Ultra-low frequency electromagnetic monitoring of earthquakes within the Earthscope Plate Boundary Observatory

Stanford University, Stanford CA

Investigators

Abstract

On an annualized basis, earthquakes in California kill several people and cause several billions of dollars of property damage every year. There is a natural societal interest in understanding whether earthquakes are predictable; and if they are predictable, developing ways to obtain even a few minutes of warning. We are attempting to address this societal need by studying electromagnetic emissions as one of the most promising potential pre-seismic transients. Our network of monitoring stations complements commercial "earthquake prediction" services operating magnetic monitoring stations in California by providing an independent source of data for the public. Part of the mission of the National Science Foundation's Plate Boundary Observatory (PBO) is to learn whether earthquakes are predictable. We are building a network of three ultra-low frequency electromagnetic (ULF-EM) recording sites in northern California in order to understand earthquake physics and to detect pre-seismic transients if they exist. Our electromagnetic sensors include ultra-sensitive magnets to detect tiny changes in Earth's magnetic field, and low-noise electric cables to detect tiny electric currents inside the Earth. Our ULF-EM sensors are collocated with existing PBO sites, so that any anomalies observed on our electromagnetic sensors can be cross-checked against more-conventional earthquake detection instruments: strain meters and seismographs. All our data is transmitted to a public data archive on a continuous basis, 24 hours a day. We monitor our instruments on a daily basis to confirm that they are recording valid data, and we run the data through a set of standardized programs to search for possible anomalies. It is possible that we will recognize a major change in signal levels or character before an earthquake, but it is more likely that after a major earthquake we will have a valuable dataset that we can use to test hypotheses about the way earthquakes initiate, and to help us understand future earthquake cycles. We are recording ULF-EM data because ultra-low frequency (0.01-10 Hz) magnetic field anomalies prior to and following large earthquakes (magnitude greater than 6.5) 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. We are collocating our magnetic sensors with PBO strain-meters and seismometers in order to allow signals from earthquakes to be separated from artifacts related to sensor tilt. We are recording with identical, spatially separated recorders in order to exclude other potential sources of magnetic signals (including man-made signals). Continuous recording of the magnetic and electric field before, during and after, and spatially close to, a significant earthquake, is required to demonstrate the reality or absence of ULF-EM signals. We are locating our ULF-EM recording sites along the San Andreas and other major faults in San Francisco Bay Area, because these faults have a large potential for a major earthquake in the next decade.