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http://www.livescience.com/environment/080630-china-quake-rare.html

China Earthquake Was Rare Type

By Andrea Thompson, Senior Writer

posted: 30 June 2008 12:35 pm ET



A woman grieves for her lost mother in Beichuan, in China's southwest Sichuan province, Sunday May 18, 2008. Credit: AP Photo/Greg Baker.

The major earthquake that devastated China last month was something of a seismological oddity, seismologists report in a new analysis. The faults that caused the temblor rarely rumble.

More than 69,000 people are confirmed to have died from the magnitude 7.9 quake that struck China's Sichuan province around noon on May 12, leveling school buildings and other poorly-constructed structures.

The earthquake was also a complete surprise to scientists. MIT seismologists who had been operating an array of 25 seismograph stations in the region for more than a year had found no hints that a large temblor might hit.

"Nobody was thinking there would be a major seismological event," in that area, said MIT's Leigh Royden. "This earthquake was quite unusual."

The region is extremely unusual geographically, Royden said, because of the very steep slopes at the boundary between the Sichuan Basin to the east and the Tibetan Plateau to the west. The elevation rises by more than two miles (about 3.5 kilometers) in a span of just 30 miles (50 kilometers).

The area is at the boundary between the Indian and Asian tectonic plates, which are engaged in an ongoing collision that has created the Himalayas and the Tibetan plateau.

In central and eastern Tibet, unlike most other areas where two continental plates are colliding, the movement of the crust is hidden. Instead of folding and faulting, the surface of the eastern Tibetan plateau is undeformed and being lifted upward by the thickening of a weak crustal layer more than 9 miles (15 km) below the surface.

The crust in this layer is flowing rapidly eastward away from central Tibet. But in the area where the earthquake occurred, it is obstructed by a major obstacle, the Sichuan Basin.

"The crust and mantle beneath the basin appears to form a hard, cold knot," Royden said, which forces the flow to "wrap around the knot." The huge elevation differences between the surface of the plateau and the Sichuan Basin provided the underlying stress that led to the quake, she added.

Similar events in the area occur only once in every 2,000 to 10,000 years on average, the researchers say, though they caution that because earthquakes can sometimes occur in clusters, residents and officials should still be wary of another possible large-scale earthquake.

The new analysis is detailed in the July issue of GSA Today, a publication of the Geological Society of America. The research was funded by the National Science Foundation.

MAP 7.5 2008/07/05 02:12:03 53.890 153.034 605.4 SEA OF OKHOTSK

I really don't have time to get into anything deep here. I did want to mention I was just headed out the door yesterday for our 4th of July get together when I began to see, what I call, cystral flashings...for a migraine headache. I stopped in my tracks and went for Niacin. It went to work right away, but I was left feeling strange and not quite myself. So, between that TL S- spike and the headache start up, I wasn't surprised to see the 7.7 when I got home. I decided to stay in and not go out to see the fireworks...which I think was a good thing. My dog was scared to death this year. She stayed right at my feet shaking. The cats took them in stride, but I think that was because I was here and if big mama cat isn't worried, they are not about to show they are. This was, by far, the biggest year for fireworks. They were all around us and they just kept going on and on. Must have been something to see coming into LAX.

I'm off to my dad's house. Had another fire scare there yesterday...nine miles away from the house. They got on top of this fire and had it contained before I got back here. They said this fire got started when a mobile home exploded and started others on fire. No word on what caused the explosion. No matter...I have been thanking the Lord ever since.

Hope the rest of the weekend goes well for everyone.