Review of Electric and Magnetic Fields Accompanying Seismic and Volcanic Activity
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New observations of magnetic, electric and electromagnetic field variations, possibly related to recent volcanic and seismic events, have been obtained on Mt. Unzen in Japan, Reunion Island in Indian Ocean, the Long Valley volcanic caldera in California, and for faults in China and Russia, California and several other locations. For volcanic events, contributions from different physical processes can be identified during the various eruption stages. Slow processes (weeks to months) include near-surface thermal demagnetization effects, piezomagnetic effects, and effects from rotation/displacement of magnetized material. Rapid processes (seconds to days) include piezomagnetic effects from instantaneous stress redistribution with explosive eruptions and electrokinetic effects from rupture of high pressure fluid compartments commonly encountered in volcanic regions. For seismic events, the observed coseismic offsets are instantaneous, provided care has been taken to ensure sensors are insensitive to seismic shaking and are in regions of low magnetic field gradient. Simple piezomagnetic dislocation models based on geodetically and seismically determined fault parameters generally match the observed signals in size and sign. Electrokinetic effects resulting from rupture of fluid filled compartments at hydrostatic to lithostatic pore pressures can generate transient signals in the frequency band 100 Hz to 0.01 Hz. However, large-scale fluid driven processes are not evident in near-field measurements in the epicentral region minutes to weeks before large earthquakes. The subset of ionospheric disturbances generated by trapped atmospheric pressure waves (also termed gravity waves and/or acoustic waves, traveling ionospheric disturbances or TID's) that are excited by earthquakes and volcanic eruptions are common and propagate to great distances. These are known and expected consequences of earthquakes, volcanic explosions (and other atmospheric disturbances), that must be identified and their effects removed from VLF/ULF electromagnetic field records before associating new observations of ionospheric disturbances with earthquake activity.
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