Electromagnetic and seismoacoustic signals revealed in karst caves (central Italy)

Since 1988-89 equipment for detecting electric, magnetic and seismoacoustic signals has been running inside the Amare cave. The Amare cave is placed on the southern slope of the Gran Sasso chain, that is one of the largest karst areas of the Italian Apennines. In 1992, a similar equipment was installed inside the Cervo cave. This cave is located in another karst area of the Central Apennines, at about 50 km southwestwards of the Amare cave. In both these measurements sites, the signals are recorded every ten minutes in a digital form; the equipment is able to record signals, the frequency of which ranges from some hundred Hz to some hundred kHz. The data collected up to now seem to identify two different states that we call ''quiet'' and ''perturbed'' state. In the quiet state only electric and magnetic signals with the highest frequencies appear. These signals are connected with radio broadcastings and with the general lightnings activity of the Earth. A perturbed state is characterized by the sudden appearance of seismoacoustic signals coupled with electric and magnetic ones. This phenomenology is connected with local processes. Rainfall, atmospheric-pressure variations and some thermal effects are responsible for these local processes. A possible model is proposed to justify the observed phenomenology: micromovements of the limestone blocks that constitute the roof of the caves are invoked for the production of seismoacoustic signals. The electrification generated by these movements is invoked for the production of electric and magnetic signals.