Review of magnetic and electric field effects near active faults and volcanoes in the U.S.A.
Synchronized measurements of geomagnetic field have been recorded along 800 km of the San Andreas fault and in the Long Valley caldera since 1974, and during eruptions on Mount St. Helens since 1980. For shorter periods of time, continuous measurements of geoelectric field measurements have been made on Mount St. Helens and near the San Andreas fault where moderate seismicity and fault slip frequently occurs. Significant tectonic and volcanic events for which nearby magnetic and electric field data have been obtained include: (1) two moderate earthquakes (ML > 5.8) for which magnetometers were close enough to expect observable signals (about three source lengths), (2) one moderate earthquake (MS 7.3) for which magnetometers were installed as massive fluid outflow occurred during the post-seismic phase, (3) numerous fault creep events and moderate seismicity, (4) a major explosive volcanic eruption and numerous minor extrusive eruptions, and (5) an episode of aseismic uplift. For one of the two earthquakes with ML > 5.8, seismomagnetic effects of -1.3 and -0.3 nT were observed. For this event, magnetometers were optimally located near the epicenter and the observations obtained are consistent with simple seismomagnetic models of the event. Similar models for the other event indicate that the expected seismomagnetic effects are below the signal resolution of the nearest magnetometer. Precursive tectonomagnetic effects were recorded on two independent instruments at distances of 30 and 50 km from a ML 5.2 earthquake. Longer-term changes were recorded in one region in southern California where a moderate ML 5.9 earthquake has since occurred. Surface observations of fault creep events have no associated magnetic or electrical signature above the present measurement precision (0.25 nT and 0.01%, respectively) and are consistent with near-surface fault failure models of these events. Longer-term creep is sometimes associated with corresponding longer-term magnetic field perturbations. Correlated changes in gravity, magnetic field, areal strain, and uplift occurred during episodes of aseismic deformation in southern California primarily between 1979 and 1983. Because the relationships between these parameters agrees with those calculated from simple deformation and tectonomagnetic models, the preferred explanation appeals to short-term strain episodes independently detected in each data set. An unknown source of meteorologically generated noise in the strain, gravity, and uplift data and an unknown, but correlated, disturbance in the absolute magnetic data might also explain the data. No clear observations of seismoelectric or tectonoelectric effects have yet been reported. The eruption of Mount St. Helens generated large oscillatory fields and 9 ?? 2 nT offset on the only surviving magnetometer. A large-scale traveling magnetic disturbance passed through the San Andreas array from 1 to 2 h after the eruption. Subsequent extrusive eruptions generated small precursory magnetic changes in some cases. These data are consistent with a simple volcanomagnetic model, magneto-gas dynamic effects, and a blast excited traveling ionospheric disturbance. Traveling ionospheric disturbances (TIDs), also generated by earthquake-related atmospheric pressure waves, may explain many electromagnetic disturbances apparently associated with earthquakes. Local near-fault magnetic field transients rarely exceed a few nT at periods of a few minutes and longer. ?? 1989.