GNGTS 2017 - 36° Convegno Nazionale

92 GNGTS 2017 S essione 1.1 SEISMIC AND ASEISMIC SLIP BEHAVIOUR OF THE ALTOTIBERINA LOW-ANGLE NORMAL FAULT SYSTEM (NORTHERN APENNINES) THROUGH HIGH-RESOLUTION EARTHQUAKE LOCATIONS L. Valoroso 1 , L. Chiaraluce 1 , R. Di Stefano 1 , G. Monachesi 1 1 Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy Low-angle normal faults (LANF, dip < 30°) are geologically widely documented in most extensional settings. They are considered responsible for accommodating large-magnitude crustal extension within the brittle crust, although their mechanical behaviour and seismogenic potential are still enigmatic (e.g., Davis and Lister, 1988; Axen, 2004; Chiaraluce et al. , 2007; Collettini, 2011; and references therein). In fact, these geological structures represent a paradox in a brittle, elastic homogeneous crust, since Anderson-Byerlee frictional fault reactivation theory predicts no slip on normal faults dipping less than 30° in an extending crust characterized by vertical sigma-1 developed in rocks with a friction coefficient in the range of 0.6–0.85. In this study, we shed light on the anatomy, slip-behavior and seismic potential of an actively slipping low-angle normal fault, named Altotiberina fault (ATF), located in the Northern Apennines (Italy), which is capable of generating a M7 event. We computed a high-resolution earthquake catalogue composed by ∼ 37,000 events with M W <3.9 and covering a time window of 4.5-years (2010-2014). Earthquakes have been recorded by a seismic network belonging to the multidisciplinary research infrastructure of the Altotiberina Near Fault Observatory (TABOO). The TABOO seismic network consists of a dense array of sensitive seismographs, installed both at surface and in boreholes, that allow us to reach a very low threshold in seismic event detection (M L around -0.2) and a very low completeness magnitude of the catalogue (M C = 0.5 M L ). The microseismic activity defines the anatomy of the fault system dominated at depth by the Altotiberina low-angle (15°-20°) normal fault extending for ∼ 50km along-strike and between 4-5 to 16 km at depth. The ATF-seismicity hosts 10% of the recorded earthquakes (M L <2.4) released, at an almost constant rate of ∼ 2.2 events/day, within a 1.5-km-thick fault zone composed by multiple sub-parallel slipping planes. The remaining earthquakes occur in the ATF hanging-wall, defining a complex network of high-angle (50° to 60°) synthetic and antithetic faults located between 1 and 6 km depth, that can be traced along-strike for a maximum length of 35-km. These faults have been activated through a series of minor, sometimes long- lasting (several months), seismic sequences associated to progressive migration of multiple mainshock-aftershock sequences with multiple mainshocks of M W >3. These episodes of seismicity migration along contiguous fault segments have been interpreted as diffusion processes in response to pore-fluid-pressure changes in the source volume, as well as episodes of static stress interaction processes that may have contributed in driving the deformation. The footwall of the ATF is instead almost aseismic. Events occurring along theATF (ATF-seismicity) show a seismicity pattern that is consistent with a mixed-mode (stick-slip and stable-sliding) slip-behavior. Seismic moment released by the ATF-seismicity accounts for a small portion (30%) of the geodetic one, implying that part of the deformation was released through aseismic slip. Consistently, we found in the ATF-seismicity, 97 clusters of repeating earthquakes, located within the geodetically detected uncoupled fault portions and surrounding the main locked patch (Anderlini et al. , 2016). Clusters of repeating earthquakes are mostly composed by doublets occurring within short (hours) inter-event-time. The rate of occurrence of repeating earthquakes seems to be synchronous with theATF-hanging- wall seismic release, thus suggesting that the creeping along the LANF may guide the strain partitioning along the ATF-hanging-wall faults.  We better investigated the slip-behaviour of the ATF, by comparing the ATF fault zone structure with the Zuccale fault (ZF), a low-angle normal fault exposed on the easternmost