Disturbances in LF radio signals and the Umbria-Marche (Italy) seismic sequence in 1997-1998

The electric field strength of three broadcasting stations - CLT (189 kHz, Sicily, Italy), MCO (216 kHz, France) and of CZE (270 kHz, Czech Republic) - has been monitored since 1997 with a sampling frequency of ten minutes at two sites, named AS and PE, located in the Central Apennines (Central Italy). The strongest seismic activity occurring in Central Italy during the last decade was the 1997-1998 Umbria-Marche seismic sequence; over a period of eight months, seven earthquakes with magnitudes greater than 5.0 occurred. The distances of the epicentres from the radio receivers were over 100 km. Unfortunately, at the PE site there were problems with both the power supply and saturation of the recorded signals so we could not use the data from this receiver in this study. At the AS measurement site, no anomalies were evident in the signals received from the three broadcasting stations before the main seismic event (M=5.9) of September 26, 1997. However, during February 1998, we observed a significant increase in the electric field strength of the CZE broadcasting station. On March 26, 1998 a strong earthquake (M=5.5) occurred in the same zone, starting the final phase of the Umbria-Marche seismic sequence. The seismic sequence was shallow with focal depths ranging from I kin to a maximum of 10 km; on the contrary the March 1998 earthquake was unusual because of its great focal depth (similar to 50 km). The path from the CZE station to the receiver crosses the epicentral zone so it is possible that the increase in signal strength detected could be a precursor of the March 1998 earthquake, possibly related to an ionospheric effect. On the basis of a single observation, however, one can only tentatively suggest this as a precursor. (C) 2001 Elsevier Science Ltd. All rights reserved.