NHESS Natural Hazards and Earth System Sciences NHESS Nat. Hazards Earth Syst. Sci. 1684-9981 Copernicus Publications Göttingen, Germany 10.5194/nhess-8-573-2008 On the ionospheric perturbation associated with the 2007 Niigata Chuetsu-oki earthquake, as seen from subionospheric VLF/LF network observations Hayakawa M. 1 Horie T. 1 Yoshida M. 1 Kasahara Y. 1 Muto F. 1 Ohta K. 2 Nakamura T. 3 Department of Electronic Engineering and Research Station on Seismo Electromagnetics, The University of Electro-Communications, 1-5-1 Chofugaoka, Chofu Tokyo 182–8555, Japan Department of Electronic and Information Engineering, Chubu University, Kasugai Aichi, Japan Department of Physics, Kochi University, Kochi Kochi, Japan 17 06 2008 8 3 573 576 Copyright: © 2008 M. Hayakawa et al. 2008 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://nhess.copernicus.org/articles/8/573/2008/nhess-8-573-2008.html The full text article is available as a PDF file from https://nhess.copernicus.org/articles/8/573/2008/nhess-8-573-2008.pdf

In order to investigate any precursory effect of ionospheric perturbations associated with the large 2007 Niigata Chuetsu-oki earthquake (16 July 2007) (magnitude, 6.8), we have made full use of our VLF/LF network observation in Japan by examing the four propagation paths; JJI transmitter (Kyusyu, Ebino)-MSR (Moshiri, Hokkaido), JJY transmitter (Fukushima)-MSR, JJY-KOC (Kochi) and JJI-CBA (Tateyama, Chiba). For the former two paths of JJI-MSR and JJY-MSR, we have observed significant propagation anomalies (both a decrease in nighttime average amplitude and an enhancement in nighttime amplitude fluctuation both satisfying the 2σ (σ: standard deviation) criteria) on 8–9 July, about one week before the earthquake. However, the lack of observation for the path from JJY-KOC, has enabled us to make no conclusion for this path. On the other hand, the path from JJI-CBA does not seem to be perturbed. Although this earthquake is very big and shallow, the amount of VLF anomaly for this earthquake is not so pronounced as expected, probably because the epicenter is not unfortunately located within the sensitive areas of any propagation paths. These observational facts suggest that the lower ionosphere above the epicenter is perturbed definitely prior to the earthquake, with radius of a few hundred kilometers.

 References Dobrovol'sky, I. P., Zubkov, S. I., and, Miachkin, V. I.: Estimation of the size of earthquake preparation zones, 117, 1025–1044, 1979. Hayakawa, M., Molchanov, O. A., Ondoh, T., and Kawai, E.: The precursory signature effect of the Kobe earthquake on VLF subionospheric signals, J. Comm. Res. Lab., Tokyo, 43, 169–180, 1996. Hayakawa, M.: Atmospheric and Ionospheric Electromagnetic Phenomena Associated with Earthquakes, Terra Sci. Pub. Co., 996p, Tokyo, 1999. Hayakawa, M. and Molchanov, O. A. Editors, Seismo Electromagnetics: Lithosphere – Atmosphere – Ionosphere Coupling, TERRAPUB, 477p, Tokyo, 2002. Hayakawa, M., Molchanov, O. A. and NASDA/UEC team, Summary report of NASDA's earthquake remote sensing frontier project, Phys. Chem. Earth, 29, 617–625, 2004. Hayakawa, M., Shvets, A. V., and Maekawa, S.: Subionospheric LF monitoring of ionospheric perturbations prior to the Tokachi-oki earthquake and a possible mechanism to lithosphere – ionosphere coupling, Adv. Polar Upper Atmos. Res., 19, 42–54, 2005. Hayakawa, M., Ohta, K., Maekawa, S., Yamauchi, T., Ida, Y., Gotoh, T., Yonaiguchi, N., Sasaki, H., and Nakamura, T.: Electromagnetic precursors to the 2004 Mid Niigata Prefecture earthquake, Phys. Chem. Earth,31, 356–364, 2006. Hayakawa, M. and O. A., Molchanov, Seismo-electromagnetics as a new field of radiophysics: Electromagnetic phenomena associated with earthquakes, Radio Sci. Bull., 320, 8–17, 2007. Horie, T., Maekawa, S., Yamauchi, T., and Hayakawa, M.: A possible effect of ionospheric perturbations associated with the Sumatra earthquake, as revealed from subionospheric very-low-frequency (VLF) propagation (NWC-Japan), Int'l J. Remote Sensing, 28, 13, 3133–3139, 2007a. Horie, T., Yamauchi, T., Yoshida, M., and Hayakawa, M.: The wave-like structures of ionospheric perturbation associated with Sumatra earthquake of 26 December 2004, as revealed from VLF observation in Japan of NWC signals, J. Atmos. Solar-terr. Phys., 69, 1021–1028, 2007b. Maekawa, S., Horie, T., Yamauchi, T., Sawaya, T., Ishikawa, M., Hayakawa, M., and Sasaki,H.: A statistical study on the effect of earthquakes on the ionosphere, based on the subionospheric LF propagation data in Japan, Ann. Geophysicae, 24, 2219–2225, 2006. Molchanov, O. A., Hayakawa, M., and Miyaki, K.: VLF/LF sounding of the lower ionosphere to study the role of atmospheric oscillations in the lithosphere-ionosphere coupling, Adv. Polar Upper Atmos. Res., 15,146–158, 2001. Molchanov, O. A., and Hayakawa, M.: Seismo-electromagnetics and Related Phenomena: History and Latest Results, TERRAPUB, Tokyo, 189p., 2008. Rozhnoi, A., Solovieva, M. S.,Molchanov, O. A., and Hayakawa, M.: Middle latitude LF (40 kHz) phase variations associated with earthquakes for quiet and disturbed geomagnetic conditions, Phys. Chem. Earth, 29, 589–598, 2004. Ruzhin, Y. Y. and Depueva, K. A.: Seismoprecursors in space as plasma and wave anomalies, J. Atmos. Electr., 16, 271–288, 1996. Shvets, A. V., Hayakawa, M., and Maekawa, S.: Results of subionospheric radio LF monitoring prior to the Tokachi (m=8, Hokkaido, 25 September 2003) earthquake, Natural Hazards Earth System Sci., 4, 647–653, 2004. Sorokin, V. M., Yaschenko, A. K., Chmyrev, V. M., and Hayakawa, M.: DC electric field formation in the mid-latitude ionosphere over typhoon and earthquake regions, Phys. Chem. Earth, 31, 454–461, 2006. Yamauchi, T., Maekawa, S., Horie, T., Hayakawa, M., and Soloviev, O.: Subionospheric VLF/LF monitoring of ionospheric perturbations for the 2004 Mid-Niigata earthquake and their structure and dynamics, J. Atmos. Solar-terr. Phys., 69, 793–802, 2007.