Anomalous time variation of the self-potential in the fractured zone of an active fault preceding the earthquake occurrence.

Abstract

Anomalous variation of the electric self-potential difference between carbon rod electrodes fixed in an underground geophysical observation tunnel took place preceding a considerably strong earthquake (M=5.6) which occurred at an epicentral distance of 3.2km. The precursor-like changes ranging 40 mY at maximum began about 40 days before and also 55 hours before the earthquake occurrence and the recovery began co-seismically coming back to the initial level after 13 hours. The geophysical observation tunnel is located in the fractured and electrically conductive zone of the Yamasaki active fault, a target field for the earthquake prediction study.It was elucidated from the results of observation of the electric self-potential differences with the components of various directions and electrodes spacing and from the measurements of the self-potential across the fault and in the tunnel that the observed anomalous variation does not imply the variation of the earth-current observed due to the voltage drop through the ohmic resistance between the electrodes, but implies the decrease of the self-potential of the electrode, fixed in the fractured zone of the fault, itself, to that of another which served as a reference.As for the cause of the decrease of the self-potential, there may be two possibilities: (1) it was due to the time variation of the contact electric potential difference between the electrode and the surroundings at each electrode, (2) it was caused by the time variation of the self-potential of the fractured rock or of the water in the cracks of the fault where the electrode is fixed.As the electrodes used are carbon rod electrodes, non-equilibrium balanced type electrodes, the first possibility may be plausible. However, as all electrodes had already aged and become stable enough at that time and also considering the other observational results, the second possibility is likely to be more probable.

Modeling of seismo-electromagnetic phenomena

“…An example of an apparent source would be the apparent electrotelluric field arising from a change, for whatever reason, of the chemical composition of pore water during measurement of the field, since such a measurement is done using electrodes placed just below the surface [Miyakoshi, 1986]. Another example is a change in the apparent local orientation of a detector due to local displacement of the crust.…”

“…This search has been conducted in many areas of high seismic activity worldwide (Japan [Fukutomi, 1934;Miyakoshi, 1986;Noto, 1933;Ozima et al, 1989;Shiratori, 1925;Uyeda, et al, 2000], the former Soviet Union [Sobolev et al, 1981], Greece [Lighthill, 1996;Varotsos and Alexopoulos, 1984a, 1984bVarotsos et al, 1993], China [Qian et al, 1990;Raleigh et al, 1977], Bulgaria [Ralchovsky and Komarov, 1987], USA [Sornette and Sornette, 1990]). Several hundred cases have been reported which relate such anomalies to strong nearby quakes.…”

Earthquake Prediction Research in Japan.

“…As an example of these results, an anomalous change in geoelectric potential prior to an earthquake of M5.6 along the Yamasaki fault in western Honshu is shown in Fig. 2 (Miyakoshi, 1986). 1901solid lines, extension (from GSI, 1991b).…”

“…2. Anomalous change in geoelectric potential prior to an earthquake of M5.6 along the Yamasaki fault in western Honshu (from Miyakoshi, 1986). …”

Observation of earth-potential using telegraphic facilities and analysis with BAYTAP-G

“…Many authors have claimed that "anomalous changes" in earth-potential prior to earthquakes were precursors (e.g., MYACHKIN et al , 1972;CORwIN and MORRISON, 1977;KOYAMA and HONKURA, 1978;VAROTSOS and ALEXOPOULOS , 1984a, b;MIYAKOSHI, 1986). However, we think it very debatable whether or not those anomalous changes were truly precursors, for the following reasons .…”