HIGH RESOLUTION MICROSEISMICITY DATA CONSTRAINING THE ACTIVITY OF THE ALTOTIBERINA LOW-ANGLE NORMAL FAULT NORTHERN APENNINES (CENTRAL ITALY)

Whether normal faults may be seismically active at very low-dips (d<30°) in the upper continental crust is very controversial. Dip estimates, from positively discriminated focal mechanisms, show no moderate and large normal-slip earthquakes on faults dipping less than 30 degrees. In marked contrast three low-angle dips (10°-30°) have been proposed for three normal-fault mechanisms in the Papua New Guinea region, though without positive discrimination of the true rupture plane. In addition a microseismically active detachment, dipping ~15° north, has been proposed beneath the Gulf of Corinth in Greece, but recently this interpretation has been questioned and the microseismicity has been interpreted to be related to the brittle-ductile transition. As a further contribution to the low-angle normal fault debate, we present high resolution microseismicity data constraining the geometry and kinematics of the Altotiberina fault, ATF, located in the Umbria region (Central Italy) the active extending area in the Northern Apennines. In the area (2800 km²) during the time period 2000-2001 (8 months), 30 digital three-component stations recorded 2200 local earthquakes with Ml<3.2. The microseismicity, including many tens of multiple events, has been relocated using the Double-Difference location algorithm and for the best located events, we compute focal mechanisms by using polarity data. The microearthquakes distribution highlight the geometry of the ATF that cuts the first 15 km of the crust for a length of 40 km. In addition the ATF hanging-wall block is affected by small fault-segments, mostly confined in the first 6 km of crust and exhibiting a pure normal faulting mechanism. At depths greater than 8 km, the microearthquakes are only located along the ATF trace that cuts across the Phyllilitc basement. The inversion of the focal mechanisms (150 events) defines a stress field characterized by a NE-trending direction of extension that is consistent with the regional stress computed for Quaternary striated fault planes, large earthquakes and borehole breakouts.