HR: 0800h
AN: T51B-0448    [Abstracts]
TI: Anomalous Magnetic Field Pulses, Ground Currents, and the Build-up of Stress prior to the Chi-Chi Earthquake
AU: * Hall, C G
EM: hall_c_gabriel@yahoo.com
AF: San Jose State University, Department of Physics, San Jose, CA 95192-0106 United States
AU: * Hall, C G
EM: hall_c_gabriel@yahoo.com
AF: Lockheed Martin Space Systems, 100 Campus Dr., Newtown, PA 18940 United States
AU: Yen, H Y
EM: yenhy@earth.ncu.edu.tw
AF: Central National University, Department of Geophysics, Chung-Li, 320 Taiwan
AU: Chen, H C
EM: nonono@url.com.tw
AF: Central National University, Department of Geophysics, Chung-Li, 320 Taiwan
AU: Takeuchi, A
EM: takeuchi@science.sjsu.edu
AF: San Jose State University, Department of Physics, San Jose, CA 95192-0106 United States
AU: Takeuchi, A
EM: takeuchi@science.sjsu.edu
AF: NASA Goddard Space Flight Center, Geodynamics Branch MS 921, Greenbelt, MD 20771 United States
AU: Lau, B W
EM: blau@science.sjsu.edu
AF: San Jose State University, Department of Physics, San Jose, CA 95192-0106 United States
AU: Lau, B W
EM: blau@science.sjsu.edu
AF: NASA Goddard Space Flight Center, Geodynamics Branch MS 921, Greenbelt, MD 20771 United States
AU: Freund, F
EM: ffreund@core2.gsfc.nasa.gov
AF: San Jose State University, Department of Physics, San Jose, CA 95192-0106 United States
AU: Freund, F
EM: ffreund@core2.gsfc.nasa.gov
AF: NASA Goddard Space Flight Center, Geodynamics Branch MS 921, Greenbelt, MD 20771 United States
AU: Freund, F
EM: ffreund@core2.gsfc.nasa.gov
AF: NASA Ames Research Center, Earth System Science and Technology, MS 242-4, Moffett Field, CA 94035-1000 United States
AB: Before the Sept. 21, 1999 Chi-Chi earthquake in Taiwan and during the period of aftershocks local magnetic field anomalies (up to 200 nT) were recorded at two stations of the Taiwan magnetometer network. The magnetic pulses each lasted for several hours. They arrived in week-long bunches and extend over more than three months. Powerful ground currents are required to generate such strong local magnetic fields, in the order of 10$^{6}$ Amp at peak intensity. The seismic energy released by small earthquakes (earthmurmur) during the weeks before the main shock shows a similar time-dependent evolution, which correlates with the magnetic field anomalies. This suggests that the ground currents are generated when the regional stresses increase as signaled by an increase in the frequency of small earthquakes. We have measured in the laboratory the stress-induced electrical currents generated in igneous rocks (granite and anorthosite). Our experiments show that these rocks exhibit a battery-like behavior, i.e. they produce currents, which flow out of the stressed rock volume into the surrounding unstressed rock. The charge carriers are electronic and positively charged. They are believed to be positive holes (p-holes), i.e. defect electrons in the valence band of the otherwise insulating rocks. The number of outflowing charge carriers is in the order of 106 cm$^{-3}$ of stressed rock. Scaling up to the dimensions of the Chi-Chi event and assuming that the compressed rock volume was 100 x 10 x 50 km$^{3}$ (length of the surface rupture in the N-S direction x thickness x width in the E-W direction) we find that the number of charge carriers activated in such a large "source volume" would suffice to produce outflow currents in the order of 10$^{6}$ Amps over an extended period of time.
DE: 8045 Role of fluids
DE: 8164 Stresses--crust and lithosphere
DE: 5109 Magnetic and electrical properties
DE: 5194 Instruments and techniques
DE: 1645 Solid Earth
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting