Sensitivity of ionosonde detection of atmospheric disturbances induced by seismic Rayleigh waves at different latitudes
Received: 1 October 2016
Accepted: 11 January 2017
Published: 30 January 2017
Abstract
Ionospheric disturbance was observed in ionograms at Kazan, Russia \((55.85^\circ \hbox {N}, 48.81^\circ \hbox {E})\), associated with the M8.8 Chile earthquake in 2010 \((35.91^\circ \hbox {S}, 72.73^\circ \hbox {W})\). The disturbance was caused by infrasound waves that were launched by seismic Rayleigh waves propagating over 15,000 km along Earth’s surface from the epicenter. This distance was extremely large compared with the detection limit of similar ionospheric disturbances that were previously studied at lower latitudes over Japan. The observations suggest that the sensitivity of ionograms to coseismic atmospheric disturbances in the infrasound range differs at different locations on the globe. A notable difference in the geophysical condition between the Russian and Japanese ionosonde sites is the magnetic inclination (dip angle), which affects the ionosphere–atmosphere dynamical coupling and radio propagation of vertical incidence ionosonde sounding. Numerical simulations of atmospheric–ionospheric perturbation were conducted, and ionograms were synthesized from the disturbed electron density profiles for different magnetic dip angles. The results showed that ionosonde sounding at Kazan was sensitive to the atmospheric disturbances induced by seismic Rayleigh waves compared with that at Japanese sites by a factor of \(\sim\)3.
Keywords
Background
Large earthquakes are known to cause appreciable ionospheric disturbances through lithosphere–atmosphere–ionosphere coupling. The vertical ground motion of seismic waves launches infrasonic acoustic waves (infrasound) into the atmosphere, and the excited waves propagate upward. The amplitude of waves increases with height owing to conservation of momentum, because the atmosphere is rarefied exponentially with height. The neutral particle motion of the acoustic waves induces alternating enhancements and depletions of plasma density through the neutral–ion collisions at ionospheric heights (Maruyama and Shinagawa 2014). The resulting ionospheric perturbation is detected by various radio techniques. The trans-ionospheric radio propagation of signals transmitted from Global Positioning Satellite System (GPS) satellites is used to observe the coseismic effect on the total electron content (TEC) along the ray path (Calais and Minster 1995; Tsugawa et al. 2011; Rolland et al. 2011). The Doppler sounding with continuous high-frequency (HF) radio waves detects the periodic fluctuation of phase length associated with the electron density perturbation near the reflection level (Yuen et al. 1969; Wolcott et al. 1984; Tanaka et al. 1984; Artru et al. 2004; Chum et al. 2012, 2016). The pulsed HF radar tracks horizontal propagation of the perturbation (Nishitani et al. 2011; Ogawa et al. 2012). A combination of the TEC measurements, HF Doppler sounding, magnetometer, and ground infrasonic sounder tracks the vertical propagation of the perturbation, since each technique is sensitive to the disturbances at different height (Liu et al. 2016). The vertical incidence radio sounding (ionosonde) observes the distortion of echo traces (Leonard and Barnes 1965; Yuen et al. 1969; Liu and Sun 2011; Maruyama et al. 2011, 2012, 2016a, b; Maruyama and Shinagawa 2014; Berngardt et al. 2015) and is a technique dealt with in this paper, as described further below.
At remote distances from the epicenter, Rayleigh waves are the most important source of coseismic infrasound because they propagate along Earth’s surface without significant attenuation of the amplitude due to geometrical spreading (Lay and Wallace 1995). The ionospheric anomaly exhibits a characteristic of traveling ionospheric disturbances following the Rayleigh wave propagation (Liu and Sun 2011; Maruyama et al. 2012). Short-period Rayleigh waves in the range of 15–50 s, near the Airy phase, yield ionospheric density fluctuations with vertical wavelengths of 7.5–50 km, because the sound speed is 500–1000 m/s at ionospheric heights. The vertical wavelength is less than the bottom-half thickness of the ionosphere, and several cycles of alternating enhancements and depletions of electron density cause distortion of the ionograms characterized as a multiple cusp signature (MCS) (Maruyama et al. 2011, 2016a, b; Maruyama and Shinagawa 2014). Each cusp is related to a density ledge, and the vertical separation of the ledges is the wavelength of the infrasound propagating in the thermosphere (Maruyama et al. 2012, 2016a). Thus, MCS ionograms are considered to be a wave snapshot. Note that this type of ionospheric anomaly is not detected by TEC measurement because the alternating enhancements and depletions of electron density along the ray path offset the contribution to the TEC changes.
Maruyama et al. (2012) examined earthquakes that occurred worldwide with a seismic magnitude of 8.0 or greater and ionograms observed at five ionosonde sites over Japan during the period from 1957 to 2011 and concluded that the detection of MCSs was limited to epicentral distances shorter than \(\sim\)6000 km. Contrary to this, however, MCSs were observed at several sites in Russia (Kaliningrad and Kazan) and Finland (Sodankylä) with long epicentral distances of 9000–15,000 km after several large earthquakes (Yusupov and Akchurin 2015). One of those events observed at Kazan, Russia, after the M8.8 Chile earthquake in 2010 was examined in detail by Maruyama et al. (2016a, b). A notable difference between Kazan and Japanese ionosonde sites is the inclination of Earth’s magnetic field (dip angle) \(I\), i.e., \(I=72^\circ\) at Kazan and \(I=38, 45, 49, 53, \text {and } 59^\circ\) at Japanese sites (Okinawa, Yamagawa, Kokubunji, Akita, and Wakkanai, respectively). Maruyama et al. (2016a) briefly discussed magnetic inclination effects that potentially affect the sensitivity of ionosondes to upper atmospheric disturbances induced by upward propagating infrasound. Other differences are the topology of Earth’s surface and jet streams, which may distort the wave fronts of acoustic waves propagating from the ground to the ionospheric level, but these are outside the scope of the current paper. The magnetic inclination effects are discussed quantitatively in this paper. Here we present numerical simulations of infrasonic acoustic waves and electron density perturbation induced by vertical ground motion of seismic Rayleigh waves. Ionograms were synthesized from the simulated electron density profiles for different dip angles. Examples of MCS ionograms are presented in the next section. Magnetic inclination effects, numerical simulations, and synthetic ionograms follow. The results are summarized in the last section.
Observations
Ionospheric disturbance caused by 2010 M8.8 Chile earthquake. a Ionogram showing multiple cusp signature observed at Kazan (epicentral distance 15,148 km), Russia, and b seismogram observed at Obninsk (epicentral distance 14,375 km), Russia
A similarly significant MCS was observed at Yamagawa (\(31.2^\circ \hbox {N}\), \(130.62^\circ \hbox {E}\); epicentral distance 1124 km), Japan, after the M7.7 aftershock (\(36.12^\circ \hbox {N}\), \(141.25^\circ \hbox {E}\); 0615:34 UTC) of the massive M9.0 Tohoku-Oki earthquake in 2011, as shown in Fig. 2a. The maximum amplitude of the Rayleigh wave responsible for this MCS was 5.0 mm/s at Tashiro \((31.19^\circ \hbox {N}, 130.91^\circ \hbox {E})\), near Yamagawa (difference in epicentral distances of 23 km), as shown by the horizontal bar at \(\sim\) 0621:30 UTC in Fig. 2b. Figure 3a shows another example of an MCS ionogram observed at Yamagawa during the same earthquake. In this ionogram, the amplitude of the deformation was small, showing wiggles at frequencies of 3.5–6 MHz, which is almost the detection limit of coseismic ionogram deformation. Figure 3b shows the ground motion at Tashiro. The large-amplitude signals before \(\sim\)0556 UTC (goes off-scale in the plot) were the Rayleigh waves excited by the M9.0 main shock. The ground motion responsible for the MCS ionogram in Fig. 3a is shown by the horizontal bar in Fig. 3b and was most probably excited by the M6.6 aftershock at 0558 UTC; the amplitude was 0.5–1 mm/s. (Note that the ionosonde was operated at each quarter-hour and no ionogram corresponding to the main shock was obtained.)
Ionospheric disturbance caused by M7.7 aftershock of 2011 Tohoku-Oki earthquake. a Ionogram showing multiple cusp signature observed at Yamagawa (epicentral distance 1124 km), Japan, and b seismogram observed at Tashiro (epicentral distance 1101 km), Japan
Ionospheric disturbance caused by M6.6 aftershock of 2011 Tohoku-Oki earthquake. a Ionogram showing wiggles observed at Yamagawa (epicentral distance 1261 km), Japan, and b seismogram observed at Tashiro (epicentral distance 1240 km), Japan
Magnetic inclination effects
Neutral–ion coupling
Radio propagation
Frequency dependence of group refractive index for different dip angles (\(I\)). Vertical incidence rays are assumed, and the angle between the ray direction and the magnetic field is \(\pi /2-I\)
Dip angle \((I)\) dependence of group refractive index for different values of \(X\ (=f_p^2/f^2)\). Vertical incidence rays are assumed, and the angle between the ray direction and the magnetic field is \(\pi /2-I\). MCS ionograms were also observed at Kaliningrad, Russia, and Sodankylä, Finland, after the 2012 Sumatra earthquake (epicentral distances \(\sim\)9000 km) (Yusupov and Akchurin 2015)
Numerical simulations
The numerical study on the sensitivity of ionograms to atmosphere–ionosphere perturbation induced by ground motion followed two steps. The simulation of acoustic wave propagation and associated electron density perturbation was first conducted. Then ionograms were synthesized from the perturbed electron density distribution. The calculations were conducted for \(I=45\) and \(72^\circ\) corresponding to Yamagawa and Kazan, respectively.
Unperturbed ionospheric profiles used for simulation of acoustic wave-induced anomaly for \(I=72^\circ\) (orange) and \(45^\circ\) (green)
Amplitude of acoustic wave propagating upward
Perturbation amplitude as a percentage of the background electron density induced by acoustic waves for \(I=72^\circ\) (orange) and \(45^\circ\) (green)
Synthesized ionograms (O-mode only) for \(I=72^\circ\) (orange) and \(45^\circ\) (green) corresponding to ionospheric disturbances in Fig. 8. The virtual height at each frequency is plotted by a symbol “ | ” representing the sounder pulse width so that the plots look like actual ionograms
Summary
Seismic waves cause ionospheric disturbances at remote distances. Rayleigh waves with a period of 15–50 s induce multiple cusp signatures (MCSs) or wiggles in ionogram traces. In the previous observations, this type of irregularity was found to be significant at higher latitudes, such as Kazan, Russia, than Japan’s latitudes. By numerical simulations, it was shown that the sensitivity of ionograms to seismic ground motion varies depending on the magnetic inclination. At higher latitudes with a large dip angle, coupling between the acoustic waves launched by seismic ground motion and the ionospheric plasma is strong, inducing a large amplitude of perturbation in the electron density than that at lower latitudes with a small dip angle. Radio pulses transmitted by ionosondes are sensitive to the electron density perturbation at a small angle between the ray direction and the magnetic fields at higher latitudes. Thus, the combined effect of atmosphere–ionosphere coupling and radio wave propagation at high latitudes yields a high sensitivity of the ionosonde detection of seismic ground motion, which explains the observations of coseismic ionospheric irregularities at a large epicentral distance of 15,000 km such as at Kazan, after the 2010 Chile earthquake.
Declarations
Authors' contributions
TM analyzed the ionograms and seismograms and synthesized the ionograms. HS simulated atmosphere–ionosphere coupling. KY and AA developed and operated the ionosonde at Kazan and searched MCS ionograms at long distances from epicenters. All authors read and approved the final manuscript.
Acknowledgements
Broadband seismograms were obtained from the National Research Institute for Earth Science and Disaster Prevention (NIED), Japan, and Incorporated Research Institutions for Seismology (IRIS). The work at Kazan Federal University was performed in accordance with the Russian Government Program of Competitive Growth of Kazan Federal University.
Competing interests
The authors declare that they have no competing interests.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
Authors’ Affiliations
References
- Artru J, Farges T, Lognonné P (2004) Acoustic waves generated from seismic surface waves: propagation properties determined from Doppler sounding observations and normal-mode modelling. Geophys J Int 158:1067–1077View ArticleGoogle Scholar
- Berngardt OI, Kotovich GV, Mikhailov SY, Podlesnyi AV (2015) Dynamics of vertical ionospheric inhomogeneities over Irkutsk during 06:00–06:20 UT 11/03/2011 caused by Tohoku earthquake. J Atmos Solar Terr Phys 132:106–115View ArticleGoogle Scholar
- Calais E, Minster JB (1995) GPS detection of ionospheric perturbations following the January 17, 1994, Northridge earthquake. Geophys Res Lett 22(9):1045–1048View ArticleGoogle Scholar
- Capon J (1970) Analysis of Rayleigh-wave multipath propagation at LASA. Bull Seism Soc Am 60(5):1701–1731Google Scholar
- Chum J, Hruska F, Zednik J, Lastovicka J (2012) Ionospheric disturbances (infrasound waves) over the Czech Republic excited by the 2011 Tohoku earthquake. J Geophys Res 117:A08319. doi:10.1029/2012JA017767 Google Scholar
- Chum J, Liu JY, Laštovička J, Fišer J, Mošna Z, Baše J, Sun YY (2016) Ionospheric signatures of the April 25, 2015 Nepal earthquake and the relative role of compression and advection for Doppler sounding of infrasound in the ionosphere. Earth Planets Space 68:24. doi:10.1186/s40623-016-0401-9 View ArticleGoogle Scholar
- Davies K (1969) Ionospheric radio waves. Blaisdell Publishing Company, WalthamGoogle Scholar
- Hedin AE (1991) Extension of the MSIS Thermosphere Model into the middle and lower atmosphere. J Geophys Res 96(A2):1159–1172. doi:10.1029/90JA02125 View ArticleGoogle Scholar
- Lay T, Wallace TC (1995) Modern global seismology. Academic Press, San DiegoGoogle Scholar
- Leonard RS, Barnes RA (1965) Observation of ionospheric disturbances following the Alaska earthquake. J Geophys Res 70(5):1250–1253View ArticleGoogle Scholar
- Liu JY, Sun YY (2011) Seismo-traveling ionospheric disturbances of ionograms observed during the 2011 \(\text{M}_{{\rm w}}9.0\) Tohoku Earthquake. Earth Planets Space 63:897–902View ArticleGoogle Scholar
- Liu JY, Chen CH, Sun YY, Chen CH, Tsai HF, Yen HY, Chum J, Lastovicka J, Yang QS, Chen WS, Wen S (2016) The vertical propagation of disturbances triggered by seismic waves of the 11 March 2011 M9.0 Tohoku earthquake over Taiwan. Geophys Res Let 43. doi:10.1002/2015GL067487
- Maruyama T, Shinagawa H (2014) Infrasonic sounds excited by seismic waves of the 2011 Tohoku-oki earthquake as visualized in ionograms. J Geophys Res 119:4094–4108. doi:10.1002/2013JA019707 View ArticleGoogle Scholar
- Maruyama T, Tsugawa T, Kato H, Saito A, Otsuka Y, Nishioka M (2011) Ionospheric multiple stratifications and irregularities induced by the 2011 off the Pacific coast of Tohoku Earthquake. Earth Planets Space 63:869–873View ArticleGoogle Scholar
- Maruyama T, Tsugawa T, Kato H, Ishii M, Nishioka M (2012) Rayleigh wave signature in ionograms induced by strong earthquakes. J Geophys Res 117:A08306. doi:10.1029/2012JA017952 View ArticleGoogle Scholar
- Maruyama T, Yusupov K, Akchurin A (2016a) Ionosonde tracking of infrasound wavefronts in the thermosphere launched by seismic waves after the 2010 M8.8 Chile earthquake. J Geophys Res Space Phys 121:2683–2692. doi:10.1002/2015JA022260 View ArticleGoogle Scholar
- Maruyama T, Yusupov K, Akchurin A (2016b) Interpretation of deformed ionograms induced by vertical ground motion of seismic Rayleigh waves and infrasound in the thermosphere. Ann Geophys 34:271–278. doi:10.5194/angeo-34-271-2016 View ArticleGoogle Scholar
- Nishitani N, Ogawa T, Otsuka Y, Hosokawa K, Hori T (2011) Propagation of large amplitude ionospheric disturbances with velocity dispersion observed by the SuperDARN Hokkaido radar after the 2011 off the Pacific coast of Tohoku Earthquake. Earth Planets Space 63:891–896View ArticleGoogle Scholar
- Ogawa T, Nishitani N, Tsugawa T, Shiokawa K (2012) Giant ionospheric disturbances observed with the SuperDARN Hokkaido HF radar and GPS network after the 2011 Tohoku earthquake. Earth Planets Space 64:1295–1307View ArticleGoogle Scholar
- Rees MH (1989) Physics and chemistry of the upper atmosphere. Cambridge University Press, CambridgeView ArticleGoogle Scholar
- Rolland LM, Lognonné P, Astafyeva E, Kherani EA, Kobayashi N, Mann M, Munekane H (2011) The resonant response of the ionosphere imaged after the 2011 off the Pacific coast of Tohoku Earthquake. Earth Planets Space 63:853–857View ArticleGoogle Scholar
- Tanaka T, Ichinose T, Okuzawa T, Shibata T, Sato Y, Nagasawa C, Ogawa T (1984) HF-Doppler observations of acoustic waves excited by the Urakawa-Oki earthquake on 21 March 1982. J Atmos Terr Phys 46(3):233–245View ArticleGoogle Scholar
- Tsugawa T, Saito A, Otsuka Y, Nishioka M, Maruyama T, Kato H, Nagatsuma T, Murata KT (2011) Ionospheric disturbances detected by GPS total electron content observation after the 2011 off the Pacific coast of Tohoku Earthquake. Earth Planets Space 63:875–879View ArticleGoogle Scholar
- Wolcott JH, Simons DJ, Lee DD, Nelson RA (1984) Observations of an ionospheric perturbation arising from the Coalinga earthquake of May 2, 1983. J Geophys Res 89(A8):6835–6839View ArticleGoogle Scholar
- Yuen PC, Weaver PF, Suzuki RK, Furumoto AS (1969) Continuous, traveling coupling between seismic waves and the ionosphere evident in May 1968 Japan earthquake data. J Geophys Res 74(9):2256–2264View ArticleGoogle Scholar
- Yusupov K, Akchurin A (2015) Incredibly distant ionospheric responses to earthquake. Geophys Res Abs 17:EGU2015-15198-1Google Scholar