Decrease in deformation rate as a possible precursor to the next Parkfield earthquake

CRUSTAL deformations along fault zones are monitored because they show the accumulation and possible premonitory release of strain energy before major earthquakes. On the Parkfield segment of the San Andreas fault the next mainshock (local magnitude ML=5.7) is expected to occur between 1983 and 1993, based on the relatively regular recurrence of characteristic earthquakes in 1857, 1881, 1901, 1922, 1934 and 19661. A concerted monitoring effort is therefore underway at Parkfield, with the aim of detecting precursory changes in observable quantities that can be measured reliably2. Here we report data from two geodetic lines crossing the San Andreas fault near Parkfield which show that the rate of shortening decreased by ???20% starting in August 1986. This change coincided with a rate decrease of 45% for earthquakes of ML???2.1 in the area3,4. Both phenomena may have been caused by temporary locking of the fault surface, or a decrease in strain accumulation rate, and they are interpreted as intermediate-term precursors to the next Parkfield earthquake.