Barometric and magnetic observations of vertical acoustic resonance and resultant generation of field-aligned current associated with earthquakes
© 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. 2013
Received: 2 October 2012
Accepted: 8 February 2013
Published: 17 September 2013
Three rare occasions are introduced, where the excitation of vertical acoustic resonance between the ground and the ionosphere, and the resultant generation of a field-aligned current, just after earthquakes are observationally confirmed. In the case of two inland earthquakes, barometric observations very close to the epicenters (i.e., only 30 km apart) were available, and they showed a sharp spectral peak which appeared within one hour after the origin time and lasted a few hours. The observed periods of the spectral peaks around 260 seconds are close to the period of the theoretically-expected fundamental mode of the resonance. On the other hand, magnetic observations on the ground showed a dominant period at 220–230 seconds which corresponds to the first overtone among theoretically-expected major resonance peaks. In the third case, i.e., during the 2010 Chile earthquake, a long-period magnetic oscillation in the east-west direction, which has two major resonance periods at 265 and 190–195 seconds, was observed on the night-side magnetic dip equator in Peru, where the distance is more than 2600 km from the epicenter, under a very quiet geomagnetic condition. The oscillation was interpreted as the effect of field-aligned current generated through a dynamo process in the ionosphere over the epicenter caused by the resonance.
During the 2004 Sumatra earthquake, a peculiar Pc5 magnetic pulsation and GPS-TEC pulsation were observed about 12 minutes after the origin time in the north-east part of Thailand, Phimai, where the distance from the epicenter was about 1500 km (Iyemori et al., 2005; Choosakul et al., 2009). Because both phenomena have a sharp spectral peak at about 4 minutes (to be exact, 3.6 minutes), they were explained as the results of vertical acoustic resonance between the ocean surface and the ionosphere, excited by the sudden vertical movement of the ocean surface at the epicenter. In fact, a similar oscillation in the GPS-TEC data has been observed during the 2011 Off the Pacific Tohoku Earthquake (Saito et al., 2011; Tsugawa et al., 2011), and a numerical simulation study on atmospheric gravity waves, including the acoustic mode, strongly supports the excitation of vertical acoustic resonance (see Matsumura et al., 2011).
Although vertical acoustic resonance is the most plausible mechanism of the 4-minute oscillation observed after the earthquakes, there has been no direct evidence of acoustic resonance by atmospheric pressure observations. As for the case of volcanic eruption, there exist some reports on atmospheric pressure oscillation with a long period. During the 1991 Mt. Pinatubo eruption, a long-period seismic wave having a period about 4 minutes (to be exact, 3.7 minutes) was observed worldwide, and the phenomenon was explained as the effect of vertical acoustic resonance (Kanamori and Mori, 1992; Kanamori et al., 1994). However, there was no digital recording of atmospheric pressure near the volcano available for spectral analysis.
In this paper, we show the barometric data obtained at the sites close to the epicenter of the two inland type (i.e., caused by movement of an active fault) earthquakes that occurred in Japan, and compare their spectra with those of geomagnetic observations. The comparison strongly supports the excitation of vertical acoustic resonance over the epicenter just after the earthquakes, and the generation of electric currents in the ionosphere.
Evidence of the generation of a field-aligned current associated with acoustic resonance is also shown from magnetic observations in Peru during the 2010 Chile earthquake.
2. Method to Identify the Vertical Acoustic Resonance
After the 2004 Sumatra earthquake, we began microbarometric observations at stations in Japan and outside of Japan. The pressure sensor used is the Vaissala PTB210 and the 1-second-averaged data from 490 samples/s A/D output have been recorded. The actual resolution of the pressure variation is about 1 Pascal.
In general, the pressure variation caused by the acoustic mode atmospheric waves is smaller than that of internal gravity waves having a longer period (i.e., typically more than 6 or 7 minutes) in the lower atmosphere, and it is not easy to find out from the time series plot of the pressure data of our observations because of the limitation of resolution and low S/N ratio. However, it is possible to detect by spectral analysis with high-frequency (period) resolution, because the vertical acoustic resonance has 3 major resonance peaks theoretically well defined (calculated) with a realistic atmospheric model. That is, it is well established by several authors that the major 3 resonant peaks exist at around 260–270 seconds, 220–230 seconds, and 190–200 seconds (e.g., Tahira, 1995; Lognonne et al., 1998; Nishida et al., 2000; Shinagawa et al., 2007) although the period may vary for some extent depending on the atmospheric model, boundary condition, etc. If one or more spectral peaks close to the expected period of resonance appear just after the origin time of an earthquake with an amplitude larger than that of the normal condition or that before the earthquake, we may conclude with a high probability that this indicates the excitation of the resonance by the earthquake. To analyze this with high-frequency resolution, we adopt the MEM (Maximum Enthoropy Method) in this paper.
From the numerical simulation study by, for example, Shinagawa et al. (2007) or Matsumura et al. (2011), the region of vertical acoustic resonance in the initial stage is limited to the area over the epicenter, and it is expected that the barometric variation associated with the resonance can be observed in the area close to the epicenter, say within 100 km depending on the scale of the earthquake. On the other hand, the ionospheric current effect, i.e., the magnetic variation, may be observed over a wider area because the electric current circuit can extend outside the resonance region by the return current and/or the field-aligned current, as discussed in a later section.
After we started barometric observation in 2006, two inland-type earthquakes occurred with rather a large magnitude near the barometric observation sites, and the distance between the epicenters and the observation site was only 30 km or less. Because of the sparse barometric observation sites, these two cases are very rare events.
In the paper by Iyemori et al. (2005), an ionospheric dynamo process, and associated field-aligned current generation by the resonance, were proposed, and the Pc5 magnetic pulsation observed in Thailand just after the Sumatra earthquake was explained as the effect of an ionospheric current system with a field-aligned current. Since the conductivity parallel to the magnetic field is very high, the polarization electric field is easily mapped to the conjugate points, and thus a part of the return current flows as the field-aligned current and closes through the opposite hemisphere. The
same process is expected to occur in general, including the case of inland-type earthquakes. Therefore, we also analyze geomagnetic data observed on the ground. To show the existence of a field-aligned current generated by the dynamo process, an event associated with the 2010 Chile earthquake which occurred off the coast of central Chile on February 27, 2010, at 06:34 UTC, having a magnitude of 8.8 on the moment magnitude scale, is also introduced.
3.1 2008 Iwate-Miyagi Nairiku earthquake
An inland earthquake with magnitude 7.2 occurred on June 13, 2008, 23:43 UT. The epicenter was at (39.0N, 140.9E) and the depth was 8 km. As shown in Fig. 1, the epicenter was close to the NAO/Mizusawa (MIZ) observatory at (39.11N, 141.20E) where a superconducting gravimeter was running. The distance between the epicenter and the site MIZ is only about 30 km. The gravity measurement itself was stopped by the earthquake; however, a barometer attached to the gravimeter continued the measurement. It had a similar sensitivity as our barometric observations at several sites, including Shigaraki used in this paper. In this section, we analyze the MIZ barometric data and geomagnetic data obtained at Memambetsu (MMB) at (43.91N, 144.19E) and the Kakioka (KAK) magnetic observatory at (36.23N, 140.19E). The time resolution of these data isone second, and they are high-pass filtered with a cutoff period around 10 minutes, which is not very sharp because of the Gaussian type filter. After the filtering, a MEM (Maximum Entropy Method) with a lag of 1200 seconds is applied for each 1-hour interval, shifting 30 minutes, to get the power spectral density (PSD) with a high frequency (period) resolution to identify the resonance period. The PSD is calculated at every period with a 5-second interval. The same filter and MEM parameters are applied for other data used in this paper.
The peaks at 265 and 275 seconds in the magnetic data may also be the fundamental resonance period. However, they slightly differ from that of the pressure data, i.e., 260 seconds. This difference suggests that the location of the resonance which caused the magnetic variation at Memambetsu was different from that of the pressure data observed near the epicenter. This point is mentioned again in the Discussion section.
3.2 2007 Mieken-Chubu earthquake
An inland earthquake with magnitude 5.4 occurred at 03:19 UT on April 15, 2007. The epicenter was at (34.8N, 136.4E) and the depth was 10 km. One of our barometric observation sites, Shigaraki (SGA) at (34.85N, 136.11E), was very close to this epicenter. The distance between the epicenter and the site SGA was only about 30 km.
3.3 2010 Chile earthquake
One remarkable characteristic is that the oscillation is observed only in the D (declination) component, i.e., in the East-West direction at Huancayo, although a smaller oscillation is seen also in the Z (vertical) component at Ancon. Because of the local time before dawn, under very quiet geomagnetic activity as seen in the AE indices, i.e., hourly AE index values of about 30 nT or less (see http://wdc.kugi.kyoto-u.ac.jp/dstae/index.html) with the lowest Kp index, 00, and under a rather stable solar wind condition, i.e., a solar wind speed around 320 km/s and a density around 4.5 protons/cm3 (see http://omniweb.gsfc.nasa.gov/form/omni_min.html), the possibility of a magnetospheric disturbance, such as a substorm onset or direct solar wind disturbance effect, is very low. It is also not plausible that the oscillation comes from the sensor oscillation caused by low-frequency seismic waves, such as the Rayleigh or Love waves, because of the much longer period (i.e. about 4 minutes) than that of the Rayleigh or Love waves (i.e., their major period is normally shorter than 30 seconds or at most 1 minute or so—e.g. Selby et al., 2005). This is also confirmed, for example, by a seismometer record at Tungurahua, Ecuador.
4. Summary and Discussion
The emergence of clear spectral peaks both in the barometric data near the epicenter, and in the geomagnetic data, strongly supports the causal relationship between the earthquake and the excitation of the vertical acoustic resonance near the epicenter. The peaks have a period close to the major modes of the resonance that has been predicted theoretically (e.g., Tahira, 1995; Lognonne et al., 1998; Nishida et al., 2000; Shinagawa et al., 2007), and their magnetic effects, through the ionospheric dynamo process, have also been expected since 2004 Sumatra earthquake (Iyemori et al., 2005).
As for the cases of the two inland earthquakes introduced in this paper, the sharp spectral peaks in the barometric data have a period close to the fundamental mode (i.e., at 265 s and 250 s, respectively), although the main spectral peaks in the geomagnetic data have periods that are close to other resonance periods around 220–230 s (i.e., the first overtone) or 190 s (i.e., the second overtone) rather than that of the fundamental mode. This tendency was also observed during the 2004 Sumatra earthquake, where the first overtone (3.6 minutes) dominated in the magnetic data during the initial one or two hours. The spectral peaks of the barometric variation at the 2008 Iwate-Miyagi Nairiku earthquake show the first overtone (at 210 s) and the second overtone (at 175 s) during the interval which is 30 minutes earlier than the period when the highest peak appears in Fig. 3 (green line), and these peaks are consistent with the magnetic signature indicated in Figs. 4 and 5 although there exists a small (i.e., about 10 seconds) difference in the period.
The periods of the resonance peaks of the magnetic field variation are not necessarily the same with those of the pressure variation, as seen in the case of the 2008 Iwate-Miyagi Nairiku earthquake. That is, as seen in Fig. 3, the pressure data shows the fundamental mode with a period of 260 seconds. On the other hand, the Memambetsu geomagnetic data show the spectral peaks at a period of 275 or 265 seconds. The difference suggests that the resonance occurs not only over the epicenter, but also at a different location with a slightly different resonance period as the seismic wave, or the atmospheric gravity waves, propagate from the epicenter.
To investigate the cause of the difference, we need a collocated observation of magnetic field and pressure variation as well as multi-points observation.
On the other hand, the 2010 Chile earthquake occurred-before dawn and the PSD of the geomagnetic oscillation observed in Peru (HUA and ANC) shows spectral peaks at 265 s and 190–195 s and no peak at 220–230 s. This difference could be explained as the altitude difference of the dynamo layer between the dayside and nightside. That is, the mode having a large amplitude at the ionospheric dynamo layer, which is around 110–120 km on the dayside and a slightly higher altitude on the nightside, could dominate. To confirm this, and also to solve the problem of the small difference in the period mentioned in the previous paragraph, the height dependence of resonance oscillation and horizontal movement of the resonance region (or horizontal propagation of the acoustic/internal gravity waves) should be simulated with a realistic model.
For the 2010 Chile earthquake, the closure depicted in Fig. 11(a) may be the case because it happened on the night-side. The tracing of geomagnetic lines of force with the IGRF model indicates that the geomagnetic line of force from 130 km above the epicenter passes the longitude and altitude at 71.5W and 1132 km, respectively, on the magnetic dip equator. That is, it is 3.8 degrees east of Huancayo and 5.7 degrees east of Ancon. Taking into account the resonance region with a diameter of about a few hundred km, the Huancayo observatory could be under the west side field-aligned current depicted in Fig. 11(a), where no vertical magnetic effect appears. On the other hand, the Ancon observatory could be on the west of the west side field-aligned current, and, hence, it could have a small oscillation also in the vertical component. The phase relation between the D (eastward positive) and the Z (downward positive) of the variation at Ancon shown in Fig. 9(b) has a positive and linear correlation, which is consistent with the above explanation.
For the two inland earthquakes, the closure depicted in Fig. 11(b) may be the case, because they occurred on the dayside where the Hall conductivity is larger than the Pedersen conductivity. On the dayside, the ionospheric Hall current which circulates in the ionosphere mainly contributes to the magnetic variation on the ground (e.g., Tamao, 1986; Fujita and Tamao, 1988). For the current closure of the Fig. 11(b) type, the primary Hall current in the north-south direction closes the current circuit through the field-aligned current. On the other hand, the Hall current which closes in the ionosphere flows mainly in the east-west direction by the polarized electric field in the north-south direction. This polarized electric field is caused by the primary Hall current in the north-south direction driven by the dynamo electric field in the east-west direction, because the dynamo region is limited both in latitude and longitude and the magnetically-conjugate ionosphere in another hemisphere has a finite resistivity. This polarized electric field drives the secondary Hall current in the east-west direction (not shown in Fig. 11(b)). The resultant magnetic disturbance is in the north-south direction on the ground, i.e., in the H component, because the magnetic field caused by the ionospheric current in the north-south direction is mostly cancelled by the field-aligned currents (Fukushima, 1976). On the other hand, the magnetic field caused by the secondary Hall current is not cancelled and observed on the ground. As seen in Figs. 4, 5, and 8, the tendency that the resonance peaks appear mainly in the H component rather than in the D component is consistent with this expectation.
These qualitative interpretations should be confirmed quantitatively with a three-dimensional realistic numerical simulation of atmospheric gravity waves, including the ionospheric dynamo process, in future, as well as with direct observation by low-altitude magnetic satellites such as CHAMP or SWARM.
The geomagnetic data from Memambetsu and Kakioka were provided by the JMA/Kakioka Magnetic Observatory. The barometric data at Mizusawa were provided by the NAOJ/Mizusawa VLBI Observatory. Barometric observations at Shigaraki are maintained by the RISH/Shigaraki MU Radar Site, Kyoto University. This study was partly supported by the 2011 Kyoto University “Core Stage Backup” program and the 2012 Kyoto University “Zengaku Keihi” program.
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