New insights into the High Agri Valley deep structure revealed by magnetotelluric imaging and seismic tomography (Southern Apennine, Italy)
Introduction
The High Agri Valley, (hereinafter HAV), a NW-SE trending intra-mountain basin of Southern Apennines, is one of the areas of Italy with the highest seismogenic potential as demonstrated by the historical Mw 7.0 strong earthquake occurred in 1857 (Mallet, 1862). Moreover, the impoundment of an artificial water reservoir (Pertusillo Lake) and the intensive hydrocarbon exploitation of the Val d'Agri oilfield makes this area affected by induced seismic events as well. Since 2006, the coproduced saline water formation by oil exploitation has been disposed, in the interval between 2890 and 3096 m depth (b.s.l.), through its injection into an unproductive well (Costa Molina 2, hereinafter CM2, at 1045 m a.s.l.), located in the marginal sector of the Val d'Agri oilfield. This activity is causing fluid-induced microseismicity (Stabile et al., 2014a; Improta et al., 2017), well confirmed by the fact that the onset of the seismicity occurred only after June 2006 when the injection operations began (Stabile et al., 2014a). In addition, a second cluster of continuous induced seismicity has been observed SW of the Pertusillo Lake and it has been associated to the seasonal fluctuations of the reservoir's water level (Valoroso et al., 2009; Stabile et al., 2014b; Stabile et al., 2015; Telesca et al., 2015).
Since early Pleistocene, brittle tectonics have strongly controlled the formation and evolution of the HAV, which is bounded to the eastern flank by SW-dipping transtensional faults (Eastern Agri Fault System, EAFS) and to the western flank by NW-SE trending and NE-dipping Monti della Maddalena Fault System (MMFS), (Maschio et al., 2005). The seismogenic fault system causative of the 1857 earthquake is still debated. According to Benedetti et al. (1998) and Cello et al. (2000) the active fault system should be the EAFS, whereas Maschio et al. (2005) attributed the main seismogenic potential of the area to the MMFS.
While in the Northern Apennines the good quality of seismic profiling (e.g., Barchi et al., 1998; Boncio et al., 2000) provides a clear geometry of deep active faults, in the Southern Apennines less data are available. Indeed, this area is characterized by a low magnitude/poor instrumental seismicity (Mazzotti et al., 2000; Mazzoli et al., 2005) and the seismogenic sources for large historical earthquakes are unknown (Ercoli et al., 2020). The assessment of the active structures is very complicated but, in some cases, important results concerning the deep structure of subsoil are kept hidden by oil enterprise for strategic reasons.
Despite several tens of years of oil and gas exploration activity, most of the subsurface geophysical data, specially coming from seismic lines (Butler et al., 2004; Shiner et. al., 2004; Nicolai and Gambini, 2007; Mazzoli et al., 2013; Candela et al., 2015), magnetotelluric surveys (Dell'Aversana and Morandi, 2002), and data wells (D'Andrea et al., 1993; Monaco et al., 1998; Finetti et al., 2005), are not public. Then, the deep crustal structure of the Southern Apennine in the sector of the HAV is still a matter of discussion.
Therefore, this orogenic segment is well suited for applying non-invasive investigation techniques such as magnetotellurics, in order to contribute to a more holistic framework.
Generally, magnetotelluric (MT) exploration can provide valuable information on the structures and processes at lithospheric level thanks to high capability of penetration depths and its sensitivity to image lateral and vertical electrical conductivity variations. As known, fault zones are characterized by significant permeability contrast due to their fracturing level. Deep crustal fluids could strongly affect conductivity specially if interconnected over kilometer scale distances (Wannamaker et al., 2002; Bedrosian et al., 2004). High electrical conductivity could be indicators of rheological weak zones in the crust within which fracturing are concentrated, against low electrical conductivity values typical of the undeformed zones (Tank et al., 2005).
Unfortunately, most of geophysical inverse problems must face a twofold order of problems: the spatially varying resolution of obtained images and the non-uniqueness of the mathematical solution. These factors constitute a huge limitation for the right analysis of a geophysical model, and it is especially true when the interpretation is based on images describing a single physical parameter, such as resistivity. These problems are very often overcome by integrating the information provided by separated geophysical approaches and by their joint interpretation, which may be carried out in both a qualitative (Stanley et al., 1990) and a quantitative way (Bedrosian et al., 2007; Shaharabi et al., 2016). To this purpose, several works in literature combined seismic and electrical methods for the subsoil characterization and description (e.g. Eberhart-Phillips et al., 1995; Bedrosian et al., 2004; Tank et al., 2005).
In this paper, we present the results of a 35 km-long MT survey, crossing a large sector of the Lucanian Apennine and traversing the HAV in correspondence of the two clusters of induced seismic events. The MT profile, oriented at N40 direction and with an investigation depth down to 12 km, has the objective to obtain a resistivity model of the deep structure of our investigated area.
The MT model results have been integrated with a 3-D elastic crustal model, well resolved down to 6–8 km depth and retrieved by also inverting unprecedented seismic data belonging to a very dense seismic network installed in the area (Stabile et al., 2020). The obtained elastic model is described in terms of images of P-wave velocity (VP) and of the ratio between P-and S-wave velocities (VP/VS).
Section snippets
Geological setting
The study area is situated in the central sector of the Southern Apennines and it extends from the HAV to the front of the chain (Tempa Rossa oil field, Fig. 1). The Southern Apennine arc is a key segment of the circum-Mediterranean mountain belts and it consists of a fold-and-thrust belt developed from the Upper Oligocene-Lower Miocene onwards as a result of tectonic accretion toward NE (Doglioni et al., 1996; Gueguen et al., 1998; Patacca and Scandone, 2007 and references therein) of
MT data collection
The MT method uses the natural fluctuations of the earth's magnetic field as signal source inducing secondary electromagnetic field in the ground. On the surface two components of the electric field (Ex, Ey) and magnetic field (Hx, Hy) are measured in two directions. It is customary to have the x direction pointing to the magnetic north, so our measurement system was set considering the x-component in the north-south direction.
The MT data used in the present paper have been collected according
MT data analysis
For each of the 26 acquired soundings, time series were firstly visually inspected to filter out segments clearly contaminated by cultural noise and then processed to obtain impedance estimates by using the robust processing code of Egbert (1997). Robust single station (SS) processing and robust remote reference (RR) processing were applied. In the last case, the Tramutola MT monitoring permanent observatory was used as remote station. It was noted that both SS and RR processing scheme produced
Seismic tomography inversion
In this study seismicity data of the HAV collected from January 2002 to December 2018 were used to obtain a 3-D elastic structure of the area described in terms of VP and VP/VS images. The dataset used for the tomographic inversion was mainly recorded by stations of three seismic networks: a permanent monitoring network made up of 15 triggering mode stations operated by ENI Oil Company (Stabile et al., 2014b), a denser temporary network installed in the period May 2005–June 2006 by the INGV and
Discussion
In this paragraph we compare and jointly interpret electrical resistivity and P-wave seismic velocity in order to better understand the subsurface geological units and their physical properties (fractures, permeability, water contents etc.). Resistivity and velocity models are independent images with different resolution in space. A quantitative comparison in the distribution of the physical parameters along the profile requires a common set of grid points, therefore we restrict the comparison
Conclusions
Summing up, in this study we have provided both a resistivity and an elastic imaging of the subsurface of HAV. The former depicts the first 12 km depth of a SW-NE trending profile cutting the southern part of the HAV and, in particular, its main seismicity clusters; the latter depicts the subsoil of a 28 km × 28 km area centred close to the Pertusillo Lake down to a depth of 6–8 km. The two geophysical models have put in evidence several lithological, tectonic and rheological features of the
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We are grateful to both the anonymous reviewers for their comments and constructive suggestions that greatly improved the manuscript and we would like also to extend thanks to the the editor Ramon Carbonell for his precious support. The seismic data of the Eni network (2002-2014) were provided by the Basilicata Region.This research has been partially supported by the INSIEME project of the Italian SIR-MIUR programme (grant no. RBSI14MN31) and partially by the project “Detection and tracking of
References (120)
- et al.
Extracting quantitative dynamics in Earth’s apparent resistivity time series by using the detrended fluctuation analysis
Phys. A
(2007) - et al.
Geophysical images of the creeping segment of the San Andreas fault: implications for the role of crustal fluids in the earthquake process
Tectonophys.
(2004) Anatomy of a siliciclastic turbidite basin: the Gorgoglione Flysch, Upper Miocene, southern Italy: physical stratigraphy, sedimentology and sequence-stratigraphic framework
Sediment. Geol.
(1997)- et al.
Finite element modelling of stress field perturbations and interseismic crustal deformation in the Val d'Agri region, southern Apennines, Italy
Tectonophysics
(2015) - et al.
Occurence of palygorskite and sepiolite in upper Paleocene – middle Eocene marine deep sediments of the Lagonegro basin (Southern Apennines – Italy): paleoenvironmental and provenante inferences
Sediment. Geol.
(2011) - et al.
Fault zone characteristics and scaling properties of the Val d’Agri Fault System (Southern Apennines, Italy)
J. Geodyn.
(2000) - et al.
2D Self-Potential tomographies for studying groundwater flows in the Varco d’Izzo landslide (Basilicata, southern Italy)
Eng. Geol.
(2006) - et al.
On the post-25 Ma geodynamic evolution of the western mediterranean
Tectonophysics
(1998) - et al.
Stratigraphical and structural constraints in the Lucanian Apennines (Southern Italy): tools for reconstructing the geological evolution
J. Geodyn.
(2002) - et al.
Geometry, segmentation pattern and displacement variations along a major Apennine thrust zone, Central Italy
J. Struct. Geol.
(2005)
Structural evolution of the Lucanian Apennines, Southern Italy
J. Struct. Geol.
Thin-skinned versus thick-skinned structural models for Apulian carbonate reservoirs: constraints from the Val d’Agri Fields, S Apennines, Italy
Mar. Petrol. Geol.
Three-dimensional magnetotelluric inversion: data-space method
Phys. Earth Planet. Inter.
A new look at the statistical model identification
IEEE Trans. Autom. Control
Present-day stress field and active tectonics in southern peninsular Italy
Geophys. J. Int.
Stability analysis of apparent resistivity measurement in the seismically active area of Val d’Agri (Southern Italy)
Nat. Hazards Earth Syst. Sci.
Electric and magnetic field changes observed during a Seismic Swarm in Pollino Area (Southern Italy)
Bull. Seismol. Soc. Am.
Magnetotelluric investigation in the High Agri Valley (southern Apennine, Italy)
Nat. Hazards Earth Syst. Sci.
The Pollino 2011-2012 seismic swarm (southern Italy): first results of the ML=3.6 aftershock recorded by co-located electromagnetic and seismic stations
Boll. Geofis. Teor. Appl.
Extensional tectonics in the northern Apennines (Italy): evidence from the CROP03 deep seismic reflection line
Mem. Soc. Geol. Ital.
Stratigraphic revision of the Eocene Albidona Formation in the type locality (Calabria, Southern Italy)
Riv. Ital. Paleontol. Stratigr.
MT+, integrating magnetotellurics to determine earth structure, physical state, and processes
Surv. Geophys.
Lithology-derived structure classification from the joint interpretation of magnetotelluric and seismic models
Geophys. J. Int.
Surface rupture of the 1857 Southern Italian earthquake
Terra Nova
Geological and geophysical characterization of the Brindisi di Montagna Scalo landslide (Basilicata, Southern Italy)
Geomatics Nat. Hazards Risk
Stratigrafia di dettaglio del «Flysch di Gorgoglione» (Lucania)
Mem. Soc. Geol. Ital.
Architecture and seismotectonics of a regional low-angle normal fault zone in Central Italy
Tectonics
The magnetotelluric phase tensor: a critical review
Geophys. J. Int.
3D Model of the active extensional fault system of the high Agri River Valley, Southern Appennines, Italy
J. Virtual Explor.
Geological map of the northeastern sector of the high Agri Valley, Southern Apennines (Basilicata, Italy)
J. Maps
Frontal collapse during thrust propagation in mountain belts: a case study in the Lucanian Apennines, Southern Italy
J. Geol. Soc. Lond.
The history of the Southern Apennines of Italy preserved in the geosites along a geological itinerary in the High Agri Valley
Geoheritage
Applying thick-skinned tectonic models to the Apennine thrust belt of Italy: limitations and implications
Inversion of inherited thrusts by waste- water injection induced seismicity at the Val d’Agri oilfield (Italy)
Sci. Rep.
The magnetotelluric phase tensor
Geophys. J. Int.
Late Quaternary activity along the Scorciabuoi Fault (Southern Italy) as inferred from electrical resistivity tomographies
Ann. Geophys.
Presentazione della carta geologica del Bacino del Fiume Agri (Basilicata)
Mem. Soc. Geol. Ital.
Neogene-Quaternary tectonic evolutionof the southern Apennines
Tectonics
Geochemical and mineralogical approaches to assessing provenance and deposition of shales: a case study
Clay Miner.
Illite-smectite mixed layers in sicilide shales and piggy-back deposits of the Gorgoglione Formation (Southern Apennines): geological inferences
Boll. Soc. Geol. Ital. (Ital. J. Geosci.)
P-T constraints from phyllosilicates of the Liguride Complex of the Pollino area (Southern Apennines, Italy): geological inferences
Ofioliti
Post-depositional history of the Miocene Gorgoglione Formation (Southern Apennines, Italy): inferences from mineralogical and structural analyses
Boll. Soc. Gol. France
Etude fine de la sismicité en zone de collision continentale aumoyen d’un réseau de stations portables: La région Hindu-Kush Pamir. [Detailed Study of Seismicity in an Area of Continental Collision by Using a Network of Portable Seismic Stations: Hindu-Kush Pamir Region]
Local pattern of stress field and seismogenic sources in the Pergola-Melandro basin and the Agri valley (southern Italy)
Geophys. J. Int.
Complete seismic release of tectonic strain and earthquake recurrence in the Apennines (Italy)
Geophys. Res. Lett.
Geological model, advanced methods help unlock oil in Italy’s Apennines
Oil Gas J.
High-precision differential earthquake location in 3-D models: evidence for a rheological barrier controlling the microseismicity at the Irpinia fault zone in southern Apennines
Geophys. J. Int.
Interpretation of seismic, MT and gravity data in a thrust belt interpretation
First Break
Integration loop of “global offset” seismic, continuous profiling magnetotelluric and gravity data
First Break
Depth model building by constrained magnetotelluric inversion
Ann. Geophys.
Cited by (4)
Structural control of gas migration pathways in the hydrocarbon-rich Val d'Agri basin (Southern Apennines, Italy)
2023, Marine and Petroleum GeologyLateral Variations of P-Wave Velocity from Deep Borehole Data in the Southern Apennines, Italy
2023, Pure and Applied Geophysics