Physics
Scientific paper
Oct 1989
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1989pepi...57...45m&link_type=abstract
Physics of the Earth and Planetary Interiors, Volume 57, Issue 1-2, p. 45-46.
Physics
2
Scientific paper
The appearance of strong periodic anomalies, with a period of 24 h, in the electrotelluric field before two large imminent earthquakes is discussed. The precursor occurs as a periodic component superimposed on the large variation of potential difference which starts a couple of days before the earthquake. An attempt is made to find an appropriate geophysical model for the anomaly. It is suggested that changes in electromagnetic induction may take place, as a result of resistivity changes in the earthquake fault area. We account also for possible anisotropy effects of the resistivity response to a stress system. Anisotropy effects related to the T-axis of the fault-plane solution are expected, as resistivity changes are related to opening and closing of cracks and to water transport between cracks. These phenomena are highly sensitive to stress orientation. Consequently, any model of resistivity variations can be modified according to anisotropy of a cracked medium. Thus, we consider sectors of dilatancy and compression; dilatancy in dry or partly saturated rocks leads to a gradual resistivity increase which is followed by a sharp decrease (Fig. 1). This decrease is due to a percolation threshold which can be related to formation of a fracture plane. In saturated rocks the situation may be apparently different. In that case we can expect water to be squeezed out from a sector of compression, and the resistivity of a rock complex would consequently also change before an earthquake. Resistivity anisotropy effects are also expected and are related to the P-axis direction. Thus, changes in the electromagnetic induction are related to the orientation of the fault plane, and corresponding changes (anomalies) in the telluric field have preferred geometrical directions. The apparent resistivity response to resistivity changes in a narrow fault zone demonstrates local extreme values along the zone (Fig. 2). Extreme changes in the electromagnetic induction (and hence in the telluric field) are strongly dependent upon the location of the recording site (compare, for instance, the situation at recording sites A and B in Fig. 2).
Meyer Klaus
Teisseyre Roman
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