VR Sreeraman on  April 4, 2008 at 7:30 PM Environmental Health   - G J E 4
Weak Geological Barrier may Cause Severe Earthquakes in US
The discovery that a devastating earthquake and tsunami in the Solomon Islands one year before broke through a geological province previously thought to form a barrier to earthquakes, could mean other sites in northwestern North America have the potential for more severe earthquakes than once thought.
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The find was made by geoscientists from the University of Texas at Austin's Jackson School of Geosciences.

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They reported that the rupture that produced an 8.1 magnitude earthquake, killing 52 people and displacing more than 6,000, broke through a geological province previously thought to form a barrier to earthquakes.

This could mean that other sites with similar geological barriers, such as the Cascadia Subduction Zone in northwestern North America, have the potential for more severe earthquakes than once thought.

According to scientists, the rupture started on the Pacific seafloor near a spot where two of Earth's tectonic plates are subducting, or diving below, a third plate.

The two subducting plates-the Australian and Woodlark plates-are also spreading apart and sliding past one another.

he boundary between them, called Simbo Ridge, was thought to work as a barrier to the propagation of a rupture because the two plates are sliding under the overriding Pacific plate at different rates, in different directions, and each is likely to have a different amount of built-up stress and friction with the overlying rock.

But, the boundary did not stop the rupture from spreading from one plate to the other.

"Both sides of that boundary had accumulated elastic strain," said Fred Taylor, a researcher at the university's Institute for Geophysics and principal investigator for the project.

"Those plates hadn't had an earthquake for quite a while and they were both ready to rupture. When the first segment ruptured, there was probably stress transferred from one to the other," he added.

According to Taylor, "What our work shows is that this is a barrier, but not a reliable one." In other words, it resists rupturing, but not insurmountably.

The last great earthquake along the Cascadia Subduction Zone was in the year 1700. The intensity of the quake has been estimated at around magnitude 9. If it happened today, it could be devastating to people living in the northwestern U.S. and western Canada.

The research work also has implications for earthquakes in other parts of the world.

Source: ANI
SRM/V
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