ABSTRACT

Rn-222 and total helium contained in saline waters and associated gaseous fluids from a series of mud volcanoes occurring along the northeastern border of the Apennine chain (northern Italy) have been determined as possible precursors of local earthquakes. Investigations spanned over about one year (May 1986-May 1987) during which a series of low- to medium-magnitude (M = 3-4.5) local earthquakes occurred with distinct focus. Spike-like Rn anomalies were almost systematically observed a short time (hours to few days) before and after the main shocks. These anomalies were frequently accompanied by slight but significant short-term changes in water chemistry. A unique very strong He anomaly was recorded in coincidence with a shock in an area very close to the focus. The significance of the observed short-term anomalies of Rn and hydrochemical parameters (chlorinity, alkaline earths content) is explained in terms of an upsetting of the equilibrium between two chemically distinct (pellicular and normal) connate waters in confined reservoirs. Pellicular water trapping radiogenic Rn would be abruptly liberated into normal connate water by a tectonic stress change. In most cases, helium seems to be insensitive to seismic events, this probably being due to the shallowness of the foci. The data confirm that mud volcanoes represent very sensitive natural systems for predicting local earthquakes.