Attenuation imaging of the High Agri Valley (Southern Italy) revealed by peak delay and coda Q analysis

In addition to the more common elastic crustal imaging, the investigation of attenuation properties is a pivotal task for gaining insights into the geological complexities of a study area. Indeed, the availability of scattering and absorption crustal images may allow for defining the extension of both highly fractured regions in seismogenic volumes and the presence of fluids, along with their possible role in the seismic activity of the target areas. Furthermore, the possible imaging of fluid saturated rock volumes could be a significant clue about the geothermal potential of target areas.

Our study is placed in this scientific framework, thus providing an unprecedented complete attenuation image of the High Agri Valley (HAV, Southern Italy). The latter is a NW-SE elongated basin located in southern Apennines, hosting the largest onshore oil field in Western Europe. Hydrocarbons are not the only fluids of interest in the area, as a meaningful amount of sulphureous hypothermal water and gases have been found in the Tramutola site, on the western side of the valley.

The HAV is bordered by two oppositely dipping fault systems: the Eastern Agri Fault System (EAFS) to the east, and the Monti della Maddalena Fault System (MMFS) to the west. In the area, one of the strongest earthquakes in Italy occurred (the 1857 Mw 7.0 earthquake) making the High Agri Valley a region affected by a very high seismic hazard. Furthermore, in the region there is a well-documented induced seismicity due to: 1) the combined effects of the water level oscillations of the Pertusillo lake, the regional tectonics, and likely the poroelastic/elastic stress due to aquifers in the carbonate rocks; 2) the injection, through the Costa Molina 2 well, of the wastewater produced by the exploitation of the Agri Valley oilfield.

The dataset adopted for our aims consists in the seismic signals recorded in the period 2016 – 2019 by a virtual network composed of the seismic stations belonging to INGV, ENI, and INSIEME seismic networks (Stabile et al., 2020). A total number of 650 earthquakes were recorded, with hypocentral depths ranging from 0 down to 10 km. The scattering and absorption imaging were retrieved by adopting an approach combining peak delay and coda-Q methodologies, already implemented in the open source code MuRAT (De Siena et al., 2016). The preliminary results show high scattering and high absorption at depth between 2 and 3.5 km featuring Costa Molina 2 injection well location, in agreement with the interpretation of a volume fractured by high-pressure injected fluids. Other high intrinsic attenuation anomalies have been found at similar depths in the Tramutola and Pertusillo Lake areas, while a deeper strong absorption has been found in the northern HAV at greater depth (4.5 km).