Creeping in Hollister

January 06, 2005

Hollistercreep

Provided by: Thomas McGuire, Textbook Author/Educator
Summary authors & editors: Thomas McGuire

When we think of faults, we usually associate them with sudden movements that result in earthquakes. But the Calaveras Fault, which runs through the village of Hollister, California, is characterized by slow and continuous creeping motion. The rate of relative motion averages about 10 mm (less than 1/2 inch) per year. As a result, structures built across this fault are slowly bent out of shape. Curbs, sidewalks, roads and even houses are slowly deformed by this continuous fault motion. Hollister is located about an hour's drive south of San Francisco Bay, and east of Salinas.

The Calaveras Fault is a splay fault of the San Andreas Fault, which separates the North American and Pacific tectonic plates. The main trace of the San Andreas Fault is about 8 km (5 miles) west at this point. This small fault absorbs some of the northward motion of the Pacific Plate. As a result, this wall and the sidewalk can be seen shifting to the right across the fault.

The wall is about 25 years old.

In addition to the obvious bending of the wall, note the following other features. The alignment of the sidewalk is even more distorted than the wall, although the distant parts of the sidewalk are straight. This shows the rigidity of the plates away from these fault boundaries. Note also the light gray cement patch on the sidewalk at the lower left and the opening cracks near the center of the photograph. Fault zone or not, the beautiful flowers and well tended lawn show the obvious pride this home owner has in his ever shifting property.

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