Mixed-Mode Slip Behavior of the Altotiberina Low-Angle Normal Fault System (Northern Apennines, Italy) through High-Resolution Earthquake Locations and Repeating Events

We generated a 4.5-year-long (2010–2014) high-resolution earthquake catalogue, composed of
~37,000 events with ML < 3.9 and MC = 0.5 completeness magnitude, to report on the seismic activity of the
Altotiberina (ATF) low-angle normal fault system and to shed light on the mechanical behavior and seismic
potential of this fault, which is capable of generating a M7 event. Seismicity defines the geometry of the fault
system composed of the low-angle (15°–20°) ATF, extending for ~50 km along strike and between 4 and
16 km at depth showing an ~1.5 km thick fault zone made of multiple subparallel slipping planes, and
a complex network of synthetic/antithetic higher-angle segments located in the ATF hanging wall (HW)
that can be traced along strike for up to 35 km. Ninety percent of the recorded seismicity occurs along the
high-angle HW faults during a series of minor, sometimes long-lasting (months) seismic sequences with
multiple MW3+ mainshocks. Remaining earthquakes (ML < 2.4) are released instead along the low-angle ATF
at a constant rate of ~2.2 events per day. Within the ATF-related seismicity, we found 97 clusters of repeating
earthquakes (RE), mostly consisting of doublets occurring during short interevent time (hours). RE are
located within the geodetically recognized creeping portions of the ATF, around the main locked asperity.
The rate of occurrence of RE seems quite synchronous with the ATF-HW seismic release, suggesting that
creeping may guide the strain partitioning in the ATF system. The seismic moment released by the ATF
seismicity accounts for 30% of the geodetic one, implying aseismic deformation. The ATF-seismicity pattern is
thus consistent with a mixed-mode (seismic and aseismic) slip behavior.