MICROSEISMICITY ALONG LOW-ANGLE NORMAL FAULTS TRIGGERED BY FLUID OVERPRESSURE: AN EXAMPLE FROM THE ALTOTIBERINA FAULT (CENTRAL ITALY)

In the Northern Apennines extension has been migrating with time from west (Tuscany) to east (Umbria). The map of CO2 Earth degassing derived from the carbon of deep provenance dissolved in the main springs of the Northern Apennines shows a widespread anomaly (1.4* 10¹¹mol/y) in Tuscany. This anomaly seems to vanish in correspondence of the active extensional front located in Umbria. Here the active extension is SSW-NNE oriented and accomplished by a set of NNW-SSE trending, SW dipping normal faults, where the strongest instrumental (5≤M≤6) and historical (intensity≤XI) earthquakes occur. Recently the CROP03 deep seismic reflection profile showed that the area is also affected by a crustal scale east-dipping low angle normal fault, the Altotiberina fault (ATF). Commercial seismic reflection profiles have been integrated with the CROP03 to constrain the geometry of the ATF over an area of 150 km². The fault shows a constant NNW-SSE trend and an average eastward dip of about 20° up to 15 km of depth. The easternmost splay of the ATF bounds the Tiber basin which is infilled by Upper Pliocene-Quaternary sediments: the age of the syntectonic sediments and the displacement of the basin-bounding fault, ~2 km, define a time average long-term slip rate of ~1 mm/yr. Two temporary microseismic networks operated in the time period 1987 and 2001-2002 recording microseismicity with a great number of microearthquakes coinciding with the ATF trace and focal mechanisms defining an extensional stress field. To explain movements on the ATF, severely misoriented for reactivation within the extensional stress field, we propose that the microearthquakes are induced by fluid overpressure. In the area characterised by CO2 anomaly, fluid overpressures are particularly likely in the actively extending crust, where rocks are less fractured making fluid escape more difficult. This is supported by two boreholes of the area, that recorded PCO2 of about 100 and 70 MPa at depth of ~5000 and 4000 m respectively, corresponding to a pore fluid pressure at 80% of the lithostatic load. CO2-rich crustal fluids on their ascent would be trapped by stratigraphycal or structural seals, as the ATF, promoting local short-lived attainment of fluid overpressures and favouring small ruptures.