PHYSICS OF THE SOLID EARTH, English Translation, VOL. 30, NO. 9, APRIL 1995

Russian Edition: SEPTEMBER 1994

On the technique for tracking brief precursors of strong earthquakes in the low-frequency telluric field of Kamchatka

Yu. F. Moroz Institute of Volcanic Geology and Geochemistry, Far East Division of the Russian Academy of Sciences, Petropavlovsk-Kamchatskiy, Kamchatka, Russia

Natural electromagnetic fields always arouse great interest in geophysical studies on earthquake prediction. Numerous experiments held in many countries have shown that anomalous changes may originate in the telluric field prior to a strong earthquake [Sobolev and Morozov, 1970; Corwin and Morrison, 1977; Noritomi, 1978; Varotsos and Alexopoulos, 1984]. However, it is difficult to detect these changes in a complex telluric field. This paper is one more attempt to solve this ambiguous problem.

Telluric currents are known to correlate with various electric processes both in the ionosphere and in the Earth. At the present state of investigation, it is expedient to incorporate certain constraints. We shall assume that anomalous variations in the telluric field prior to a strong earthquake are caused mainly by physical processes in the earthquake source area. In this case, regular variations of telluric currents due to ionospheric electric processes would be just noise. These variations would cause a rather high level of noise, in particular during magnetic storms. The intensity of this noise may be determined by electric conductivity of the medium at the point of observation. To exclude such a noise, a technique of mathematical filtration has been applied. However, it does not seem possible to exclude this noise due to the irregular character of these variations. This hinders the recognition of signals from internal sources. Another approach can be also considered.

Let us consider the problem as it relates to Kamchatka, where seismic activity is high. There, much experience has been gained in using the data on telluric currents in geological problems. In the course of these studies, the following regularities in electric-telluric field behavior have been revealed [Moroz, 1991]:

1. Compared to the magnetic field, the telluric field is characterized by higher susceptibility to the variations in the rock's conductivity. So, if conductivity increases by 1000 S m, the telluric field stress changes by 3-4 orders of magnitude versus 20% in the magnetic field.

2. Intensity and polarization of the telluric field deCopyright 1995 by the American Geophysical Union. 1069-3513/95/3009-0011$18.00/1

pends largely on the electric conductivity and degree of inhomogeneity of the medium at the point of observation.

3. Components of the telluric field have higher resolution capacity along the axes of geoelectric symmetry of the medium, and therefore they can be used to determine more precisely the distribution of electric conductivity in the Earth.

On the basis of these features, a technique has been selected to search for brief precursors of earthquakes in a low-frequency telluric field. We know the ratio between the electric and magnetic fields on the Earth's surface: E = [Z]H, where E and H are the stresses of the corresponding fields and [Z] is the impedance tensor (input complex resistance of the medium). We can see from this ratio that the expected stress of regular telluric field variations at the point of observation can be determined from the known impedance tensor and magnetic field stress. These variations would be noise. Noise can be reduced maximally if we choose the right receiving line. At the same time it is necessary to preserve the maximal susceptibility of the instrument system to the telluric signal generated by any strong earthquake. The receiving lines should be oriented along the axes of geoelectric symmetry of the medium. This is a necessary prerequisite. Thus, the telluric field components would have higher susceptibility to the excited geoelectric inhomogeneous medium. So, the results of the work on geoelectric monitoring in coal mines are evidence that the ratio between the natural field potentials along the axes of geoelectric inhomogeneity is the most sensitive parameter to tectonic stresses [Tarasov et al., 1983].

The impedance tensor can be determined from magnetotelluric sounding data. A pole diagram of the principal impedance can be calculated from the tensor components; this diagram is similar to the ellipse of the telluric field [Moroz, 1985]. It can be used to determine the orientation of the axes of geoelectric symmetry of the medium.

Earlier we mentioned that the magnetic field situated in a low-frequency area depends little upon conductivity at the point of observation. Therefore, to estimate the stress state of the magnetic field components, we can

830

moroz: on the technique for tracking brief precursors 831 use the generalized data for all of Kamchatka obtained from the network of stations for the last several years. Figure 1 presents the statistical data on the magnetic field variations for 1989-1992. We generally included in the data processing those days when the magnetic field was excited (magnetic storms). Therefore, curves plotted in Figure 1 show high intensity of the magnetic field.

This technique has been tested in the settlement of Topolovo on Kamchatka (Figure 2). Prior to the testing, a deep magnetotelluric sounding was performed there in the period range from 1000 to 10,000 s. For processing, we used a program incorporating spectral analysis of the field based on the theory of stochastic processes [Semenov, 1985]. As a result, we obtained the impedance tensor and pole diagrams of impedances. In most cases, the main axes of the principal impedance diagrams have sublatitudinal (big axis) and sublongitudinal (small axis) orientation. The principal impedance is 0.6 M\Omega in the meridional direction during the period equal to one hour when the telluric field intensity is maximum. In the latitudinal direction, the principal impedance is 0.4 M\Omega for the same period. Intensity of the latitudinal and meridional components of the magnetic field for this period is 50 and 40 A km

\Gamma 1

, respectively. Consequently, the telluric field stress maximum due to an external source would be about 40 mV km

\Gamma 1

in the meridional direction, and about 24 mV km

\Gamma 1

in

the latitudinal one. If the receiving lines are several tens of meters long, this field effect can be reduced considerably. At Topolovo, the meridional length of the receiving line was 50 m. With such a base we can expect a reliable registration of signals over 80 mV km

\Gamma 1

. Experiments held in other countries showed that signals of such intensity and greater can be expected only in the case of strong earthquakes [Varotsos and Alexopoulos, 1984].

Figure 1. Mean intensity of the magnetotelluric field components at Kamchatka. D and H are longitudinal and meridional components of the magnetic field; E

x

and E

y

are latitudinal and meridional components of

the telluric field; T is period.

Figure 2. Registered differences of the telluric field potentials at Topolovo, Kamchatka. (a) Location scheme of earthquake epicenters; (b) graphs of difference variation in the telluric field potentials; 1, earthquake epicenters; 2, sites of the telluric field registration; and 3, moment of earthquake.

Continuous measurements of the difference in the telluric field potentials in the north-south direction were carried out at Topolovo during 1992. Lead electrodes were used for grounding. They were situated in the boreholes at a depth of 30 m. This reduced considerably the effect of meteorological factors. Continuous registration of the difference in the telluric field potentials was performed by an H-3101 microamperemeter. In 1992, there were two significant variations of the telluric field prior to the strongest earthquakes registered for that period. These are shown in Figure 1. Other variations of the field did not exceed 1-2 mV, which is the instrument's limit of precision.

832 moroz: on the technique for tracking brief precursors

Table 1.

Date Time

Latitude

degrees

Longitude

degrees

Depth

km

Class

Key

symbol March 5 1439 52.71 159.95 30 14 A March 25 2104 54.38 160.2 140 13 B

From the presented data we can see that an anomaly with intensity up to 9 mV was registered on March 5, 1992, 2 h 55 min before the class 14 earthquake (1439 UT). Anomalous fluctuations of the telluric field continued for 21 min (Figure 2).

On March 25, 1992, another anomaly with intensity up to 10 mV was registered 7 min before the class 13 earthquake (2145 UT). Earthquakes smaller than class 12 were manifested in the telluric field scarcely, if at all (within the instrumental precision).

We should note that brief precursors of earthquakes were registered at only one receiving line and at only one site. Therefore, this result can be regarded as preliminary by all means. A network of three observation points as a minimum should be used for such experiments. One of these sites should be located near a seismofocal zone. The difference between the telluric field potentials should be registered at each site along two orthogonal directions. Only such a network can provide for further development of the proposed technique.

References Corwin, R. F., and H. F. Morrison, Self-potential variations

preceding earthquakes in central California, Geophys. Res. Lett., 4, 171-174, 1977. Moroz, Yu. F., Technique for determining a horizontal inhomogeneity in geoelectric cross-section from TT method, Explor. Geophys., 101, 82-86, 1985. Moroz, Yu. F., Electric Conductivity of the Earth's Crust

and Upper Mantle, 181 pp., Nauka, Moscow, 1991. Noritomi, K., Application of precursory geoelectric and geomagnetic phenomena to earthquake prediction in China, Report by Japanese Seismological Society Delegation to the People's Republic of China, Seism. Soc. Japan, 57- 87, 1978 (in Japanese; for English translation see Chinese Geophys., Am. Geophys. Union, 1 (2), 377-391, 1978). Semenov, V. Yu., Processing of Magnetotelluric Sounding

Data, 132 pp., Nedra, Moscow, 1985. Sobolev, G. A., and A. N. Morozov, Local electric field

disturbance at Kamchatka as related to earthquakes, in Physical Principles in the Search for Earthquake Prediction Technique, 110 pp., Nauka, Moscow, 1970. Tarasov, B. G., V. V. Dyrdin, and V. V. Ivanov, Geoelectric

Monitoring of the Rock Massive State, 215 pp., Nedra, Moscow, 1983. Varotsos, P., and K. Alexopoulos, Physical properties of the

variations of the electric field of the Earth preceding earthquakes, Tectonophys., 110, 99-125, 1984.