HR: 09:15h
AN: S41D-06    [Abstracts]
TI: How can Stresses in the Earth's Crust Lead to Bursts of Electromagnetic Signals?
AU: * Freund, F T
EM: ffreund@mail.arc.nasa.gov
AF: NASA Ames Research Center, 242-4, Moffett Field, CA 94035-1000, United States
AU: * Freund, F T
EM: ffreund@mail.arc.nasa.gov
AF: San Jose State University, Department of Physics, San Jose, CA 95192-0106, United States
AU: * Freund, F T
EM: ffreund@mail.arc.nasa.gov
AF: Carl Sagan Center SETI Institute, 515 N Whisman Road, Mountain View, CA 94043, United States
AB: It is by now quite well established that igneous and high-grade metamorphic rocks in the Earth's crust generate electric currents when subjected to deviatoric stresses. The reason is that these rocks contain dormant electronic charge carriers in the form of peroxy links. Peroxy links are sites in the crystal structures of the constituent minerals where oxygen anions have converted from their common 2- valence state to the 1- valence state, O3X- OO-XO3 with X=Si4+, Al3+ etc. As rocks are stressed and dislocations sweep through the mineral grains, the peroxy links break up, activating electrons and pholes ("phole" is an abbreviation for "positive hole", a defect electron on the oxygen sublattice, chemically O– in a matrix of O2–). While the electrons are confined to the stressed rock volume, the pholes are mobile electronic charge carriers that can spread out of the stressed rock into the surrounding unstressed rocks. They travel via energy levels at the upper edge of the valence bands. They flow readily through 3+ m of rocks in the laboratory and are believed to be able to flow through kilometers of rocks, probably 10s of kilometers in the field. Laboratory experiments suggest that every cubic kilometer of stressed rock has the capacity to generate currents on the order of 10,000-100,000 amps, flowing for hours and days. The situation is akin to a battery, where the stressed rock takes on the role of the anode, while the unstressed rocks act as the electrolyte. However, stressing rocks does not yet produce a battery current. An additional requirement is that a conductive path be established which enables electrons from the stressed rock volume to also flow out, meet and reconnect with the pholes at the cathode. In the laboratory this condition is easily met by running a wire from a Cu contact attached to the stressed rock to a Cu contact attached to the unstressed rock. In Nature, however, this closure of the battery circuit has to be achieved by different means. Two possibilities are: (i) the phole current flows into an aquifer or brine-saturated gouge of a fault, (ii) the stressed rock volume extends downward into the hot, electronically conductive lower crust, allowing electrons to flow out. Which of the two scenarios applies, and at which point in time the circuit closes, will determine whether or not, when and¬ how large battery currents can flow. The fact that not every major earthquake produces detectable EM signals attests to the complexity of the system.
DE: 7223 Earthquake interaction, forecasting, and prediction (1217, 1242)
SC: Seismology [S]
MN: 2007 Fall Meeting