R. L. Wesson W.L. Ellsworth R. Robinson 1974 Residuals for P-wave traveltimes at a seismograph station near Bear Valley, California, for small, precisely located local earthquakes at distances of 20 to 70 kilometers show a sharp increase of nearly 0.3 second about 2 months before a magnitude 5.0 earthquake that occurred within a few kilometers of the station. This indicates that velocity changes observed elsewhere premonitory to earthquakes, possibly related to dilatancy, occur along the central section of the San Andreas fault system. application/pdf 10.1126/science.184.4143.1281 en American Association for the Advancement of Science Variation of P-wave velocity before the Bear Valley, California, earthquake of 24 February 1972 article