Tuesday, May 27, 2008

Monster Quakes Set Off Global Tremors

May 26, 2008

Massive earthquakes—such as the magnitude 9 quake that sparked the deadly Indian Ocean tsunami in 2004—can set off smaller tremors around the globe, a new study shows.

Traditional aftershocks occur close to the time of the original earthquake—often within days or a few weeks—as the earth adjusts to changes caused by the slippage along the original fault.

But smaller, more distant earthquakes can be triggered as low-frequency vibrations—somewhat like ocean swells—pass over faults. Such waves can't be felt by people standing on the ground.

It's too early to know if the recent deadly earthquake in China also triggered its own swarm of distant tremors, but "it wouldn't surprise me in the least," said study lead author Tom Parsons of the U.S. Geological Survey in Menlo Park, California.

Fault Stress

Vibrations from a big quake momentarily reduce the locking pressures on faults, allowing them to slip, Parsons said.

"That puts a temporary stress on a fault," Parsons added. "That's small, but [sometimes] appears to be enough to trigger an earthquake."

(Set off your own earthquake in an interactive simulation.)

To figure out how often this happens, Parsons and colleagues examined records of 15 major earthquakes with magnitudes of 7 or greater.

The scientists were able to correlate distant vibrations passing through various parts of the globe with the occurrence times of smaller, local earthquakes in 12 of the 15 events studied.

Flood Threat Prompts Evacuations in Quake-Hit China

Audra Ang in Mianyang, China
Associated Press
May 27, 2008

Potentially catastrophic flooding prompted emergency evacuations in China's Sichuan Province on Tuesday, even as aftershocks continued to batter the region and the threat of disease loomed for millions of refugees.

About 80,000 people were evacuated downstream of Tangjiashan, an unstable earthquake-created dam that is threatening to collapse.

The lake, near the town of Beichuan, is the largest of some 35 new bodies of water created by river-blocking rubble after a magnitude 7.9 earthquake struck Sichuan May 12.

Some rising floodwaters have already swallowed villages, though only Tangjiashan posed a risk of another big catastrophe.

Hundreds of troops were working around the clock to dig a channel that would divert the rising waters before they breach the top of the rubble wall.

Officials fear the loose soil and debris wall could crumble easily if the water starts cascading over the top, sending a torrent flooding down into the valley below.

Tangjiashan now holds 34 billion gallons (130 billion liters) of water and was rising by more than three feet (a meter) every 24 hours, the official Xinhua News Agency reported.