Identification and Observations of Dynamically Triggered Earthquakes near the Alto Tiberina Fault in the central Apennines, Italy

Its mixed-mode slip behavior and seismic potential due to its spatial extent makes the Alto Tiberina Fault (ATF), a 60 km long, low angle normal fault in Central Italys Apennines, a tectonic structure of substantial research. Significant regional extension is accommodated by the ATF, though mainly microseismicity has been recorded along the fault. The hanging wall of the fault contains a number of synthetic and antithetic faults which, in contrast to the main fault, produce episodic swarms of microseismicity and moderately sized earthquakes sequences. The presence of CO2 fluid-over pressurization confirmed by borehole measurements several kilometers deep in the ATF footwall, have been proposed as the cause of the high microseismicity rate. If the presence of fluids is responsible for the high release of microseismic events in the ATF hanging wall, that may also increase the likelihood of dynamic earthquake triggering. Analyzing two high resolution seismic catalogs, from January 2010 to December 2014, with events with magnitudes ranging from ~0.00 < M < 4.05, we identify periods of statistically significant increases and decreases in seismicity rate that we interpret as dynamically triggered seismicity and fault clamping. We analyze the spatial and temporal distribution of the triggered activity and make systematic characterizations of the traditional parameters associated with dynamic triggering such as PGV, peak dynamic stress, as well as, the incidence angle, distance, amplitude, frequency content, and magnitude of the triggering of the event. Our observations provide the first significant evidence for dynamic triggering in the ATF and offer new insight about the mechanics of earthquake triggering in extensional systems.