Notes on Generation and Propagation of Seismic Transient Electric Signals
Mechanical Engineering Laboratory, Agency of
Industrial Science and Technology, MITI Namiki 1-2,Tsukuba, Ibaraki 305
Japan c signals
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
experiments on fracture-induced electric signals
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).|
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.
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
· coarse-grained granite and
o fine- grained granite in their natural state.
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.
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.
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
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.|
hypotheses for various observations of SEMS
|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.|
and propagation mechanisms of SEMS
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.
date, various models have been proposed to explain the generation
mechanism of SES and SEMS. They can be classified into two categories:
2) Physicochemical interaction
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.
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