Gas flow anomalies in seismogenic zones in the Upper Tiber Valley, Central Italy

Abstract

Cold CO2 gas emission sites in rainwater-filled pools, so called mofettes, are widely distributed all over Italy. Their gas reservoirs, mostly having a high CO2 content, have a magmatic and/or metamorphic origin. Temporal variations in fluid expulsions were observed at the mofettes of Caprese Michelangelo during the period from 2002 to 2005. These observations were made possible by using a new approach: photographic time-series. A first interpretation of these fluid expulsions was based on meteorological/hydrogeological explanations. However, our long-term observations show that these processes may merely be a side effect. The probable main reason for the anomalous emissions is the long-term variation in the long-distance fluid transport process from the reservoir induced by the local tectonic settings. In the northern part of the Alto Tiberina Fault, a fault intersection was reactivated by a seismic sequence which started on 2001 November 26, and continued for approximately four months. The magnitude of the main shock was MW = 4.6. As revealed by the drilling of a deep borehole, dug in the direct vicinity, overpressurized fluids trapped at a depth of 3700 m could be activated as a consequence of the improved transport conditions, that is, the fracture apertures that materialized as a result of the rupture process. A migration of the hypocentres towards the surface provides hints of a possible pore pressure diffusion process. The consequence is an increased fluid transport to the mofettes. The first indications of anomalous fluid expulsions at the mofettes of Caprese Michelangelo were detected 18 months after the seismic events.