Anomalies in VLF radio signals related to the seismicity during November–December 2004: A comparison of ground and satellite results

Abstract

A previous study of the intensity of VLF signals radiated by two ground transmitters located in Germany ( f = 16.6 kHz) and France ( f = 18.3 kHz) and received on board the French satellite DEMETER, revealed an intensity drop in the time interval November 23–December 12, 2004, probably connected with the occurrence of earthquakes ( M = 5.4–5.5) in central Europe. From 2002 a receiver has been operating at Bari (Italy) and the intensity and the phase of the VLF/LF radio signals radiated by GB ( f = 16 kHz, United Kingdom), FR ( f = 20.9 kHz, France), GE ( f = 23.4 kHz, Germany), IC ( f = 37.5 kHz, Island) and IT ( f = 54 kHz, Sicily, Italy) are being monitored with a 5 s sampling rate. In order to confirm the results obtained by the satellite measurements, both the intensity and the phase data collected by the Bari receiver from October 2004 to January 2005 have been analysed. As concerns the intensity, drops of the GB, FR, GE and IC signals were revealed before the middle of November and a drop of the IT signal appeared before December 10. The geomagnetic activity gives satisfactory justification of the November drops; on the contrary no correspondence stood up with the drop in December and seismic effects can be claimed. As concerns the phase data, at first the wavelet analysis was applied and clear anomalies were revealed on the FR and IC signals at the end of November. These anomalies appear as precursors with respect to all the mentioned seismic activity. Then, the standard deviation of the phase data sets was investigated and a clear drop was revealed on the GE signal on 23–24 November, that is just before one of the previous large earthquakes. So, not only an agreement of the ground results with those obtained by the satellite appears, but also a more precise definition of the connection between the radio anomalies and the earthquakes stands up using the ground data.