USGS diagram of San Andreas Fault
USGS diagram

“Geologists Thomas Fumal and Timothy Dawson dug trenches across the San Andreas Fault in the Santa Cruz Mountains, about 5 miles (8 km) northwest of Watsonville and discovered traces of four large past earthquakes. Broken sediment revealed quakes occur more often than earlier estimated, and two historic earthquakes reached further south than found before. In the trenches, Dawson and Fumal found a clear record of the great San Francisco quake of 1906. The three additional earthquakes hit in 1522, 1686 and either 1748 or 1838, give or take a few decades, which means this segment of the San Andreas Fault averages a big shaker every 125 years. That’s twice as often as estimates calculated by the Working Group On California Earthquake Probabilities, the group responsible for official earthquake forecasts.”