The slower, the safer

Geologists in the US have discovered that earthquakes can also occur slowly causing minimal damage

Published: Saturday 30 November 1996

earthquakes hit an area with a ground-shaking intensity. Therefore, a 'slow earthquake' seems a contradiction of sorts. Yet, us geologists have detected a quake on California's San Andreas fault that lasted 10 days ( New Scientist , Vol 151, No 2047).

Occurring when rocks slip suddenly, earthquakes trigger sharp vibrations. As the quake advances, the fault surfaces slip by several metres. The more slowly the fault slips, the less efficiently it generates seismic waves. If the fault slip is slow enough, no waves are radiated and the earthquake is silent. If all earthquakes were slow they would not be very hazardous.

Researchers led by Alan Linde of the Carnegie Institution, Washington dc , studied data from strain gauges placed five km apart in San Juan Bautista, about 20 km inland from Monterey Bay in California. On the north of this region, the fault remains locked and inactive for years on end. On the south of San Juan Bautista, tiny quakes are caused due to the smooth movement of the tectonic plates past one another. According to Linde, the whole pattern makes up a single, slow earthquake. It released as much energy as a powerful quake of 4.8 magnitude.

Linde and his team were equipped with borehole strain gauges at the time the slow quakes occurred. The fault slipped by just a few centimetres, which could hardly be detected by the most powerful satellite positioning systems. He believes that some slow tremors may be precursors to major earthquakes. But until someone collects systematic data on slow quakes, Linde says, "I don't know if there is a real connection".

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