Field strength variations of LF radio waves prior to earthquakes in central Italy

The electric field strength of the LF radio broadcasting RMC (Principality of Monaco) which operates at 216 kHz has been recorded since January 1991 by two receivers in central Italy. During the monitoring period we observed two evident attenuations of the field strength in one receiver, with durations of 6-10 days. The geomagnetic and ionospheric observations carried out in the same time interval do not seem able to explain the attenuation of the radio signal. An analysis of the seismic activity occurring in the area between transmitter and receiver has revealed that some days after the attenuations the energy released by earthquakes reaches a maximum. The observed attenuations might therefore be precursors of earthquakes. We also checked meteorological conditions and found that advections of warm air occurred during both the two anomalous periods. It stems possible that these conditions can help the action of preseismic effects in generating irregularities in the vertical gradient of the tropospheric radio refractivity able to produce defocusing of LF radiobroadcast propagation. (C) 1998 Elsevier Science B.V