Notes on Generation and Propagation of Seismic Transient Electric Signals


Yuji ENOMOTO

Mechanical Engineering Laboratory, Agency of Industrial Science and Technology, MITI Namiki 1-2,Tsukuba, Ibaraki 305 Japan

Laboratory experiments on fracture-induced electric signals
Working hypotheses for various observations of SEMS
Generation and propagation mechanisms of SEMS
Summary

Although much attention in this critical review of VAN has focused on statistical assessments of VAN'S prediction, it is evident that the seismic electromagnetic activities have been observed not only by the VAN method, but also by the other different methods. Important matters are vital to understanding the nature of seismic electromagnetic signals (SEMS) in greater detail, and to searching for sensitive methods of observation that may be applicable in populated and industrialized areas to detect seismic precursor signals and, if possible, use these to detect imminent seismic precursor signals. From this point of view, this paper discusses the following three points, with emphasis on the comparability of the VAN signals (SES):
1) Possible existence of SEMS, as suggested by laboratory experiments on fracture-induced electromagnetic signals,
2) Working hypotheses regarding various observations of seismic electromagnetic signals, including those from VAN,
3) Possible source mechanism for SEMS.

Laboratory experiments on fracture-induced electric signals
Laboratory- experiments which simulate the precursor stage leading to final fracture of the Earth's crust (earthquake) are important for understanding the nature of SEMS. Various experiments have been conducted to detect electromagnetic signals associated with the fracturing of rocks. The results, however, have not always been consistent. A typical difference is that some experiments attributed the origin of SEMS to piezoelectric effect of quartz crystals in quartz-bearing rocks such as granite, while others did not. For electric signals propagated through rock media, the relation of signal intensity to the distance from the focal area to an observing site has not been confirmed.

To resolve some of these uncertainties, we conducted experiments to detect transient electric signals induced by mechanical and thermal stimulation of rocks using a new charge detector that can detect rapidly fluctuating electric charge signals. This feature allows it to record the burst-like electric currents emitted from mechanically or thermally stimulated areas of rocks. The following four series of experiments were conducted.

The first series of fracture experiments of rock subjected to uniaxial compression, as shown in the inset of Fig.1, showed that pronounced transient electric signals could be detected in the final stage of accelerated deformation leading to the final failure. Typical results for granite are given in Fig. 1. It should also be noted that the signals have been detected not only from quartz-bearing rocks such as granites but also from quartz-free rocks like serpentinite, although the intensity of signals from quartz-free rocks was about one-fourth as large as those of from granites.
Fig. 1 Schematical view of experimental set-up (inset) and typical records of (a) electric charge signal and (b) axial compression stress and photoemission signals from fracturing granite. Time scale is magnified in (c) and (d).

In a second series of experiments, a long rectangular bar made of granite was subjected to a guillotine-type fracture at one end while a copper electrode was positioned at another end to detect electric signals. Figure 2 shows the transient electric current as a function of the distance r (mm) of the detector electrode from the fracture zone.
The following conclusions can be drawn from Fig.2,
1) The transient electric current generated due to the fracture is 10-10 ~ 10-8 A/cm2. This value is comparable to that estimated by Prof. Varotsos, as necessary to cause detectable SES at the ground level.
2) The current signal intensity is proportional to 1/r in these measurements.
3) The current intensity for coarse-grained granite having electrical resisitivity of 8.4 x 106 ohm cm is 17.5 times larger than that of fine-grained granite having electrical resistivity of 0.49 x 106 ohm-cm. This suggests that the difference between the current intensity of coarse-grained granite and that of fine-grained granite is comparable to the difference in electric conductivity.

Fig.2 Integrated intensity of fracture induced transient current signal during the fracture period of granite as a function of the distance r of the electrode from the fracture zone.
· coarse-grained granite and
o fine- grained granite in their natural state.

A third series of experiment was devised to determine whether the signal is detectable in the field-scale. To this end, a larger bar of coarse-grained granite of 3 x 3 x 110 cm in size was used. The signals were detected at two positions 5 cm and 105 cm distant from the fracture point. It is noted, as shown in Fig. 3, that the signal caused by a fracture more than 3 cm square is detectable even 100 cm from the fracture zone.

If a test 100,000 times larger in scale were conducted, it would involve a fracture of the earth's crust of 3 km square, perhaps comparable to the earthquake of a magnitude 4 ~ 4.5 on the Richter scale. The implication of 1) in the second series of experiment suggests that the transient electric current in the focal zone would 10-10 ~ 10-8 A/cm2 x (3x105 cm) 2 = 9 ~ 900 A. 

The results of the third series of experiments indicate that the transient electric current can propagate through rock media over a distance of about 100 km from the focal zone. This estimate is simple but encouraging for the conduct of field measurements to detect seismic electric signals. By this method as described later, anomalous electric signal could actually be detected prior to the earthquakes which occurred near Tsukuba, north of Tokyo. 

Although there is a dispute as to whether SES detected by the VAN method is real or not, the author believes from these laboratory experiments that detection of SES is promising way of detecting the precursor stage to faulting.

Fig 3 Typical measurements at room temperature of fracture induced transient current at an electrode bias zero at (a) 5 cm and (b) 105 cm from the fracture zone.

Working hypotheses for various observations of SEMS
Seismic electromagnetic phenomena may appear in various different forms, such as electric currents, radio waves, electrostatic induction, electrification and electric discharge/lightning. This means that various types of observation may be used to detect the phenomena. Several methods of observation have, in fact, been made to detect seismic precursory electromagnetic signals, as schematically shown in Fig. 4.

Fig. 4 Schematical view of various detection for SEMS and that of propagation mode of electromagnetic disturbance from a focal zone.
The VAN method has been used to observe the electrical potential difference between two horizontal electrodes at a frequency of 0.1 Hz. The underground electric field variation of vertical components has been observed by Fujinawa and Takahashi using a bore-hole antenna at VLF and ULF ranges. Antennas on the ground have been utilized to detect the radio waves by several investigators. In contrast, Enomoto and Hashimoto have observed the earth current of highly fluctuating components, effectively at frequencies higher than 1 MHz. This observation is related to laboratory experiments as described above. Figure 5 shows a typical record in which anomalous signals appeared before a magnitude 4.7 earthquake that occurred near Tsukuba, northeast of Tokyo.
Fig. 5 Electric charge signal as measured 10 - 20 May 1993. An earthquake with Ms =5.4 and with an epicenter at (36 03'N, 139 54"E) occurred at a focal depth of 61 km at 11:36 JST on  21 May 1993. AESs indicate anomalous electric signals appeared prior to the earthquake.
All of the methods described above have detected anomalous electromagnetic signals (AES) associated with earthquakes. It should be noted that the duration and the time lag of the anomalous signals that have been detected, are very similar to those of the SES observed by VAN method. At present, however, no seismic anomalous signals have been detected simultaneously on more than one instrument, including VAN. A programme to examine in more detail the comparability of our method with that of VAN and to better understand the nature of SES and SEMS will be carried out in Japan.

Generation and propagation mechanisms of SEMS
The observations described above are related to a working hypothesis regarding the propagation/generation mechanisms of seismic electro-magnetic activities; so, the conditions may be better understood by comparing these working hypotheses. This will probably clarify VAN approach.

In general, there are three ways that the seismic electromagnetic disturbances generated in the focal area are propagated through crust media: 1) electric conduction current, 2) electromagnetic wave propagation, and 3) electrostatic induction or electromagneticinduction.

Professor Varotsos argues for a piezostimulated transient electric current generation, and propagation as electric current through a conducting channel of the earth's crust, which falls into category 1). Fujinawa and Takahashi's observation is related to an argument that radio waves having frequency below 3 kHz suffer less attenuation when they are propagated through the earth's crust. The ground surface may be transiently electrified after the transmission of some seismic electromagnetic disturbance from the focal zone. Then, during the discharge process, fast fluctuations of electric charges at the ground surface may occur. We can probably detect such a highly fluctuating electric charge disturbance.

To date, various models have been proposed to explain the generation mechanism of SES and SEMS. They can be classified into two categories:
1) Mechanical interaction

1-a) piezoelectricity

1-b) piezo-stimulated transient current

1-c) stress-induced movements of charge dislocations or charge carriers.

2) Physicochemical interaction

2-a) electrokinetic effects of fluid flow,

2-b) gas electrification or discharge due to thermally stimulated exoelectron emissions from new cracks.

It should be noted that SES detected by the VAN method have always appeared during the preseismic period. The lack of coseismic SES suggests that the main mechanism should be attributed to physicochemical interactions rather than mechanical interactions.

The mechanism of 2-b) has not been discussed in detail to date and therefore it is discussed briefly in the following. Over a long period of geologic time, rocks have been subjected to irradiation by the a, b, g rays from decaying radioactive species such as 238U. Lone pairs of electrons produced by irradiation are often trapped in lattice defects or at the impurities in minerals. When such rock is thermally stimulated, electrons at higher than critical temperature are emitted from the trapped site. This is called thermally stimulated exoelectrons emission (TSEE). The peak temperature for TSEE of granite is about 400 °C. At the depth near the focal zone, the environmental temperature is probably slightly below the peak temperature. Therefore, stimulation of the trapped centers by energy corresponding to temperature changes of, say, fifty or more degrees centigrade can generate TSEE, which may electrify the gas molecules trapped in cracks or pores. Violent gas flows are induced by the pressure difference of coalescing cracks in the precursor stage to an earthquake and the electrical potential difference generated in the gas environment causes abrupt electric discharges. When the final slip occurs, these differences of the electric potential disappear. Therefore, SEMS is mostly preseismic.

Summary
It is evident that the VAN method opens the door for the short-term earthquake prediction, although there is still a dispute on the statistical assessment on VAN'S prediction. Further studies are needed to develop an acceptable model for SEMS and then look for more sensitive methods to detect imminent seismic precursor signals.

 




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