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Cluster clue to seismic build up

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CLUSTERS OF identically repeating micro-earthquakes on a stretch of the San Andreas fault could reveal the rate at which the Pacific and North American tectonic plates are grinding past each other. And the timing of these 'ticking' clusters may provide a new way to monitor the build- up of fault strain associated with larger earthquakes.

Geophysicists at the US Depart- ment of Energy's Lawrence Berkeley National Laboratory Robert Nadeau and Thomas McEvilly reported their findings in a recent issue of Science.

Using data collected in the Park- field region of central California since 1987, they discovered that between 1857 and 1966, magnitude 6 earthquakes occurred every 22 years on average. Since then there have been no events of that size, although there was a build up of activity from October 1992 through 1994.

'We have found a highly organised relationship between the intervals of micro- earthquakes in clusters, the occurrence of the larger events, and changes in fault slip on the surface,' Nadeau said.

'The rate at which earthquakes recur on any given asperity (small, strongly locked regions where strain repeatedly builds up and is released) indicates the average loading from slippage. Earthquakes recurring faster mean slippage is accelerating, and the load is being released more often,' said Nadeau.

He claimed there is good agree- ment between seismic data and direct measurement of surface slippage. Where large parts of the fault are locked, as in the 8km stretch, an increase in the repetition rate of events indicates that strain load is building faster. When it reaches a critical level, a swarm of medium-sized earth- quakes could dissipate the load, as could a single larger event.

It is unclear whether this correlation exists in other fault zones but preliminary results on the Hayward fault in the San Francisco Bay area 'are showing promise', said Nadeau.

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