Fault slip episodes near Hollister, California: Initial results using a multiwavelength distance-measuring instrument

The most precise measurements of strain on long base lines (1--11 km) are currently being made in central California near Hollister. Base lines radiating from Hollister have been measured daily, except when measurements are limited by poor visibility, since September 1975. Strain episodes have been observed that are attributed to slip at depth on the major faults in the area. There is a suggestion that this slip begins several kilometers down and propagates upward to the surface. Many of these strain episodes have coincided with magnitude 2.5--3.5 earthquakes in the region. Another strain episode indicates that the eastern end of the Sargent Fault may continue past the Calaveras Fault for approximately 2 km.