Geoscience Reference
In-Depth Information
Chapter 4
Pre-earthquake Signals
F. T. Freund
INTRODUCTION
Earthquakes are feared because they often strike so suddenly. Yet, there are innumerable
reports of pre-earthquake signals. Widespread disagreement exists in the geoscience
community how these signals can be generated in the Earth's crust and whether they
are early warning signs, related to the build-up of tectonic stresses before major seis-
mic events. Progress in understanding and eventually using these signals has been
slow because the underlying physical process or processes are basically not under-
stood. This has changed with the discovery that, when igneous or high-grade meta-
morphic rocks are subjected to deviatoric stress, dormant electronic charge carriers are
activated: electrons and defect electrons. The activation increases the number density
of mobile charge carriers in the rocks and, hence, their electric conductivity. The de-
fect electrons are associated with the oxygen anion sublattice and are known as posi-
tive holes or pholes for short. The boundary between stressed and unstressed rock acts
a potential barrier that lets pholes pass but blocks electrons. Therefore, like electrons
and ions in an electrochemical battery, the stress-activated electrons and pholes in the
“rock battery” have to flow out in different directions. When the circuit is closed, the
battery currents can flow. The discovery of such stress-activated currents in crustal
rocks has far-reaching implications for understanding pre-earthquake signals.
Seismologists use earthquakes as “fl ash lights” to illuminate the interior of the
Earth. Information extracted from the propagation of seismic waves has produced
great insights into the hidden structure of our dynamic planet. Unfortunately, earth-
quakes are erratic “fl ash lights” that seem to go off at unpredicted times and at un-
predicted places. Since they often lead to destruction and death, it is understandable
that seismologists have endeavored to fi nd ways to predict—within limits as narrow
as possible—time, place, and magnitude of major seismic events (Gokhberg et al.,
1995; Lomnitz, 1994; Milne, 1899, Rikitake, 1976 #452; Sykes et al., 1999; Turcotte,
1991; Wyss and Dmowska, 1997). However, using the “fl ash lights” is different from
understanding when and where a “fl ash” might go off. The two require different skills
and different tools.
Though the seismological models have become ever more sophisticated and tend
to take into account ancillary information (Holliday et al., 2005; Keilis-Borok, 2002;
Rundle et al., 2003), the tools of seismology are blunt when it comes to recognizing
the build-up of stress before the rupture.
At the same time it has been known for a long time that the Earth sends out a bewil-
dering array of non-seismic signals before major events (Tributsch, 1984). Understanding
