Assessing fault-earthquake relationships for low-grade seismic sequences (ML<4.5): examples from the extensional belt of central Italy

In northern Umbria (central Italy), the region between the Tiber Valley, Gubbio, and the main Apennine ridge is affected by persistent microseismicity (M_L < 3.0), occurring at an average rate of ~3 events per day. A significant portion of this activity is associated with the Alto Tiberina Fault (ATF), a ~60 km-long, low-angle normal fault that has been active since the Late Pliocene–Early Pleistocene. Within this tectonic framework, we analyse seven low-magnitude seismic sequences (M_L < 4.5) that occurred between 2010 and 2023 within the ATF hanging wall. These sequences are not linked to surface-exposed faults, raising questions about the nature and distribution of the seismogenic sources.

The main objectives of this study are to: (1) determine whether the observed seismicity is concentrated along discrete fault planes or instead distributed within fractured rock volumes; and (2) define the geometry and kinematics of the causative faults and assess their correspondence with structures imaged in available 2D seismic reflection profiles. Earthquakes were relocated using a high-resolution 3D velocity model and projected onto depth-converted seismic reflection sections.  Consequently, this work presents a methodological framework for analyzing low-magnitude seismic sequences by integrating active and passive seismic data.

Our results indicate that most ruptures occurred on high-angle normal faults that branch upward from the ATF detachment. The geometry of these faults is consistently constrained by both the depth distribution of relocated seismicity and the corresponding reflectors imaged in the seismic profiles, while their kinematic behaviour is compatible with that inferred for the mainshocks. The aftershock areas range from ~1 to 15 km², suggesting that the mainshocks ruptured only limited portions of larger fault segments. Additionally, the behaviour of these minor sequences, particularly in terms of rupture localization and aftershock spatial patterns, closely mirrors that observed for higher-magnitude sequences in the same region, indicating that similar seismotectonic processes operate across different magnitude scales.