Slow earthquakes and low frequency tremor along the Apennines, Italy

This paper reviews the main observations on slow earthquakes and low frequency tremor made along the Apen- nines, the main seismic active zone of Italy. These observations have been made using a geodetic interferome- ter system operating since 1994 in the underground tunnel of Gran Sasso, central Italy, and an underground seis- mic array (UNDERSEIS) operating since 2002 in the same environment. The observations made in recent years indicate that both phenomena are quite rare and apparently uncorrelated. Slow earthquakes, mainly recorded in 1997 and occasionally later, have probably been caused by the activity of a shallow fault system located near the interferometers. Until now only one tremor episode characterized by low frequency content and duration of several hours has been detected in January 2004, without any correlations with the occurrence of slow or regular earthquakes. The signal to noise ratio of this event is very low, but the results of our detailed analysis show that its frequency contents and wave field characteristics are compatible with a low frequency non volcanic tremor.