Hollister Calaveras Fault Trace - South Street
Hollister, California
In the San Francisco Bay area there are three major faults, from west to east the San Andreas, the Hayward, and the Calaveras; all are part of the San Andreas fault system. All of these are "right-lateral strike-slip faults," which means that the motion is predominantly horizontal, with the land on the west side of the fault moving north.
South of the Bay Area the Hayward and Calaveras merge into the San Andreas. Hollister is located just north of where this happens, right on top of the southern end of the Calaveras fault.
What makes Hollister particularly interesting is that from San Juan Bautista to just north of Parkfield the faults in the San Andreas system are not "stuck": instead of moving only during major earthquakes, they continuously "creep." As a result of this creep, Hollister is being slowly ripped in two, for the most part along a remarkably narrow zone running right through the middle of town.
The rate of creep is inconsistent, and has been measured at between 6 and 15mm per year at various times throughout the 20th century. Underground pipes, road paving, curbing, and foundations all show signs of being gradually shifted apart. Notice how the distortion always bends structures to the right- no matter from which side of the fault the photo was taken. Also notice that the motion is horizontal: the ground is remaining level as it moves. Together, these two observations define right-lateral strike-slip motion.
Hollister Calaveras Fault Trace - South Street
Hollister, California
In the San Francisco Bay area there are three major faults, from west to east the San Andreas, the Hayward, and the Calaveras; all are part of the San Andreas fault system. All of these are "right-lateral strike-slip faults," which means that the motion is predominantly horizontal, with the land on the west side of the fault moving north.
South of the Bay Area the Hayward and Calaveras merge into the San Andreas. Hollister is located just north of where this happens, right on top of the southern end of the Calaveras fault.
What makes Hollister particularly interesting is that from San Juan Bautista to just north of Parkfield the faults in the San Andreas system are not "stuck": instead of moving only during major earthquakes, they continuously "creep." As a result of this creep, Hollister is being slowly ripped in two, for the most part along a remarkably narrow zone running right through the middle of town.
The rate of creep is inconsistent, and has been measured at between 6 and 15mm per year at various times throughout the 20th century. Underground pipes, road paving, curbing, and foundations all show signs of being gradually shifted apart. Notice how the distortion always bends structures to the right- no matter from which side of the fault the photo was taken. Also notice that the motion is horizontal: the ground is remaining level as it moves. Together, these two observations define right-lateral strike-slip motion.