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<p>We have been monitoring the Very Low Frequency (VLF) signal of the Indian Navy transmitter VTX (17&#xa0;kHz) at Vijaynarayanam (8° 23′ 13.25" N, 77° 45′ 9.94" E) from the VLF receiving station at the Malda (25° 0′ 39" N, 88° 8′ 28" E) branch of the Indian Centre for Space Physics. Long before the great Nepal Earthquake of 25th April, 2015, we observed usual behaviour of sunrise and sunset terminators, but the anomalous behaviour started on 20th April, 2015 and persisted until 30th April, 2015, in terms of the following crucial VLF parameters—(i) sunrise terminators (SRT), (ii) sunset terminators (SST), (iii) D-layer preparation time (DLPT), (iv) D-layer destruction time (DLDT), (v) the difference between sunrise terminator derived from VLF data and local sunrise time (SRT-LSRT) and (vi) the difference between sunset terminator derived from VLF data and local sunset time (SST-LSST). It is to be remembered that the devastating earthquake (7.8&#xa0;M) occurred on the 25th April, 2015 with several aftershocks at a level of around 6.0 on the Richter scale in the next few days that followed. The signal returned to normal a few days after 25th April, 2015. We believe that the abnormal behaviour could be interpreted to be precursors to the earthquake. While the actual physical processes leading to such effects remain elusive, this appears to be a case of lithosphere-ionosphere coupling. Low-level movement of tectonic plates in the earthquake preparation zone causes the release of radioactive radon gas, which ionizes both the lower and upper atmosphere, thereby perturbing the ionosphere. Moreover, atmospheric gravity waves and radio emissions generated from the lithosphere further disturb the ionosphere and, consequently, the VLF signal.</p>

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Unusual behaviour of VLF signal parameters before the devastating Nepal earthquake of 25th April, 2015

  • 期刊论文

  • 发表日期：

    2025年6月10日

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  • Asit K. Choudhury,
  • Sandip K. Chakrabarti,
  • Md. Washimul Bari,
  • Achintya K. Chatterjee

摘要

We have been monitoring the Very Low Frequency (VLF) signal of the Indian Navy transmitter VTX (17 kHz) at Vijaynarayanam (8° 23′ 13.25" N, 77° 45′ 9.94" E) from the VLF receiving station at the Malda (25° 0′ 39" N, 88° 8′ 28" E) branch of the Indian Centre for Space Physics. Long before the great Nepal Earthquake of 25th April, 2015, we observed usual behaviour of sunrise and sunset terminators, but the anomalous behaviour started on 20th April, 2015 and persisted until 30th April, 2015, in terms of the following crucial VLF parameters—(i) sunrise terminators (SRT), (ii) sunset terminators (SST), (iii) D-layer preparation time (DLPT), (iv) D-layer destruction time (DLDT), (v) the difference between sunrise terminator derived from VLF data and local sunrise time (SRT-LSRT) and (vi) the difference between sunset terminator derived from VLF data and local sunset time (SST-LSST). It is to be remembered that the devastating earthquake (7.8 M) occurred on the 25th April, 2015 with several aftershocks at a level of around 6.0 on the Richter scale in the next few days that followed. The signal returned to normal a few days after 25th April, 2015. We believe that the abnormal behaviour could be interpreted to be precursors to the earthquake. While the actual physical processes leading to such effects remain elusive, this appears to be a case of lithosphere-ionosphere coupling. Low-level movement of tectonic plates in the earthquake preparation zone causes the release of radioactive radon gas, which ionizes both the lower and upper atmosphere, thereby perturbing the ionosphere. Moreover, atmospheric gravity waves and radio emissions generated from the lithosphere further disturb the ionosphere and, consequently, the VLF signal.