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Simultaneous infrasonic, seismic, magnetic and ionospheric observations in an earthquake epicentre
Journal of Atmospheric and Solar-Terrestrial Physics 72 (2010) 1231–1240
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Simultaneous infrasonic, seismic, magnetic and ionospheric observations in an earthquake epicentre ˇ ´ rˇova´ a, J. Hora´lek b, J. Zednı´k b, V. Krasnov c J. Laˇstovicˇka a,n, J. Baˇse a, F. Hruˇska a, J. Chum a, T. Sindela a
Institute of Atmospheric Physics, ASCR, Bocni II, 14131 Prague, Czech Republic Institute of Geophysics, ASCR, Bocni II, 14131 Prague, Czech Republic c Saint Petersburg, Russia b
a r t i c l e in fo
abstract
Article history: Received 25 March 2010 Received in revised form 2 August 2010 Accepted 3 August 2010 Available online 14 August 2010
Various pre-seismic and co-seismic effects have been reported in the literature in the solid Earth, hydrosphere, atmosphere, electric/magnetic field and in the ionosphere. Some of the effects observed above the surface, particularly some of the pre-seismic effects, are still a matter of debate. Here we analyze the co-seismic effects of a relatively weak earthquake of 28 October 2008, which was a part of an earthquake swarm in the westernmost region of the Czech Republic. Special attention is paid to unique measurements of infrasonic phenomena. As far as we know, these have been the first infrasonic measurements during earthquake in the epicentre zone. Infrasonic oscillations ( 1–12 Hz) in the epicentre region appear to be excited essentially by the vertical seismic oscillations. The observed oscillations are real epicentral infrasound not caused by seismic shaking of the instruments or by meteorological phenomena. Seismo-infrasonic oscillations observed 155 km apart from the epicentre were excited in situ by seismic waves. No earthquake-related infrasonic effects have been observed in the ionosphere. Necessity to make vibration tests of instruments is pointed out in order to be sure that observed effects are not effects of mechanical shaking of the instrument. & 2010 Elsevier Ltd. All rights reserved.
Keywords: Earthquake Infrasonic effects Ionospheric effects Magnetic effects
1. Introduction Earthquakes excite various effects in the solid Earth, hydrosphere, atmosphere and even ionosphere. Some effects have also been observed prior to earthquakes and some authors have been trying to use them as earthquake precursors. Here we will only deal with co-seismic effects, not with possible earthquake precursors. The ground-based electric and magnetic measurements and model calculations detect earthquake-related changes in geomagnetic pulsations and electric fields (e.g., Iyemori et al., 2005; Meloni et al., 2001; Sorokin et al., 2005). In the hydrosphere, tsunamis are excited by earthquakes. Oceanic waves are known to excite infrasound (e.g., Arendt and Fritts, 2002; Daniels, 1962). In the atmosphere, acoustic, infrasonic and probably shortperiod gravity waves are excited near the surface and propagate both upwards and horizontally (e.g., Arrowsmith et al., 2009; Che. et al., 2007), sometimes changes of surface atmospheric temperature are observed (e.g., Hayakawa et al., 2004), atmospheric anomalies of VHF radio wave propagation appear (e.g., Fujiwara et al., 2004), and radon emanation affects lower atmosphere ionization and, thus, the global electric circuit (e.g., Pulinets,
n
Corresponding author. Tel.: +420 267103055. E-mail address: [email protected] (J. Laˇstovicˇka).
1364-6826/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2010.08.005
2007; Pulinets and Dunajecka, 2007; Omori et al., 2009). In the ionosphere, various effects in electron density, total electron content, electron temperature and electromagnetic waves have been observed (e.g., Bhattacharya et al., 2009; Calais and Minster, 1995; DasGupta et al., 2006; Hobara and Parrot, 2005; Liu et al., 2006; Lognonne et al., 2006; Nˇemec et al., 2009; Oyama et al., 2008; Pulinets, 2007). Some of them are associated with the acoustic-gravity waves including infrasound (e.g.., Afraimovich et al., 2001; Artru et al., 2004; Kherani et al., 2009; Choosakul et al., 2009). The Cheb region in the westernmost part of the Czech Republic (Central Europe) was hit by a relatively intense earthquake swarm in October 2008. Earthquake swarms of comparable intensity appear in this region typically once per 15–25 years (Hora´lek et al., 2009). Seismic station Novy´ Kostel (NKC; 50.231N, 12.451E) is located just in the epicentral area of this earthquake swarm. Another seismic station of a local network is in the nearby location Studenec (STC; 50.261N, 12.521E), several kilometres apart from NKC. Seismometers of different frequency responses, acoustic sensors, microbarograph and magnetic field measurements were deployed at NKC station, some of them permanently (e.g., seismometers), and some of them temporarily (e.g., microbarographs), all located quite close each other. Some instruments (seismometer and about 90 m separated microbarograph) were deployed also at STC. Fig. 1 shows positions of these two stations
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with respect to the epicentre of the studied earthquake. One ionospheric Doppler sounding path is located quite nearby (quasivertical path with reflection point about 10 km from the NKC station). Here we shall analyze the event of 28 October 2008 with special attention paid to infrasonic phenomena recorded by microbarographs. As far as we know, these have been the first infrasonic measurements during earthquake in the epicentre zone. Section 2 provides basic seismic information about the swarm and event of 28 October 2008. Section 3 presents basic meteorological information necessary for evaluation of infrasonic measurements. Section 4 is the key part of the paper; it describes infrasonic measurements by microbarographs and their interpretation. Section 5 deals with ionospheric effects of seismoinfrasonic bursts, and Section 6 with magnetic and electric measurements. Paper is closed by short conclusions in Section 7.
Fig. 1. Positions of the epicentre and the two nearby stations (microbarograph + seismograph at each station).
2. Basic seismic information The westernmost part of the Czech Republic is an exceptional European region due to the recent geodynamics expressed mainly by a periodic occurrence of intraplate earthquake swarms and a high fluid activity in the upper crust. The most recent known volcanic activity occurred there about 0.3–0.5 Ma ago. It is very likely that crustal fluids, which are of a post-volcanic origin, play an important role in triggering and driving earthquake swarms in the region concerned. The 2008 swarm ranks among the most intensive local earthquake activities. It took place in the Novy´ Kostel focal area and lasted 28 days. In this course more than 20,000 events were recorded by stations of the seismic network Webnet (Hora´lek et al., 2009). The central station of WEBNET, Novy´ Kostel (NKC), is located just in the epicentral zone. The swarm was exceptional by its rapidity. It started on October 6, the most intensive phase occurred already in the evening of October 9 and lasted two days; this phase included three shocks of magnitude ML 43.5 (stronger than 3.51 on the Richter scale). Then, a fairly intensive swarm activity persisted till October 15. During this time, a pronounced swarm phase included a strong shock in October 12 with the magnitude of ML E4.0). The period between October 15 and 27 was characterized by a gradual swarm activity decay, which was broken by an abrupt swarm activity increase dominated by a shock of ML E 4.0, which produced the greatest ground motions in the epicentral area (Hora´lek et al., 2009). Hypocenters were located at the depths of 6.5–10.5 km (shallow earthquakes) on a fault segment of 12 km2 in the area near to the termination of the Maria´nske´ La´znˇe fault. Very rough estimates of the size of the fracture surface and of the slip along this surface for the strongest earthquakes of ML E 4.0 are 600 600 m2 and 0.15 m, respectively. Broadband seismometers have been installed at NKC and STC ¨ stations, NKC—Guzalp-40 T (80–0.033 Hz), STC—CM-3 (80– 0.5 Hz), respectively. They measure the ground velocity, not displacement, in units of mm s 1.
Fig. 2. Seismic record from station NKC, event onset at 08:30:11 UT. Top—vertical component; middle—N–S component; bottom—E–W component. Numbers in the upper right corner of each component record are values of the maximum observed amplitude for the given record: 1.9 mm s 1 for vertical component, 2.6 mm s 1 for N–S component, 4.7 mm s 1 for E–W component.
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We shall deal in more detail with the earthquake of 28 October, 08:30:11 UT, M¼3.6, hypocenter depth 7870 m, when other measurements are available. As Fig. 2 shows for NKC, the event begins with much larger vertical amplitude, because the event onset is the P-wave almost in the epicentre.
Table 1 Results of vibration tests of microbarograph at TIRA Vibration controller SVC01 for vertical vibrations. Arrangement of measuring system (sensor and input hose), speed of vertical motion and the measured amplitude of pressure pulses are shown. Sensor and input hose
3. Meteorological information Microbarographs measuring infrasound in the atmosphere have anti-wind shielding, nevertheless strong winds could affect the infrasonic measurements and, moreover, a strong wind in hilly terrain covered partly by forest excites local infrasonic oscillations, which could interfere with and/or overlap the infrasonic oscillations of seismic origin. The general meteorological situation on October 28, 2008 was formed by a very slowly moving weather front over central Europe with a pressure field with small pressure gradients, i.e. general conditions which do not support strong winds in the whole region. The local meteorological station Cheb reported weak winds of 1–2 m/s, temperatures 5–7 1C, sea-level pressure oscillating around 1010 hPa, cloudy to overcast and weak discontinuous rain. These meteorological conditions do not make possible the excitation of local infrasonic oscillations/noise of meteorological origin. Also other meteorological stations in the western part of the Czech Republic reported only weak winds (www.ogimet.com/synops.phtml.en). Microbarograph measurements before the earthquake displayed microbaroms but very little other infrasonic noise. Thus infrasonic measurements have not been affected by local/regional infrasonic oscillations of meteorological origin.
4. Infrasonic measurements by microbarographs We use differential microbarograph—infrasound gage ISGM03 with an input hose (to reduce/remove effects of wind and rain) for transmitting acoustic signal to the inlet of sensor. The sensor measures the pressure fluctuations in the frequency range from 0.001 to 10(12) Hz. The pressure fluctuations are measured with a 0.006 Pa resolution in the operating range of 725 Pa related to a steady state value. The data are stored with 25 Hz sampling period into the internal memory with a capacity for 2 months and can be downloaded using an Internet connection. To minimize the influence of temperature variations on the measured values, the sensor is thermally isolated from the environment. It is necessary to mention that the frequency range of microbarographs is due to their use in the standard regime mainly as supporting measurements to ionospheric Doppler system measurements. The sensor itself has a shape of a cylinder. It consists of two chambers and a thin membrane. The pressure fluctuations cause displacements of a thin membrane, which is located between the two chambers. The first chamber has a direct entrance (inlet) to the ambient air. The second chamber is connected with the first one via a very narrow capillary tube. The time constant to balance the pressure inside the second chamber with the first one is 1000 s. The pressure fluctuations are thus measured as the displacement of the membrane from its central position. It is very important to be sure that pressure oscillations measured during an earthquake are real pressure oscillations, not effect of seismic shaking of microbarographs. Therefore the microbarographs were tested in the vibration test equipment TIRA Vibration controller SVC01 used for testing satellite instruments. Table 1 show in its upper part the results of vibration testing from January 2008 for vibrations in the frequency range of 10–12 Hz, i.e. at the upper boundary of the range of frequencies
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Test for f¼ 10–12 Hz Sensor vertical, closeda Sensor vertical, hose freeb Sensor vertical, opened, no hose Sensor horizontal, hose fixed c Sensor horizontal, hose fixedc
Speed of motion (mm s 1)
Pressure amplitude (Pa)
50 10 50 50 10
0.03 0.05 0.05 0.10 0.02
Microbarograph observations at NKC during earthquake (Fig. 2)—peak around 2–4 Hz Sensor vertical, hose fixedc 1.9 0.30 Test for f ¼3–6 Hz Sensor vertical, closedc Sensor vertical, hose fixedc
20 20
0.35 1.0
a Sensor can measure either in vertical or in horizontal position (input hose is essentially horizontal). b The input hose was free. c The input hose in real observations at NKC and STC was fixed to surface, in vibration tests it was fixed to vibrating plate.
observed (the vibration equipment was unable to produce lower frequencies). The sensor itself is quite insensitive to simulated seismic oscillations (the test on the first line of Table 1). The sensor with an input hose, when the input hose is not fixed, reveals a response substantially large than that for the case of the hose fixed to vibrating surface. However, it was necessary to test microbarographs at frequencies of their peak observed effect (2–4 Hz). For this purpose it was necessary to modify the test equipment TIRA Vibration controller SVC01 to allow tests at frequencies below 10 Hz, which was finished in March 2010. The tests were performed on 23 and 25 March 2010 at frequencies 10, 9, 8, 7, 6, 5, 4, 3, 2, 1 and 0.5 Hz. The bottom part of Table 1 shows the result for the peak of sensitivity of microbarographs at 3–6 Hz. The response within the range of frequencies 3–6 Hz was similar, whereas it was remarkably weaker at higher or lower frequencies. Tests were performed for the vertical installation of microbarograph (as at stations) and for vertical shaking, as observations show that horizontal seismic motions do not affect the microbarographs significantly; the effect of vertical seismic motion is quite dominant (see Figs. 3 and 5). To assure good accuracy of test measurements, the mechanical shaking was ten times as strong as the maximum vertical seismic signal. For the closed sensor the response for ten times stronger testing is the same as the observed signal (middle part of Table 1), that means that the sensor itself is only little sensitive to mechanical/seismic shaking. For the sensor with a fixed hose as at stations NKC and STC, the response to ten times stronger mechanical shaking is about three times as large as signal observed at NKC. Our test experiments show that the response is proportional to mechanical shaking; this means that the response to the mechanical shaking is three times weaker than the total response, which is thus predominantly of infrasonic origin. However the total response observed in shaking tests is a sum of response to mechanical shaking and infrasound excited by vibrating test equipment, i.e. the real response to mechanical shaking is even somewhat lower. Therefore we can assume that the observed oscillations are very predominantly a real infrasound excited by the earthquake, only rather little contaminated by earthquake shaking of microbarographs and input hoses. The vibration tests also showed that the response of the closed sensor itself is almost ten times smaller for
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Fig. 3. Infrasonic record at NKC on 28 October 2008. Pressure fluctuations in the frequency range of 0.1–12 Hz. Time 0 is 08:30:00 UT. First burst occurred at about 08:30:13, main burst near 08:30:15. Top panel–pressure fluctuations in the range 0.1–12 Hz. Bottom panel—wavelet spectrum of pressure fluctuations.
Fig. 4. Pressure fluctuations (essentially microbaroms) in the frequency range of 0.1–12 Hz, station NKC, 20 October 2008, 08:30:00-08:31:30 UT. Pressure scale is the same as in Fig. 3.
horizontal sensor installations than for vertical sensor installations, so in future earthquake-related measurements (if any) we should use the horizontal installation of microbarographs. On 14 October, a microbarograph was moved from its permanent position and installed at NKC, and a few days before the 28 October event another microbarograph was moved from its permanent position and installed at a nearby location Studenec (STC) several kilometres apart. Their distance from the 28 October earthquake epicentre and azimuth are: 2.33 km and 12.471, and 6.89 km and 41.941, respectively, i.e. the NKC microbarograph recorded infrasound almost in the epicentre. The third microbarograph remained at its permanent position at Panska´ Ves (50.531N, 14.571E), distance to epicentre 155.2 km, azimuth 76.521. The wavelet spectrum presented in the bottom panel in Fig. 3 shows for the main burst a large increase of power at frequencies between 2 and 12 Hz observed between 8:30:13 and 8:30:15 respectively. Origin of smaller bursts observed later at 2–4 Hz is also seismic; they all are associated with aftershock seismic activity, i.e. rather weak aftershocks of the main shock. The
waveform of acoustic signal presented in the upper panel shows that short-period earthquake-related oscillations are observed on the background of longer-period (4–5 s) oscillations called microbaroms, which are supposed to be produced by a nonlinear interaction of ocean waves with the atmosphere (e.g., Daniels, 1962; Posmentier, 1967; Arendt and Fritts, 2002). Fig. 4 illustrates for the same time of the day and eight days before the event under quiet seismic and regional meteorological conditions but with strong winds over Atlantic that the microbaroms are typical dominant source of infrasonic noise, being present almost continuously. However, their periods are much longer than the periods of infrasonic oscillations of seismic origin and their amplitudes are substantially smaller, hence they do not interfere with the observations of infrasound of seismic origin. A good coincidence of the main infrasonic and seismic oscillations is well visible in Fig. 5, namely for the vertical seismic component (red line). The coincidence with horizontal components is much worse; the main green peak just before 34 s in Fig. 5 and the strong blue and green peaks just before 36 s have no corresponding response in the infrasonic magenta line. The vertical
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Fig. 5. Joint infrasonic and seismic record for station NKC, 28 October 2008, time ¼0 at 08:29:40 UT (starting time t ¼ 30 corresponds to 08:30:10). Bold magenta—infrasound; red—vertical seismic component; blue—N–S seismic component; green—E–W seismic component. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
motion of the surface during the earthquake excites infrasonic waves much more efficiently than horizontal motion, as expected. In case of ideally flat surface, only the vertical motion can make alternating compression and depression of the atmosphere. Fig. 5 reveals a positive correlation between seismic vertical and infrasonic oscillations that are in phase. An upward seismic motion is accompanied by an increase in atmospheric pressure measured by the microbarograph, and a downward motion with a decrease in pressure. This means that the upward surface motion compresses the atmosphere, and the downward motion rarefies the atmosphere. If the observed infrasonic signal is caused by the upward (downward) seismically-induced motion of the microbarometer, the sign of the phase would be opposite as the microbaromter is moved towards less dense (or denser) air. This is other evidence in support of the seismic wave origin of the observed infrasonic signals. Simultaneous seismic and microbarograph measurements at NKC allow consider the behaviour of the seismic-pressure transfer function. Both measurements and the theoretical transfer function calculations by Watada et al. (2006) reveal for a strong earthquake observed in Japan and infrasonic measurements a few hundred kilometres apart an approximately constant transfer function (independent of frequency) for periods lower than 40–50 Hz. Our seismic event is much weaker; we measure practically in the epicentre and with different type of microbarograph. Nevertheless Fig. 6 reveals also roughly constant ratio (at least after some smoothing) of the seismic power (top panel) to the pressure (infrasonic) power (bottom panel) in the frequency range of 1–12 Hz used in our analysis. At lower frequencies the observations cannot be simply used for estimating the transfer function as the pressure spectrum at periods of several seconds is dominated by microbaroms, not by seismic oscillations as it is revealed by Fig. 3. Fig. 6, top panel, also shows that the seismic vertical component power density reached a broad maximum in the analysed interval of 1–12 Hz.
The excess pressure Dp in a homogeneous fluid medium caused by the vertical motion is given by (e.g., Watada et al., 2006)
Dp ¼ rcs w
ð1Þ
where r is air density, cs the sound velocity and w the vertical velocity of fluid motion (equal to seismic vertical velocity). The maximum w (Fig. 2, Table 1) for NKC is 1.9 mm s 1 for the first seismic pulse. Then Eq. (1) provides Dp ¼0.76 hPa, whereas the observed value is only 0.3 hPa (Fig. 3). However, the frequency of the seismic pulse is about 10 Hz; at such a frequency the sensitivity of microbarograph is only about a half of sensitivity at 3–4 Hz, i.e. the difference between theoretical and observed Dp is caused mainly by the low sensitivity of microbarograph. For the second and third main pulses, observed at 08:30:13.0 and 08:30:14.0, respectively, (Fig. 2), the seismic frequency is lower and, therefore, the difference is smaller; 0.26 versus 0.22, the observed/theoretical ratio 0.85 and 0.46 versus 0.34, ratio 0.74, respectively. The observed values of Dp are slightly smaller but comparable with theoretical values. The comparison with (common measurements) microbarographs of the French CTBT team shows that at frequencies above about 1–2 Hz the sensitivity of our microbarographs is slightly lower than that of French microbarographs; the difference is increasing with increase in frequency. This also contributes to the theoretical versus observed signal difference. The microbarograph is fixed to the shaking surface and, therefore, it records also a static pressure change. However, with the observed maximum vertical displacement for NKC of 0.06 mm the static pressure change is about 0.0007 hPa, which is by more than two orders of magnitude smaller value than the observed changes of pressure. Such a change in the static pressure is quite negligible. Fig. 7 shows that at STC the main earthquake and infrasonic burst occur almost simultaneously with that at NKC due to little
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Fig. 6. Power spectrum of the vertical seismic component oscillations (top panel) and of infrasonic oscillations (bottom panel) at NKC, 08:30–08:31 UT.
Fig. 7. Joint infrasonic and seismic record for nearby station STC, 28 October 2008, time 0 at 08:30:06 UT (starting time t ¼4 corresponds to 08:30:10). Bold magenta—infrasound; red—vertical seismic component; blue—N–S seismic component; green—E–W seismic component. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
difference in distances from the epicentre. The first burst is missing and the main burst is stronger at STC than at NKC both in seismic and infrasonic records, which might be caused by a typical strong azimuthal dependence of emission of seismic waves from the earthquake source combined with the quite different azimuth of stations with respect to epicentre, 12.471
versus 41.941, and by the azimuthal difference of the P and S wave emission diagrams, with addition of some effect by smallscale structuring of local geology. A good coincidence of main infrasonic and seismic oscillations is again observed for the vertical seismic component (red line); the coincidence with horizontal components is much worse.
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Fig. 8 shows the microbarograph record at Panska´ Ves, horizontal distance from the epicentre of 155.2 km. Notice the above-background pulse at time 60 s in Fig. 8, i.e. at 08:31:00 with ‘‘period’’ 3–4 s, and a relatively weak short-period oscillations near 4 Hz in wavelet spectrum. These appear to be effects of the earthquake. To confirm this statement, Fig. 9 shows seismic record from Panska´ Ves. The extraordinary infrasonic pulse and short-period oscillations
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(both around 08:31:00) coincide in time with the maximum seismic burst associated with short-period oscillations. This means that the observed infrasonic effect is excited by seismic waves ‘‘in situ’’. Model calculations show that Panska´ Ves is in the infrasonic ‘‘shadow’’ (silence) zone for the infrasound excited in the epicentre. These calculations were performed by a standard acoustic ray calculation technique used also by others; the technique is partly
Fig. 8. Infrasonic record at Panska´ Ves, horizontal distance from the epicentre 155 km, time 0 at 08:30:00. Top panel—pressure fluctuations in the range of 0.1–12 Hz. Bottom panel—wavelet spectrum of pressure fluctuations.
LastCmd: Set Time Wdw
28-OCT-2008 --:--:--.--- >0.86< Filter: SHM_BP_1.7S_6HZ_3
Computing...
3: PVCC E
2: PVCC N
Lg
Pg
1: PVCC Z
08:30:35
08:30:45
08:30:55
08:31:05
08:31:15
Fig. 9. Copy of seismic record from station PVCC (Panska´ Ves), 28 October 2008, maximum of event around 08:31:00. Top—vertical component; middle—N–S component; bottom—E–W component.
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described by Krasnov et al. (2006). Moreover the epicentral infrasound is not very strong. These are the reasons why no other infrasonic effect associated with this earthquake has been observed including absence of epicentral infrasound. It should be mentioned that the Japanese network of microbarographs observed infrasonic atmospheric pressure changes excited by a strong 2003 Tokachi-Oki earthquake (M¼8.3) over the whole part of Japan territory covered by the network. These pressure changes were observed simultaneously with the seismic wave arrival; they were therefore explained by the acoustic coupling between the atmosphere and the ground just beneath the sensors (Watada et al., 2006). This finding agrees with our observations. On the other hand we cannot exclude possibility that with some events or over some distances the infrasound of epicentral origin propagating through the atmosphere is the dominant component, as published interpretation of various observations suggests.
5. Ionospheric effects of seismo-infrasound bursts Ionospheric effects of the infrasound of tropospheric origin ˇ ´ rˇova´ et al., 2009) or short-period ionospheric oscillations (Sindela of geomagnetic origin (Chum et al., 2009) occurring in the infrasonic frequency range can be observed using our Doppler system measurements. The Doppler system currently consists of five transmitters at frequencies near 3.59 MHz shifted by 4 Hz each other, and a central receiver in Prague. For the case of seismic swarm observations in infrasound one more receiver has been installed at Kvˇetna, quite nearby to the NKC station. Technical description of transmitters, receivers and antennas may be found at http://www.ufa.cas.cz/html/upperatm/M_Dop pler_system.pdf. One transmitter has been located at Vackov, about 5 km apart from the NKC station. No ionospheric effects of earthquake-related infrasound were observed, as Fig. 10 reveals for two Doppler measuring paths, one of them is local for the earthquake area and the other with
ionospheric reflection point at a distance of about 80 km from the epicentre. The earthquake occurred in 15 min of Fig. 10. Reflection heights for the Doppler system frequency vary around 160 km between 8:00 and 8:45 according to the electron density heightprofiles derived from measurements of the digisonde at Pruhonice (horizontal distance about 160 km). Time to reach the ionospheric Doppler measurement heights is therefore about 8 min, i.e. about 23 min of Fig. 10 for the Vackov–Kvˇetna path and slightly more (up to 9 min depending on the way of infrasonic signal penetration to reflection point) for the Vackov–Prague path due to horizontal distance of reflection point ( 80 km). No earthquake-related disturbances are seen in this time on Doppler shift measurements, the pattern on records is quite regular around 23–24 min in Fig. 10. There are three reasons to account for absence of ionospheric seismo-infrasonic signals: (1) Any kind of ionospheric signals of earthquakes have been reported in the literature only for the earthquakes with magnitude near or higher than 5.0, while the event of 28 October 2008 is of M¼ 3.6. (2) More important, infrasonic signals excited by the earthquake of 28 October 2008 were at frequencies, which were too high to penetrate into the ionosphere (e.g., Blanc, 1985; Krasnov et al., 2006, 2007). Typical ionospheric infrasound (periods tens of seconds to a few minutes) was not excited by this earthquake; this part of spectrum revealed undisturbed pattern. (3) Doppler measuring system is unable to identify oscillations at frequencies higher than 1 Hz, which were excited by this earthquake.
6. Measurements of magnetic and electric fields A magnetic search coil ( 1 m long) with permaloy core ( 1.5 m long) was used to measure magnetic field oscillations.
Fig. 10. Ionospheric Doppler measurements on 28 October 2008. Upper record—path Vackov–Prague. Lower record—path Vackov–Kvˇetna (distance o 10 km, thus ground wave is seen).
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The same electronics as that used by microbarographs was deployed in these measurements. The magnetic search coil was installed at NKC on 14 October 2008. Measurements were made in a frequency range similar to that of oscillations observed by microbarographs— 0.1–10 Hz. Unfortunately, the magnetic signal was observed not in the time of quake in the hypocentre. In case of seismic origin of the observed oscillations it should be observed in the same time as the quake occurs in the hypocentre due to the high speed of magnetic signal propagation. The magnetic signal was observed in the time of arrival of seismic waves to surface (time difference is about 1.5 s from the hypocentre time). This supports the seismic shaking of magnetic sensor, not a real magnetic signal, as explanation of observations. On the other hand, spectrum of oscillations was quite different from the seismic spectrum; periods recorded by the magnetic coil were substantially longer. This might be a quasi-resonant response of measuring system to seismic pulses, but it needs to be confirmed by further testing. Similar results were achieved also for a few other events. It was not possible to test the magnetic instrument in the vibration test equipment TIRA vibration controller SVC01 due to its large size. However, shaking the instrument in hands resulted in substantial ‘‘magnetic’’ oscillations, which supports the hypothesis of the seismic shaking origin of the observed oscillations. A mechanical shaking of the magnetic search coil in the stable Earth’s magnetic field has to produce ‘‘artificial’’ magnetic signal. Magnetic measurements by a magnetometer located 230 and 440 km (quake M¼8.6) from epicentres of two strong spring 2005 Sumatra earthquakes identified two different types of earthquake-related magnetic signal (Hasbi et al., 2009). Shortly after the earthquake, oscillations with periods of about 4–5 s were observed. These oscillations were ascribed to the seismic shaking of instrument in the Earth’s magnetic field. This is result consistent with our observations. About 11 min after the earthquake, pulsations with period of 4.8 min were observed, not observed at distant magnetic stations (i.e. not of geomagnetic micropulsation origin). Hasbi et al. (2009) ascribed them to seismic-induced effects in the ionospheric E-region (dynamo region) caused by the penetration of earthquake-induced acoustic waves of periods 3–5 min. We did not observe such an effect due to the dominance of much shorter periods in seismic spectrum of the earthquake analysed in this paper. Sensor for electric field oscillation measurements has been installed near the end of the earthquake swarm, after the event of 28 October. Moreover, quality of measurements was rather insufficient. Therefore these data have not been used at all in analyzing seismic swarm dataset.
7. Conclusions As far as we know, our unique infrasonic measurements are the first ground-based infrasonic measurements in the epicentre of an earthquake. This earthquake was relatively weak earthquake (M¼3.6); strong earthquake would probably destroy the measuring equipment. The results may be summarized as follows: 1. The infrasonic oscillations ( 1–12 Hz) in the epicentre region appear to be excited by the vertical seismic oscillations, as expected. The infrasonic oscillations correlate well with the seismic vertical oscillations, as shown in Figs. 5 and 7, and are well expressed. They are excited by the mechanical vertical motion of the surface during earthquake, which makes the oscillatory compression and depression of the atmosphere. The microbarograph-measured oscillations are not caused by
2.
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5.
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seismic shaking of measuring instrument; they are essentially a real infrasound, as proved by vibration tests of microbarographs. They are not affected by infrasound oscillations of meteorological origin due to quiet meteorological conditions locally and regionally. Infrasonic oscillations observed at a distance of 155 km from the epicentre appear to be seismically excited in situ, they do not represent the infrasound coming from the epicentre region. No infrasonic effects of this earthquake were observed in the ionosphere due to the low earthquake magnitude and too short periods of excited infrasound to be capable to excite ionospheric effects and to be identified by the Doppler measuring system. The observed magnetic effects were rather the effects of seismic shaking of magnetic sensor than real earthquake effects. It is very important to check, which observed apparent earthquake effects are real effects, because some of them may be a consequence of seismic shaking of measuring equipment.
The results clearly demonstrate the dominant role of vertical component of seismic motions in exciting infrasonic waves. The finding that at least at some distances the infrasound excited in situ by seismic waves and not the epicentral infrasound may be dominant infrasonic signal of earthquake origin might be useful for seismo-infrasonic studies and may be of some relevance for the CTBT network aimed at remote detection of underground nuclear explosions. The atmospheric–ionospheric coupling via seismically-induced atmospheric waves will be treated in more detail in another paper.
Acknowledgement This work was supported by the Grant Agency of the Czech Republic through grant 205/07/1367. References Afraimovich, E.I., Perevalova, N.P., Plotnikov, A.V., Uralov, A.M., 2001. The shockacoustic waves generated by earthquakes. Annales Geophysicae 19, 395–409. Arendt, S., Fritts, D., 2002. Acoustic radiation by ocean surface waves. Journal of Fluid. Mechanics 415, 1–21. Arrowsmith, S.J., Burlacu, R., Whitaker, R., Randall, G., 2009. A repeating secondary source of infrasound from the Wells, Nevada, earthquake sequence. Geophysical Research Letters 36, L11817. doi:10.1029/2009GL038363. Artru, J., Farges, T., Lognonne, P., 2004. Acoustic waves generated from seismic surface waves: propagation properties determined from Doppler sounding observations and normal-modes modelling. Geophysical Journal International 158, 1067–1077. Bhattacharya, S., Sarkar, S., Gwal, A.K., Parrot, M., 2009. Electric and magnetic field perturbations recorded by DEMETER satellite before seismic events of the 17th July 2006 M 7.7 earthquake in Indonesia. Journal of Asian Earth Sciences 34, 634–644. Blanc, E., 1985. Observations in the upper atmosphere of infrasonic waves from natural or artificial sources: a summary. Annales Geophysicae 3, 673–688. Calais, E., Minster, B., 1995. GPS detection of ionospheric perturbations following the January 17, 1994, Northridge earthquake. Geophysical Research Letters 22, 1045–1048. Che, I.Y., Lee, H.-I., Jeon, J.-S., Kang, T.-S., 2007. An analysis of the infrasound signal from the Miyagi-Oki earthquake in Japan on 16 August 2005. Earth Planets and Space 59, e9–e12. Choosakul, N., Saito, A., Iyemori, T., Hashizume, M., 2009. Excitation of 4 min periodic ionospheric variations following the great Sumatra Andaman earthquake in 2004. Journal of Geophysical Research 114, A10313. doi:10.1029/ 2008JA013915. ˇ ´ rˇova´, T., Hejda, P., Bochnı´cˇek, J., 2009. Chum, J., Hruˇska, F., Bureˇsova´, D., Sindela Ionospheric oscillations caused by geomagnetic Pi2 pulsations and their observations by multipoint continuous Doppler sounding; first results. Advances in Space Research 44, 667–676.
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Daniels, F.B., 1962. Generation of infrasound by ocean waves. Journal of Acoustic Society of America 34, 352–353. DasGupta, A., Das, A., Hui, D., Bandhopadhyay, K.K., Srinivas, M.R., 2006. Ionospheric perturbations observed by the GPS following the December 2004 Sumatra–Andaman earthquake. Earth Planets and Space 58, 167–172. Fujiwara, H., Kamogawa, M., Ikeda, M., Liu, J.Y., Sakata, H., Chen, Y.I., Ofuruton, H., Muramatsu, S., Chuo, Y.J., Ohtsuki, Y.A., 2004. Atmospheric anomalies observed during earthquake occurrences. Geophysical Research Letters 31, L17110. doi:10.1029/2004GL019865. Hasbi, A.M., Momani, M.A., Ali, M.A.M., Misran, N., Shiokawa, K., Otsuka, Y., Yumoto, K., 2009. Ionospheric and geomagnetic disturbances during the 2005 Sumatran earthquakes. Journal of Atmospheric and Solar-Terrestrial Physics 71, 1824–1834. Hayakawa, M., Molchanov, O.A., NASDA/UEC team, 2004. Summary report of NASDA’s earthquake remote sensing frontier project. Physics and Chemistry of the Earth 29, 617–625. Hobara, Y., Parrot, M., 2005. Ionospheric perturbations linked to a very powerful seismic event. Journal of Atmospheric and Solar-Terrestrial Physics 67, 677–685. Hora´lek, J., Fischer, T., Bouˇskova´, A., Micha´lek, J., Hrubcova´, P., 2009. The West Bohemian 2008-earthquake swarm: when, where, what size and data. Studia Geophysica et Geodaetica 53, 351–358. Iyemori, T., Nose, M., Han, D., Gao, Y., Hashizume, M., Choosakul, N., Shiganawa, H., Tanaka, Y., Utsugi, M., Saito, A., McCreadie, H., Odagi, Y., Yang, F., 2005. Geomagnetic pulsations caused by the Sumatra earthquake on December 26, 2004. Geophysical Research Letters 32, L20807. doi:10.1029/2005GL024083. Kherani, A.E., Lognonne, P., Kamath, N., Crespon, F., Garcia, R., 2009. Response of the ionosphere to seismic triggered acoustic waves: electron density and electromagnetic fluctuations. Geophysical Journal International 176, 1–13. Krasnov, V.M., Drobzheva, Ya.V., Laˇstovicˇka, J., 2006. Recent advances and difficulties of infrasonic wave investigation in the ionosphere. Surveys in Geophysics 27, 169–209. Krasnov, V.M., Drobzheva, Ya.V., Laˇstovicˇka, J., 2007. Acoustic energy transfer to the upper atmosphere from sinusoidal sources and a role of non-linear processes. Journal of Atmospheric and Solar-Terrestrial Physics 69, 1357–1365. Liu, J.Y., Tsai, Y.B., Chen, S.W., Lee, C.P., Chen, Y.C., Yen, H.Y., Chang, W.Y., Liu, C., 2006. Giant ionospheric disturbances excited by the M9.3 Sumatra earthquake
of 26 December 2004. Geophysical Research Letters 33, L02103. doi:10.1029/ 2005GL023963. Lognonne, P., Artru, J., Garcia, R., Crespon, F., Ducic, V., Jeansou, E., Occhipinti, G., Helbert, J., Moreaux, G., Godet, P.-E., 2006. Ground based GPS imaging of ionosphere post-seismic signal. Planetary and Space Science 54, 528–540. Meloni, A., di Mauro, D., Mele, G., Palangio, P., Ernst, T., Teisseyre, R., 2001. Evolution of magnetotelluric, total magnetic field, and VLF field parameters in Central Italy: relations to local seismic activity. Annali di Geofisica 44, 383–394. Nˇemec, F., Santolı´k, O., Parrot, M., 2009. Decrease of intensity of ELF/VLF waves observed in the upper ionosphere close to earthquakes: a statistical study. Journal of Geophysical Research 114, A04303. doi:10.1029/2008JA013972. Omori, Y., Nagahama, H., Kawada, Y., Yasuoka, Y., Ishikawa, T., Tokonami, S., Shinogi, M., 2009. Preseismic alterations of atmospheric electric conditions due to anomalous radon emanation. Physics and Chemistry of the Earth 34, 435–440. Oyama, K.-I., Kakinami, Y., Liu, J.-Y., Kamogawa, M., Kodama, T., 2008. Reduction of electron temperature in low-latitude ionosphere at 600 km before and after large earthquakes. Journal of Geophysical Research 113, A11317. doi:10.1029/ 2008JA013367. Posmentier, E., 1967. A theory of microbaroms. Geophysical Journal of Royal Astronomical Society 13, 487–501. Pulinets, S.A., 2007. Natural radioactivity, earthquakes, and the ionosphere. EOS 88 (20), 217–218. Pulinets, S.A., Dunajecka, M.A., 2007. Specific variations of air temperature and relative humidity around the time of Michoacan earthquake M8.1 September 19, 1985 as a possible indicator of interaction between tectonic plates. Tectonophysics 431, 221–230. ˇSindela´rˇova´, T., Bureˇsova´, D., Chum, J., Hruˇska, F., 2009. Doppler observations of infrasonic waves of meteorological origin at ionospheric heights. Advances in Space Research 43, 1644–1651. Sorokin, V.M., Chmyrev, V.M., Yaschenko, A.K., 2005. Theoretical model of DC electric field information in the ionosphere by seismic activity. Journal of Atmospheric and Solar-Terrestrial Physics 67, 1259–1268. Watada, S., Kunugi, T., Sugioka, H., Nishida, K., Segikuchi, S., Oikawa, J., Tsuji, Y., Kanamori, H., 2006. Atmospheric pressure change associated with the 2003 Tochaki-Oki earthquake. Geophysical Research Letters 33, L24306. doi:10.1029/2006GL027967.
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Journal of Atmospheric and Solar-Terrestrial Physics journal homepage: www.elsevier.com/locate/jastp
Simultaneous infrasonic, seismic, magnetic and ionospheric observations in an earthquake epicentre ˇ ´ rˇova´ a, J. Hora´lek b, J. Zednı´k b, V. Krasnov c J. Laˇstovicˇka a,n, J. Baˇse a, F. Hruˇska a, J. Chum a, T. Sindela a
Institute of Atmospheric Physics, ASCR, Bocni II, 14131 Prague, Czech Republic Institute of Geophysics, ASCR, Bocni II, 14131 Prague, Czech Republic c Saint Petersburg, Russia b
a r t i c l e in fo
abstract
Article history: Received 25 March 2010 Received in revised form 2 August 2010 Accepted 3 August 2010 Available online 14 August 2010
Various pre-seismic and co-seismic effects have been reported in the literature in the solid Earth, hydrosphere, atmosphere, electric/magnetic field and in the ionosphere. Some of the effects observed above the surface, particularly some of the pre-seismic effects, are still a matter of debate. Here we analyze the co-seismic effects of a relatively weak earthquake of 28 October 2008, which was a part of an earthquake swarm in the westernmost region of the Czech Republic. Special attention is paid to unique measurements of infrasonic phenomena. As far as we know, these have been the first infrasonic measurements during earthquake in the epicentre zone. Infrasonic oscillations ( 1–12 Hz) in the epicentre region appear to be excited essentially by the vertical seismic oscillations. The observed oscillations are real epicentral infrasound not caused by seismic shaking of the instruments or by meteorological phenomena. Seismo-infrasonic oscillations observed 155 km apart from the epicentre were excited in situ by seismic waves. No earthquake-related infrasonic effects have been observed in the ionosphere. Necessity to make vibration tests of instruments is pointed out in order to be sure that observed effects are not effects of mechanical shaking of the instrument. & 2010 Elsevier Ltd. All rights reserved.
Keywords: Earthquake Infrasonic effects Ionospheric effects Magnetic effects
1. Introduction Earthquakes excite various effects in the solid Earth, hydrosphere, atmosphere and even ionosphere. Some effects have also been observed prior to earthquakes and some authors have been trying to use them as earthquake precursors. Here we will only deal with co-seismic effects, not with possible earthquake precursors. The ground-based electric and magnetic measurements and model calculations detect earthquake-related changes in geomagnetic pulsations and electric fields (e.g., Iyemori et al., 2005; Meloni et al., 2001; Sorokin et al., 2005). In the hydrosphere, tsunamis are excited by earthquakes. Oceanic waves are known to excite infrasound (e.g., Arendt and Fritts, 2002; Daniels, 1962). In the atmosphere, acoustic, infrasonic and probably shortperiod gravity waves are excited near the surface and propagate both upwards and horizontally (e.g., Arrowsmith et al., 2009; Che. et al., 2007), sometimes changes of surface atmospheric temperature are observed (e.g., Hayakawa et al., 2004), atmospheric anomalies of VHF radio wave propagation appear (e.g., Fujiwara et al., 2004), and radon emanation affects lower atmosphere ionization and, thus, the global electric circuit (e.g., Pulinets,
n
Corresponding author. Tel.: +420 267103055. E-mail address: [email protected] (J. Laˇstovicˇka).
1364-6826/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2010.08.005
2007; Pulinets and Dunajecka, 2007; Omori et al., 2009). In the ionosphere, various effects in electron density, total electron content, electron temperature and electromagnetic waves have been observed (e.g., Bhattacharya et al., 2009; Calais and Minster, 1995; DasGupta et al., 2006; Hobara and Parrot, 2005; Liu et al., 2006; Lognonne et al., 2006; Nˇemec et al., 2009; Oyama et al., 2008; Pulinets, 2007). Some of them are associated with the acoustic-gravity waves including infrasound (e.g.., Afraimovich et al., 2001; Artru et al., 2004; Kherani et al., 2009; Choosakul et al., 2009). The Cheb region in the westernmost part of the Czech Republic (Central Europe) was hit by a relatively intense earthquake swarm in October 2008. Earthquake swarms of comparable intensity appear in this region typically once per 15–25 years (Hora´lek et al., 2009). Seismic station Novy´ Kostel (NKC; 50.231N, 12.451E) is located just in the epicentral area of this earthquake swarm. Another seismic station of a local network is in the nearby location Studenec (STC; 50.261N, 12.521E), several kilometres apart from NKC. Seismometers of different frequency responses, acoustic sensors, microbarograph and magnetic field measurements were deployed at NKC station, some of them permanently (e.g., seismometers), and some of them temporarily (e.g., microbarographs), all located quite close each other. Some instruments (seismometer and about 90 m separated microbarograph) were deployed also at STC. Fig. 1 shows positions of these two stations
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with respect to the epicentre of the studied earthquake. One ionospheric Doppler sounding path is located quite nearby (quasivertical path with reflection point about 10 km from the NKC station). Here we shall analyze the event of 28 October 2008 with special attention paid to infrasonic phenomena recorded by microbarographs. As far as we know, these have been the first infrasonic measurements during earthquake in the epicentre zone. Section 2 provides basic seismic information about the swarm and event of 28 October 2008. Section 3 presents basic meteorological information necessary for evaluation of infrasonic measurements. Section 4 is the key part of the paper; it describes infrasonic measurements by microbarographs and their interpretation. Section 5 deals with ionospheric effects of seismoinfrasonic bursts, and Section 6 with magnetic and electric measurements. Paper is closed by short conclusions in Section 7.
Fig. 1. Positions of the epicentre and the two nearby stations (microbarograph + seismograph at each station).
2. Basic seismic information The westernmost part of the Czech Republic is an exceptional European region due to the recent geodynamics expressed mainly by a periodic occurrence of intraplate earthquake swarms and a high fluid activity in the upper crust. The most recent known volcanic activity occurred there about 0.3–0.5 Ma ago. It is very likely that crustal fluids, which are of a post-volcanic origin, play an important role in triggering and driving earthquake swarms in the region concerned. The 2008 swarm ranks among the most intensive local earthquake activities. It took place in the Novy´ Kostel focal area and lasted 28 days. In this course more than 20,000 events were recorded by stations of the seismic network Webnet (Hora´lek et al., 2009). The central station of WEBNET, Novy´ Kostel (NKC), is located just in the epicentral zone. The swarm was exceptional by its rapidity. It started on October 6, the most intensive phase occurred already in the evening of October 9 and lasted two days; this phase included three shocks of magnitude ML 43.5 (stronger than 3.51 on the Richter scale). Then, a fairly intensive swarm activity persisted till October 15. During this time, a pronounced swarm phase included a strong shock in October 12 with the magnitude of ML E4.0). The period between October 15 and 27 was characterized by a gradual swarm activity decay, which was broken by an abrupt swarm activity increase dominated by a shock of ML E 4.0, which produced the greatest ground motions in the epicentral area (Hora´lek et al., 2009). Hypocenters were located at the depths of 6.5–10.5 km (shallow earthquakes) on a fault segment of 12 km2 in the area near to the termination of the Maria´nske´ La´znˇe fault. Very rough estimates of the size of the fracture surface and of the slip along this surface for the strongest earthquakes of ML E 4.0 are 600 600 m2 and 0.15 m, respectively. Broadband seismometers have been installed at NKC and STC ¨ stations, NKC—Guzalp-40 T (80–0.033 Hz), STC—CM-3 (80– 0.5 Hz), respectively. They measure the ground velocity, not displacement, in units of mm s 1.
Fig. 2. Seismic record from station NKC, event onset at 08:30:11 UT. Top—vertical component; middle—N–S component; bottom—E–W component. Numbers in the upper right corner of each component record are values of the maximum observed amplitude for the given record: 1.9 mm s 1 for vertical component, 2.6 mm s 1 for N–S component, 4.7 mm s 1 for E–W component.
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We shall deal in more detail with the earthquake of 28 October, 08:30:11 UT, M¼3.6, hypocenter depth 7870 m, when other measurements are available. As Fig. 2 shows for NKC, the event begins with much larger vertical amplitude, because the event onset is the P-wave almost in the epicentre.
Table 1 Results of vibration tests of microbarograph at TIRA Vibration controller SVC01 for vertical vibrations. Arrangement of measuring system (sensor and input hose), speed of vertical motion and the measured amplitude of pressure pulses are shown. Sensor and input hose
3. Meteorological information Microbarographs measuring infrasound in the atmosphere have anti-wind shielding, nevertheless strong winds could affect the infrasonic measurements and, moreover, a strong wind in hilly terrain covered partly by forest excites local infrasonic oscillations, which could interfere with and/or overlap the infrasonic oscillations of seismic origin. The general meteorological situation on October 28, 2008 was formed by a very slowly moving weather front over central Europe with a pressure field with small pressure gradients, i.e. general conditions which do not support strong winds in the whole region. The local meteorological station Cheb reported weak winds of 1–2 m/s, temperatures 5–7 1C, sea-level pressure oscillating around 1010 hPa, cloudy to overcast and weak discontinuous rain. These meteorological conditions do not make possible the excitation of local infrasonic oscillations/noise of meteorological origin. Also other meteorological stations in the western part of the Czech Republic reported only weak winds (www.ogimet.com/synops.phtml.en). Microbarograph measurements before the earthquake displayed microbaroms but very little other infrasonic noise. Thus infrasonic measurements have not been affected by local/regional infrasonic oscillations of meteorological origin.
4. Infrasonic measurements by microbarographs We use differential microbarograph—infrasound gage ISGM03 with an input hose (to reduce/remove effects of wind and rain) for transmitting acoustic signal to the inlet of sensor. The sensor measures the pressure fluctuations in the frequency range from 0.001 to 10(12) Hz. The pressure fluctuations are measured with a 0.006 Pa resolution in the operating range of 725 Pa related to a steady state value. The data are stored with 25 Hz sampling period into the internal memory with a capacity for 2 months and can be downloaded using an Internet connection. To minimize the influence of temperature variations on the measured values, the sensor is thermally isolated from the environment. It is necessary to mention that the frequency range of microbarographs is due to their use in the standard regime mainly as supporting measurements to ionospheric Doppler system measurements. The sensor itself has a shape of a cylinder. It consists of two chambers and a thin membrane. The pressure fluctuations cause displacements of a thin membrane, which is located between the two chambers. The first chamber has a direct entrance (inlet) to the ambient air. The second chamber is connected with the first one via a very narrow capillary tube. The time constant to balance the pressure inside the second chamber with the first one is 1000 s. The pressure fluctuations are thus measured as the displacement of the membrane from its central position. It is very important to be sure that pressure oscillations measured during an earthquake are real pressure oscillations, not effect of seismic shaking of microbarographs. Therefore the microbarographs were tested in the vibration test equipment TIRA Vibration controller SVC01 used for testing satellite instruments. Table 1 show in its upper part the results of vibration testing from January 2008 for vibrations in the frequency range of 10–12 Hz, i.e. at the upper boundary of the range of frequencies
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Test for f¼ 10–12 Hz Sensor vertical, closeda Sensor vertical, hose freeb Sensor vertical, opened, no hose Sensor horizontal, hose fixed c Sensor horizontal, hose fixedc
Speed of motion (mm s 1)
Pressure amplitude (Pa)
50 10 50 50 10
0.03 0.05 0.05 0.10 0.02
Microbarograph observations at NKC during earthquake (Fig. 2)—peak around 2–4 Hz Sensor vertical, hose fixedc 1.9 0.30 Test for f ¼3–6 Hz Sensor vertical, closedc Sensor vertical, hose fixedc
20 20
0.35 1.0
a Sensor can measure either in vertical or in horizontal position (input hose is essentially horizontal). b The input hose was free. c The input hose in real observations at NKC and STC was fixed to surface, in vibration tests it was fixed to vibrating plate.
observed (the vibration equipment was unable to produce lower frequencies). The sensor itself is quite insensitive to simulated seismic oscillations (the test on the first line of Table 1). The sensor with an input hose, when the input hose is not fixed, reveals a response substantially large than that for the case of the hose fixed to vibrating surface. However, it was necessary to test microbarographs at frequencies of their peak observed effect (2–4 Hz). For this purpose it was necessary to modify the test equipment TIRA Vibration controller SVC01 to allow tests at frequencies below 10 Hz, which was finished in March 2010. The tests were performed on 23 and 25 March 2010 at frequencies 10, 9, 8, 7, 6, 5, 4, 3, 2, 1 and 0.5 Hz. The bottom part of Table 1 shows the result for the peak of sensitivity of microbarographs at 3–6 Hz. The response within the range of frequencies 3–6 Hz was similar, whereas it was remarkably weaker at higher or lower frequencies. Tests were performed for the vertical installation of microbarograph (as at stations) and for vertical shaking, as observations show that horizontal seismic motions do not affect the microbarographs significantly; the effect of vertical seismic motion is quite dominant (see Figs. 3 and 5). To assure good accuracy of test measurements, the mechanical shaking was ten times as strong as the maximum vertical seismic signal. For the closed sensor the response for ten times stronger testing is the same as the observed signal (middle part of Table 1), that means that the sensor itself is only little sensitive to mechanical/seismic shaking. For the sensor with a fixed hose as at stations NKC and STC, the response to ten times stronger mechanical shaking is about three times as large as signal observed at NKC. Our test experiments show that the response is proportional to mechanical shaking; this means that the response to the mechanical shaking is three times weaker than the total response, which is thus predominantly of infrasonic origin. However the total response observed in shaking tests is a sum of response to mechanical shaking and infrasound excited by vibrating test equipment, i.e. the real response to mechanical shaking is even somewhat lower. Therefore we can assume that the observed oscillations are very predominantly a real infrasound excited by the earthquake, only rather little contaminated by earthquake shaking of microbarographs and input hoses. The vibration tests also showed that the response of the closed sensor itself is almost ten times smaller for
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Fig. 3. Infrasonic record at NKC on 28 October 2008. Pressure fluctuations in the frequency range of 0.1–12 Hz. Time 0 is 08:30:00 UT. First burst occurred at about 08:30:13, main burst near 08:30:15. Top panel–pressure fluctuations in the range 0.1–12 Hz. Bottom panel—wavelet spectrum of pressure fluctuations.
Fig. 4. Pressure fluctuations (essentially microbaroms) in the frequency range of 0.1–12 Hz, station NKC, 20 October 2008, 08:30:00-08:31:30 UT. Pressure scale is the same as in Fig. 3.
horizontal sensor installations than for vertical sensor installations, so in future earthquake-related measurements (if any) we should use the horizontal installation of microbarographs. On 14 October, a microbarograph was moved from its permanent position and installed at NKC, and a few days before the 28 October event another microbarograph was moved from its permanent position and installed at a nearby location Studenec (STC) several kilometres apart. Their distance from the 28 October earthquake epicentre and azimuth are: 2.33 km and 12.471, and 6.89 km and 41.941, respectively, i.e. the NKC microbarograph recorded infrasound almost in the epicentre. The third microbarograph remained at its permanent position at Panska´ Ves (50.531N, 14.571E), distance to epicentre 155.2 km, azimuth 76.521. The wavelet spectrum presented in the bottom panel in Fig. 3 shows for the main burst a large increase of power at frequencies between 2 and 12 Hz observed between 8:30:13 and 8:30:15 respectively. Origin of smaller bursts observed later at 2–4 Hz is also seismic; they all are associated with aftershock seismic activity, i.e. rather weak aftershocks of the main shock. The
waveform of acoustic signal presented in the upper panel shows that short-period earthquake-related oscillations are observed on the background of longer-period (4–5 s) oscillations called microbaroms, which are supposed to be produced by a nonlinear interaction of ocean waves with the atmosphere (e.g., Daniels, 1962; Posmentier, 1967; Arendt and Fritts, 2002). Fig. 4 illustrates for the same time of the day and eight days before the event under quiet seismic and regional meteorological conditions but with strong winds over Atlantic that the microbaroms are typical dominant source of infrasonic noise, being present almost continuously. However, their periods are much longer than the periods of infrasonic oscillations of seismic origin and their amplitudes are substantially smaller, hence they do not interfere with the observations of infrasound of seismic origin. A good coincidence of the main infrasonic and seismic oscillations is well visible in Fig. 5, namely for the vertical seismic component (red line). The coincidence with horizontal components is much worse; the main green peak just before 34 s in Fig. 5 and the strong blue and green peaks just before 36 s have no corresponding response in the infrasonic magenta line. The vertical
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Fig. 5. Joint infrasonic and seismic record for station NKC, 28 October 2008, time ¼0 at 08:29:40 UT (starting time t ¼ 30 corresponds to 08:30:10). Bold magenta—infrasound; red—vertical seismic component; blue—N–S seismic component; green—E–W seismic component. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
motion of the surface during the earthquake excites infrasonic waves much more efficiently than horizontal motion, as expected. In case of ideally flat surface, only the vertical motion can make alternating compression and depression of the atmosphere. Fig. 5 reveals a positive correlation between seismic vertical and infrasonic oscillations that are in phase. An upward seismic motion is accompanied by an increase in atmospheric pressure measured by the microbarograph, and a downward motion with a decrease in pressure. This means that the upward surface motion compresses the atmosphere, and the downward motion rarefies the atmosphere. If the observed infrasonic signal is caused by the upward (downward) seismically-induced motion of the microbarometer, the sign of the phase would be opposite as the microbaromter is moved towards less dense (or denser) air. This is other evidence in support of the seismic wave origin of the observed infrasonic signals. Simultaneous seismic and microbarograph measurements at NKC allow consider the behaviour of the seismic-pressure transfer function. Both measurements and the theoretical transfer function calculations by Watada et al. (2006) reveal for a strong earthquake observed in Japan and infrasonic measurements a few hundred kilometres apart an approximately constant transfer function (independent of frequency) for periods lower than 40–50 Hz. Our seismic event is much weaker; we measure practically in the epicentre and with different type of microbarograph. Nevertheless Fig. 6 reveals also roughly constant ratio (at least after some smoothing) of the seismic power (top panel) to the pressure (infrasonic) power (bottom panel) in the frequency range of 1–12 Hz used in our analysis. At lower frequencies the observations cannot be simply used for estimating the transfer function as the pressure spectrum at periods of several seconds is dominated by microbaroms, not by seismic oscillations as it is revealed by Fig. 3. Fig. 6, top panel, also shows that the seismic vertical component power density reached a broad maximum in the analysed interval of 1–12 Hz.
The excess pressure Dp in a homogeneous fluid medium caused by the vertical motion is given by (e.g., Watada et al., 2006)
Dp ¼ rcs w
ð1Þ
where r is air density, cs the sound velocity and w the vertical velocity of fluid motion (equal to seismic vertical velocity). The maximum w (Fig. 2, Table 1) for NKC is 1.9 mm s 1 for the first seismic pulse. Then Eq. (1) provides Dp ¼0.76 hPa, whereas the observed value is only 0.3 hPa (Fig. 3). However, the frequency of the seismic pulse is about 10 Hz; at such a frequency the sensitivity of microbarograph is only about a half of sensitivity at 3–4 Hz, i.e. the difference between theoretical and observed Dp is caused mainly by the low sensitivity of microbarograph. For the second and third main pulses, observed at 08:30:13.0 and 08:30:14.0, respectively, (Fig. 2), the seismic frequency is lower and, therefore, the difference is smaller; 0.26 versus 0.22, the observed/theoretical ratio 0.85 and 0.46 versus 0.34, ratio 0.74, respectively. The observed values of Dp are slightly smaller but comparable with theoretical values. The comparison with (common measurements) microbarographs of the French CTBT team shows that at frequencies above about 1–2 Hz the sensitivity of our microbarographs is slightly lower than that of French microbarographs; the difference is increasing with increase in frequency. This also contributes to the theoretical versus observed signal difference. The microbarograph is fixed to the shaking surface and, therefore, it records also a static pressure change. However, with the observed maximum vertical displacement for NKC of 0.06 mm the static pressure change is about 0.0007 hPa, which is by more than two orders of magnitude smaller value than the observed changes of pressure. Such a change in the static pressure is quite negligible. Fig. 7 shows that at STC the main earthquake and infrasonic burst occur almost simultaneously with that at NKC due to little
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Fig. 6. Power spectrum of the vertical seismic component oscillations (top panel) and of infrasonic oscillations (bottom panel) at NKC, 08:30–08:31 UT.
Fig. 7. Joint infrasonic and seismic record for nearby station STC, 28 October 2008, time 0 at 08:30:06 UT (starting time t ¼4 corresponds to 08:30:10). Bold magenta—infrasound; red—vertical seismic component; blue—N–S seismic component; green—E–W seismic component. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
difference in distances from the epicentre. The first burst is missing and the main burst is stronger at STC than at NKC both in seismic and infrasonic records, which might be caused by a typical strong azimuthal dependence of emission of seismic waves from the earthquake source combined with the quite different azimuth of stations with respect to epicentre, 12.471
versus 41.941, and by the azimuthal difference of the P and S wave emission diagrams, with addition of some effect by smallscale structuring of local geology. A good coincidence of main infrasonic and seismic oscillations is again observed for the vertical seismic component (red line); the coincidence with horizontal components is much worse.
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Fig. 8 shows the microbarograph record at Panska´ Ves, horizontal distance from the epicentre of 155.2 km. Notice the above-background pulse at time 60 s in Fig. 8, i.e. at 08:31:00 with ‘‘period’’ 3–4 s, and a relatively weak short-period oscillations near 4 Hz in wavelet spectrum. These appear to be effects of the earthquake. To confirm this statement, Fig. 9 shows seismic record from Panska´ Ves. The extraordinary infrasonic pulse and short-period oscillations
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(both around 08:31:00) coincide in time with the maximum seismic burst associated with short-period oscillations. This means that the observed infrasonic effect is excited by seismic waves ‘‘in situ’’. Model calculations show that Panska´ Ves is in the infrasonic ‘‘shadow’’ (silence) zone for the infrasound excited in the epicentre. These calculations were performed by a standard acoustic ray calculation technique used also by others; the technique is partly
Fig. 8. Infrasonic record at Panska´ Ves, horizontal distance from the epicentre 155 km, time 0 at 08:30:00. Top panel—pressure fluctuations in the range of 0.1–12 Hz. Bottom panel—wavelet spectrum of pressure fluctuations.
LastCmd: Set Time Wdw
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3: PVCC E
2: PVCC N
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08:30:35
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Fig. 9. Copy of seismic record from station PVCC (Panska´ Ves), 28 October 2008, maximum of event around 08:31:00. Top—vertical component; middle—N–S component; bottom—E–W component.
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described by Krasnov et al. (2006). Moreover the epicentral infrasound is not very strong. These are the reasons why no other infrasonic effect associated with this earthquake has been observed including absence of epicentral infrasound. It should be mentioned that the Japanese network of microbarographs observed infrasonic atmospheric pressure changes excited by a strong 2003 Tokachi-Oki earthquake (M¼8.3) over the whole part of Japan territory covered by the network. These pressure changes were observed simultaneously with the seismic wave arrival; they were therefore explained by the acoustic coupling between the atmosphere and the ground just beneath the sensors (Watada et al., 2006). This finding agrees with our observations. On the other hand we cannot exclude possibility that with some events or over some distances the infrasound of epicentral origin propagating through the atmosphere is the dominant component, as published interpretation of various observations suggests.
5. Ionospheric effects of seismo-infrasound bursts Ionospheric effects of the infrasound of tropospheric origin ˇ ´ rˇova´ et al., 2009) or short-period ionospheric oscillations (Sindela of geomagnetic origin (Chum et al., 2009) occurring in the infrasonic frequency range can be observed using our Doppler system measurements. The Doppler system currently consists of five transmitters at frequencies near 3.59 MHz shifted by 4 Hz each other, and a central receiver in Prague. For the case of seismic swarm observations in infrasound one more receiver has been installed at Kvˇetna, quite nearby to the NKC station. Technical description of transmitters, receivers and antennas may be found at http://www.ufa.cas.cz/html/upperatm/M_Dop pler_system.pdf. One transmitter has been located at Vackov, about 5 km apart from the NKC station. No ionospheric effects of earthquake-related infrasound were observed, as Fig. 10 reveals for two Doppler measuring paths, one of them is local for the earthquake area and the other with
ionospheric reflection point at a distance of about 80 km from the epicentre. The earthquake occurred in 15 min of Fig. 10. Reflection heights for the Doppler system frequency vary around 160 km between 8:00 and 8:45 according to the electron density heightprofiles derived from measurements of the digisonde at Pruhonice (horizontal distance about 160 km). Time to reach the ionospheric Doppler measurement heights is therefore about 8 min, i.e. about 23 min of Fig. 10 for the Vackov–Kvˇetna path and slightly more (up to 9 min depending on the way of infrasonic signal penetration to reflection point) for the Vackov–Prague path due to horizontal distance of reflection point ( 80 km). No earthquake-related disturbances are seen in this time on Doppler shift measurements, the pattern on records is quite regular around 23–24 min in Fig. 10. There are three reasons to account for absence of ionospheric seismo-infrasonic signals: (1) Any kind of ionospheric signals of earthquakes have been reported in the literature only for the earthquakes with magnitude near or higher than 5.0, while the event of 28 October 2008 is of M¼ 3.6. (2) More important, infrasonic signals excited by the earthquake of 28 October 2008 were at frequencies, which were too high to penetrate into the ionosphere (e.g., Blanc, 1985; Krasnov et al., 2006, 2007). Typical ionospheric infrasound (periods tens of seconds to a few minutes) was not excited by this earthquake; this part of spectrum revealed undisturbed pattern. (3) Doppler measuring system is unable to identify oscillations at frequencies higher than 1 Hz, which were excited by this earthquake.
6. Measurements of magnetic and electric fields A magnetic search coil ( 1 m long) with permaloy core ( 1.5 m long) was used to measure magnetic field oscillations.
Fig. 10. Ionospheric Doppler measurements on 28 October 2008. Upper record—path Vackov–Prague. Lower record—path Vackov–Kvˇetna (distance o 10 km, thus ground wave is seen).
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The same electronics as that used by microbarographs was deployed in these measurements. The magnetic search coil was installed at NKC on 14 October 2008. Measurements were made in a frequency range similar to that of oscillations observed by microbarographs— 0.1–10 Hz. Unfortunately, the magnetic signal was observed not in the time of quake in the hypocentre. In case of seismic origin of the observed oscillations it should be observed in the same time as the quake occurs in the hypocentre due to the high speed of magnetic signal propagation. The magnetic signal was observed in the time of arrival of seismic waves to surface (time difference is about 1.5 s from the hypocentre time). This supports the seismic shaking of magnetic sensor, not a real magnetic signal, as explanation of observations. On the other hand, spectrum of oscillations was quite different from the seismic spectrum; periods recorded by the magnetic coil were substantially longer. This might be a quasi-resonant response of measuring system to seismic pulses, but it needs to be confirmed by further testing. Similar results were achieved also for a few other events. It was not possible to test the magnetic instrument in the vibration test equipment TIRA vibration controller SVC01 due to its large size. However, shaking the instrument in hands resulted in substantial ‘‘magnetic’’ oscillations, which supports the hypothesis of the seismic shaking origin of the observed oscillations. A mechanical shaking of the magnetic search coil in the stable Earth’s magnetic field has to produce ‘‘artificial’’ magnetic signal. Magnetic measurements by a magnetometer located 230 and 440 km (quake M¼8.6) from epicentres of two strong spring 2005 Sumatra earthquakes identified two different types of earthquake-related magnetic signal (Hasbi et al., 2009). Shortly after the earthquake, oscillations with periods of about 4–5 s were observed. These oscillations were ascribed to the seismic shaking of instrument in the Earth’s magnetic field. This is result consistent with our observations. About 11 min after the earthquake, pulsations with period of 4.8 min were observed, not observed at distant magnetic stations (i.e. not of geomagnetic micropulsation origin). Hasbi et al. (2009) ascribed them to seismic-induced effects in the ionospheric E-region (dynamo region) caused by the penetration of earthquake-induced acoustic waves of periods 3–5 min. We did not observe such an effect due to the dominance of much shorter periods in seismic spectrum of the earthquake analysed in this paper. Sensor for electric field oscillation measurements has been installed near the end of the earthquake swarm, after the event of 28 October. Moreover, quality of measurements was rather insufficient. Therefore these data have not been used at all in analyzing seismic swarm dataset.
7. Conclusions As far as we know, our unique infrasonic measurements are the first ground-based infrasonic measurements in the epicentre of an earthquake. This earthquake was relatively weak earthquake (M¼3.6); strong earthquake would probably destroy the measuring equipment. The results may be summarized as follows: 1. The infrasonic oscillations ( 1–12 Hz) in the epicentre region appear to be excited by the vertical seismic oscillations, as expected. The infrasonic oscillations correlate well with the seismic vertical oscillations, as shown in Figs. 5 and 7, and are well expressed. They are excited by the mechanical vertical motion of the surface during earthquake, which makes the oscillatory compression and depression of the atmosphere. The microbarograph-measured oscillations are not caused by
2.
3.
4.
5.
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seismic shaking of measuring instrument; they are essentially a real infrasound, as proved by vibration tests of microbarographs. They are not affected by infrasound oscillations of meteorological origin due to quiet meteorological conditions locally and regionally. Infrasonic oscillations observed at a distance of 155 km from the epicentre appear to be seismically excited in situ, they do not represent the infrasound coming from the epicentre region. No infrasonic effects of this earthquake were observed in the ionosphere due to the low earthquake magnitude and too short periods of excited infrasound to be capable to excite ionospheric effects and to be identified by the Doppler measuring system. The observed magnetic effects were rather the effects of seismic shaking of magnetic sensor than real earthquake effects. It is very important to check, which observed apparent earthquake effects are real effects, because some of them may be a consequence of seismic shaking of measuring equipment.
The results clearly demonstrate the dominant role of vertical component of seismic motions in exciting infrasonic waves. The finding that at least at some distances the infrasound excited in situ by seismic waves and not the epicentral infrasound may be dominant infrasonic signal of earthquake origin might be useful for seismo-infrasonic studies and may be of some relevance for the CTBT network aimed at remote detection of underground nuclear explosions. The atmospheric–ionospheric coupling via seismically-induced atmospheric waves will be treated in more detail in another paper.
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