Big quake, tiny shake

April 09, 2003

While a quake's score on the Richter scale measures the overall movement of the ground, it is the jittery high-frequency shaking that is most likely to make buildings collapse. Data from the best-measured quake to date shows that the more the ground moves, the less shaking you get.

The Chi-Chi earthquake that hit Taiwan in 1999 - 7.6 on the Richter scale - was so big it created cliffs 12 metres high in just seconds, and the ground moved at the highest speeds ever recorded.

Yet while the greatest ground displacement along the fault happened in the north, the shaking there was much less severe than in the south.

"People originally thought it might have been something funny with the local geology," says Emily Brodsky, a geophysicist at the University of California Los Angeles. But then the same thing happened during a 7.9 quake in Denali, Alaska in 2002- massive ground displacement was seen with very little shaking.

Now Brodsky has come up with an explanation. High-frequency shaking occurs when rough spots on the adjoining fault surfaces rub against each other. So Brodsky wondered whether the exceptionally fast plate movement during Chi-Chi was enough to increase the fluid pressure of clay particles and groundwater within the fault. That would push the plates apart, reducing contact between rough spots and limiting the destructive vibrations.

Judging by the slip distances and speeds seen during Chi-Chi, she calculates that the plates could have been pushed between 5 and 8 millimetres apart- enough to significantly reduce friction (Geophysical Research Letters, vol 30, p 1244). The researchers say this lubricating effect should come into play for all quakes greater than 7 on the Richter scale.

"I don't know of any other viable explanations," says Paul Somerville, a seismologist with engineering firm URS in Pasadena. He says scientists have not noticed the effect before because of a lack of monitoring networks and because buildings are often so poorly built that they topple even from a small amount of shaking.

Well-constructed buildings should have a surprisingly good chance of making it through a large quake. "That's very good news," says Somerville.
Written by Nicola Jones

New Scientist issue: 12 April 2003

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