P.F. Biagi 1, L. Castellana 1, T. Maggipinto 1, R. Piccolo 1, A. Minafra 1, A. Ermini 2, V. Capozzi 3, M. Solovieva 4, A. Rozhnoi 4, O.A. Molchanov 4, and M. Hayakawa 5 (1) Department of Physics, University of Bari, Italy; (2) Department of Engineering of Enterprise, University of Roma Tor Vergata,Italy; (3) Department of Biomedical Sciences, University of Foggia, Italy; (4) United Institute of the Earth’s Physics, Russian Academy of Science, Moscow, Russia; (5) Department of Electronic Engineering, The University of Electro-Communications, Tokyo, Japan ANOMALIES IN VLF RADIO SIGNALS RELATED TO SEISMICITY DURING NOVEMBER-DECEMBER 2004: A COMPARISON OF GROUND AND DEMETER SATELLITE RESULTS

The VLF/LF (10-60 kHz) radio signals are used for standard frequency and time broadcasts as well as navigation system. Many transmitters are into operation all over the world. In next map the stars indicate the location of the powerful transmitters in the eastern hemisphere.

The electric field strength (intensity) of some of the quoted VLF/LF radio signals is clearly recorded by a receiver on board of the DEMETER satellite.

Here is an example of registration on board of the satellite in the kHz range during one orbit. The horizontal lines are the signals from the VLF/LF transmitters. f = 19.8 kHz NWC The strongest one is at the top.

We analysed the intensities of the FTU (f = 18.3 kHz) and the DFY (f = 16.6 kHz) radio signals recordered on board of the satellite during October 25, 2004 – January 31, The transmitters are located in France and in Germany, respectively.

At first we obtained the signal to noise ratio for the selected signals. Then the data collected at night time were separated and only these data were considered for the analysis. Finally, in order to obtain statistically significant results, the data were averaged on long time intervals. Taking into account some periods of missing data, the following time intervals were selected: October 25-November 22, 2004 November 23-December 12, 2004 December 26, 2004 – January 31, 2005

The results can be presented in this form where the line represents the orbit of the satellite and the different color and size of the rings indicate the “intensity” of the radio signal in an arbitrary scale. The “intensity” is the signal to noise ratio at night time averaged on one of the previous time intervals. Note that the violet represents the greatest values.

Then, we identified the largest earthquakes occurred during the three previous time intervals in Europe and they are here indicated. Note that their occurrence is in the second of the three time intervals, that is November 23- December 12.

Now I am going to present the results for the FTU radio signal in the three quoted time intervals.

October 25-November 22, 2004 November 23-December 12, 2004 December 26, 2004 – January 31, 2005

The main effect is a decrease of the “intensity” of the radio signal in a zone rougly defined by the red circle during the second time interval, that is before or during the occurrence of the quoted large earthquakes.

November 23-December 12, 2004 December 26, 2004–January 31, 2005 Looking at the DFY radio signal the effect is also evident, as here is shown.

So, the data (electric field strength) from the DEMETER satellite have revealed a “seismic decrease” in a zone centered on central-southern Italy, during: N NN November 23-December 12, 2004 Due to the fact that the radio signals data are averaged on all the period we cannot define if the effect is a precursor or not of some of the large earthquakes occurred in Europe during this period and previously shown. But, in any case, the effect exists.

We examined in datails the quoted earthquakes. According to our opinion, the main candidate for producing disturbances is the November 25 event that was really the beginning of an intense seismic sequence lasting more than one month.

On February 2002, in the framework of a scientific cooperation among Japanese, Russian and Italian teams, a receiver able to measure the electric field strength (intensity) and the phase of five VLF/LF radio signals was put into operation at Bari (South Italy).

In order to compare the ground results with the previous ones obtained by the DEMETER data, at first, the intensity data collected by the Bari receiver have been analysed.

The raw data are sampled by the receiver each 5 s and so a large amount of data exists. In order to reduce this amount, one datum each 10 min, averaged between the raw data 5 min before and 5 min after the datum, was taken. Here is an example. raw 10 min avg

Now I am going to present the 10 min averaged data of the five radio signals from October 2004 to January 2005.

October 25-November 22 December 26-January 31

In order to have a clearer scene we calculated for the different radio signals the values averaged on the three time intervals. Here is the result. October 25-November 22 December 26-January 31

October 25-November 22

So, the geomagnetic activity gives the most convincing explanation of the decrease (GE) we presented. The same explanation is valid for the other three (GB,IC,FR) decreases pointed out. No effect appears on the IT signal.

Note that the IC, GB, GE and FR paths practically have the direction of the lines of the geomagnetic field, while the IT path is perpendicular to these lines.

So, for the IT radio signal a seismic effect could be claimed and a result in agreement with the satellite results stands up.

With the ground data we can be more precise as it regards the location of the effect. In fact it can be noted that the time occurrence of the decrease we presented is post seismic with respect to all the earthquakes previously indicated, in particular with respect to the main candidate for producing disturbances that is the seismic sequence in north Adriatic sea started on November 25, 2004.

Note that the path of the IT radio signal is in part inside the possible disturbed zone in ionosphere indicated by the DEMETER data.

In conclusion, as it concerns the radio signals intensity, the analysis of the ground data has revealed one case in agreement with the satellite results and four cases in discrepancy.

In order to compare the ground results with those obtained by the DEMETER data, in a second step, the phase data collected by the Bari receiver from October 2004 to January 2005, have been analysed.

At first the wavelet analysis was applied on the phase data. I am going to present the results of this analysis.

FR November 23-December 12

IC November 23-December 12

Adriatic sea sequence North Europe earthquake

Finally the standard deviation σ of the phase data sets was investigated in the period from October 2004 to January 2005.

By this analysis a clear short drop was revealed on November 23-24, 2004.

GE This anomaly could be a short term precursor of the November 25 earthquake in north Adriatic sea.

The analysis of the ground VLF/LF data, as it concerns the phase, indicates seismic effects. Particularly, if we make reference to the Adriatic sea sequence (started on November 25, 2004), postseismic effects and a precursor could be claimed.

CONCLUSIONS The VLF/LF radio signals present disturbances related to seismicity. The VLF/LF data are very important for increasing the knowledge of the earthquakes precursors. The starting up of ground networks for monitoring VLF/LF radio signals is hoped for, as well as the improvement of the satellite research.