GEOPHYSICS: Still Waiting After All These Years

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Science  19 Jul 2002:
Vol. 297, Issue 5580, pp. 305b
DOI: 10.1126/science.297.5580.305b

Six successive magnitude 6 earthquakes had occurred at Parkfield, California, on the San Andreas fault, regularly every 21 to 24 years from the middle of the 19th century to 1966. It thus seemed a perfect place to collect detailed data toward understanding how earthquakes happen, and the region was instrumented in 1985 in anticipation of the next big one.

The expected earthquake has not taken place, however. Toda and Stein provide an explanation for at least part of the long delay: Earthquakes in 1983 near Coalinga, east of the San Andreas fault, were oriented so that they reduced the stress on the Parkfield part of the San Andreas fault. The predicted stress reduction can account for a delay of the Parkfield earthquake by about 10 years and for increases in activity on other parts of the San Andreas fault that were loaded by these earthquakes. This model explains why the Parkfield quake did not occur in the 1980s, but why it hasn't occurred by now is unclear. Still, seismicity has increased at Parkfield over the past several years, so it may be returning to its normal cycle. — BH

J. Geophys. Res. 107, 10.1029/2001JB000172 (2002).

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