Satellite sees how the earth moved: Pictures taken before and after quakes allow scientists to improve prediction of tremors and landslips

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SATELLITE pictures have captured how the earth moved during two quakes that struck California last year. It is the first time that scientists have been able to survey from space the displacement caused by an earthquake.

The European remote sensing satellite, ERS-1, used its synthetic aperture radar to measure ground deformations caused by the Landers earthquake in southern California on 28 June 1992 which had a magnitude of 7.3 on the Richter scale, and the smaller Big Bear earthquake three hours later.

Images taken before and after the earthquakes show how the ground slipped sideways by up to seven metres along the central fault lines. The image on the left, showing an area 100 kilometres square, was produced by a computer on the basis of the satellite data. It is analogous to a contour map: each line represents a land slippage of about 28 millimetres; where the lines cluster most thickly, the movement of the earth was greatest. The shading is a computer graphical effect.

The satellite images, published in today's Nature, are the work of researchers from France's National Centre for Space Studies in Toulouse. The head of the team, Didier Massonet, said the research could lead to better predictions of not only earthquakes, but volcanic eruptions and glacial landslides. 'We are currently surveying volcanoes and glacial flows with the technique,' he said yesterday.

The satellite took radar images of the California earthquake region near to the San Andreas fault in the April before the earthquake and in the August following it. Because of the radar's powerful resolution the scientists were able to discern the precise movements of the earth to within 34 millimetres.

(Photograph omitted)