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

Abstract

Abstract. VLF signals are long thought to give away important information about the lithosphere-ionosphere coupling. In order to establish co-relations, if any, between the ionospheric activities and the earthquakes, we need to understand what the reference signals are, throughout the year. The best opportunity to do this is during the period of solar minimum where the number of flares and sunspots are negligible and the data would be primarily affected by the sun and variation would be due to normal sunset and sunrise effects. In this paper, we present the result of the sunrise and sunset terminators as a function of the day of the year for a period of four years, viz, 2005-2008 when the solar activity was very low. The terminators are for the 18.2 KHz VTX signal of the Indian Navy as observed from Indian Centre for Space Physics receiving station located in Kolkata. A total of 624 days of data have been used to obtain the mean plot. Any deviation of observations from this so-called the standardized calibration curve would point to influences by terrestrial (such as earthquakes) and extra-terrestrial events (such as solar activities). We present examples of deviations which occur in a period of 16 months and show that the correlation with seismic events is significant and typically the highest deviation takes place up to a couple of days prior to the seismic event. Simultaneous observations of such deviations from more than one station could improve the predictability of earthquakes.

Studies of seismo-ionospheric correlations using anomalies in phase of very low frequency signal

“…For a constant long -term statistical analysis, one needs to choose a proper convention to select the terminator time. Say for our previous publications, , Sasmal and Chakrabarti (2009) and as mentioned in Section 1, we have chosen the first minima in sunrise and the last minima during sunset as sunrise and sunset terminator times, respectively. For the phase data, the terminators are highly distinct and there are no secondary minima due to low modal interference.…”

“…This correlation between unusual values of DLPT and DLDT with seismic magnitude shows that the maximum anomaly in the signal occurs on one day before the seismic events. Meanwhile, a statistical study was carried out by Sasmal and Chakrabarti (2009) using the unusual shifts in terminator time by the analysis of a quantity named 'VLF day-length' which is the difference between sunrise and sunset terminator time. In Sasmal and Chakrabarti (2009), it has been reported that the VLF day-length becomes anomalously higher and sometime lower than the normal values during seismic events.…”

A study of the behavior of the terminator time shifts using multiple VLF propagation paths during the Pakistan earthquake (<i>M</i> = 7.2) of 18 January 2011

“…Just after the 2004 devastating earthquake of Indonesia, the seismo-ionospheric correlation was examined by Chakrabarti et al (2005), who came to the conclusion that a definite shift towards the night is present a few days before the earthquake. Subsequently, using the VLF signal from the Indian Navy station VTX (latitude 8.43 • N, longitude 77.73 • E) to Indian Centre for Space Physics (ICSP), Kolkata (latitude 8.43 • N, longitude 77.73 • E), Sasmal et al (2009) showed that the "VLF day length" (defined as the time difference between the two terminator times) becomes anomalously high typically two days before the earthquakes. Subsequently, Ray et al (2010) reported that the "VLF day length" obtained from the VLF signals for VTX-Malda propagation path become anomalously high one day before the earthquake.…”

Studies of VLF signal anomalies due to earthquake

“…Very Low Frequency (VLF)/Low Frequency (LF) radio sounding is one of the most successful tools for this research. There are several independent findings of anomalies in ELF/VLF/LF signal prior to seismic hazards (e.g., Gokhberg et al, 1989;Hayakawa et al, 1996Hayakawa et al, , 2003Molchanov et al, 1998;Molchanov and Hayakawa, 1998;Clilverd et al, 1999;Hayakawa and Molchanov, 2000;Chakrabarti et al, 2005Sasmal et al, 2009Ray et al, , 2012Ray et al, , 2013. Indian Centre for Space Physics (ICSP) (Lat.…”