Abstract
Since 2009 a network of VLF (20 - 60 kHz) and LF (150 - 300 kHz) radio receivers is operating in Europe in order to study the disturbances produced by the earthquakes on the propagation of these signals. In 2011 the network was formed by nine receivers, of which three are located in Italy and one is in Austria, Greece, Portugal, Romania, Russia and Turkey. On May 19, 2001 an earthquake (Mw = 5.7) occurred in western Turkey, that is inside the “sensitive” area of the network. The radio data collected during April-May 2011 were studied using the Wavelet spectra, the Principal Component Analysis and the Standard Deviation trends as different methods of analysis. Evident anomalies were revealed both in the signals broadcasted by the TRT transmitter (180 kHz) located near Ankara and in a VLF signal coming from a transmitter located in Western Europe and collected by the receiver TUR of the network located in eastern Turkey. Evident precursor phases were pointed out. Some differences in the efficiency of the three analysis methods were revealed.
Highlights
VLF radio signals lie in the 10 - 60 kHz frequency band
The radio data collected during April-May 2011 were studied using the Wavelet spectra, the Principal Component Analysis and the Standard Deviation trends as different methods of analysis
This study has confirmed that the VLF and LF radio signals are able to give information on the preparatory phase of earthquakes with Mw greater than 5.5
Summary
VLF radio signals lie in the 10 - 60 kHz frequency band These radio signals are used for worldwide navigation support, time signals and for military purposes. They are propagated in the earth-ionosphere wave-guide mode along great circle propagation paths. LF signals lie in 150 - 300 kHz frequency band They are used for long way broadcasting by the few (this type of broadcasting is going into disuse) transmitters located in the world. These radio signals are characterized by the ground wave and the sky wave propagation modes [1]. As already reported by many previous studies [2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18] the disturbances are classified as anomalies and different methods of analysis as the residual dA/ dP [15], the terminator time TT [9], the Wavelet spectra and the Principal Component Analysis have been used [6,7]
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