Disturbances in LF radio-signals as seismic precursors

Low Frequency (LF) radio signals lie in the band 30-300 kHz. Monitoring equipment able to measure the electric strength of such signals, at field sites with very low noise levels, were designed and assembled in Italy. From 1993 onwards, the electric Field strength of the MCO (216 kHz, France) broadcasting station has been measured at two sites in Central Italy. At the end of 1996, radio signals from the CLT (189 kHz, Italy) and the CZE (270 kHz, Czech Republic) broadcasting stations were included in the measurements. During this monitoring period, evident attenuation of the electric field strength in some of the radio signals was observed at some of the receivers. The duration of the attenuation observed was several days and so it could have been related to particular meteorological conditions. On the other hand, this phenomenon could also represent precursors of moderate (3.0 less than or equal to M less than or equal to 3.5) earthquakes that occurred near the receivers (within 50 km) along the transmitter-receiver path. In this case it is possible that some local troposphere defocusing of the radio signals, produced by the pre-seismic processes, might have occurred, These observations were related only to moderate earthquakes and in these cases it may be that suitable meteorological conditions are needed to observe the effect. During February-March 1998 at one measuring site, we observed a significant increase in the CZE electric field strength. Unfortunately, the data of the other receiver could not be used in this case because of frequent interruptions in the recordings. The increase might have been a precursor of a strong earthquake (M = 5.3) that occurred on March 26, 1998 in the Umbria-Marche zone at a location over 100 km from the receiver, but which lay along the transmitter-receiver path. In this case, it is possible that an ionospheric disturbance, produced by the pre-seismic processes, might have occurred. If this pre-seismic behaviour of the LF signals could be confirmed then this type of precursor would seem capable of giving information on the direction, and perhaps even the rough location, of a forthcoming earthquake.