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In the Apennine Mountains of the Italian Peninsula, GPS data display 3–4 mm/yr of divergent motion oriented N50°E between the Adriatic and Tyrrhenian coastlines. However, the mechanisms driving this extension remain debated and along-strike variations of extension within the actively deforming belt remain poorly constrained. Here, we derived the first large-scale extensional and vertical velocity field for the Apennines by multi-temporal InSAR analysis of 7 years of Sentinel-1 data at the scale of the entire range, improving the spatial resolution and vertical accuracy of existing GPS measurements. The results reveal along-strike variations of extensional rates and gradients, with extension concentrated on single fault systems in the north, consistent with the loci of seismicity and recent moderate earthquakes, and distributed throughout the central Apennines, where the range is widening. Vertical surface displacements do not resolve any active long-wavelength uplift of the orogenic belt and, on average, show more subsidence than uplift relative to the Tyrrhenian and Adriatic coasts. This work provides the first InSAR-based geodetic map of differential extension and uplift within the Italian Peninsula. Our results are compatible with a pure shear extensional model of the crust, driven by both boundary and gravitational forces. • Lateral variations of extension and uplift quantified using Sentinel-1 SAR data. • Strain rates are higher across the easternmost set of mapped normal fault systems. • Extension within the central Apennines is distributed into several faultsystems. • Regional long-wavelength uplift is not observed in this sector of the Apennines. [ABSTRACT FROM AUTHOR]

The frontal thrusts and folds of the northern Apennines - Italy - are mainly covered under the alluvial deposits of the Po Plain. Some of these structures show geological evidence of Late Quaternary activity, thus posing the need for an accurate seismic hazard assessment due to widespread housing settlements, industries, lifeline infrastructures, and large towns. We present new morphostructural, geophysical, and seismological data to discuss the recent activity of the Broni-Sarmato fault, an 18 km-long outcropping section of the north-verging Stradella thrust, located 50 km south of Milan, along the Pede-Apennine thrust front (PTF) in the rear of the Emilia Arc thrust system. The new geoelectrical surveys across the fault scarp show deformation of the shallow deposits. The outcropping deformations, with a fault scarp ranging up to 25.8 m, are investigated within the seismotectonic framework of the PTF and the Emilia Arc. The analysis of the associated seismicity and new focal mechanisms highlight two seismogenic contractional volumes dipping at low-angle southwest-ward, at upper (<12 km) and lower crustal depths (~20–30 km). The shallow seismicity partially illuminates the Stradella thrust and its along-strike southeastward prosecution along the extent of the Stradella-Salsomaggiore Arc. Subordinately, it also illuminates some of the Emilia Arc thrust planes. The deeper seismogenic volume shows large patches of the basal thrust of the Emilia Arc fault system. We interpret the above multi-scale data as evidence of ongoing tectonic activity of the outer fronts of the Emilia Arc under a regional NNE-oriented compressional stress field, with some evidence of thrust involvement along the Pede-Apennine front. In our 3D fault-model reconstruction, all the analyzed thrust structures appear as expressions of a thick-skinned deformation that controls earthquake release at different structural levels. • Multi-scale data confirm activity of the Stradella thrust at upper crust depths. • Earthquake data illuminate the Emilia Arc Basal Thrust at lower crust depths. • Multi-source data constrain a crust-scale 3D non-planar seismotectonic fault model. • A thick-skinned geometry at the outer front of the Apennine chain is outlined. • Crustal layering controls seismic release along discrete shear planes. [ABSTRACT FROM AUTHOR]

An integrated investigation including geological, geomorphological, geophysical and structural survey, tephra analyses, 14 C and 40 Ar/ 39 Ar dating, as well as paleoseismic trenching along the N-Matese fault system is presented. The study allowed the characterization of the tectonic mobility of this structure as well as the associated Bojano basin sedimentary-tectonic evolution since the early Middle Pleistocene, providing also new clues concerning the fault historical activity and the associated Mw > 6.5 earthquakes. We have found lines of evidence for > 1 mm/yr slip rate along the presently buried Bojano fault during the mid Middle Pleistocene, and similar rates for the main fault segments paralleling the Matese flanks. The buried Bojano fault significantly slowed down during the last 300 kyr, ceasing its activity before the Holocene. In turn, the segments outcropping along the Matese flanks reactivated at the onset of Late Pleistocene, after a long period of quiescence (480–110 ka), with robust slip rates that would seem even accelerating in post LGM times. Paleoseismic data suggest the occurrence of four Mw > 6.6 earthquakes in the past 3 ka, three of which match the little known 280 BC event, and the devastating 1456 and 1805 earthquakes. [ABSTRACT FROM AUTHOR]

In the middle of the Mediterranean, the partly still active Apennines subduction system has been usually defined using tomographic images and available shear wave splitting measurements. In this paper we describe the new seismic anisotropy dataset for Central Italy, the region where the transition between Northern and Southern Apennines occurs. The new measurements show NW-SE fast polarisation directions beneath the belt, due to the retreat of the slab, NN E -SSW orientations from proper Adriatic mantle sources, and E-W directed anisotropy, attributed to mantle convection flow at the Thyrranian side. Additionally, the new data suggest the presence of a toroidal mantle flow through a tear in the Apenninic slab, from the Adria to the Tyrrhenian side. However, mantle circulation and flows, identified by the pattern of shear wave splitting results, seem different from what was proposed in previous geodynamic models. Indeed, our results support the presence of a vertical slab tear with limited dimension. In the geodynamic model we propose, the tear acts to accommodate a differential slab retreat. The slab partitioning results in a different pattern and strength of seismic anisotropy traced from the Central Apennines with respect to the adjacent Northern and Southern Apennines. • New SKS data consistent with previously defined mantle deformation patterns • Geodynamic model for Central Mediterranean from new seismic anisotropy data • NW-SE Apenninic mantle anisotropy interrupted by a narrow well developed mantle flow • Seismic anisotropy consistent with shallow horizontal slab tear beneath central Italy [ABSTRACT FROM AUTHOR]

This study investigates the geometry of the crust–mantle boundary along the Northern Apennines (Italy) and its relationship to the tectonic evolution of a retreating subduction system. Using receiver function analysis from teleseismic records, the Moho depth was computed beneath 26 seismic stations located in key positions along the orogen. The results show a relatively shallow (~ 20 km) Moho in the Tyrrhenian side, as expected from an extended overriding plate, and a deeper Moho (~ 40 km) in the subducted Adriatic plate, SW-dipping 15–45° beneath the chain. Stations deployed on the Northeastern edge of the Plio-Quaternary extensional basins provide maximum Moho depth estimates, thus supporting the hypothesis that these basins define the zone where the two plates separate at depth. In addition, an abrupt change in the Moho depth is recognized along strike of the orogen, attributed to a 45 km-wide, trench-normal deformation zone that crosses the Northern Apennines in the proximity of the Livorno–Sillaro lineament. This zone is interpreted as an incipient lithospheric tear fault, which accommodates varying rates of trench retreat along the length of the subduction zone. [ABSTRACT FROM AUTHOR]

Abstract: Seismological and geodetic data provide key information about the kinematics and active tectonics of plate margins. Focal solutions enable the determining of the directions in which the current tectonic stress acts when fault rupturing occurs; GPS measurements provide information on the crustal velocity field and on current interseismic strain rates. The comparison of the strain rates resulting from the two datasets provides further insight into how large an area is affected by aseismic deformation, which is a valuable indicator for seismic hazard mitigation and estimation of the seismic potential. In this work, we investigate both seismic and geodetic strain rates and the combined field resulting from the joint inversion of the geodetic and seismic datasets, providing a picture of the overall deformation field and its variation during the last decades. In this way, we try to give an overview of the seismic potential distribution across the Apennines and southern Italy, as a qualitative analysis of space-time variations in the released seismic strain rate, compared to the space-time distribution of the cumulated geodetic strain rate. The results show a variable distribution of the seismic efficiency over Italy. The Southern Apennines shows the greatest seismic potential, highlighting a significantly lower seismicity in the last two decades over an area affected by the highest total strain rates. The Messina Straits and eastern Sicily have a significant seismic potential, together with the Calabrian arc (from the Tindari–Letojanni and central Aeolian islands to the Mt. Pollino area), as a result of seismic gaps with respect to the combined strain rates in the investigated period. This long gap highlights the longer recurrence periods for the strongest earthquakes on this area. The central–northern Apennines and off-shore northern Sicily, show a lower seismic potential than central–southern Apennines, probably due to the more recent seismicity affecting these areas. [Copyright &y& Elsevier]

According to the most common interpretation, the Apennines developed in Neogene and Quaternary times in the hanging wall of a west directed subduction zone. Seismic tomography is the most powerful tool to investigate large volume of Earth at depth, and it has been extensively applied to shed light on the geometry and shape of the subduction under the Italian peninsula. The various experiments were able to display the slab under the Southern Apennines, but even the most recent tomographic images were non-uniquely interpretable and left open questions about the characteristics of the subduction in the Northern-Central sector of the chain. We here present the results of an improved inversion experiment focused on the Northern and Central Apennines. The results do not show any pronounced subduction slab and the most evident anomaly is a low velocity body extending down to 100 km depth, located in a relatively small area under the western Tuscany. On the basis of accurate synthetic tests, we assess that, if established, a subduction like geometry should be visible in our tomographic images. We then conclude that no subduction is imaged in the Northern and Central Apennines. We thus interpret this anomaly as an asthenospheric flow. However, we cannot exclude that our result is due to intrinsic limitations of the methodology. In fact in response to the original question about the capability of local earthquake tomography to settle the matter about subduction, we underline that the absence of deep earthquakes to illuminate the model from below, the existence of seismic gaps in some sectors of the area under study even at shallow depth and the non uniqueness of interpretation of the tomographic images make local tomography unable to give alone definitive information on the deep structure of the Northern and Central Apennines. [ABSTRACT FROM AUTHOR]

Abstract: We studied the motion of the Adriatic microplate using Eurasian-referenced GPS-derived velocities from Istria Peninsula (Slovenia, Croatia) and Po Plain (Italy) sites and earthquake slip vectors around its edges from a Regional Centroid Moment Tensor catalogue. We explored kinematic parameters by inverting GPS velocities using a variety of site combinations and comparing results. Our best-fitting GPS Adria–Eurasia angular velocity vector (Euler pole) comes from 7 Istria Peninsula (Slovenia, Croatia) and 10 Po Plain (Italy) sites; it locates at 45.03°N, 6.52°E, with a 0.297±0.116°/Myr counterclockwise rotation rate. This new GPS-derived pole locates and overlaps with our earthquake slip-vector-derived pole. An Adriatic microplate interpretation is at odds with Neogene geologic features that indicate recent convergence across the Apennines and Alps. The neotectonics–geology mismatch probably signals the recent birth of the Adria microplate upon termination of the Nubia–Eurasia Alpine collison and Adria slab break-off beneath the Apennines. [Copyright &y& Elsevier]

Abstract: In this paper we present and discuss an improved picture of the seismicity distribution of the Umbria–Marche–Abruzzi Apennines as obtained through the integration of the national and the regional seismic networks operating from 2002 to 2006. During this period, both the Istituto Nazionale di Geofisica e Vulcanologia (INGV) National Seismic Network and the regional networks have been greatly improved. We compare the results of the integrated catalogue obtained in this study with the Catalogue of the Italian Seismicity between 1981 and 2001 [Castello, B., Selvaggi, G., Chiarabba, C., Amato, A., 2006. CSI Catalogo della sismicità italiana 1981–2002, versione 1.1. INGV-CNT, Roma.http://legacy.ingv.it/CSI )], confirming the basic known features of the seismic activity in the region, but also evidencing some original and interesting results. In particular, the new data set allows us to better define the geometry and kinematics of the crustal seismicity, which is confined to the upper 20 km and shows a clear general deepening from west to east. In the crust, we find additional evidence of extensional seismicity below the central portion of the belt and thrust/reverse faulting mechanisms at the outer fronts of the Apennines. Looking at the seismicity along the belt, it is also possible to observe aseismic regions, which could be due to either locked or creeping portions of the Apenninic fault system. At greater depth, the west-dipping seismicity distribution down to about 70 km confirms the hypothesis of a slab of Adriatic lithosphere subducted below the Apennines, but also suggests that there are strong lateral heterogeneities and possibly tears in the slab. [Copyright &y& Elsevier]

Abstract: The integration of seismic reflection profiles with well-located earthquakes shows that the mainshocks of the 1997–1998 Umbria–Marche seismic sequence nucleated at a depth of ∼6 km on normal faults within the Triassic Evaporites. In the same lithology, made of interbeds of dolomites and anhydrites, two deep boreholes (∼4–5 km), drilled northwest of the epicentral area, encountered CO2 at near lithostatic pressure. In order to investigate the deformation processes operating at depth in the source region, we have combined field studies on a major evaporite-bearing normal fault (the Roccastrada fault) with mechanical and permeability data from rock deformation experiments on anhydrite rocks. The Roccastrada fault is a mature normal fault (inferred displacement >100 m and exhumation from depths >1 km) that dips in the range 40°–45°. The fault zone structure is characterized by a 5–6 m thick fault core, which appears to be zoned. The inner fault core is made of fine-grained fault rocks (∼1 m thick), with deformation localized along continuous and straight slip surfaces associated with a dolomite-rich cataclasite (brittle deformation). The outer fault core is mainly characterized by distributed deformation accommodated by a fault parallel fabric consisting of interbeds of cataclastic dolostones and foliated Ca-sulphate rocks. The damage zone consists of foliated Ca-sulphate rocks (foliation almost perpendicular to the fault zone) and heavily fractured and boudinaged dolostones. Mechanical data obtained from triaxial loading tests on borehole-recovered anhydrites with different grain size and mesoscopic fabric, show that the transition from localized to distributed deformation occurs at effective pressures of about 20 MPa. The permeability measured under hydrostatic stress conditions, before loading, is generally low and ranges between 10− 21 ≤ k ≤10−19 m2. During sample loading, the permeability increases up to 3 (prior to brittle localized failure) and 2 (prior to distributed ductile failure) orders of magnitude, with measured k values of 10−17 m2 and 10−18 m2, respectively. The integration of field observations with mechanical data allows us to propose a fault evolution model where fault initiation occurs as distributed deformation within the anhydrites, along fault zones dipping at ∼45°. With increasing displacement, dolostones are concentrated into the inner fault core. The ultimate result is a fault zone structure dipping at ∼45° , made of an inner fault core affected by brittle processes surrounded by a macroscopically ductile outer fault core. This fault zone evolution can explain the low rupture dip (38°–48°) of the Umbria–Marche 1997–1998 mainshocks. The combination of field observations and permeability measurements suggests a fault zone permeability low enough for fluid overpressures to develop. Therefore in the active area of the Umbria–Marche Apennines deep-seated CO2-rich fluids can be trapped at seismogenic depths within evaporite-bearing faults and can potentially promote earthquake nucleation. [Copyright &y& Elsevier]

Abstract: We present here the new observations of seismic anisotropy obtained from SKS birefringence analysis. We studied 27 teleseismic earthquakes recorded by the temporary seismic network of RETREAT project in the Northern Apennines region. For each station–event couple we calculate the anisotropic parameters (delay time and fast-polarization direction) by minimizing the energy in the transverse component. Our measurements confirm the existence of two domains. The Tuscany domain, on the south-west with respect to the Apennines, shows mostly NW–SE fast axes directions, with a rotation toward E–W direction moving toward the Tyrrhenian Sea. The Adria domain, north-east of the Apennines orogen, shows more scattered measurements, with prevailing N–S to NNE–SSW directions, also with back-azimuthal dependence. The transition between the two domains is abrupt in the northern part of the study region but more gradual in the southern part. Measured delay times (1.8 s on average) suggest that the detected anisotropy is located principally in the asthenosphere. Beneath the Adria domain, where the presence of a double-layer structure seems consistent, a lithospheric contribution is plausible. An interpretation in terms of ongoing mantle deformation suggests a differential evolution of the trench-retreat process along the Northern Apennines orogen. The orogen-parallel anisotropy in the study region is beneath the inner part of the belt instead of beneath its crest and no orogen-normal measurements are found in the Tuscany side. Compared to the anisotropy pattern of the typical slab retreat seen in southern part of the Northern Apennines, in the northernmost one the anisotropy suggests that an oblique trench-retreat has occurred, possibly linked to Northern Apennines retreat since 5 Ma. [Copyright &y& Elsevier]

Abstract: We present two examples of statistical analysis of seismicity conducted by integrating geological, geophysical and seismological data with the aim to characterize the active stress field and to define the spatio-temporal distribution of large earthquakes. Moreover, our data will help to improve the knowledge of the “seismogenic behavior” of the areas and to provide useful information for seismic hazard evaluation. The earthquakes are described by two non-parametric statistical procedures integrating also tectonic-physical parameters to study the spatio-temporal variability. The results show that the areas are characterized by: 1) a stress regime with mainly extensional kinematics; 2) tectonic structures mainly oriented with the active stress field (S hmin =N44°±18° in the southern Apennines and S hmin =N50°±17° in the central Apennines); 3) cluster distribution of seismicity and 4) a high probability of earthquake occurrence (M >5.5) in the next 10 years. [Copyright &y& Elsevier]

Abstract: We estimate current vertical movements along the Apennines (Italy) through repeatedly measured high precision levelling routes. In order to highlight regional crustal deformation the analysis of a geodetic database, with a minimum benchmark density of 0.7 bm/km (1943–2003 time period), is carried out. We evaluate systematic and random error and their propagation along the levelling routes. Tests on original raw height data have been carried out to define error propagation. The computed relative vertical rates stand significantly above error propagation. A series of traverses along and across the Apennines and a map of relative vertical velocities reveal a geodetic signal characterised by values up to 2.5–3.0 mm/a and by wavelengths up to 100 km. [Copyright &y& Elsevier]

Abstract: We carried out paleoseismological analyses in Norcia, one of the oldest town of central Italy. Four trenches were dug in late Pleistocene–Holocene deposits, across an unmapped, antithetic splay of the Norcia Fault System. The investigated fault runs through the recent settlement of the town, brushing against the middle-age city walls. We found evidence of repeated surface ruptures in the past 20 ky, the last one dated to a period fitting with the 1703 AD, catastrophic earthquake (M =6.8). Our data (i) show definitively the late Pleistocene–Holocene activity of the Norcia Fault System, (ii) strengthen the historical accounts describing surface ruptures during the 1703 event in Norcia, (iii) cast light on the seismogenic behavior of the 70-km-long fault system between L''Aquila and Norcia (central Italy) and (iv) predict the occurrence of normal surface faulting inside the municipality of Norcia during future M ≥6 earthquakes. [Copyright &y& Elsevier]

Combined structural and geophysical investigations were carried out in the Matese Massif at the boundary between the Central and the Southern Apennines. The aim of this research was to define the structural geometry and the kinematics of Neogene deformation in order to validate the applicability of either thin-skinned or thick-skinned compressive styles in the Apennines of Italy.On the basis of two surface N–S cross-sections, integrated with recently acquired magnetotelluric data, the Matese structure appears as a single thrust sheet limited at the bottom by a low-angle thrust fault with sense-of-displacement towards the North. The structure was later deformed by footwall thrusts carrying Molise–Sannio Pelagic Basin Units and Apulian Carbonate Platform Units.The structure of the Matese Massif is composed mainly of Mesozoic carbonates of inner platform to by-pass and slope facies, separated within the thrust sheet by high-angle faults with low reverse horizontal displacement. These faults acted as normal faults in the Mesozoic and Cenozoic and controlled the carbonate facies distribution. During Neogene compression, these structures were tilted and then truncated by the basal short-cut thrust fault that cut from pre-Triassic units to Jurassic dolostone towards the foreland.Finally, Quaternary extensional tectonics dissected the whole structure with fault-related displacement values of up to 1000 m. This resulted in the present-day structural setting of the Matese Massif as a horst bounded to the SW and NE by fault-controlled basins, filled with continental deposits.This revised structural scenario is discussed within the framework of the structural style of external zones of the Apennines. [Copyright &y& Elsevier]

Late Pliocene–Pleistocene tectonic evolution of the Apennines is driven by progressive eastward migration of extensional downfaulting superposed onto the Late Miocene–Early Pliocene compressional thrust belt. This process has led to distinct structural domains that show decreasing transcrustal permeability from conditions of pervasive mixing between deep and surface fluids in the hinterland (west) to conditions of restricted fluid circulation and overpressuring in the foreland (east). At present, the highest rates of normal faulting and the strongest seismicity occur in the area bounded by stretched, highly permeable crust to the west and thick, poorly permeable crust to the east. In this area, the seismogenic sources of the largest earthquakes (5

During the Neogene and Quaternary, the western Mediterranean geodynamics was apparently dominated by the nearly eastward migration of the Apenninic arc and the associated opening (spreading) of the back-arc basin (Tyrrhenian Sea). However, during the last 5 My, the collision of the arc with the Apulian platform led to a dramatic change in the tectonic setting of the area. As geological processes require a long period of time to register the displacements of the different blocks, it is indispensable to take into account the present-day motion given by space geodesy data analysis in order to better constrain the geological models.Geodetic motions were derived from Global Positioning System (GPS), Satellite Laser Ranging (SLR) and Very Long Baseline Interferometry (VLBI) observations collected from different networks. All the geodetic solutions have been computed and combined at the Centre of Space Geodesy (CGS), at Matera, Italy.The geodetic results show a NNE motion of the Adriatic plate with a small component of counter-clockwise rotation, in good agreement with the geological and geophysical observations.In the southern Tyrrhenian area, the lengthening of the Matera–Cagliari baseline should imply that convergence cannot be considered as the driving mechanism for the Apenninic subduction process. The estimated motion of Noto is in quite good agreement with the estimated motion of the African plate. [Copyright &y& Elsevier]

In 2016–2017, a destructive sequence of earthquakes affected a wide portion of Central Italy, activating a complex, 80-km long system of SW-dipping normal faults and causing impressive surface faulting and widespread damage. Former studies providing reconstructions of the fault systems activated during this sequence, are mostly based on high-resolution seismological and geodetic data. In this paper, we integrate surface and subsurface geological data with the ones obtained by an irregular network of seismic reflection profiles, aimed at providing a comprehensive reconstruction of the subsurface lithologies and structures in this area. We have constructed a set of five geological cross-sections, passing through the mainshock epicentral areas (Mw > 5.5) of the seismic sequence. The cross-sections are extrapolated down to a depth of ca. 12 km, along which we have plotted relocated seismicity. Combined geological and seismological data support a new 3D seismotectonic model, illustrating the propagation through time and space of the seismic ruptures during the sequence. Our results show that the litho-mechanical stratigraphy exerted a primary control on the distribution of seismicity, as it is mostly hosted in the more competent lithologies (i.e. the Late Triassic-Paleogene succession, consisting of carbonates and evaporites). In addition, we illustrate the crucial role played by the inherited compressional structures in determining the lateral and vertical variations of the rheological properties of the upper crust and, eventually, the overall geometry and segmentation of the seismogenic extensional system. The workflow proposed here can be applied to other seismogenic zones throughout the world, since reliable seismotectonic models require an accurate reconstruction of the subsurface geological setting, based on a close integration of geological, geophysical and seismological data. • The litho-mechanical stratigraphy of the upper crust controls the seismicity distribution. • The 4 mainshocks nucleated within 4 different tectonic units. • The pre-existing thrusts affect the segmentation of the seismogenic faults. [ABSTRACT FROM AUTHOR]