Case 11: Anomalous Time Variation of the Self-Potential in the Fractured Zone of an Active Fault Preceding the Earthquake Occurrence
- Max Wyss
Published Online: 3 APR 2013
Copyright 1991 American Geophysical Union.
Evaluation of Proposed Earthquake Precursors
How to Cite
Miyakoshi, J. (1991) Case 11: Anomalous Time Variation of the Self-Potential in the Fractured Zone of an Active Fault Preceding the Earthquake Occurrence, in Evaluation of Proposed Earthquake Precursors (ed M. Wyss), American Geophysical Union, Washington, D. C.. doi: 10.1029/SP032p0068
- Published Online: 3 APR 2013
- Published Print: 1 JAN 1991
Print ISBN: 9780875907840
Online ISBN: 9781118667873
- Earthquake prediction—Evaluation
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 3.2 km. The precursor-like changes ranging 40 mV 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 electrode 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, or (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.