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Lessons from a quake that came 16 years late

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Most earthquakes make history for the damage they inflict. But last week's quake at Parkfield, Calif., is likely to be remembered for insights that could improve earthquake-hazard assessments, quake-resistant construction techniques, and, perhaps, efforts to forecast significant temblors.

Over the past 18 years, scientists turned the area, which straddles the San Andreas fault, into one of the most heavily instrumented fault segments on the planet. As a result, they have amassed enormous amounts of information.

When the quake happened Sept. 28, researchers recorded it as it happened. Now they are poring over all the data to unravel the specific factors that led to the quake.

Past efforts to test different theories about earthquakes and gauge their usefulness for forecasting have been stymied "because our instruments were too far from the earthquake to see anything," notes Lucile Jones, scientist in charge of the US Geological Survey's southern California earthquake-hazards team. "We were in the Catch-22 position of needing to predict an earthquake to collect the data to be able to learn how to predict earthquakes. Many of those questions may be answered" with insights from last week's event, she says.

For example, she notes that buildings near faults are designed for earthquake resistance using computer simulations of ground motion 10 to 100 kilometers from a quake's epicenter. Results vary widely inside 10 kilometers because instruments have never recorded an actual quake inside that radius. "We just got 100 records from within 10 kilometers" of the Parkfield quake's epicenter, Ms. Jones says.

In addition, quake experts hope to gain a better understanding of how quickly a fault releases its energy. Caltech seismology and earthquake engineering professor Tom Heaton had proposed that a sudden break in a fault such as the San Andreas can generate "fault fling" - a single large pulse of motion that can shove a building's foundation into the building itself. It appears as though fault fling occurred at Parkfield, giving engineers a close look at another source of stress to compensate for as they design new buildings.

Parkfield has been a mecca for seismology ever since researchers noted that the hamlet (pop. 37) has endured a magnitude 6.0 earthquake on average every 22 years since 1857. The culprit: the San Andreas fault, which forms the boundary between two large crustal plates on the Earth's surface - the Pacific and North American plates.

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