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M=3.5 earthquake strikes near creeping San Andreas and Calaveras Faults

At 9:35 p.m. local time last night (January 30), a small M=3.5 earthquake shook the city of Hollister, CA, and was felt as far away as Santa Cruz and Monterey. According to the USGS ShakeMap, only light shaking was felt, and around 300 people recorded feeling the quake on the USGS website.

This earthquake occurred between the creeping sections of the San Andreas and Calaveras faults. The fact that they are creeping means that there is very slow continuous motion along the faults. Areas such as this do not tend to have large earthquakes as stress does not build up as much as it does on locked sections.

Even though this earthquake occurred close to the San Andreas and Calaveras faults, based its focal mechanism, it appears to have occurred along a secondary fault. We would expect an earthquake on either the San Andreas or Calaveras faults to have almost pure strike-slip motion. While this quake was primarily strike-slip, it did also have a large component of extensional motion. Nonetheless, because of its location, it merits a closer look.

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