A Large Fault Partially Reactivated During Two Contiguous Seismic Sequences in Central Italy: the Role of Geometrical and Frictional Heterogeneities

Several seismic sequences have occurred in the normal fault system of Central Italy over the past few years. In particular, in the Campotosto area, the Gorzano fault with a dip of about 50° and a length of about 25 km, potentially capable of hosting events of magnitude about 6.5 was partially reactivated by two major sequences. On 18 January 2017, the Amatrice- Visso-Norcia (AVN) sequence, located north of the Campotosto area, partially reactivated the northernmost portion of Gorzano fault with four events of 5.0 < Mw < 5.5 in three hours. Similarly, the 2009 L'Aquila sequence, south of the Campotosto area, reactivated the southernmost part of the Gorzano fault with three events of 5.0 < Mw < 5.5. To study the behavior of this fault more in detail, and to constrain its geometry, kinematics and stress field, in this work we recalculated the tensor moments (TDMT) of 136 events with M > 3 occurring in this area since 2009. By integrating these solutions with high-resolution seismicity catalogs of the two sequences and with the slip distribution on the fault derived from strong motion and GPS data available in the literature, we have seen how the seismicity distribution of recent years has reactivated the whole structure at seismogenic depths between 2 and 10-12 km depth. In particular, the listric geometry evidenced by seismicity in the southernmost part of the Gorzano fault is consistent with the geometry obtained from TDMT solutions. The presence of slip at shallower depths and seismicity at greater depths is compatible with a different slip behaviors of the rocks due to a variation of the friction properties. Also the stress field inversion of the M > 5 events that occurred in the whole area of the two sequences shows great compatibility with the regional stress field and great consistency with the TDMT solutions of even the smallest events along the Gorzano fault. In this paper we merge the seismological data with the inferred friction properties of the fault to develop a mechanical model that we use to discuss the possible slip behavior of the Gorzano fault.