Reviewing the 1997 Umbria-Marche seismic sequence: a fresh look from the integration of new seismological and subsurface data

The central-northern Apennines represent a high seismic hazard area characterized in the last decades by multiple seismic sequences (1997 Umbria-Marche, 2009 L’Aquila, 2016-2017 Amatrice-Visso-Norcia) related to the post-orogenic extension.

After the recent Amatrice-Visso-Norcia seismic sequence, the large availability of subsurface geological data and the dense seismological and geodetic networks allowed for better imaging of the shallow crust structural setting and the relationship with the occurred seismic sequences. 

Recent advances in those areas focused on comprehending the role of inherited structures (namely the large thrust faults related to the building up of the Apennines orogen) in compartmentalizing both horizontally and vertically the seismic sequences. Also, they suggested that major compressive structures may play an active role in seismogenesis through their kinematic inversion into the current extensional regime.

Such behavior was already debated after the 1997 Umbria-Marche seismic sequence, characterized by six main shocks with 5 < Mw < 6. All the large shocks originated on adjacent and parallel NW trending normal faults whose extent varies between 5 and 10 km at a hypocentral depth of 5 –6 km.

Our work presents a review of the data available for the 1997 Umbria-Marche seismic sequence. Using a combined dataset of seismic reflection profiles and deep boreholes, as well as detailed data from geological surveys, we present a new 3D geological and velocity model of the area. We also re-analyzed the passive seismic data recorded by both the temporary and permanent seismic networks. As a result, we computed a new 1-D relocation catalog based on the 3-D geological and geophysical imaging of the shallow portion of the crust in the target area.

The comparison of the geological model and the relocated seismicity shows a substantially vertical and horizontal compartmentation of the shallow crust due to the action of the thrusts. The seismicity distribution is strictly conditioned by the organization of crustal volumes separated by major thrusts and is concentrated within the same structural and stratigraphic levels, both on normal faults and pre-existing thrusts, possibly reactivated during the sequences.

The integrated analysis of seismological and geological subsurface data shed light on the open questions related to the interference between Quaternary normal faults and Tertiary thrusts and on the geometry of the causative faults of the 1997 seismic sequence. In addition, they help to define a more robust seismotectonic behavior and to assess the seismic hazard of those areas.