SPATIAL AND TEMPORAL SEISMICITY CLUSTERING IN CENTRAL-NORTHERN APENNINES: FLUIDS AND SEISMICITY

The Northern Apennines (NAs) are composed by a NE verging thrust-fold belt formed as result of the collision (Oligo-Miocene) between the European plate (Sardinia-Corsica block) and the Adriatic microplate, once the westward subduction of the Tethyan oceanic lithosphere was comple-ted (Alvarez, 1972; Reutter et al., 1980; Argnani, 2002). This mountain chain is dominated by con-comitant extension and compression in two adjacent areas: inner zone of the belt and outer zone (Frepoli and Amato, 1997; Collettini and Barchi, 2002). Our study area is located in the inner zone characterised by Pliocene-Quaternary sedimentation developed in grabens and half-grabens borded by normal faults. This tectonic extension process is associated whith diffuse CO2 degassing (Chiodini et al., 2004). The question whether the CO2 has a metamorphic and/or magmatic upper crust origin is more debated (Minissale et al., 2000; Chiodini et al., 2004; Minissale, 2004; Heinicke et al., 2006). Instrumental seismicity is concentrated in a relatively shallow layer that deepens from the internal to external areas (Chiarabba and Amato, 2003). High fluid pressures (85 % of the litho-static pressure) encountered at shallow crustal depth suggest that deep fluids from deeper layers could play a key role in triggering earthquakes (Chiodini et al., 2004; Antonioli et al., 2005) and seem to control also the spatio-temporal evolution of the seismicity (Piccinini and Antonioli, 2007). Our study area is confined by the Upper Tiber Valley (NSE), Casentino (W), Mugello (NW) and the Montefeltro seismic area (NE). We analysed the seismic events recorded both by the National Seismic Network (1981-2001, CSI 1.1; data-set was extracted online