Analysis of Ultralow-Frequency Electromagnetic Field Measurements Associated with the 1999 M 7.1 Hector Mine, California, Earthquake Sequence

We installed two electromagnetic (EM) monitoring systems in the im- mediate aftermath of the 16 October 1999 Hector Mine earthquake to search for possible continuing ultralow-frequency (ULF) EM activity due to the mainshock as well as for any precursory or coseismic EM signals that might be associated with large aftershocks. We installed the first portable monitoring system 2.5 days after the M 7.1 Hector Mine earthquake at a location 16 km southeast of the epicenter and 2 km east of the surface rupture. A second system was installed on 29 October 1999, 10 km northwest of the epicenter and within 100 m of the surface rupture. Our continuous measurements of multiple-component magnetic field, electric field, and ground motion span the low frequencies appropriate for recording possible EM sig- nals generated at seismogenic depths and were carried out during 3 months following the mainshock. Continuous magnetic-field measurements at observatory EM stations operating in California are used as remote-reference sites to remove global atmo- spheric signals, which helps isolate local terrestrial sources of interest. Our analysis of preseismic ULF-EM variations, the coseismic response, the 2-month-long magnetic-field power spectra, and electric-field polarization shows no anomalous behavior clearly associated with seismic activity.