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Tsunami-driven ionospheric perturbations associated with the 2011 Tohoku earthquake as detected by subionospheric VLF signals

Tsunami-driven ionospheric perturbations associated with the 2011 Tohoku earthquake as detected... Geomatics, Natural Hazards and Risk, 2014 Vol. 5, No. 4, 285–292, http://dx.doi.org/10.1080/19475705.2014.888100 Tsunami-driven ionospheric perturbations associated with the 2011 Tohoku earthquake as detected by subionospheric VLF signals A. ROZHNOIy, M. SOLOVIEVAy, M. HAYAKAWA*z, H. YAMAGUCHIx, Y. HOBARAz, B. LEVIN{ and V. FEDUNj yInstitute of Physics of the Earth, Russian Academy of Sciences, 10 B. Gruzinskaya, Moscow, 123995 Russia zDepartment of Electronic Engineering, University of Electro-Communications, 1-5-1 Chofugaoka, Chofu Tokyo 182-8585, Japan xFuji Security Systems Co. Ltd., 9 Iwato-cho, Shinjuku-ku Tokyo, 160-0832, Japan {Institute of Marine Geology and Geophysics, Far East Branch of Russian Academy of Sciences, Yuzhno-Sakhalinsk, 693022, Russia jDepartment of Automatic Control and Systems Engineering, University of Sheffield, Sheffield S1 3JD, UK (Received 25 June 2013; accepted 17 January 2014) The subionospheric data from a Japanese very low frequency/low frequency (VLF/LF) receiving station at Moshiri, Hokkaido, are used to detect the response of the lower ionosphere to the tsunami triggered by the 2011 Tohoku earthquake. Disturbances in the phase and amplitude of VLF signals propagating from the transmitter in Hawaiian Islands are observed during the tsunami wave passage, and these effects in the ionosphere are compared to the in situ sea-level global positioning system (GPS) measurements near Japan. The frequency of the maximum spectral amplitude both for the VLF and GPS data is found to be in the range of periods of 8–50 min, which is likely to correspond to the period of the internal gravity waves generated by the tsunami. 1. Introduction Recent progress in the detection of ionospheric perturbations has shown that the iono- sphere is sensitive to seismic phenomena such as earthquakes and tsunamis. Experi- mental observations of tsunami-driven effects in the ionosphere were based on theoretical studies made on internal gravity waves (IGWs) by Hines (1972), Najita et al. (1974), and Peltier and Hines (1976). Tsunami-generated IGWs propagate upward into the atmosphere where they are amplified because the air density decreases with increasing height. When they reach the ionospheric heights they dissipate and produce perturbations in the plasma density, which can be detected by electromagnetic waves such as subionospheric very low frequency/low frequency (VLF/LF) signals. The first tsunami-related ionospheric observation was made by Artru et al. (2005) during the trans-Pacific tsunami generated by the 2001 Peru earthquake, and they used measurements of the total electron content (TEC) from the very dense Japanese GPS network. To corroborate the tsunamigenic hypothesis of these disturbances in *Corresponding author. Email: hayakawa@hi-seismo-em.jp 2014 Taylor & Francis 286 A. Rozhnoi et al. the ionospheric plasma Occhipinti et al. (2006) reproduced, with a 3D numerical modelling of the ocean-atmosphere-ionosphere coupling, the tsunami signature in TEC for the 2004 Sumatra tsunami. Similar observations in TEC were performed during the 2006 Kuril, the 2009 Samoa, the 2010 Chile, and the 2011 Tohoku tsuna- mis (Rolland et al. 2010; Galvan et al. 2011; Komjathy et al. 2012). Rolland et al. (2010) compared spectral signature of TEC variations in data recorded by a GPS net- work in Hawaii to the in situ sea-level measurements confirming the hypothesis of the tsunami signature in GPS data. Mostly, ionospheric perturbations produced by tsunamis were observed far away from the epicentres of tsunamigenic earthquakes where these perturbations can be clearly seen. In the recent work of Occhipinti et al. (2013) the early tsunami signature in the ionosphere was first detected in TEC data in the epicentral area. It was demonstrated that acoustic-gravity waves generated at the epicentre by the direct vertical displacement of the source rupture and the gravity waves coupled with the tsunami can be discriminated with the theoretical support. Another technique that can detect tsunami-driven IGWs is based on over-the- horizon (OTH) radars. OTH radars operate in high frequency (3–30 MHz) band and sound the bottom side ionosphere. The possibility to detect tsunami-generated IGWs in the low ionosphere by an OTH radar was explored theoretically by Coisson et al. (2011). As for the Tohoku 2011 tsunami, Makela et al. (2011), using an imaging system, have observed the airglow layers in the ionosphere during the passage of the tsunami. The results have been modelled and explained by Occhipinti et al. (2011). In our previous work by Rozhnoi et al. (2012) we presented the first observations of tsunami-induced phase and amplitude perturbations of subionospheric VLF sig- nals using the data from two receivers in Petropavlovsk–Kamchatsky and Yuzhno– Sakhalinsk (Russia). The analysis was made for tsunamis caused by the Kuril 2006 and the Tohoku 2011 earthquakes. Specific perturbations in the phase and amplitude of VLF signals starting about an hour and a half after the occurrence of earthquakes have been found. A qualitative interpretation of the observed effects has been sug- gested in terms of the interaction of IGWs with the lower ionosphere. Here as a con- tinuation of our analysis of the Tohoku tsunami from subionospheric VLF signals, we use measurements from a Japanese receiving station in Moshiri, Hokkaido and recordings of GPS sensor buoys of the Japanese Nationwide Ocean Wave Informa- tion Network for Ports and Harbours (NOWPHAS). 2. Results of analysis The earthquake with magnitude Mw ¼ 9.0 occurred on March 11, 2011 at 05:46 UT near Tohoku. The epicentre of the main shock was located at the geographic coordi- nates, 38.322 N and 142.369 E with a focal depth of 24 km (NEIC/USGS). The tsunami with heights 10–15 m (in some places 20 m) severely affected the north-east coast of the Honshu island and caused serious casualties and extensive destructions including a technogenic disaster at the Fukushima-1 nuclear power plant. This tsunami was recorded by the tide gauges of Tsunami Warning Centers throughout the Pacific Ocean (http://wcatwc.arh.noaa.gov/about/tsunamimain.php). In this work the analysis of the tsunami is based on the data recorded by the VLF receiver located in Moshiri (Japan). For comparison we also show the recordings of the VLF signal in Yuzhno–Sakhalinsk (Russia) station, which were presented in our previous paper (Rozhnoi et al. 2012). These VLF/LF receivers measure simultaneously Geomatics, Natural Hazards and Risk 287 the amplitude and phase of signals from the transmitters located in Japan (JJY and JJI), and also in Australia (NWC) and Hawaiian Islands (NPM) with time resolution 20 sec (Hayakawa & Hobara 2013). We compare our VLF data with the measure- ments from NOWPHAS GPS buoys. Figure 1 shows the position of the VLF receivers in Yuzhno–Sakhalinsk (YSH) and Moshiri (MSR) and the Hawaiian transmitter NPM (21.4 kHz), two Japanese transmitters JJY (40 kHz) and JJI (22.2 kHz) together with the position of the deep water DART buoys and NOWPHAS GPS buoys in the region under analysis. The ellipses for each path in figure 1 show the sensitivity zones, which correspond to the fifth Fresnel zone. All four sensitivity zones are shown for the receiver in Yuzhno–Sakhalinsk and for the receiver in Moshiri only NPM–MSR path is shown. To analyse the VLF signal variations observed after the earthquake the subionospheric paths NPM–YSH and NPM–MSR were used because they extend along the propagation direction of the tsunami (they are shown by the filled ellipses in the figure). Positions of the other propagation paths are not found to coincide with the tsunami direction (hollow ellipses). The phase and amplitude perturbations of the NPM signal recorded in Yuzhno– Sakhalinzk and Moshiri stations on March 11, 2011 (red line) together with the monthly averaged signal (blue line) are shown in figure 2. The latter was calculated using the data from undisturbed days. A vertical line in the figure shows the occur- rence time of the earthquake on 11 March 2011. Horizontal green rectangles show an interval of the tsunami-related perturbation in the VLF signal. In both stations the VLF signal exhibits a significant decrease in amplitude (about 10–15 dB) together with the phase variations of up to 40 degrees relative to the monthly averaged signal. It should be noted that the signals from other transmitters (JJY, JJI and NWC) recorded on the same day did not reveal any appreciable perturbations. The Figure 1. A map showing the position of two VLF receivers in Yuzhno–Sakhalinsk (YSH) and Moshiri (MSR) (two green dots) and the transmitters NPM (21.4 kHz) in Hawaii, JJY (40 kHz) and JJI (22.2 kHz) in Japan together with the position of the deep water DART sta- tions (21, 401, 51, 407) and NOWPHAS GPS buoys (807, 802) in the region under analysis. The solid brown circle shows the position of the earthquake epicentre on 11 March 2011 (from USGS/NEIC http://neic.usgs.gov/neis/epic/epic_global.html). The ellipses are projections of the wave sensitivity zone (defined by the fifth Fresnel zone) on the earth’s surface. 288 A. Rozhnoi et al. Figure 2. Records of phase (left) and amplitude (right) of the subionospheric signals from the NPM (21.4 kHz) transmitter measured in Yuzhno–Sakhalinsk and Moshiri on 11 March 2011. Blue and red lines are the averaged and observed signals, respectively. Dotted black lines are confidence interval ( sigma). The vertical dotted violet line shows the occurrence time of the earthquake on 11 March 2011. Horizontal green rectangles show an interval of the tsunami-related perturbation in the VLF signal. anomalies in figure 2 are observed during all local night, which lasts from 8 to 16 UT (LT ¼ 17–25 h) for the amplitude and they are a little shorter for the phase. So, the delay between the earthquake and observed effects is 3 hours. This fact was discussed in details in our previous work. As is seen from figure 2 the Tohoku earthquake occurred at a time coincident with a strong signal perturbation caused by the evening terminator. As a result, the actual onset of the signal anomaly may be hidden, and it was confirmed by the spectral analysis of the signal. The VLF signal exhibits a diurnal effect with a strong change in the signal level during the periods of sunset and sunrise when the altitude of lower ionosphere changes abruptly [so-called terminator times (e.g. Hayakawa et al. 1996)]. Waves in the VLF frequency range reflect from the D region of the ionosphere at altitudes of about 85 km at night and from the sporadic layer in the daytime (60 km), which is generated by solar radiation. Therefore the condition of the day lower ionosphere is determined solely by the activity of the sun. As a result the VLF signal is very stable Geomatics, Natural Hazards and Risk 289 in daytime and unaffected by external factors (including magnetic storms) except by X-rays emitted during solar flares. Night-time, however, provides the optimal condi- tions for the detection of ionospheric disturbances (caused by magnetic storms, earthquakes, tsunami, etc.) by the VLF signals. In our previous paper (Rozhnoi et al. 2012) we compared fluctuation periods in the spectra of the amplitude and phase of VLF signal with periods observed in the data recorded by the DART sensor buoys. The maximum spectral amplitude of VLF sig- nal recorded in Yuzhno–Sakhalinsk station on 11 March 2011 was found to be in a range of periods of 8–30 min, which is consistent with periods of the tsunami. Periods of 14 and 26 minutes were found in the spectral analysis of data from the buoy 51407 as shown by Makela et al. (2011). Here we compare the spectra of VLF data from Moshiri station and spectra of the sea level oscillations recorded by the GPS buoys located along the north cost of Honshu as shown in figure 1. We used the data from GPS buoys 802 and 807 in figure 1 which are the nearest to the MSR station. Figures 3 and 4 show the recording of the tsunami on 802 and 807 buoys and the cor- responding wavelet spectra of the signal. The tsunami was recorded at about 6 UT at the buoy 802 and about 6:15 at the buoy 807 that is 15 and 30 min, respectively, after Figure 3. Top panel shows the sea-level oscillations recorded by the GPS buoy 807 of the Japanese Nationwide Ocean Wave Information Network for Ports and Harbours (NOW- PHAS) on 11 March 2011 for the period 5 to 17 UT. The bottom panel shows the correspond- ing wavelet spectra of the signal. 290 A. Rozhnoi et al. Figure 4. The same as in Figure 3 but for the GPS buoy 802. the earthquake. The spectral maximum of the signal recorded at the buoy 807 in figure 3 is located from 25 min to about 1 hour with weaker energy in the range of 8–20 min. The maximum energy of the signal recorded at the buoy 802 is in the inter- val 15–55 min and it has a higher-frequency component of 6–15 min. Figure 5 shows the waveforms for the amplitude (left) and phase (right) of the night-time data recorded along the NPM–MSR propagation path. The top panels show the complete waveforms, and those in the middle are the waveforms filtered in the frequency range 0.3–15 mHz (55–1 min). The lower panels show the correspond- ing wavelet spectrograms of the data. The frequency of the maximum spectral ampli- tude is found to lie in the range of 8–50 min for the amplitude of the signal and in the range 10–40 min for the phase. These periods are likely to correspond to the range of periods for IGWs and they are also in compliance with the periods observed in data recorded by the GPS buoys near Japan. Both in the VLF data and in the GPS buoys data periods of about 40–50 min have been revealed along with shorter periods of 8–25 min. It differs slightly from spectral composition of the signal recorded in DART boy 51407 located near Hawaiian Islands (Makela et al. 2011). It can be seen from figure 5 that the IGWs start to interact with the lower iono- sphere approximately 1–1.5 hours after the earthquake during the passage of the evening terminator. Therefore, this interaction is not so effective and the maximum in the VLF signal anomaly is observed with a delay of about two hours, when there Geomatics, Natural Hazards and Risk 291 Figure 5. Top panels show the amplitude (left) and phase (right) of the subionospheric signal from the NPM (21.4 kHz) transmitter recorded on 11 March 2011 in Moshiri. Dotted red lines are the averaged signals. The middle panels illustrate the corresponding signals filtered in the range 0.3–15 mHz. The bottom panels show the wavelet spectra of the filtered signals. Arrows in the middle panels show the occurrence time of the earthquake (indicated by EQ) and arrival times of the tsunami at the DART buoys 21,401 and 51,407. begins the local night. Interaction of IGWs with the lower ionosphere in 1–1.5 hours after the earthquake found from the VLF observations at distances more than 500 km is close enough to the theoretical works on propagation of tsunami-generated IGWs (Occhipinti et al. 2013). Blue arrows in figure 5 mark the arrival times of the tsunami at the DART buoys, which are located at each end of the VLF propagation paths under consideration. Disturbances in the VLF signal continue all the night during tsunami propagation along the path NPM–MSR. 3. Conclusion In this paper we have presented observations of tsunami-induced lower ionospheric perturbations from the analysis of subionosheric VLF signal recorded in Moshiri sta- tion in Japan. The obtained results confirmed our earlier analysis for Tohoku tsunami (Rozhnoi et al. 2012). Periods between 8 and 50 min, coherent with the tsunami, have been found in the observed VLF signals. These periods are also in good agreement with the periods of IGWs. Our observations have confirmed that the tsunami generates ionospheric perturbations that can be detected. The ion- ospheric monitoring by different methods including GPS, airglow observations, sub- ionospheric VLF propagation, etc., can be useful in the development of tsunami monitoring and early warning systems. As mentioned in the Introduction, the thorough interpretation of the observed effects based on the interaction of IGWs with the lower ionosphere was made in our previous paper (Rozhnoi et al. 2012). The observational results presented in the pres- ent paper are in agreement with the theoretical approach. 292 A. Rozhnoi et al. Acknowledgements The authors are grateful to Fuji Security Systems Co. Ltd., Japan, for their partial support. Funding The work was supported by Russian Foundation for Basic Research (RFBR) [grant number 11-05-00155-a] and a joint grant between the UK and Russia [grant number 13-05-92602-KO]. References Artru J, Ducic V, Kanamori H, Lognonne P, Murakami M. 2005. Ionospheric detection of gravity waves induced by tsunamis. Geophys J Int. 160:840–848. doi:10.1111/j.1365- 246X.2005.02552.x Coisson P, Occhipinti G, Lognonne PJ, Molinie P, Rolland L. 2011. Tsunami signature in the ionosphere: a simulation of OTH radar observations. Radio Sci. 46:RS0D20. doi:10.1029/2010RS004603 Galvan DA, Komjathy A, Hickey MP, Mannucci AJ. 2011. The 2009 Samoa and 2010 Chile tsunamis as observed in the ionosphere using GPS total electron content. J Geophys Res. 116:A06318. doi:10.1029/2010JA016204 Hayakawa M, Hobara Y. 2013. Seismo electromagnetic studies in UEC. In: Hayakawa M, editor. Earthquake prediction studies: seismo electromagnetics. Tokyo: TERRAPUB; p. 57–80. Hayakawa M, Molchanov OA, Ondoh T, Kawai E. 1996. The precursory signature effect of the Kobe earthquake on VLF subionospheric signals. J Comm Res Lab Tokyo. 43:169–180. Hines CO. 1972. Gravity waves in the atmosphere. Nature. 239:73–78. Komjathy A, Galvan DA, Stephens P, Butala MD, Akopian V, Wilson B, Verkhoglyadova O, Mannucci AJ, Hickey M. 2012. Detecting ionospheric TEC perturbations caused by natural hazards using a global network of GPS receivers: the Tohoku case study. Earth Planets Space. 64:1287–1294. Makela J, Lognonne P, Hebert H, Gehrels T, Rolland L, Allgeyer S, Kherani A, Occhipinti G, Astafyeva E, Coısson P, et al. 2011. Imaging and modeling the ionospheric airglow response over Hawaii to the tsunami generated by the Tohoku earthquake of 11 March 2011. Geophys Res Lett. 38:L00G02. doi:10.1029/2011GL047860 Najita K, Weaver P, Yuen P. 1974. A tsunami warning system using an ionospheric technique. Proc IEEE. 62(5):563–577. Occhipinti G, Coisson P, Makela JJ, Allgeyer S, Kherani A, Hebert H, Lognonne P. 2011. Three-dimensional numerical modeling of tsunami-related internal gravity waves in the Hawaiian atmosphere. Earth Planets Space. 63:847–851. Special Issue Tohoku. Occhipinti G, Lognonne P, Kherani EA, Hebert H. 2006. Three dimensional waveform model- ing of ionospheric signature induced by the 2004 Sumatra tsunami. Geophys Res Lett. 33:L20104. doi:10.1029/2006GL026865 Occhipinti G, Rolland L, Lognonne P, Watada S. 2013. From Sumatra 2004 to Tohoku-Oki 2011: the systematic GPS detection of the ionospheric signature induced by tsunami- genic earthquakes. J Geophys Res Space Phys. 118. doi:10.1002/jgra.50322 Peltier WR, Hines CO. 1976. On the possible detection of tsunamis by a monitoring of the ion- osphere. J Geophys Res. 81:1995–2000. Rolland LM, Occhipinti G, Lognonne P, Loevenbruck A. 2010. Ionospheric gravity waves detected offshore Hawaii after tsunamis. Geophys Res Lett. 37(17):101. doi:10.1029/ 2010GL044479 Rozhnoi A, Shalimov S, Solovieva M, Levin BW, Hayakawa M, Walker SN. 2012. Tsunami- induced phase and amplitude perturbations of subionospheric VLF signals. J Geophys Res. 117:A09313. doi:10.1029/2012JA017761 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png "Geomatics, Natural Hazards and Risk" Taylor & Francis

Tsunami-driven ionospheric perturbations associated with the 2011 Tohoku earthquake as detected by subionospheric VLF signals

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Abstract

Geomatics, Natural Hazards and Risk, 2014 Vol. 5, No. 4, 285–292, http://dx.doi.org/10.1080/19475705.2014.888100 Tsunami-driven ionospheric perturbations associated with the 2011 Tohoku earthquake as detected by subionospheric VLF signals A. ROZHNOIy, M. SOLOVIEVAy, M. HAYAKAWA*z, H. YAMAGUCHIx, Y. HOBARAz, B. LEVIN{ and V. FEDUNj yInstitute of Physics of the Earth, Russian Academy of Sciences, 10 B. Gruzinskaya, Moscow, 123995 Russia zDepartment of Electronic Engineering, University of Electro-Communications, 1-5-1 Chofugaoka, Chofu Tokyo 182-8585, Japan xFuji Security Systems Co. Ltd., 9 Iwato-cho, Shinjuku-ku Tokyo, 160-0832, Japan {Institute of Marine Geology and Geophysics, Far East Branch of Russian Academy of Sciences, Yuzhno-Sakhalinsk, 693022, Russia jDepartment of Automatic Control and Systems Engineering, University of Sheffield, Sheffield S1 3JD, UK (Received 25 June 2013; accepted 17 January 2014) The subionospheric data from a Japanese very low frequency/low frequency (VLF/LF) receiving station at Moshiri, Hokkaido, are used to detect the response of the lower ionosphere to the tsunami triggered by the 2011 Tohoku earthquake. Disturbances in the phase and amplitude of VLF signals propagating from the transmitter in Hawaiian Islands are observed during the tsunami wave passage, and these effects in the ionosphere are compared to the in situ sea-level global positioning system (GPS) measurements near Japan. The frequency of the maximum spectral amplitude both for the VLF and GPS data is found to be in the range of periods of 8–50 min, which is likely to correspond to the period of the internal gravity waves generated by the tsunami. 1. Introduction Recent progress in the detection of ionospheric perturbations has shown that the iono- sphere is sensitive to seismic phenomena such as earthquakes and tsunamis. Experi- mental observations of tsunami-driven effects in the ionosphere were based on theoretical studies made on internal gravity waves (IGWs) by Hines (1972), Najita et al. (1974), and Peltier and Hines (1976). Tsunami-generated IGWs propagate upward into the atmosphere where they are amplified because the air density decreases with increasing height. When they reach the ionospheric heights they dissipate and produce perturbations in the plasma density, which can be detected by electromagnetic waves such as subionospheric very low frequency/low frequency (VLF/LF) signals. The first tsunami-related ionospheric observation was made by Artru et al. (2005) during the trans-Pacific tsunami generated by the 2001 Peru earthquake, and they used measurements of the total electron content (TEC) from the very dense Japanese GPS network. To corroborate the tsunamigenic hypothesis of these disturbances in *Corresponding author. Email: hayakawa@hi-seismo-em.jp 2014 Taylor & Francis 286 A. Rozhnoi et al. the ionospheric plasma Occhipinti et al. (2006) reproduced, with a 3D numerical modelling of the ocean-atmosphere-ionosphere coupling, the tsunami signature in TEC for the 2004 Sumatra tsunami. Similar observations in TEC were performed during the 2006 Kuril, the 2009 Samoa, the 2010 Chile, and the 2011 Tohoku tsuna- mis (Rolland et al. 2010; Galvan et al. 2011; Komjathy et al. 2012). Rolland et al. (2010) compared spectral signature of TEC variations in data recorded by a GPS net- work in Hawaii to the in situ sea-level measurements confirming the hypothesis of the tsunami signature in GPS data. Mostly, ionospheric perturbations produced by tsunamis were observed far away from the epicentres of tsunamigenic earthquakes where these perturbations can be clearly seen. In the recent work of Occhipinti et al. (2013) the early tsunami signature in the ionosphere was first detected in TEC data in the epicentral area. It was demonstrated that acoustic-gravity waves generated at the epicentre by the direct vertical displacement of the source rupture and the gravity waves coupled with the tsunami can be discriminated with the theoretical support. Another technique that can detect tsunami-driven IGWs is based on over-the- horizon (OTH) radars. OTH radars operate in high frequency (3–30 MHz) band and sound the bottom side ionosphere. The possibility to detect tsunami-generated IGWs in the low ionosphere by an OTH radar was explored theoretically by Coisson et al. (2011). As for the Tohoku 2011 tsunami, Makela et al. (2011), using an imaging system, have observed the airglow layers in the ionosphere during the passage of the tsunami. The results have been modelled and explained by Occhipinti et al. (2011). In our previous work by Rozhnoi et al. (2012) we presented the first observations of tsunami-induced phase and amplitude perturbations of subionospheric VLF sig- nals using the data from two receivers in Petropavlovsk–Kamchatsky and Yuzhno– Sakhalinsk (Russia). The analysis was made for tsunamis caused by the Kuril 2006 and the Tohoku 2011 earthquakes. Specific perturbations in the phase and amplitude of VLF signals starting about an hour and a half after the occurrence of earthquakes have been found. A qualitative interpretation of the observed effects has been sug- gested in terms of the interaction of IGWs with the lower ionosphere. Here as a con- tinuation of our analysis of the Tohoku tsunami from subionospheric VLF signals, we use measurements from a Japanese receiving station in Moshiri, Hokkaido and recordings of GPS sensor buoys of the Japanese Nationwide Ocean Wave Informa- tion Network for Ports and Harbours (NOWPHAS). 2. Results of analysis The earthquake with magnitude Mw ¼ 9.0 occurred on March 11, 2011 at 05:46 UT near Tohoku. The epicentre of the main shock was located at the geographic coordi- nates, 38.322 N and 142.369 E with a focal depth of 24 km (NEIC/USGS). The tsunami with heights 10–15 m (in some places 20 m) severely affected the north-east coast of the Honshu island and caused serious casualties and extensive destructions including a technogenic disaster at the Fukushima-1 nuclear power plant. This tsunami was recorded by the tide gauges of Tsunami Warning Centers throughout the Pacific Ocean (http://wcatwc.arh.noaa.gov/about/tsunamimain.php). In this work the analysis of the tsunami is based on the data recorded by the VLF receiver located in Moshiri (Japan). For comparison we also show the recordings of the VLF signal in Yuzhno–Sakhalinsk (Russia) station, which were presented in our previous paper (Rozhnoi et al. 2012). These VLF/LF receivers measure simultaneously Geomatics, Natural Hazards and Risk 287 the amplitude and phase of signals from the transmitters located in Japan (JJY and JJI), and also in Australia (NWC) and Hawaiian Islands (NPM) with time resolution 20 sec (Hayakawa & Hobara 2013). We compare our VLF data with the measure- ments from NOWPHAS GPS buoys. Figure 1 shows the position of the VLF receivers in Yuzhno–Sakhalinsk (YSH) and Moshiri (MSR) and the Hawaiian transmitter NPM (21.4 kHz), two Japanese transmitters JJY (40 kHz) and JJI (22.2 kHz) together with the position of the deep water DART buoys and NOWPHAS GPS buoys in the region under analysis. The ellipses for each path in figure 1 show the sensitivity zones, which correspond to the fifth Fresnel zone. All four sensitivity zones are shown for the receiver in Yuzhno–Sakhalinsk and for the receiver in Moshiri only NPM–MSR path is shown. To analyse the VLF signal variations observed after the earthquake the subionospheric paths NPM–YSH and NPM–MSR were used because they extend along the propagation direction of the tsunami (they are shown by the filled ellipses in the figure). Positions of the other propagation paths are not found to coincide with the tsunami direction (hollow ellipses). The phase and amplitude perturbations of the NPM signal recorded in Yuzhno– Sakhalinzk and Moshiri stations on March 11, 2011 (red line) together with the monthly averaged signal (blue line) are shown in figure 2. The latter was calculated using the data from undisturbed days. A vertical line in the figure shows the occur- rence time of the earthquake on 11 March 2011. Horizontal green rectangles show an interval of the tsunami-related perturbation in the VLF signal. In both stations the VLF signal exhibits a significant decrease in amplitude (about 10–15 dB) together with the phase variations of up to 40 degrees relative to the monthly averaged signal. It should be noted that the signals from other transmitters (JJY, JJI and NWC) recorded on the same day did not reveal any appreciable perturbations. The Figure 1. A map showing the position of two VLF receivers in Yuzhno–Sakhalinsk (YSH) and Moshiri (MSR) (two green dots) and the transmitters NPM (21.4 kHz) in Hawaii, JJY (40 kHz) and JJI (22.2 kHz) in Japan together with the position of the deep water DART sta- tions (21, 401, 51, 407) and NOWPHAS GPS buoys (807, 802) in the region under analysis. The solid brown circle shows the position of the earthquake epicentre on 11 March 2011 (from USGS/NEIC http://neic.usgs.gov/neis/epic/epic_global.html). The ellipses are projections of the wave sensitivity zone (defined by the fifth Fresnel zone) on the earth’s surface. 288 A. Rozhnoi et al. Figure 2. Records of phase (left) and amplitude (right) of the subionospheric signals from the NPM (21.4 kHz) transmitter measured in Yuzhno–Sakhalinsk and Moshiri on 11 March 2011. Blue and red lines are the averaged and observed signals, respectively. Dotted black lines are confidence interval ( sigma). The vertical dotted violet line shows the occurrence time of the earthquake on 11 March 2011. Horizontal green rectangles show an interval of the tsunami-related perturbation in the VLF signal. anomalies in figure 2 are observed during all local night, which lasts from 8 to 16 UT (LT ¼ 17–25 h) for the amplitude and they are a little shorter for the phase. So, the delay between the earthquake and observed effects is 3 hours. This fact was discussed in details in our previous work. As is seen from figure 2 the Tohoku earthquake occurred at a time coincident with a strong signal perturbation caused by the evening terminator. As a result, the actual onset of the signal anomaly may be hidden, and it was confirmed by the spectral analysis of the signal. The VLF signal exhibits a diurnal effect with a strong change in the signal level during the periods of sunset and sunrise when the altitude of lower ionosphere changes abruptly [so-called terminator times (e.g. Hayakawa et al. 1996)]. Waves in the VLF frequency range reflect from the D region of the ionosphere at altitudes of about 85 km at night and from the sporadic layer in the daytime (60 km), which is generated by solar radiation. Therefore the condition of the day lower ionosphere is determined solely by the activity of the sun. As a result the VLF signal is very stable Geomatics, Natural Hazards and Risk 289 in daytime and unaffected by external factors (including magnetic storms) except by X-rays emitted during solar flares. Night-time, however, provides the optimal condi- tions for the detection of ionospheric disturbances (caused by magnetic storms, earthquakes, tsunami, etc.) by the VLF signals. In our previous paper (Rozhnoi et al. 2012) we compared fluctuation periods in the spectra of the amplitude and phase of VLF signal with periods observed in the data recorded by the DART sensor buoys. The maximum spectral amplitude of VLF sig- nal recorded in Yuzhno–Sakhalinsk station on 11 March 2011 was found to be in a range of periods of 8–30 min, which is consistent with periods of the tsunami. Periods of 14 and 26 minutes were found in the spectral analysis of data from the buoy 51407 as shown by Makela et al. (2011). Here we compare the spectra of VLF data from Moshiri station and spectra of the sea level oscillations recorded by the GPS buoys located along the north cost of Honshu as shown in figure 1. We used the data from GPS buoys 802 and 807 in figure 1 which are the nearest to the MSR station. Figures 3 and 4 show the recording of the tsunami on 802 and 807 buoys and the cor- responding wavelet spectra of the signal. The tsunami was recorded at about 6 UT at the buoy 802 and about 6:15 at the buoy 807 that is 15 and 30 min, respectively, after Figure 3. Top panel shows the sea-level oscillations recorded by the GPS buoy 807 of the Japanese Nationwide Ocean Wave Information Network for Ports and Harbours (NOW- PHAS) on 11 March 2011 for the period 5 to 17 UT. The bottom panel shows the correspond- ing wavelet spectra of the signal. 290 A. Rozhnoi et al. Figure 4. The same as in Figure 3 but for the GPS buoy 802. the earthquake. The spectral maximum of the signal recorded at the buoy 807 in figure 3 is located from 25 min to about 1 hour with weaker energy in the range of 8–20 min. The maximum energy of the signal recorded at the buoy 802 is in the inter- val 15–55 min and it has a higher-frequency component of 6–15 min. Figure 5 shows the waveforms for the amplitude (left) and phase (right) of the night-time data recorded along the NPM–MSR propagation path. The top panels show the complete waveforms, and those in the middle are the waveforms filtered in the frequency range 0.3–15 mHz (55–1 min). The lower panels show the correspond- ing wavelet spectrograms of the data. The frequency of the maximum spectral ampli- tude is found to lie in the range of 8–50 min for the amplitude of the signal and in the range 10–40 min for the phase. These periods are likely to correspond to the range of periods for IGWs and they are also in compliance with the periods observed in data recorded by the GPS buoys near Japan. Both in the VLF data and in the GPS buoys data periods of about 40–50 min have been revealed along with shorter periods of 8–25 min. It differs slightly from spectral composition of the signal recorded in DART boy 51407 located near Hawaiian Islands (Makela et al. 2011). It can be seen from figure 5 that the IGWs start to interact with the lower iono- sphere approximately 1–1.5 hours after the earthquake during the passage of the evening terminator. Therefore, this interaction is not so effective and the maximum in the VLF signal anomaly is observed with a delay of about two hours, when there Geomatics, Natural Hazards and Risk 291 Figure 5. Top panels show the amplitude (left) and phase (right) of the subionospheric signal from the NPM (21.4 kHz) transmitter recorded on 11 March 2011 in Moshiri. Dotted red lines are the averaged signals. The middle panels illustrate the corresponding signals filtered in the range 0.3–15 mHz. The bottom panels show the wavelet spectra of the filtered signals. Arrows in the middle panels show the occurrence time of the earthquake (indicated by EQ) and arrival times of the tsunami at the DART buoys 21,401 and 51,407. begins the local night. Interaction of IGWs with the lower ionosphere in 1–1.5 hours after the earthquake found from the VLF observations at distances more than 500 km is close enough to the theoretical works on propagation of tsunami-generated IGWs (Occhipinti et al. 2013). Blue arrows in figure 5 mark the arrival times of the tsunami at the DART buoys, which are located at each end of the VLF propagation paths under consideration. Disturbances in the VLF signal continue all the night during tsunami propagation along the path NPM–MSR. 3. Conclusion In this paper we have presented observations of tsunami-induced lower ionospheric perturbations from the analysis of subionosheric VLF signal recorded in Moshiri sta- tion in Japan. The obtained results confirmed our earlier analysis for Tohoku tsunami (Rozhnoi et al. 2012). Periods between 8 and 50 min, coherent with the tsunami, have been found in the observed VLF signals. These periods are also in good agreement with the periods of IGWs. Our observations have confirmed that the tsunami generates ionospheric perturbations that can be detected. The ion- ospheric monitoring by different methods including GPS, airglow observations, sub- ionospheric VLF propagation, etc., can be useful in the development of tsunami monitoring and early warning systems. As mentioned in the Introduction, the thorough interpretation of the observed effects based on the interaction of IGWs with the lower ionosphere was made in our previous paper (Rozhnoi et al. 2012). The observational results presented in the pres- ent paper are in agreement with the theoretical approach. 292 A. Rozhnoi et al. Acknowledgements The authors are grateful to Fuji Security Systems Co. Ltd., Japan, for their partial support. Funding The work was supported by Russian Foundation for Basic Research (RFBR) [grant number 11-05-00155-a] and a joint grant between the UK and Russia [grant number 13-05-92602-KO]. References Artru J, Ducic V, Kanamori H, Lognonne P, Murakami M. 2005. Ionospheric detection of gravity waves induced by tsunamis. Geophys J Int. 160:840–848. doi:10.1111/j.1365- 246X.2005.02552.x Coisson P, Occhipinti G, Lognonne PJ, Molinie P, Rolland L. 2011. Tsunami signature in the ionosphere: a simulation of OTH radar observations. Radio Sci. 46:RS0D20. doi:10.1029/2010RS004603 Galvan DA, Komjathy A, Hickey MP, Mannucci AJ. 2011. The 2009 Samoa and 2010 Chile tsunamis as observed in the ionosphere using GPS total electron content. J Geophys Res. 116:A06318. doi:10.1029/2010JA016204 Hayakawa M, Hobara Y. 2013. Seismo electromagnetic studies in UEC. In: Hayakawa M, editor. Earthquake prediction studies: seismo electromagnetics. Tokyo: TERRAPUB; p. 57–80. Hayakawa M, Molchanov OA, Ondoh T, Kawai E. 1996. The precursory signature effect of the Kobe earthquake on VLF subionospheric signals. J Comm Res Lab Tokyo. 43:169–180. Hines CO. 1972. Gravity waves in the atmosphere. Nature. 239:73–78. Komjathy A, Galvan DA, Stephens P, Butala MD, Akopian V, Wilson B, Verkhoglyadova O, Mannucci AJ, Hickey M. 2012. Detecting ionospheric TEC perturbations caused by natural hazards using a global network of GPS receivers: the Tohoku case study. Earth Planets Space. 64:1287–1294. Makela J, Lognonne P, Hebert H, Gehrels T, Rolland L, Allgeyer S, Kherani A, Occhipinti G, Astafyeva E, Coısson P, et al. 2011. Imaging and modeling the ionospheric airglow response over Hawaii to the tsunami generated by the Tohoku earthquake of 11 March 2011. Geophys Res Lett. 38:L00G02. doi:10.1029/2011GL047860 Najita K, Weaver P, Yuen P. 1974. A tsunami warning system using an ionospheric technique. Proc IEEE. 62(5):563–577. Occhipinti G, Coisson P, Makela JJ, Allgeyer S, Kherani A, Hebert H, Lognonne P. 2011. Three-dimensional numerical modeling of tsunami-related internal gravity waves in the Hawaiian atmosphere. Earth Planets Space. 63:847–851. Special Issue Tohoku. Occhipinti G, Lognonne P, Kherani EA, Hebert H. 2006. Three dimensional waveform model- ing of ionospheric signature induced by the 2004 Sumatra tsunami. Geophys Res Lett. 33:L20104. doi:10.1029/2006GL026865 Occhipinti G, Rolland L, Lognonne P, Watada S. 2013. From Sumatra 2004 to Tohoku-Oki 2011: the systematic GPS detection of the ionospheric signature induced by tsunami- genic earthquakes. J Geophys Res Space Phys. 118. doi:10.1002/jgra.50322 Peltier WR, Hines CO. 1976. On the possible detection of tsunamis by a monitoring of the ion- osphere. J Geophys Res. 81:1995–2000. Rolland LM, Occhipinti G, Lognonne P, Loevenbruck A. 2010. Ionospheric gravity waves detected offshore Hawaii after tsunamis. Geophys Res Lett. 37(17):101. doi:10.1029/ 2010GL044479 Rozhnoi A, Shalimov S, Solovieva M, Levin BW, Hayakawa M, Walker SN. 2012. Tsunami- induced phase and amplitude perturbations of subionospheric VLF signals. J Geophys Res. 117:A09313. doi:10.1029/2012JA017761

Journal

"Geomatics, Natural Hazards and Risk" – Taylor & Francis

Published: Oct 2, 2014

References

  • Ionospheric detection of gravity waves induced by tsunamis
  • Tsunami signature in the ionosphere: a simulation of OTH radar observations
  • The 2009 Samoa and 2010 Chile tsunamis as observed in the ionosphere using GPS total electron content
  • Seismo electromagnetic studies in UEC
  • The precursory signature effect of the Kobe earthquake on VLF subionospheric signals
  • Gravity waves in the atmosphere
  • Detecting ionospheric TEC perturbations caused by natural hazards using a global network of GPS receivers: the Tohoku case study
  • Imaging and modeling the ionospheric airglow response over Hawaii to the tsunami generated by the Tohoku earthquake of 11 March 2011
  • A tsunami warning system using an ionospheric technique
  • Three-dimensional numerical modeling of tsunami-related internal gravity waves in the Hawaiian atmosphere
  • Three dimensional waveform modeling of ionospheric signature induced by the 2004 Sumatra tsunami
  • From Sumatra 2004 to Tohoku-Oki 2011: the systematic GPS detection of the ionospheric signature induced by tsunamigenic earthquakes
  • On the possible detection of tsunamis by a monitoring of the ionosphere
  • Ionospheric gravity waves detected offshore Hawaii after tsunamis
  • Tsunami-induced phase and amplitude perturbations of subionospheric VLF signals