Seismic Activity Along a Low-Angle Normal Fault: The Case Study of the Alto Tiberina Fault (Northern Apennines, Italy).

Abstract:

Crustal faults are complex systems whose mechanical conditions and physical properties vary in space and time. The availability of long time series of high-resolution multidisciplinary data is a fundamental requirement to understand the multi-scale physical processes controlling faulting and earthquake generation processes. A crucial step for the seismological community is the availability of high-resolution catalogues of accurate earthquake locations describing the long-term brittle response of the fault activity.

We report on the seismicity recorded in a sector of the Northern Apennines undergoing active extension and where a set of high-angle normal faults is detached by the Alto Tiberina low-angle normal fault (ATF). From 2010, this area hosts a research infrastructure (the Alto Tiberina Near Fault Observatory, TABOO), consisting of a dense array of multi-sensor stations, devoted to studying the earthquakes preparatory phase and the fast and slow deformation processes along the ATF fault system.

We show the location of about 30k low-magnitude earthquakes recorded during the past 4 years (2010-2014) showing a completeness magnitude of the catalogue of M_L0.5. Data have been analyzed by combining an automatic picking procedure for P and S waves, together with cross-correlation and double-difference location methods.

The seismicity pattern shows an almost constant and high rate of seismic release along the ATF plane (r = 24.30e−04 eqks/day*km²) with clusters of similar events identified all along the fault plane. On the contrary a long series of low magnitude seismic sequences (M_max 4.0) activated minor synthetic and antithetic normal faults segments (4–5 km long) located in the main fault hanging-wall. The geometry of the fault segments described by the seismicity distribution is cinematically consistent with the focal mechanisms solutions.