Correlation between seismic events and anomalous VLF day-length for west-east and east-west propagation paths

Abstract

We present results of year-long analysis of VLF signals, both for VTX-Malda (west-east propagation path) and VTX-Pune (east-west propagation path). We analyzed whole year data of 2008 and 2009 for VTX-Malda and VTX-Pune propagation path, respectively. In both the cases we found that VLF day-length (defined as time difference between sunset terminator time and sunrise terminator time) became anomalously high 1–2 days before an earthquake. Besides this year-long study we have also done some case by case analysis. On 9th January, 2009 an earthquake of magnitude 5.5 occurred at Carlsberg Ridge (latitude 10.3° N, longitude 57.1° E). In a separate incident, on 3rd November, 2009, another earthquake of magnitude 5.4 occurred at Andaman Islands (latitude 14.1° N, longitude 93.1° E). We analyze VLF signals for VTX-Pune (east-west propagation path) propagation path around these two earthquake days and found that for both of these cases, the VLF-day-length became anomalously high two days before the event. This agrees well with statistical analysis based on year-long data for VTX-Pune path. Furthermore, during time period of June, 2010, two major earthquakes of low-depth (10Km) and high magnitude (M > 5) occurred near NWC-Salt Lake (east-west propagation path) propagation paths. First one occurred on 13th June, 2010 at Nicobar Islands, India (latitude 7.8° N, longitude 92.0° E). Its magnitude was 5.1 (in Richter scale). The second one of magnitude 6.0 (in Richter Scale) occurred on 19th June, 2010 at Andaman Islands, India (latitude 13.4° N, longitude 93° E). For both of these two earthquakes, we found that VLF-day-length, became anomalously high one day before earthquakes. We claim that these were pre-cursors of the earthquakes which occurred in Andaman and Nicober Islands.