Natural Hazards and Earth System Science www.nat-hazards-earth-syst-sci.net 1561-8633 1684-9981 10 8 2010 10.5194/nhess-10-1751-2010 http://www.nat-hazards-earth-syst-sci.net/10/1751/2010/ http://www.nat-hazards-earth-syst-sci.net/10/1751/2010/nhess-10-1751-2010.html http://www.nat-hazards-earth-syst-sci.net/10/1751/2010/nhess-10-1751-2010.pdf 1751 1757 2010-08-25 Ionospheric anomaly due to seismic activities – Part 2: Evidence from D-layer preparation and disappearance times S. K. Chakrabarti chakraba@bose.res.in S. Sasmal S. Chakrabarti S. N. Bose National Centre for Basic Sciences, JD Block, Salt Lake, Kolkata 700098, India Indian Centre for Space Physics, 43 Chalantika, Garia Station Road, Kolkata 700084, India Maharaja Manindra Chandra College, 20, Ramkanta Bose Street, Kolkata 700003, India We show evidences for anomalous ionospheric behaviour in the signal of Indian navy VLF transmitting station named VTX due to earthquakes in the South Asian region. We concentrate on the variation of the D-layer preparation time (DLPT) and D-layer disappearance time (DLDT) in a period of sixteen months and study their average behaviors. We identify those days in which DLPT and DLDT exhibit significant deviations. Separately, we compute the energy release by earthquakes during this period and show that "anomalous VLF" days are associated with anomalous energy release. We find that the anomaly and the deviation of DLPT and DLDTs from the mean are linearly correlated. We discuss the predictability in this approach and compare with the terminator shift approach using the same set of data. Chakrabarti, S., Sasmal, S., Saha, M., Khan, R., Bhowmik, D., and Chakrabarti, S. K.: Unusual behaviour of D-region Ionization time at 18.2 KHz during seismically active days, Indian J. Phys., 81(5, 6), 531–538, 2007. Clilverd, M. A., Rodger, C. J., and Thomson, N. R.: Investigating seismoionospheric effects on a long subionospheric path, J. Geophys. Res., 104(A12), 28171–28179, 1999a. Clilverd, M. A., Rodger, C. J., and Thomson, N. R.: Sunrise effects on VLF signals propagating over a long north-south path, Radio Sci., 34(4), 939–948, 1999b. Gokhberg et al.: Phys. Earth Planet. Inter., 57, 64-67, 1989. Gufeld, I. L., Rozhnoi, A. A., Tyumensev, S. N., et al.: Radiowave disturbances in period to Rudber and Rachinsk earthquakes, Phys. Solid Earth, 28(3), 267–270, 1992. Lowrie, W.: Fundamentals of Geophysics, Cambridge University Press, UK, 2007. Hayakawa, M., Molchanov, O. A., Shima, N., Shvets, A. V., and Yamamoto, N.: Seismo Electromagnetics: Lithosphere Atmosphere-Ionosphere Couplings, edited by: Hayakawa, M. and Molchanov, O. A., TERRAPUB, Tokyo, Japan, 223~pp., 2003. 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. Geophys., 24, 2219–2225, doi:10.5194/angeo-24-2219-2006, 2006. Molchanov, O. A. and Hayakawa, M.: Subionospheric VLF signal perturbations possibly related to earthquakes, J. Geophys. Res, 103, 17489–17510, 1998. Molchanov, O. A., Hayakawa, M., Ondoh, T., and Kawai, E.: Precursory effects in the subionospheric VLF Signals for the Kobe earthquake, Phys. Earth Planet. In., 105, 239–248, 1998. Rodger, C. J., Clilverd, M. A., and Thomson, N. R.: Modeling of subionospheric VLF signal perturbations associated with earthquakes, Radio Sci., 34(5), 1177–1185, 1999. Rozhnoi, A., Solovieva, M., Molchanov, O., Schwingenschuh, K., Boudjada, M., Biagi, P. F., Maggipinto, T., Castellana, L., Ermini, A., and Hayakawa, M.: Anomalies in VLF radio signals prior the Abruzzo earthquake ($M$=6.3) on 6~April 2009, Nat. Hazards Earth Syst. Sci., 9, 1727–1732, doi:10.5194/nhess-9-1727-2009, 2009. Sasmal, S. and Chakrabarti, S. K.: Ionosperic anomaly due to seismic activities - Part 1: Calibration of the VLF signal of VTX 18.2 KHz station from Kolkata and deviation during seismic events, Nat. Hazards Earth Syst. Sci., 9, 1403–1408, doi:10.5194/nhess-9-1403-2009, 2009. Soloviev, O. V. and Hayakawa, M.: Three-dimensional subionospheric VLF field diffraction by a truncated highly conducting cylinder and its application to the Trimpi effect problem, Radio Sci., 37, 1079, doi:10.1029/2001RS002499, 2002. Soloviev, O. V., Hayakawa, M., Ivanov, V. I., and Molchanov, O. A.: Seismo-electromagnetic phenomenon in the atmosphere in terms of 3D subionospheric radio wave propagation problem, Phys. Chem. Earth, 29, 639–647, 2004.