PETER J. SMITH
Department of Earth Sciences
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.)
Concentrations of zinc (Zn) have been measured in decontaminated ice cores and large snow blocks from Antarctica that range in age from a...