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Physics and Chemistry of the Earth 31 (2006) 389–396
Electric currents streaming out of stressed igneous rocks –
A step towards understanding pre-earthquake
low frequency EM emissions
Friedemann T. Freund a,b,*, Akihiro Takeuchi b,c, Bobby W.S. Lau b
a NASA Goddard Space Flight Center, Planetary Geodynamics Laboratory, Code 698 Greenbelt, MD 20771, USA
b San Jose State University, Department of Physics, San Jose, CA 95192-0106, USA
c Niigata University, Department of Chemistry, Niigata 950-2181, Japan
Accepted 6 February 2006
Available online 19 May 2006
Abstract
Transient electric currents that flow in the Earth’s crust are necessary to account for many non-seismic pre-earthquake signals, in
particular for low frequency electromagnetic (EM) emissions. We show that, when we apply stresses to one end of a block of igneous
rocks, two currents flow out of the stressed rock volume. One current is carried by electrons and it flows out through a Cu electrode
directly attached to the stressed rock volume. The other current is carried by p-holes, i.e., defect electrons on the oxygen anion sublattice,
and it flows out through at least 1 m of unstressed rock to meet the electrons that arrive through the outer electric circuit. The two out-
flow currents are part of a battery current. They are coupled via their respective electric fields and fluctuate. Applying the insight gained
from these laboratory experiments to the field, where large volume of rocks must be subjected to ever increasing stress, leads us to suggest
transient, fluctuating currents of considerable magnitude that would build up in the Earth’s crust prior to major earthquakes.
2006 Elsevier Ltd. All rights reserved.
Keywords: Seismo-electromagnetic phenomena; Uniaxial loading; Igneous rock; Positive hole (p-hole); p-type semiconductor; Battery current
1. Introduction
A recent letter in Nature (Gerstenberger et al., 2005)
begins with the words: ‘‘Despite a lack of reliable determin-
istic earthquake precursors, seismologists ha