Estimating a temblor's strength on the fly

5:58pm, January 2, 2006
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New analyses of ground motions caused by large earthquakes suggest that it may be possible to estimate the full magnitude of such quakes immediately after they start rumbling. That could enable emergency systems to better warn distant populations of a temblor before it reaches them.

Currently, it isn't possible to measure an earthquake's total magnitude until the rumbling has stopped. That's because the seismic energy that's released depends on the total slippage that occurs between two sides of a fault, says Richard M. Allen, a seismologist at the University of California, Berkeley.

He and his colleagues have discovered a quick way to estimate a quake's magnitude if there happens to be a seismometer near its epicenter.

A seismometer within 100 kilometers of the epicenter records both high- and low-frequency vibrations, whereas instruments farther away receive only low frequencies. After studying ground-motion patterns of 71 quakes recorded by nearby seismometers, Allen and his colleagues noticed that the relative amounts of energy going into the two vibration categories varied systematically with the size of the quake.

Although fault slippage in most small quakes included in the study lasted only a second or so, several of the large quakes rumbled for more than 30 seconds. Nevertheless, the researchers found that the ratio of seismic energy received at high and low frequencies during the first 4 seconds of ground motions spreading from the quake enabled the team to estimate the quake's full magnitude.

The technique may make it possible for scientists to more quickly recognize that an ongoing earthquake will be large and damaging, says Allen. He and his colleagues report their findings in the Nov. 10, 2005 Nature.

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