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PETER J. SMITH Department of Earth Sciences Open University Milton Keynes, U.K. The three largest earthquakes over the past 150 years in the San Francisco Bay area (1868, 1906, 1989) were all preceded by several decades of high activity in moderate (magnitude 5.0 to 5.9) earthquakes on surrounding faults, and followed by low levels of such activity. If the level of moderate earthquake activity is low over the next few years, another large shock is unlikely to occur for at least a decade. If

Peter J. Smith

PETER J. SMITH
Department of Earth Sciences
Open University
Milton Keynes, U.K.

The three largest earthquakes over the past 150 years in the San Francisco Bay area (1868, 1906, 1989) were all preceded by several decades of high activity in moderate (magnitude 5.0 to 5.9) earthquakes on surrounding faults, and followed by low levels of such activity. If the level of moderate earthquake activity is low over the next few years, another large shock is unlikely to occur for at least a decade. If it remains high, however, another large earthquake can be expected soon.

L.R. Sykes, S.C. Jaumé, "Seismic activity on neighboring faults as a long-term precursor to large earthquakes in the San Francisco Bay area," Nature, 348, 595-9, 13 December 1990. (Columbia University, Palisades, N.Y.)

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