A walking tour of the Calaveras fault in Hollister, California – JOE DELLINGER (PH.D Stanford)

Go to his web page for more information & copyrights. Click here for his actual tour page.

Note – All text in green and 2004 photos are by Jim Ryono (high school teacher). See his website with special emphasis on San Luis Obispo County day hikes.

Find ``Vista Park Hill'', just NorthWest of downtown. (Locust street curves around the base of the Southern end of the hill.) Begin the walking tour there.

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Introduction

Hollister, California is located South of the San Francisco Bay area.

In the 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. The USGS continuously monitors their activity. 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 to geophysicists 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, the usual pattern for faults, they continuously ``creep''. As a result of this creep, Hollister is being ripped in two, for the most part along a remarkably narrow zone running right through the middle of town.

Here is a map showing the approximate active trace of the fault for the part of town covered in our tour.

The creep rate apparently varies in an unpredictable way. From Sylvester and Crowell, 1989 (quoted from Rogers and Nason, 1971): ``Movement did not occur between 1910 and 1929, judging from the amount of offset in two sidewalks that were laid in 1910 and 1929, and in a pipeline laid in 1929. Creep commenced sometime after 1929 and averaged 8 mm/yr. Between 1961 and 1967, the slip rate was about 15 mm/yr. Since 1979, two sites have been monitored in Hollister, one showing 6.6 mm/yr and the other, only 2.3 km northwest, creeps 12 mm/yr - the fastest rate of movement measured across any fault in the San Francisco Bay Region.'' (For those not metrically inclined, 12 mm/yr is half an inch a year. I don't think any of the places I've photographed have moved quite that fast, though.)

Hollister has become rather ``yuppified'' over the last decade. (It's now considered to be within commuting range of San Jose.) Unfortunately, that means many of the features so interesting to geologists nowadays tend to get conscientiously repaired (in other words, erased). However, if you visit Hollister yourself you should expect to see for yourself most of what I show below, plus many more sights I didn't include.

Hollister is a pleasant little town, and the residents are generally friendly and often enjoy talking to visiting geologists about their famous fault... but they get justifiably annoyed at thoughtless tourists tramping around in their yards without permission. So if you decide to visit Hollister yourself, please respect private property rights and stay out of people's yards!!!

If you drive into Hollister from the North via California Highway 25, look for the small ridge to the right (West) of the highway leaving the road at a shallow angle, marking where the Calaveras fault intersects the highway. You may want to park off the shoulder there and sight down the edge of the highway, to see if you can detect the offset in the road itself.

Walking Tour

By popular request, I have split the tour up into several pages so that there aren't too many inline images on any one page. Click on a thumbnail to see a more detailed view!

Notice in all the photos how the distortion always bends curbs, walls, foundations, etc, 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.

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Other Calaveras Fault links