Emanuele Tondi

Active tectonics in the central Apennines and possible implications for seismic hazard analysis in peninsular Italy

Abstract The central Apennines fault system (CAFS) of peninsular Italy, overprints earlier structures of a Neogene fold and thrust belt and includes segments characterized by diffuse seismicity distributed within a NNW-SSE-trending zone, 50–60 km wide. The system has been analysed by means of morphotectonic and structural analysis of exposed active fault segments. The resulting fault structure consists of an interconnecting network of roughly N-S-trending, left-lateral, strike-slip segments and mostly NW-SE-oriented, transtensional to normal faults. Evidence for recent activity of CAFS structures is provided by faulted Middle Pleistocene-Holocene deposits (including 30–40-ka-old pyroclastites and 40-ka-old palustrine sediments), fresh scarps in both bedrock and Late Quaternary continental deposits, and decametric lateral offsets locally affecting the post-Wurmian drainage pattern of the area. The regional stress field responsible for the development and evolution of the CAFS, as inferred from fault slip data, is characterized by a NW-SE compression and by a NE-SW extension. The CAFS pattern and its present-day kinematics have been related to left-lateral strike-slip motion on north-south-trending crustal faults. The existence of deep-seated strike-slip faults in the central Apennines has implications for seismic hazard analysis. Motion along these structures suggests, in fact, that coseismic surface faulting is distributed, and that cumulative displacements include normal, transtensional, and strike-slip components. The seismogenic potential of CAFS structures can therefore be best described by multiple-rupture models and be better analysed in terms of partial contributions of lower-rank features constituting congruent structural associations within the system.

Reconstruction of continental margin architecture deformed by the contraction of the Lagonegro Basin, southern Apennines, Italy

Thrust propagation through previously rifted continental margins may result in fold and thrust belts whose structure is strongly controlled by the inherited basin architecture, as it occurs in southern Italy. The Lagonegro units of the southern Apennines comprise a deformed pelagic basin succession showing variable stratigraphic characteristics, mainly lateral variations in both facies and thickness, interpreted to be due to a complex basin topography related to a Triassic rifting event. In contrast to previous studies, cross-section balancing and restoration indicate that the Lagonegro units exposed in the high Agri Valley area suffered relatively limited internal shortening (8 km, i.e. 35%). Early deformation of these rocks, later incorporated into a large-displacement thrust sheet, was dominated by folding around (present-day) roughly north–south-trending axes. The attainment of a regional décollement level was favoured by an early mild inversion of the basin, producing a roughly similar structural elevation of both hanging-wall and footwall successions to Mesozoic faults. Most of the contractional deformation was accommodated by buckling of the Mesozoic syn-rift strata between synsedimentary faults, which represented major mechanical interfaces. Early strain localization in the Lagonegro Basin ahead of the active thrust front was most probably mechanically controlled by a faulted crustal segment which originally lay, within the continental margin, between two massive carbonate platforms.

Pre-orogenic tectonics in the Umbria-Marche sector of the Afro-Adriatic continental margin

Abstract The pre-orogenic deformation of the Afro-Adriatic paleomargin during Jurassic–Paleogene times is recorded in the Umbria–Marche regions of Central Italy by facies differences, lateral thickness variations and, most importantly, by faults. Structural data suggest the persistence of normal faulting not only during rifting (Late Trias–Bathonian) but also during drifting and more precisely in Late Cretaceous times. Syn- and post-rift extension caused the development of mainly NNW–SSE- and ENE–WSW-trending structures. Post break-up normal faults are found in several localities of the Apennines and generally coincide with older, syn-rift sites of stretching. One-dimensional numerical modelling of subsidence suggests that thinning during rifting was about 15%, compatible with the stretching factor reconstructed from geological sections. Late Cretaceous thinning was obviously less but still significant and estimated at ca 5%. The acquisition and analysis of the stratigraphic and structural data presented in this study allowed us to correlate the subsidence history of the Umbria–Marche basin with the tectonic events that occurred in pre-orogenic times, in this sector of the Apennines, thus deriving useful indications about the pre-Oligocene evolution of this segment of the perimediterranean mountain belt.

Fault zone characteristics and scaling properties of the Val d’Agri Fault System (Southern Apennines, Italy)

Abstract Systematic in situ analysis of active fault zones in Val d’Agri (southern Italy) suggests that the acquisition of quantitative data on fault-related structural discontinuities is fundamental for (1) discriminating between the latter and older regional features, (2) defining the architecture and related permeability structure of faults, and (3) performing simple statistics in order to evaluate the validity range within which fault characteristics may be considered to be fractals. This type of information can be integrated with regional seismotectonic analysis in order to asses the present-day conditions of deformation characterising the area, and to constrain the possible kinematics of the seismogenic structures controlling earthquake activity in this sector of the southern Apennines. The results of our study are also of interest for modelling earthquake sources, since a knowledge of the permeability structure and scaling properties of a faulted rock volume is potentially relevant for simulating the time and space dependent behaviour of fault zones during a seismic cycle.

Ground effects and surface faulting in the September-October 1997 Umbria-Marche (Central Italy) seismic sequence

The September–October 1997 seismic sequence in the Umbria–Marche regions of Central Italy (main shocks on September 26, Mw 5.7 and 6.0, and on October 14, Mw 5.6) left significant ground effects, which were mainly concentrated in the Colfiorito intermountain basin. These effects included surface faulting, ground cracks and settlements, rock falls, slides, hydrological and gas anomalies. The distribution and size of ground effects has proved useful for (1) defining the epicentral area and the location of the causative fault; (2) complementing the intensity pattern from damage distribution (this can be very useful in poorly inhabited zones); (3) integrating or testing the intensity assessment of many historical events, in order to obtain a better evaluation of the magnitude from intensity data. Of special interest was the observation of surface ruptures generated along segments of a system of normal faults already mapped as capable, with end-to-end lengths of 12 km and maximum displacements of 8 cm. Many pieces of evidence confirm that coseismic slip was not a secondary, gravity-induced, phenomenon, but had a tectonic origin. Detailed descriptions of surface faulting for moderate earthquakes are not common, being easily missed or misinterpreted; however, in this paper we emphasize that surface faulting due to the 1997 event can be used to infer the threshold magnitude for surface faulting in Central Apennines, allowing to calibrate palaeoearthquake size from fault offsets as seen in trench investigations.

The crustal fault structure responsible for the 1703 earthquake sequence of central Italy

Abstract An active crustal shear zone has been recognized in the central Apennines (Italy) by detailed investigations of the surface fault trace pattern of the axial sectors of the mountain belt. Our data suggest that a 16–18 km thick seismogenic layer rests above the ductile lower part of this shear zone and is affected by an interconnecting Late-Quaternary fault system which includes three main transcurrent (left-lateral) fault sets trending roughly north-south. Several lines of evidence indicate that the central fault set is the seismogenic structure responsible for generating the 1703 seismic sequence of central Italy. Fault slip data, morphotectonic and paleoseismological observations, together with fractal statistics, suggest that coseismic deformation within the epicentral area of the 1703 earthquake sequence is partitioned among a few linked faults, and that the ratio of vertical vs. horizontal slip rates within the active central fault set is about 1:3. Slip rate estimates also show that recurrence times for 1703 type earthquakes are in the range of a millennium. The seismogenic fault structure of the 1703 earthquake sequence may be viewed as a multiple rupture zone with an immature fractal geometry evolving within a crustal volume which deforms in response to transcurrent motion. The main results of this work support our view that the seismogenic potential of this sector of the Apennines can be evaluated by integrating, into a fractal model, the effects due to higher and lower-rank structural components of the Late Quaternary fault system of central Italy.

The COST project in Italy: analysis and monitoring of seismogenic faults in the Gargano and Norcia areas (central-southern Apennines, Italy)

In this paper we illustrate the state of the art of the European Co-operation in the Field of Scientific and Technical Research, COST Action 625 “3-D Monitoring of Active Tectonic Structures” in Italy. The project is mainly focused on detecting strain variations and/or any phenomena that may precede future seismic events, in order to evaluate the time evolution and modes of deformation of seismogenic structures during the inter-coseismic cycles. We selected two note seismic areas, (i) the Gargano promontory (southern Italy) and (ii) the Norcia basin (central Italy), for the installation of a monitoring equipment consisting of: (a) local networks of geodetic stations (benchmark for GPS and total station measurements with micrometric sensitivity) and (b) 3D monitoring devices (TM71; capable of recording micrometric displacements). In this paper we show the geo-structural analysis and detailed morphostructural studies, including high-resolution seismic reflection profiles and paleoseismological investigations, carried out across the main active faults of selected areas, which allowed us to assess the main spatial and dimensional properties of faults. As concerns the monitoring experiment, available data are not yet adequate to be used for convincing tectonic interpretations because the installation of local GPS stations and that of TM71 extensometers has been carried out only very recently. Accordingly, we only show a few examples of microdisplacement measurements carried out in both areas.

Fault properties and fluid flow patterns from Quaternary faults in the Apennines, Italy

The study of Quaternary fault zones in the High Agri Valley (southern Italy) and in the axial zones of the central Apennines, allowed us to collect information on the permeability structure, fluid characteristics, and scaling properties of the main fault zones exposed in the area. Detailed structural mapping allowed us to derive the appropriate values of the fractal dimension characterising different active fault zone patterns and to evaluate the basic parameters needed for assessing the architecture and related permeability structure of the mapped faults. Scan line and scan area analysis helped in constraining the scaling properties of some of the attributes (i.e. fault zone thickness, fracture spacing etc.) of the main fault zones and in defining their validity range. Our results suggest that, in the Apennines, (i) extrapolation of the architectural indices of a fault zone is admissible over three orders of magnitude, and (ii) the composition of the analysed fluid inclusions and related homogenisation temperatures are associated with fault-driven fluid circulation from both superficial and deep (4–6 km) levels.

Fault zone fabrics and geofluid properties as indicators of rock deformation modes

Abstract In this paper we present the results of a geostructural study on active faults in central Italy, where seismogenic fault zones occur as part of a Quaternary network dissecting and/or inverting earlier tectonic features of the central Apennines fold and thrust belt. In our work we focus on the possibility of using structurally-oriented quantitative analysis of fault fabrics and fluid inclusion studies for assessing the hydraulic properties and scaling relations of fault zones in order to evaluate the role and effects of the interaction between rock and fluids in the brittle deformation of strained crustal rock volumes. The results of our study show that this approach is appropriate for (i) assessing the structural permeability of faulted and fractured rock volumes, (ii) defining the conduit/barrier behaviour of fault zones to fluid flow, (iii) mapping spatial variations of the fluid pressure across different fault segments, (iv) evaluating the maturity of a structural network and the degree of interaction of linked structural discontinuities, (v) assessing fluid composition and the conditions of deformation by means of microstructural and fluid inclusion data.

Scaling properties of the dimensional and spatial characteristics of fault and fracture systems in the Majella Mountain, central Italy

Abstract In this paper we report on the results of a systematic study carried out on the fault and fracture systems exposed in the Majella Mountain, in the central Apennines fold and thrust belt of mainland Italy. The focus of our work was to assess the dimensional, spatial, and scaling properties of fault and fractures in carbonate rocks, in order to set up appropriate flow models for these types of potential geofluid reservoirs. The results provide information on (1) orientation, size distribution, density variations, and fractal characteristics of the fault and fracture networks affecting the Majella anticline; (2) the scaling properties and the overall architecture of different fault zone components; (3) the overprinting relationships between fault and fracture sets and the Majella fold structure. These data were used to elaborate a three-dimensional discrete fault and fracture model (DFFN model) of a ∼100 m3 geological volume, and for this to (1) evaluate the transport and storage properties of the reservoir; and (2) assess the degree of vulnerability and any possible hazard related to the exploitation and management of geofluids hosted in carbonate rock volumes.