Foreshock May Have Preceded China Quake

Because of geology and history of the region, scientists were not surprised when an earthquake struck China on Wednesday. They were, however, intrigued that the quake may have been preceded by a small foreshock.

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The earthquake that struck China yesterday has killed more than 600 people by the latest count and injured thousands more. And we'll have updates on rescue efforts elsewhere in today's program. Earth scientists have not been surprised by this quake and by the damage it caused, given the geology and the history of the region, but they say the quake was surprising in one way. It may have been proceeded by a warning.

NPR's Richard Harris reports.

RICHARD HARRIS: Before we get to the surprise warning, let's set the scene of Wednesday's quake. It struck in an arid, high plateau, crisscrossed by faults. Some thrust upward as India, to the south, collides with Tibet. And other faults move side to side, as the earth continually readjusts to the forces laid on by the moving continental plates.

Roger Bilham at the University of Colorado says the latest quakes were of the slide-slipping variety.

Professor ROGER BILHAM (University of Colorado): They're famous for it. It's known as the Zhuanglang fault. It's the Zhuanglang River fault, and these earthquakes are carrying on an extension of that fault deeper into Tibet.

HARRIS: The faults tend to carve deep valleys, which is also where the water runs, so that's also where people build monasteries, towns and roads. As a result, the greatest shaking occurs where the people are. Bilham has traveled to this region to study the faults, and he says most of the construction is adobe, poorly reinforced concrete or a combination of the two.

Prof. BILHAM: And this earthquake is beginning to look like every other earthquake that's happened in the last few months. In other words, it's bad construction that's caused most of the loss of life.

HARRIS: So scientists weren't at all surprised by the quake or the damage it caused. But they were surprised by news reports suggesting that the quake was proceeded by a foreshock.

Dr. JIAN LIN (Geophysicist, Woods Hole Oceanographic Institution): Some people actually were woke up by another earthquake, smaller earthquake, at about two hours earlier.

HARRIS: Jian Lin is a geophysicist at the Woods Hole Oceanographic Institution. He's hoping that the news reports will be borne out by Chinese seismic instruments in the region.

Dr. LIN: Any earthquakes which have foreshocks always are of great interest to scientists.

HARRIS: They certainly are the exception, not the rule, but they provide intriguing clues about the origin and nature of quakes. The most famous example, in fact, comes from China. Back in 1975, scientists in northeastern China saw a series of quakes they decided were potential foreshocks, and when these small quakes suddenly stopped, the scientists went on high alert.

Dr. LIN: Warnings went out in the morning at about 10:00 o'clock or so, and the earthquake occurred at 7:00 o'clock in the evening.

HARRIS: Lin says that warning probably saved many lives. But optimism about using foreshocks as a warning tool didn't last long.

Dr. LIN: Just one year later, in the same region in 1976, the Tangshan earthquake struck, and Tangshan earthquake had no foreshock.

HARRIS: And the Tangshan earthquake was the deadliest quake on earth in at least the past 400 years. By one estimate it killed more than 600,000 people, triple the death toll from Haiti.

Richard Harris, NPR News.

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