Spectral peaks in electric field at resonance frequencies for seismically excited motion of ions in the Earth’s magnetic field
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2011
Received: 17 September 2010
Accepted: 25 March 2011
Published: 7 September 2011
The Erratum to this article has been published in Earth, Planets and Space 2015 65:6500100057
Clear electric field variations have been observed during the passage of seismic wave. The most notable feature is circular polarization of electric field and it has been interpreted as reflecting circular motion of ions in groundwater under the Earth’s magnetic field. We have called such a mechanism ‘seismic dynamo effect’ but more convincing evidence is required to support this mechanism. Here we show strong evidence in terms of transfer function relating the electric field to the ground velocity due to seismic wave. We estimate transfer functions for electric field and ground velocity records for five aftershocks after the occurrence of M 6.9 earthquake. Then some peaks in the transfer function are found at specific frequencies where the resonance of ion motion in groundwater with the Earth’s magnetic field is expected. This result clearly supports the mechanism, which provides a method for in-situ estimation of ion contents in groundwater.
There are many studies on physical or chemical mechanisms to interpret electric field variations associated with earthquakes. Even for electric field variations associated with the passage of seismic wave, there are many examples (e.g. Thompson and Gist, 1993; Butler et al., 1996; Mikhailov et al., 1997; Beamish, 1999; Garambois and Dietrich, 2001; Haines et al., 2007) as well as theoretical studies (e.g. Pride and Morgan, 1991; Gershenzon, 1992; Pride, 1994; Pride and Haartsen, 1996; Haartsen and Pride, 1997; Garambois and Dietrich, 2002; Huang, 2002; Ren et al., 2010a, b). Appearance of unambiguous electric field changes was mostly synchronized with the arrival of seismic wave (Yamada and Murakami, 1982; Honkura et al., 2000, 2004; Mogi et al., 2000; Nagao et al., 2000; Matsushima et al., 2002; Ujihara et al., 2004). Recently we proposed a new mechanism for electric field generation by seismic wave or more specifically by ground motion, based on some observational examples (Honkura et al., 2009). Our claim is unique in that the Earth’s magnetic field plays an essential role in generation of electric field, and hence we named the underlying mechanism as ‘seismic dynamo effect’ in an analogy with the dynamo theory for generation of planetary magnetism (e.g. Takahashi et al., 2005).
One peculiar observational feature is circular polarization of horizontal electric field variation and we succeeded in explaining this feature in terms of resonance-like motion of ion in groundwater under the Earth’s magnetic field. In particular, left-handed (counterclockwise) circular polarization, if seen towards the magnetic lines of force, is found to be associated with anions with negative electric charge. Similarly, right-handed (clockwise) circular polarization is associated with cations with positive charge. The magnitude of generated electric field is obviously proportional to the magnitude of ground velocity. It also depends on the number density of ions in groundwater. However, the most striking feature is the resonance-like behavior.
2. Mechanism of Electric Field Generation
In our model, ions motion is excited only on the plane perpendicular to the direction of B0, unless additional conditions such as the boundary conditions are imposed. Hence we consider the two components on the plane and rotate the coordinate system, (x,y,z), by 90° – I counterclockwise around the y-axis, where I is the inclination of the Earth’s magnetic field (about 50° at the observation sites in this letter). Hereafter we denote the rotated system as X and Y (same as y) with new Z corresponding to the direction of B0.
3. Modification of Transfer Function
We in fact found some peaks in transfer function estimates, as expected from resonance-like behavior, but the resonance frequencies for a few kinds of ion in groundwater turned out to systematically shift towards lower frequencies, as will be shown later. There must be a factor to be further considered in our model formulation. So far we have not considered any boundary conditions which may arise in the real Earth. The most likely condition is that the vertical component of electric field should vanish at the Earth’s surface because the Earth is conducting. This condition implies that no vertical electric field should arise at the Earth’s surface, or the vertical motion of ions must be prohibited near the Earth’s surface. In view of this argument, we replace B0 with B0 sin I (= B0z) and υ0X with υ0X sin I as the x component of ground velocity.
4. Results of Analyses
To record temporal variations of the ground velocity and the electric field simultaneously for aftershocks of the M 6.9 mainshock which occurred on March 25, 2007 in central Japan, we installed a short-period seismometer of velocity type and two pairs of Pb-PbCl2 electrodes. The separation of electrodes was 25 meters in both the NS and EW directions. The sampling frequency was 200 Hz.
The absolute values of stacked transfer function estimates are shown in Fig. 2(c). It is clear that the pair of |Z11| and |Z21| is much larger than the pair of |Z12| and |Z22| for some reason and hence the former is better determined. We can see clear three peaks in |Z11| and |Z21| near the resonance frequencies for ,Cl− and Na+. It should be noted here that the resonance frequencies are shifted to lower frequencies, corresponding to the vertical component of the Earth’s magnetic field. The theoretical resonance responses, with the λ value adjusted to data, for the resonance frequency of 9 Hz for are also shown for |Z11| and |Z21|. The theoretical response is steep at the resonance frequency whereas the observed response is gradual. This is due partly to the smoothing operation, for example band averaging, inherent in spectral analysis and also partly to the fact that the ground velocity never keeps an exact resonance-frequency variation for a sufficiently long time so that the steady resonance state, as presumed in theoretical treatment, is never realized.
In the case of aftershocks shown above, the ground velocity is drastically reduced at frequencies higher than 30 Hz or so. In this respect, we already found that higher frequency components are available for the electric field and the ground velocity for artificial earthquakes generated by blasting (Honkura et al., 2009). Recently we made some observations in occasions of blasting and hence we also tried to estimate transfer functions for these records. It should be noted, however, that stacking essential for statistical properties is not possible in these cases and hence accuracy of transfer function estimates is worse than the case of aftershocks. Also, a higher sampling rate is required for transfer function estimates at higher frequencies. Recently, we introduced a data logger with 1 kHz sampling, which turned out to be sufficient for frequencies lower than 40 Hz, for example.
5. Discussion and Concluding Remarks
In all the cases examined above, |Z11| and |Z21| turned out to be dominant compared with |Z12| and |Z22|. In the theory proposed in Honkura et al. (2009), such anisotropy does not appear and hence there should be another factor to be considered in the theory. However, the present discussion on resonance-like response is little to do with this problem of anisotropy.
One may claim that the transfer function estimates should be clearer if the proposed mechanism is really in operation in groundwater. We admit such a claim but at the same time we point out one difficulty in the transfer function analysis which relates the two components of outputs (electric field) with the two components of inputs (ground velocity). One assumption is implicit in this treatment; two components of inputs are independent of each other. In reality, three components of ground velocity are somewhat correlated with each other and hence the x and y components are also correlated with each other to some extent. This problem would be overcome by stacking of many estimates, because correlations may change from one record to another. In fact, in the case of aftershocks, even stacking of five examples appreciably improved the quality of transfer function estimates. Such improvement due to stacking is in fact well known in magnetotellurics (MT).
We may now conclude that the appearance of peaks in the transfer function at the resonance frequencies, for , Cl− and Na+ contained in groundwater, confirms the basic mechanism of electric field generation in association with the passage of seismic wave. Peak values of transfer function are indicative of relative amounts of respective ions at the site. In this sense, the mechanism is expected to provide a sort of in-situ ion analysis for groundwater.
We thank Professor Hiroshi Sato of Earthquake Research Institute, University of Tokyo, for providing us with the opportunity of our work during the blasting experiments. We also thank Japan Petroleum Exploration Co., Ltd. for providing us with an opportunity for our work during its seismic exploration. We are grateful to Dr. K. Yamazaki and an anonymous reviewer for their useful comments. This study was supported by the Japan Society for Promotion of Science (JSPS) under grant-in-aid for scientific research No. 21340126.
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