HR: 0845h
AN: S41D-04    [Abstracts]
TI: The Large-Amplitude ULF Magnetic Fields Preceding Large Earthquakes: Merely the "Tip of the Iceberg"?
AU: * Fraser-Smith, A C
EM: acfs@alpha.stanford.edu
AF: Departments of Electrical Engineering and Geophysics, Stanford University, Stanford, CA 94305-9515, United States
AB: It is now around 400 years since it was first recognized that there was a slowly-time-varying "magnetic signal" emerging out of the Earth (i.e., the Earth's magnetic field), but until very recently the competition from ultra-low frequency (ULF, f ≤ 10 Hz) magnetic signals originating in the upper atmosphere has made it difficult to identify other different types of ULF signals originating in the Earth. The four independent published reports of large-amplitude ULF magnetic fields preceding large earthquakes currently available now provide evidence for another source of these fields within the Earth; the presumed earthquake-related fields being large enough to appear above the upper-atmospheric noise background. An obvious inference from this result is that there may be other processes within the Earth generating ULF magnetic signals that are not big enough to be detected above the upper atmospheric noise background. Fortunately this latter noise tends to be uniform over large distances on the Earth's surface and can be removed by simple cancellation techniques using either pairs or arrays of identical, spaced measurement systems. ULF magnetic field measurements made in 1994 will be used to demonstrate that there are indeed residual components that remain after cancellation that may well originate in the Earth. If so, the presumably earthquake-related ULF magnetic fields are merely the "tip of the iceberg" providing us a limited glimpse through an unexploited window into the Earth. Magnetometers and magnetic-field gradiometers of the greatest sensitivity and with broad frequency responses are obvious keys to exploring the processes glimpsed through this tantalizing new(?) window.
DE: 1500 GEOMAGNETISM AND PALEOMAGNETISM
DE: 1530 Rapid time variations
DE: 5109 Magnetic and electrical properties (0925)
DE: 7200 SEISMOLOGY
DE: 7223 Earthquake interaction, forecasting, and prediction (1217, 1242)
SC: Seismology [S]
MN: 2007 Fall Meeting