A 3.5 M earthquake occurred roughly 20 miles north of Prescott, Arizona, on Tuesday, 25 October 2011. The USGS put the focus – i.e. depth – at about 3.1 miles, with an CV did you feel it mapuncertainty of nearly 2 miles.

Anecdotal reports from northern Chino Valley-Prescott area, where more than 600 people reported to USGS’s “Did you feel it?, indicate that the tremor was strongly felt there. A former Californian called our office and said, “this was as big a jolt as some earthquakes I felt in California”. In the Prescott courthouse, people on the 4th floor were sufficiently concerned to consider evacuating the building. They didn’t.

Tuesday’s earthquake coincided with the recent release of two videos, products of AZGS’s AZ Shakes earthquake outreach program, describing the geometry, structure, recurrence, and the probable maximum magnitude earthquake of the Little Chino and Big Chino faults (see our blog of 22 October). The earthquake does not appear to have been on either of those faults, but it was situated a few miles from the Big Chino Fault. As Phil Pearthree (Chief of AZGS Environmental Geology) pointed out, the Big Chino fault is capable of producing an earthquake with a magnitude up to 6.5 or 7.0. It’s been at least 20,000- to 30,000-years since a major earthquake on that fault.

We are coordinating with Yavapai Counties Emergency Management Dept. on a brochure promoting earthquake preparedness in Yavapai County. Our target release date is late November.

Resources:  The Little Chino and Big Chino fault videos available at AZGS’s YouTube channel.

Mike Conway (29 October 2011)