Ultra-low Frequency Electromagnetic Monitoring of Earthquakes in the San Francisco Bay Area: Initial Results of an Earthscope PBO Project

As a necessary part of the PBO (Plate Boundary Observatory) mission we are developing a network of three ultra-low frequency electromagnetic (ULF-EM) recording sites in northern California to understand earthquake physics and to detect pre-seismic transients if such exist. Each site will have three orthogonal magnetometers and duplicate sets of orthogonal horizontal electrode pairs, recording signals from 0.01 to 20 Hz, collocated with an existing broadband seismometer. These three PBO-sponsored sites will complement two existing magnetometer sites maintained by UC Berkeley at Parkfield and Hollister in central California, and a growing California network of magnetometers managed by Quakefinder Inc. In 2005 our first station came online at Jasper Ridge Biological Preserve adjacent to the campus of Stanford University, and our data are freely available through the Northern California Earthquake Data Center web-site http://quake.geo.berkeley.edu/ncedc/. In 2006 we intend to complete installations in the North Bay and East Bay, pending continuing discussions with landowners. We present preliminary data from our Jasper Ridge station (JRSC), aimed at establishing a systematic baseline against which any future potential seismogenic anomalies may be tested. Ultra-low frequency (0.01-10 Hz) magnetic field anomalies prior to and following M > 6.5 earthquakes have been reported from a number of different regions of the world. If real, these signals contain important information about the physics of earthquakes, particularly fluid motion in and around the fault before, during and after seismic activity. Although we do not (yet) present any claims for signals related to earthquakes, we plan to continue monitoring for the duration of the project, ideally beyond the next large Bay Area earthquake.