Holocene activity of the San Andreas Fault at Wallace Creek, California

The present channel displays an offset of 125 m along the San Andreas Fault. Average rate of slip along the San Andreas fault has been 33.9 + or - 2.9 mm/yr for the past 3,700 yr and 35.8 + 5.4/-4.1 mm/yr for the past 13,250 yr. Small gullies near Wallace Creek record evidence for the amount of dextral slip during the past three great earthquakes. Slip during these great earthquakes ranged from approximately 9.5 to 12.3 m. Using these values and the average rate of slip during the late Holocene, estimation that the period of dormancy preceding each of the past 3 great earthquakes was between 240 and 450 yr. This is in marked contrast to the shorter intervals ( approximately 150 yr) documented at sites 100 to 300 km to the southeast. These lengthy intervals suggest that a major portion of the San Andreas Fault represented by the Wallace Creek site will not generate a great earthquake for at least another 100 yr. Rupture of a 90-km-long segment northwest of Wallace Creek, which sustained as much as 3.5 m of slip in 1857, is likely to generate a major earthquake by the turn of the century. The Wallace Creek slip rate is appreciably lower than the average rate of slip (56 mm/yr) between the Pacific and North American plates determined for the interval of the past 3 m.y.--Modified journal abstract.