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Monday's earthquake in China has a precedent. It was comparable to the great San Francisco earthquake of 1906. Both were the same magnitude. Both tore open about 200 hundred miles of fault line. Both shook the ground for what felt like an eternity. And both toppled structures that simply were not built to sustain that amount of movement.

Here's NPR's Richard Harris.

RICHARD HARRIS: Seismologist Lucy Jones of the U.S. Geological Survey lives in California, but she used to live in Sichuan province, near the scene of this week's quake.

Dr. LUCY JONES (U.S. Geological Survey): The earthquake occurred on the Lung-men Shan fault. It's sort of like the San Andreas for California. That's one of the really big faults to cause a lot of problems.

HARRIS: Now, the comparison isn't perfect. Movement along the San Andreas is caused by continental plates sliding past one another. Quakes on the Lung-men Shan fault are the result of a head-on collision.

Dr. JONES: The big picture is that India is moving into Eurasia and doesn't stop. And the Himalayas have been pushed up, and India still doesn't stop. So, actually, China is being squeezed out over the Pacific Ocean to make way for India to continue barreling north.

HARRIS: Every so often, the stress in the rocks becomes too much. The result: a fault slips. This earthquake was huge because, like the 1906 earthquake in San Francisco, a lot of the fault gave way at once.

Dr. JONES: And in this case, the fault's almost 200 miles long. And every point on that 200-mile-long fault is giving off energy. And it's one of the things we're trying to teach people here in California, that when we have our big San Andreas earthquake, it's going to be very different - not because it's so much more intense shaking at any moment, but in large part because so much larger an area will be involved.

HARRIS: Jones says shaking was not only widespread, but painfully long.

Dr. JONES: The bigger the earthquake, the longer it lasts. And this earthquake probably had a duration of maybe 70, 80 seconds.

HARRIS: So add that all together and Jones is not surprised to see that this was a devastating quake.

Seismologists have a way to estimate potential damage. They use a shaking-intensity scale that goes from one to 10. Anything eight or above means expect heavy damage.

Dr. JONES: In this earthquake, there were almost a million and a half people exposed to either intensity nine or 10. So there are just a huge number of people that are living right on top of really heavy levels of shaking, and the damage has to result from there.

HARRIS: Even so, Chinese-born structural engineer Chu-hong Jhao was stunned to see the images and hear the death toll from Monday's quake.

Professor CHU-HONG JHAO (Structural Engineer, University of Tennessee Knoxville): I guess one thing that kind of surprised me was the casualties.

HARRIS: Jhao is an assistant professor at the University of Tennessee in Knoxville. She's traveled often through Sichuan province. She says, on the one hand, the Chinese government recognized that the area had high earthquake potential, so it called for fairly strict building codes. But that awareness didn't trickle down to the residents.

Prof. JHAO: You have the seismic zone there, but I think the people there, they don't think that, you know, earthquake is something that they will think of in everyday life. It's not like in California, you know, you know about earthquakes, you talk about earthquakes all the time.

HARRIS: She expects that most of the damage will turn out to have been in older construction, built before strict building codes came into existence. Many structures built more than 30 or 40 years ago were made simply of brick, and those buildings do poorly in earthquakes.

Prof. JHAO: We're just guessing what's the reason, but I think really some serious investigation needs to be initiated to find out the reason.

HARRIS: Would you be interested in participating in that?

Prof. JHAO: Yes, of course.

HARRIS: Rescue is everybody's first priority. But after that's complete, Jhao hopes to go back to China to find out why so many buildings collapsed, and how to prevent future tragedies like this.

Richard Harris, NPR News.

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