Seismic quiescence at Parkfield: an independent indication of an imminent earthquake

THE Parkfield segment of the San Andreas fault midway between San Francisco and Los Angeles has experienced six moderate-sized earthquakes since 1850, with an average recurrence time of about 22 years. Based on this regular behaviour, and the occurrence of the most recent earthquake in 1966, a shock of local magnitude ML=5.7 is expected to occur by 19931,2. Here we suggest that a current lack of small earthquakes on this segment could be a precursory anomaly to the next characteristic Parkfield earthquake. Only four earthquakes of M≥2.5 have occurred on the segment since January 1986, whereas twenty would have been expected, extrapolating from the mean background rate. Comparison with other cases of precursory quiescence suggests that the M = 5.7 earthquake should occur in the next two years, rupturing the same 35-km fault segment that ruptured in 19663.

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