Abstract
The US Navy VLF/LF Transmitter’s NSY signal (45.9 kHz) transmitted from Niscemi, Sicily, Italy, and received at the Kiel Long Wave Monitor, Germany, was analyzed for the period of two months, May and June (EQ-month) of 2013. There were 12 earthquakes of magnitude greater than 4 that hit Italy in these two months, of which the earthquake of 21st June having magnitude of 5.2 and a shallow focal depth of 5 km was the major one. We studied the earthquake of 21st of June 2013, which struck Tuscany, Central Italy, (44.1713°N and 10.2082°E) at 10:33 UT, and also analyzed the effects of this earthquake on the sub-ionos- pheric VLF/LF signals. In addition, we also studied another earthquake, of magnitude 4.9, which hit the same place at 14:40 UT on 30th of June and had shallow focal depth of 10 km. We assessed the data using terminator time (TT) method and night time fluctuation method and found unusual changes in VLF/LF amplitudes/phases. Analysis of trend, night time dispersion, and night time fluctuation was also carried and several anomalies were detected. Most ionospheric perturbations in these parameters were found in the month of June, from few days to few weeks prior to the earthquakes. Moreover, we filtered the possible effects due to geomagnetic storms, auroras, and solar activities using parameters like Dst index, AE index, and Kp index for analyzing the geomagnetic effects, and Bz (sigma) index, sunspot numbers, and solar index F10.7 for analyzing the solar activities for the confirmation of anomalies as precursors.
Highlights
Earthquakes still exist as a threatening disaster
The ionospheric layer is highly influenced by geomagnetic winds, solar flares, cosmic rays and gamma-rays from the space, coronal mass ejection, so on and so forth, which can bring about variations in VLF/LF wave propagation
We analyzed the effects of the Tuscany earthquake of 21st June 2013 (5.2 M) and 30th June (4.9 M), which were the two greatest earthquakes in these months, on NSY (North Somerset Yeomanry) signals transmitted from US Navy VLF/LF Transmitter of 45.9 kHz, Niscemi, Sicily, Central Italy (37.1N/14.4E) and received at the Kiel Long Wave Monitor
Summary
Earthquakes still exist as a threatening disaster. Being able to predict earthquakes will certainly help prevent substantial loss of life and property. Due to low attenuation and long wavelength, VLF/LF signals—transmitted from different VLF/LF transmitters established for research or other application purposes—can travel long distance by undergoing consecutive reflections between lower ionosphere and surface of the Earth. This system of ionosphere and earth that guides the VLF/LF wave is called Earth-ionosphere waveguide. We assume shallow depth of the earthquake to be the reason that the seismic activities could cause disturbances in the lower ionosphere These disturbances were received by the signal before they got attenuated due to the short propagation path length and closeness of epicenters to the propagation path; and even the earthquakes of magnitude 5.2 and 4.9 could have caused considerable perturbations
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