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Since the Late Pliocene - Early Pleistocene, the Calabrian Arc (southern Italy) is affected by extensional and transcurrent tectonic superimposed on the previous collisional context. Various seismogenic sources have been proposed over time to explain such a complex structural framework, but the topic is still matter of debate. In this work we apply a multidisciplinary approach, concerning Geology, Geomorphology, Seismology and Geodesy, to develop a reliable 3D model of the Cittanova and Serre faults. These faults are considered the causative faults for the 1783 seismic sequence (M 6.5-7) as proposed by Jacques et alii (2001). We used CROP data to investigate the crustal architecture of the area and to constrain the geometry at depth of the major structures. through two schematic geological sections orthogonal to these two faults. The shallow geometric patterns of the Cittanova and Serre faults, were verified trough geological, geomorphological and structural field data. Earthquakes hypocentres were analysed and relocated in order to recognize possible cluster alignments useful to constrain the faults geometry at depth. The high-density level of crustal seismicity attests that this domain is seismically active, between 0 km and 23 km and it concentrates along the main faults. To compute the strain and velocity field of the area (time span of the last 20 years) we measured the IGM95 (Instituto Geografico Militare) benchmarks and processed several GNSS permanent stations belonging to the RING Network (http://ring.gm.ingv.it) and TopNETlive Italy Network (https://rtk.topnetlive.com/italy/networks/topnet-live-italy) using GipsyX 1.5 Strain inversion (performed through grid_strain 2D software) allowed us to define a predominant WNW-ESE extensional deformation, in agreement with previous studies). Combining all previous data, we built for the first time a reliable 3D model of the Cittanova and Serre fault planes, that are consistent with: i) fault magnitude/size empirical relations (Magnitude vs rupture Area, Magnitude vs fault length; ii) geological and geomorphological field observation (fault attitude and kinematic), iii) seismological and geodetic data. Results show that our model is compatible with the seismogenic sources of the 1783 seismic sequence.

Geophysical data provide the chance to investigate a volcano’s dynamics; in particular, considerable information can be gleaned on the stress and strain patterns accompanying the internal processes and the effect of magma ascent on the main structures triggering earthquakes. Here, we analysed in detail the seismicity recorded over the last two decades on Etna volcano, in central Mediterranean. We focused on earthquake and focal mechanism distribution and clustering; in addition, the ground deformation pattern affecting the volcanic edifice was considered to evaluate the inflation and deflation phases. Analysed data were compared in order to shed light on possible relationships with the volcanic activity and to better understand the internal dynamics of the volcano over time. Major steps during or shortly before major eruptions in the seismic strain release and ground deformation temporal series highlight a straightforward relationship between seismicity occurring at shallow level, inflation/deflation and volcanism. Furthermore, at depths greater than 5–7 km, down to about 20 km, the orientation of the P- and T-axes clearly indicate the existence of a pressure source in the central part of the volcano. All the results underline that the stress field related to the volcano plumbing system interferes with the regional field, partly overriding it.

The analysis of the b-value, i.e. the slope of the Gutenberg & Richter frequency-magnitude distribution of earthquakes, provides the chance to investigate the local stress conditions with great resolution, especially in active volcanic areas, where seismic productivity is generally high.In this work we investigated the seismicity of Mt. Etna between 2005 and 2019, focusing on one of the largest known episodes of unrest in December 2018, when most of the intruding magma aborted its ascent inside the volcano. We found a possible stress concentration zone along magma pathways that may have inhibited the occurrence of a larger, more complete eruption. The b-values time series strongly increase about 19 days before the December 2018 unrest event, while a sharp drop of b started 2 days in advance. Our results suggest that the study of the b-value, in broader correlation with other monitoring measurements, may offer an opportunity to investigate the volcano state and improve the assessment of impending volcanic eruptions.

Offshore data in the western Ionian Sea indicate that the NW-SE trending dextral shear zone of the Alfeo-Etna fault system turns to N-S direction near the Ionian coastline, where the Timpe fault system occurs. This latter deform the lower eastern slope of Mt. Etna, showing NNW-SSE to NNE-SSW orientation and resulting from E-W trending regional extension. They are seismically active having given rise to shallow and low-moderate magnitude earthquakes in the last 150 years. Morpho-structural data show that NW-SE trending right-lateral strike-slip faults connect the Timpe fault system with the upper slope of the volcano, where the eruptive activity mainly occurs along N-S to SW-NE trending fissures. As a whole, morpho-structural, geodetic and seismological data, seismic profiles and bathymetric maps suggest that similar geometric and kinematic features characterize the shear zone both on the eastern flank of the volcano and in the Ionian offshore. The Alfeo-Etna fault system probably represents a major kinematic boundary in the western Ionian Sea associated with the relative motion of Africa and Eurasia since it accommodates, by dextral transtensional kinematics, diverging motions in adjacent western Ionian compartments. Along this major tectonic alignment, crustal structures such as releasing bends, pull-apart basins and extensional horsetails occur both offshore and on-land, where they probably represent the pathway for magma uprising from depth.

A multidisciplinary work integrating structural, geodetic and seismological data was performed in the Catanzaro Trough (central Calabria, Italy) to define the seismotectonic setting of this area. The Catanzaro Trough is a structural depression transversal to the Calabrian Arc, lying in-between two longitudinal grabens: the Crati Basin to the north and the Mesima Basin to the south. The investigated area experienced some of the strongest historical earthquakes of Italy, whose seismogenic sources are still not well defined. We investigated and mapped the major WSW–ENE to WNW–ESE trending normal-oblique Lamezia-Catanzaro Fault System, bounding to the north the Catanzaro Trough. Morphotectonic data reveal that some fault segments have recently been reactivated since they have displaced upper Pleistocene deposits showing typical geomorphic features associated with active normal fault scarps such as triangular and trapezoidal facets, and displaced alluvial fans. The analysis of instrumental seismicity indicates that some clusters of earthquakes have nucleated on the Lamezia-Catanzaro Fault System. In addition, focal mechanisms indicate the prevalence of left-lateral kinematics on E–W roughly oriented fault plains. GPS data confirm that slow left-lateral motion occurs along this fault system. Minor north-dipping normal faults were also mapped in the southern side of the Catanzaro Trough. They show eroded fault scarps along which weak seismic activity and negligible geodetic motion occur. Our study highlights that the Catanzaro Trough is a poliphased Plio-Quaternary extensional basin developed early as a half-graben in the frame of the tear-faulting occurring at the northern edge of the subducting Ionian slab. In this context, the strike-slip motion contributes to the longitudinal segmentation of the Calabrian Arc. In addition, the high number of seismic events evidenced by the instrumental seismicity, the macroseismic intensity distribution of the historical earthquakes and the scaling laws relating to earthquakes and seismogenic faults support the hypothesis that the Lamezia-Catanzaro Fault System may have been responsible for the historical earthquakes since it is capable of triggering earthquakes with magnitude up to 6.9.

The south-western edge of the Calabrian Arc in southern Italy has been investigated throughout a joint analysis of field, marine and geophysical data which provided constraints on the fault pattern and on the seismotectonic potential. The study was focused on a poorly known sector of a larger belt of seismically active faults slicing across the NE corner of Sicily, the so-called Tindari Fault System. Our data pointed out that the investigated area, including the mainland and the Ionian offshore, is deformed by oblique faulting with a general NW-SE tectonic trend. Earthquake distribution and seismic profiles pointed out active deformation in the offshore while the mainland is characterized by the occurrence of a NW-SE oriented, >20 km-long, structural belt. However, scarce seismicity has been recorded in the last 30 years alongside this tectonic structure, accounting for a possible silent segment of the larger fault system. Tomographic images revealed that the Moho discontinuity is deformed by a NE-dipping lithospheric tectonic structure which has been here retained the main mode of deformation and responsible for coseismic displacement in the area. As a whole, field and geophysical data agree with a general NW-SE trend segmented pattern of recent/active faults that have the potentiality of generating magnitude 6.5–7 earthquakes.

The 2018 December 26th earthquake (MW = 4.9) at the south-eastern slope of Mt. Etna provides new insights for improving the knowledge of the kinematics of the eastern flank of the volcano. The earthquake was preceded by a seismic swarm on the upper southern-western sector of the volcano and by a short eruptive event in the summit area. The associated crustal deformation triggered seismic reactivation of tectonic structures in the eastern flank of the volcano. The seismogenic source has been localized along one of the segments cutting the south-eastern slope the volcanic edifice, the NW-SE trending Fiandaca Fault, one of the most active shear zone belonging to the upslope extension of the Timpe fault system. In the last centuries, all these faults have been the source of very shallow, low magnitude, but destructive earthquakes. In order to determine the response of the unstable eastern flank of Mt. Etna to the volcano-tectonic events, we applied a multidisciplinary approach based on: i) analysis of historical and instrumental seismicity; ii) mapping of coseismic fracturing, iii) analysis of GPS and InSAR data. This study allows to better define the seismotectonic framework of the shear zone occurring in the eastern flank of Mt. Etna, framing it in the seismogenic belt extending as far as the Ionian offshore.

Pino et al. (2021, hereinafter PIN2021) commented on the paper by Barreca et al. (2021, hereinafter B2021) titled: “The Strait of Messina: Seismotectonics and the source of the 1908 earthquake”, which was published in the journal Earth-Sciences Reviews in May 2021. PIN2021 argued both on the “source model of the 1908 EQ”, as proposed by B2021, and on the existence of the newly discovered causative fault (i.e. the B2021W-Fault). Based on “objective reading of achieved results along with other existing geophysical information…“, PIN2021 conclude: “the source mechanism for the 1908 EQ is based on incorrect assumptions, while their results are internally inconsistent and with other independent observations”. According to PIN2021, the inconsistency of the proposed “source mechanism”, which foresaw the possibility of an aseismic slip on a low-angle discontinuity preceding the 1908 mainshock (see B2021), would be mainly demonstrated by “the lack of significant variations of the relative sea level in the Messina harbor area, in the time period relevant for the levelling data (1907-1908) ……. and at least for the decade preceding the event”. Moreover, to demonstrate that the deformation is mostly coseismic, PIN2021 proposes a sea level diagram based on unreliable data from the Messina tide gauge. In this paper, we demonstrate that the comments by PIN2021 are unfounded. We strongly confirm the scientific validity of the model proposed in B2021.

A detailed 3D image of the Calabro-Ionian subduction system in the central Mediterranean was obtained by means of a seismic tomography, exploiting a large dataset of local earthquakes and computing algorithms able to build a dense grid of measure nodes. Results show that the slab is continuous below the southern sector of the Calabro-Peloritan Arc, but the deformation processes developing at its edges are leading to its progressive narrowing, influencing tectonics and magmatism at the surface, and with possible stress concentration in the tip zones. In the southwest, the deformation occurring at a free slab edge lead to propagation of a vertical lithospheric tear in the overriding plate, which extends along a NW-SE fault system (Aeolian-Tindari-Letojanni) up to about 30 km into the Ionian Sea; further southeast, the lithosphere appears only flexed and not broken yet. In the northeast, the slab seems to break progressively, parallel to the trench. Finally, northwest of Mt. Etna, the tomography highlights low VP that can be related to an upwelling of deep mantle material likely flowing laterally through a window opened by the complete slab detachment.

More than 100 years after the devasting Messina-Reggio Calabria earthquake (M = 7.1), the largest seismic event ever recorded in southern Europe in the instrumental epoch, its causative seismic source is still unknown, and the several rupture models proposed in the last decades are far from any shared solution. Data interpretation on a new dataset of sub-seafloor geophysical soundings with unprecedented resolution, relocated seismicity, and Vp model, together with morphotectonic investigations and inverse modelling of available levelling data, provide additional constraints on the deformation mechanisms and seismotectonics of the Strait of Messina area. High-resolution seismic lines in the offshore, along with displaced Quaternary marine terraces on land, point to active deformation along a previously unmapped ~34.5 km-long extensional fault. Spatial distribution of relocated earthquakes highlighted that a cut-off of the seismicity occurs within the crustal depth. The seismic discontinuity roughly delineates a foreland-dipping and low-angle discontinuity apparently confirming previous studies predicting low-angle seismogenic sources for the 1908 seismic event. However, according to the overburden stress and the attitude of the discontinuity, stress analysis suggests that a seismic slip is unlikely along it. This therefore weakens the hypothesis that a large earthquake may have nucleated along a low-angle discontinuity. Rather, aseismic creeping is instead expected since movement is allowed only by assuming a mechanical weakness of the plane. This mechanical behaviour is currently also supported by the large interseismic strain-rate recorded in the area. Both seismic tomography and crustal-to-subcrustal scale 3D-modelling strongly suggest a cause-and-effect relationship between slab retreat, mantle wedging, uplift in the upper plate block, and active extension in the Strait of Messina area. Lithospheric doming of the upper plate is here interpreted to be the main process controlling uplift in the Peloritani Mts. of Sicily and subsidence in the Strait of Messina region where deformation is mainly accommodated by the weak low-angle discontinuity. In this frame, an almost aseismical slip towards the foreland of the low-angle discontinuity is here accounted to produce stress perturbation in the area. Coulomb stress change modelling revealed that simulated normal slip on the foreland-dipping discontinuity can induce additional stress and promote failure in the overlying brittle faults. An excellent fit between calculated and observed subsidence is achieved by geodetic data inversion that resolved a normal slip on the low-angle discontinuity and a transtensional (slightly left-lateral) motion on the 34.5 km-long and previously unknown extensional fault. The fault-length along with the observed seafloor displacement make this tectonic structure as the most likely to have produced large earthquakes in the Strait of Messina area.

In the Central Mediterranean region, the production of chemically diverse volcanic products (e.g., those from Mt. Etna and the Aeolian Islands archipelago) testifies to the complexity of the tectonic and geodynamic setting. Despite the large number of studies that have focused on this area, the relationships among volcanism, tectonics, magma ascent, and geodynamic processes remain poorly understood. We present a tomographic inversion of P-wave velocity using active and passive sources. Seismic signals were recorded using both temporary on-land and ocean bottom seismometers and data from a permanent local seismic network consisting of 267 seismic stations. Active seismic signals were generated using air gun shots mounted on the Spanish Oceanographic Vessel 'Sarmiento de Gamboa'. Passive seismic sources were obtained from 452 local earthquakes recorded over a 4-month period. In total, 184,797 active P-phase and 11,802 passive P-phase first arrivals were inverted to provide three different velocity models. Our results include the first crustal seismic active tomography for the northern Sicily area, including the Peloritan-southern Calabria region and both the Mt. Etna and Aeolian volcanic environments. The tomographic images provide a detailed and complete regional seismotectonic framework and highlight a spatially heterogeneous tectonic regime, which is consistent with and extends the findings of previous models. One of our most significant results was a tomographic map extending to 14 km depth showing a discontinuity striking roughly NW-SE, extending from the Gulf of Patti to the Ionian Sea, south-east of Capo Taormina, corresponding to the Aeolian-Tindari-Letojanni fault system, a regional deformation belt. Moreover, for the first time, we observed a high-velocity anomaly located in the south-eastern sector of the Mt. Etna region, offshore of the Timpe area, which is compatible with the plumbing system of an ancient shield volcano located offshore of Mt. Etna.

The Calabrian subduction zone is one of the narrowest arcs on Earth and a key area to understand the geodynamic evolution of the Mediterranean and other marginal seas. Here in the Ionian Sea, the African plate subducts beneath Eurasia. Imaging the boundary between the downgoing slab and the upper plate along the Calabrian subduction zone is important for assessing the potential of the subduction zone to generate mega‐thrust earthquakes and was the main objective of this study. Here we present and analyze the results from a 380 km long, wide‐angle seismic profile spanning the complete subduction zone, from the deep Ionian Basin and the accretionary wedge to NE Sicily, with additional constraints offered by 3‐D Gravity modeling and the analysis of earthquake hypocenters. The velocity model for the wide‐angle seismic profile images thin oceanic crust throughout the basin. The Calabrian backstop extends underneath the accretionary wedge to about 100 km SE of the coast. The seismic model was extended in depth using earthquake hypocenters. The combined results indicate that the slab dip increases abruptly from 2‐3° to 60‐70° over a distance of ≤50 km underneath the Calabrian backstop. This abrupt steepening is likely related to the roll‐back geodynamic evolution of the narrow Calabrian slab which shows great similarity to the shallow and deep geometry of the Gibraltar slab. Plain language abstract We investigate the deep crustal structure of southern Italy and the Central Mediterranean where some of the oldest oceanic crust on Earth is actively descending (subducting) into the earth's interior (Speranza et al., 2012). This process causes much of the moderate seismicity observed in this region and may be responsible for strong historical earthquakes as well (Gutscher et al., 2006). Deep seismic data recorded during a marine geophysical expedition performed in 2014, allow us to reconstruct the 3‐D geometry of this subduction zone. Our data reveal a 1‐4 km thick evaporitic (salt bearing) layer in the 13 km thick accretionary wedge. The thin underlying crust has characteristics of oceanic crust. The adjacent onshore domains (E Sicily and SW Calabria) are composed of 25‐30 km thick crust with velocities typical of continental crust. Together with earthquake travel‐time tomography (providing images of the subducting slab down to 300 km) and gravity modeling we can for the first time image the abrupt steepening of the subducting slab, the “slab hinge”, where slab dip increases from ≤5° to >60° over a downdip distance of 50 km. This slab dip is steep compared to other subduction zones, for example in Northern Honshu Japan or Sumatra, where the slab dip remains roughly 10° down to 40 km depth and therefore may have consequences on the seismicity of the region.

We performed an integrative analysis of geological gravimetric and seismological data - Analyzed data depicted a not well-known wrench zone in the central-eastern Sicily - Our findings enabled us to interpret this shear zone as a Plio-Pleistocene STEP fault

• Detailed tectonic features are revealed in a crucial sector of the central Mediterranean • Seismic pattern pinpoints the activity of a lithospheric-scale tear fault • The complex of faults enables adjacent compressional and extensional domains

Local site effects, normally ground motion amplification, represent one of the main components when developing ground motion simulations and play an important role in the potential earthquake damage. In the framework of the UPStrat-MAFA project a stochastic finite-fault simulation method was selected for the generation of synthetic ground motion scenarios. This method uses spectral site correction functions to account for site amplification effects. These local effects may undergo significant changes due to the source–receiver configuration (i.e., distance, source depth and ray incidence). This holds in particular for reflection and transmission coefficients which may strongly vary depending on the source–receiver geometry, and may alter the characteristics of the spectral site-correction functions. A strategy is proposed to account for local site effects in the context of the regional geological structure, considering SH-waves propagating in a 1D velocity model. Spectral correction functions are derived by comparing Green’s functions obtained for general velocity models and those more detailed at shallow depths. The developed approach is applied in two of the test areas selected in the project, the Mt Etna in Italy and the Vega Baja in SE Spain. The results show the different behaviour in two environments, i.e., volcanic and tectonic, with different seismicity characteristics, and highlight the importance of performing specific site-effect studies in some regions where standard building code soil factors could have some limitations to evaluate the potential for ground motion amplification., This research was developed in the framework of the European project UPStrat-MAFA (Grant Agreement 230301/2011/613486/SUB/A5). Part of the work was supported by the Spanish projects CGL2007-62454, and CGL2010-11831-E.

In active volcanic zones, fault dynamics is considerably fast but it is often difficult to separate the pattern of nearly continuous large-scale volcanic processes (inflation/deflation processes, flank instability) from impulsive episodes such as dyke intrusions or coseismic fault displacements. At Etna, multidisciplinary studies on active faults whose activity does not strictly depend on volcanic processes, are relatively few. Here we present the case-study of the San Leonardello fault, an active structure located in the eastern flank of Mt. Etna characterised by a well-known seismic history. This fault saw renewed activity in May 2009, when pre-seismic creeping along the southern segment preceded an MW 4.0 earthquake in the northern segment, followed by some twenty-five aftershocks. Later, in March–April 2016, creep events reactivated the southern section of the same fault. Both the seismic and aseismic phenomena were recorded by the seismic and GNSS networks of INGV-Osservatorio Etneo, and produced surface faulting that left a footprint in the pattern of ground deformation detected by the InSAR measurements. We demonstrate that the integration of multidisciplinary data collected for volcano surveillance may shed light on different aspects of fault dynamics, and allow understanding how coseismic slip and creep alternate in space and time along the strike. Moreover, we use findings from our independent datasets to propose a conceptual model of the San Leonardello fault, taking into account behaviour and previous constraints from fault-based seismic hazard analyses. Although the faulting mechanisms described here occur at a very small scale compared with those of a purely tectonic setting, this case-study may represent a perfect natural lab for improving knowledge of seismogenic processes, also in other fault zones characterised by stick slip vs. stable-sliding fault behaviour.

Seismology and geodesy are generally seen as the most reliable diagnostic tools for monitoring highly active or erupting volcanoes, like Mt. Etna. From the early 1980's, seismic activity was monitored at Mt. Etna by a permanent seismic network, progressively improved in the following years. This network has been considerably enhanced since 2005 by 24-bit digital stations equipped with broad-band (40 s) sensors. Today, thanks to a configuration of 33 broad-band and 12 short-period stations, we have a good coverage of the volcanic area as well as a high quality of the collected data. In the framework of the VULCAMED project a workgroup of Istituto Nazionale di Geofisica e Vulcanologia has taken on the task of developing the seismic monitoring system, through the installation of other seismic stations. The choice of optimal sites must be clearly made through a careful analysis of the geometry of the existing seismic network. In this paper, we applied the Seismic Network Evaluation through Simulation in order to evaluate the performance of the Etna Seismic Network before and after the addition of the stations in the candidate sites. The main advantage of the adopted method is that we can evaluate the improvement of the network before the actual installation of the stations. Our analysis has permitted to identify some critical issues of the current permanent seismic network related to the lack of stations in the southern sector of the volcano, which is nevertheless affected by a number of seismogenic structures. We have showed that the addition of stations at the candidate sites would greatly extend the coverage of the network to the south by significantly reducing the errors in the hypocenter parameters estimation.

Ground motion scenarios for Mt. Etna are created using synthetic simulations with the program EXSIM. A large data set of weak motion records is exploited to identify important input parameters which govern the modeling of wave propagation effects, such as Q-values, high frequency cut-off and geometrical spreading. These parameters are used in the simulation of ground motion for earthquakes causing severe damage in the area. Two seismotectonic regimes are distinguished. Volcano-tectonic events, though being of limited magnitude (Mmax ca. 5), cause strong ground shaking for their shallow foci. Being rather frequent, these events represent a considerable threat to cities and villages on the flanks of the volcano. A second regime is related to earthquakes with foci in the crust, at depths of 10–30 km, and magnitudes ranging from 6 to 7. In our synthetic scenarios, we chose two examples of volcano-tectonic events, i.e. the October 29, 2002, Bongiardo event (I = VIII) and the May 8, 1914, Linera earthquake (I = IX–X). A further scenario regards the February 20, 1818 event, considered representative for stronger earthquakes with foci in the crust. We were able to reproduce the essential features of the macroseismic field, in particular accounting for the possibility of strong site effects. We learned that stress drop estimated for weak motion events is probably too low to explain the intensity of ground motion during stronger earthquakes. This corresponds to findings reported in the literature claiming an increase of stress drop with earthquake size.

The Gulf of Patti and its onshore sector represent one of the most seismically active regions of the Italian Peninsula. Over the period 1984–2014, about 1800 earthquakes with small-to-moderate magnitude and a maximum hypocentral depth of 40 km occurred in this area. Historical catalogues reveal that the same area was affected by several strong earthquakes such as the Mw = 6.1 event in April 1978 and the Mw = 6.2 one in March 1786 which have caused severe damages in the surrounding localities. The main seismotectonic feature affecting this area is represented by a NNW–SSE trending right-lateral strike-slip fault system called ‘‘Aeolian–Tindari–Letojanni’’ (ATLFS) which has been interpreted as a lithospheric transfer zone extending from the Aeolian Islands to the Ionian coast of Sicily. Although the large-scale role of the ATLFS is widely accepted, several issues about its structural architecture (i.e. distribution, attitude and slip of fault segments) and the active deformation pattern are poorly constrained, particularly in the offshore. An integrated analysis of field structural geology with marine geophysical and seismological data has allowed to better understand the structural fabric of the ATLFS which, in the study area, is expressed by two major NW–SE trending, en-echelon arranged fault segments. Minor NNE–SSW oriented extensional structures mainly occur in the overlap region between major faults, forming a dilatational stepover. Most faults display evidence of active deformation and appear to control the main morphobathymetric features. This aspect, together with diffused continental slope instability, must be considered for the revaluation of the seismic and geomorphological hazard of this sector of southern Tyrrhenian Sea.

Geological, geodetic and seismological data have been analyzed in order to frame the Lipari–Vulcano complex (Aeolian archipelago, southern Italy) into the geodynamic context of the southeastern Tyrrhenian Sea. It is located at the northern end of a major NNW–SSE trending right-lateral strike-slip fault system named “Aeolian–Tindari–Letojanni” which has been interpreted as a lithospheric discontinuity extending from the Aeolian Islands to the Ionian coast of Sicily and separating two different tectonic domains: a contractional one to the west and an extensional one to the north-east. Structural field data consist of structural measurements performed on well-exposed fault planes and fractures. The mesostructures are mostly represented by NW–SE striking normal faults with a dextral-oblique component of motion. Minor structures are represented by N–S oriented joints and tension gashes widespread over the whole analyzed area and particularly along fumarolized sectors. The analyzed seismological dataset (from 1994 to 2013) is based on earthquakes with magnitude ranging between 1.0 and 4.8. The hypocenter distribution depicts two major alignments corresponding to the NNW–SSE trending Aeolian–Tindari–Letojanni fault system and to the WNW–ESE oriented Sisifo–Alicudi fault system. GPS data analysis displays ∼3.0 mm/yr of active shortening between the two islands, with a maximum shortening rate of about 1.0 × 10 −13 s −1 , between La Fossa Caldera and south of Vulcanello. This region is bounded to the north by an area where the maximum values of shear strain rates, of about 0.7 × 10 −13 s −1 are observed. This major change occurs in the area south of Vulcanello that is also characterized by a transition in the way of the vertical axis rotation. Moreover, both the islands show a clear subsidence process, as suggested by negative vertical velocities of all GPS stations which exhibit a decrease from about −15 to −7 mm/yr from north to south. New data suggest that the current kinematics of the Lipari–Vulcano complex can be framed in the tectonic context of the eastward migrating Sisifo–Alicudi fault system. This is dominated by transpressive tectonics in which contractional and minor extensional structures can coexist with strike-slip motion.

We performed an in-depth analysis of the ongoing tectonics of a large sector of southern Sicily, including the Hyblean Foreland and the front of the Maghrebian Chain, as well as the Ionian Sea offshore, through the integration of seismic and GPS observations collected in the nearly two decades. In particular, a dataset consisting of more than 1100 small-to moderate-magnitude earthquakes (1.0 ≤ ML ≤ 4.6) has been used for local earthquake tomography in order to trace the characteristics of the faulting systems, and for focal mechanisms computation to resolve the current local stress field and to characterise the faulting regime of the investigated area. In addition, GPS measurements, carried out on both episodic and continuous stations, allowed us to infer the main features of the current crustal deformation pattern. Main results evidence that the Hyblean Plateau is subject to a general strike–slip faulting regime, with a maximum horizontal stress axis NW–SE to NNW–SSE oriented, in agreement with the Eurasia–Nubia direction of convergence. The Plateau is separated into two different tectonic crustal blocks by the left-lateral strike–slip Scicli–Ragusa Fault System. The western block moves in agreement with central Sicily while the eastern one accommodates part of the contraction arising from the main Eurasia–Nubia convergence. Furthermore, we provided evidences leading to consider the Hyblean–Maltese Escarpment Fault System as an active boundary characterised by a left-lateral strike–slip motion, separating the eastern block of the Plateau from the Ionian basin. All these evidences lend credit to a crustal segmentation of the southeastern Sicily.

In this manuscript we present the new friendly seismic tomography software based on joint inversion of active and passive seismic sources called PARTOS (Passive Active Ray TOmography Software). This code has been developed on the base of two well-known widely used tomographic algorithms (LOTOS and ATOM-3D), providing a robust set of algorithms. The dataset used to set and test the program has been provided by TOMO-ETNA experiment. TOMO-ETNA database is a large, high-quality dataset that includes active and passive seismic sources recorded during a period of 4 months in 2014. We performed a series of synthetic tests in order to estimate the resolution and robustness of the solutions. Real data inversion has been carried out using 3 different subsets: i) active data; ii) passive data; and iii) joint dataset. Active database is composed by a total of 16,950 air-gun shots during 1 month and passive database includes 452 local and regional earthquakes recorded during 4 months. This large dataset provides a high ray density within the study region. The combination of active and passive seismic data, together with the high quality of the database, permits to obtain a new tomographic approach of the region under study never done before. An additional user-guide of PARTOS software is provided in order to facilitate the implementation for new users.

This paper presents an overview of the seismic and volcanic activity occurred during 2014 in the region involved by the TOMO-ETNA seismic active experiment (Mt. Etna, Aeolian Islands and Peloritani-Messina Strait areas). To better characterize the seismicity over the year, three-dimensional hypocenter locations and focal mechanism solutions of a dataset of 678 selected small-to-moderate magnitude earthquakes (0.5 ≤ ML ≤ 4.3) were analyzed. In the framework of the TOMO-ETNA experiment, a temporary seismic network was installed on-land from June to November 2014, both to acquire seismic signals produced by shots and to record the local seismicity. Data collected by the temporary network were used to integrate those deriving from the permanent seismic network operated by the Istituto Nazionale di Geofisica e Vulcanologia (INGV)-Osservatorio Etneo (Etna Observatory), thus obtaining a numerically more robust dataset. In agreement with previous analysis and studies, the distribution of the hypocentral locations is well representative of the seismicity that typically characterizes this area. The selected well-constrained 42 fault plane solutions evidence two domains characterized by different motions and style of deformation. In particular, an extensional domain in the northeastern Sicily and a strike-slip regime in the northernmost part of the studied region have been observed.

To recognize possible spatial clusters and identify active seismogenic zones and structures in the Aeolian Archipelago, in the south of Italy, we analyzed the spatial pattern of seismicity between 1993 and 2010 in a selected area comprising Vulcano, Lipari, Salina and Filicudi and calculated 22 fault plane solutions (FPSs) for shocks with magnitude greater than 2.7. First, we computed a 1-D velocity model for this area including information from recorded earthquakes by a joint hypocenter-velocity inversion (Kissling et al., 1994). Successively, we applied the double-difference approach of Waldhauser and Ellsworth (2000), finding that a certain part of the scattered epicenter locations collapse in roughly linear features. Relocated seismicity evidenced three main alignments, oriented NNW–SSE and NE–SW at different depths that concur well with the known tectonic lineaments and focal mechanisms. A detailed discussion is focused on a seismogenetic structure, NE–SW oriented, 3–8 km deep, located in the northern area of Vulcano island. This recognized element could represent a link between magma accumulation zones, thus representing a possible preferential pathway along which magma may intrude. Two earthquake clusters, located south-west and east of Vulcano, with their focal mechanisms, highlight the Aeolian-Tindari-Letojanni Fault System seismic activity and the existence of a transitional zone going from the N–S compressive domain that dominates the Aeolian Islands to the NW–SE extensional domain characterizing the south-eastern Tyrrhenian.

A local seismic network, over a five-year period, recorded about 450 earthquakes in western Argentina. In this region, the geodynamics is controlled by the subduction of the Nazca plate beneath the South American lithosphere, which is characterized here by a sub-horizontal path before reassuming its downward descent. As accurate earthquake locations are of primary importance when studying the seismicity of a given area, events recorded by the local seismic network enable in-depth investigations into seismo-tectonic patterns, allowing to improve the earthquake source characterization and knowledge on the ongoing seismo-tectonics of the region. To this end, we performed a simultaneous 1-D inversion of both the velocity structure and the hypocentre location. The minimum 1-D model obtained is complemented by station corrections which lead to a first insight into the deeper 3-D structure. In addition, stability tests were performed to verify the robustness of our earthquake location results. They reveal a fairly stable hypocentre determination, demonstrating that the locations obtained by the inversion process are not systematically biased. The results show that Sierra Pie de Palo is characterized by a crustal seismogenic structure, dipping west and extending from its eastern boundary to about 30 km of depth. The study also provided new constraints on the geometry of the subducted slab. We noted a great concentration of shallower seismicity, compared to that of the surrounding areas of the Wadati–Benioff zone, at the expected position of the Juan Fernandez Ridge (JFR). Our hypocentres indicate that JFR certainly influences the subduction style along its strike, leading to the formation of a bend in the slab geometry.

Stromboli is considered one of the most active volcanoes in the world, and its persistent but moderate explosive activity is only interrupted by occasional episodes of more vigorous activity accompanied by lava flows. A new effusive eruption began in late February 2007 and was characterized by intense seismic activity throughout the whole period. The accurate seismic signals analysis showed the presence of families of events with similar waveform signatures (i.e., multiplets) located beneath the crater region. Since traditional location techniques do not allow obtaining reliable hypocenters, our analysis focused on high precision locations of the seismicity in order to better define the source geometry of the events. Hypocenters, therefore, have been relocated considering two steps: the first, based on a robust probabilistic approach, is used to find the absolute position of the clusters and the second exploits a master-event concept for the relative location of the events. Finally, the shape of the clusters and the temporal migration of the foci were correlated with the eruptive phases. The results show that the occurrence of a cluster of events is related to the opening and closure of a vent opened in the Sciara del Fuoco slope and, in particular, to the intrusion of a dike injected by central conduit in a radial direction, whereas another cluster lies in a narrow vertical volume positioned under the crater area. The geometry of the clusters suggests a source region depicting the shallower feeding system. Overall, the results highlight that the high precision locations method is an efficient and quick tool to obtain a better understanding of the magmatic processes occurring during an ongoing eruption.

The Messina Strait is the most important structural element interrupting the southernmost part of the Alpine-Apenninic orogenic belt, known as the Calabro-Peloritan Arc. It is being a narrow fan-shaped basin linking the Ionian Sea to the Tyrrhenian Sea. This region is affected by considerable seismic activity which mirrors the geodynamic processes due to the convergence between the African and the Eurasian plates. In the last four centuries, a significant number of disastrous earthquakes originated along the Arc. Among these, the most noteworthy event occurred on December 28, 1908 (known as the Reggio Calabria–Messina earthquake), in the Messina Strait area and caused a large tsunami and more than 100,000 casualties. In this research we focus on the relationships between the general tectonic setting, which characterize the Messina Strait and adjacent areas, seismicity patterns and the crustal structure. We analyzed a dataset consisting of more than 300 events occurring in the years from 1999 to 2007, having a magnitude range from 1.0 to 3.8. This dataset was exploited in a local earthquake tomography, by carrying out a simultaneous inversion of both the three-dimensional velocity structure and the distribution of seismic foci. We applied the “tomoADD” algorithm, which uses a combination of absolute and differential arrival times and a concept of self-adapting grid geometry, accounting for ray density encountered across the volume. With this method the accuracy of event locations is improved and velocity structure near the source region is resolved in more detail than standard tomography. Fault plane solutions were obtained for the major and best-recorded earthquakes. The obtained velocity images highlight vertical and lateral heterogeneities that can be associated with structural features striking from NNE–SSW to NE–SW. These results are consistent with important tectonic elements visible at the surface and the pattern delineated by earthquake locations and focal mechanisms.

The Peloritani Mts. (NE Sicily) are characterized by frequent seismicity. Between 1994 and 2006 more than 1000 earthquakes (1.0 ≤ ML ≤ 3.3) occurred, mostly as highly clustered swarms located at shallow depth near the villages of Castroreale and Rodi Milici (western part of Peloritani Mts.). The same area is also characterized by some geothermal springs and gas vents. Using a multidisciplinary approach, data were collected on the tectonic setting, seismicity pattern and geochemical characteristics of fluid emissions, with the aim of understanding the process of earthquake swarm generation beneath the investigated area. Most of the gases emitted in the study area, in terms of focused and/or diffuse gas emissions often associated with thermal fluids, is of mantle origin, as shown by their He isotopes ratio. On approaching the surface, deep gases interact strongly with local aquifers. An estimate of both the surface efflux of mantle-derived gases measured in focused emissions and of the P–T conditions of fluids in the local crust point to a pressurised gas source that would be located at depth of 7–12 km, corresponding to the range of hypocentral depths of seismic swarms. The complex network of tectonic structures in the area would act as high-permeability pathways for the migration of sub-crustal fluids towards the surface. This scenario could be compatible with a close interplay between pressurised mantle fluids at depth, nucleation of earthquakes due to higher-than-hydrostatic pore pressure and release of mantle-derived gases at the surface. This sequence would be repeated in time, thus producing the observed cycles in the local seismic activity.

SUMMARY Accurate earthquake locations are of primary importance when studying the seismicity of a given area, they allow important inferences on the ongoing seismo-tectonics. Both, for standard, as well as for earthquake relative location techniques, the velocity parameters are kept fixed to a priori values, that are assumed to be correct, and the observed traveltime residuals are minimized by adjusting the hypocentral parameters. However, the use of an unsuitable velocity model, can introduce systematic errors in the hypocentre location. Precise hypocentre locations and error estimate, therefore, require the simultaneous solution of both velocity and hypocentral parameters. We perform a simultaneous inversion of both the velocity structure and the hypocentre location in NE-Sicily and SW-Calabria (Italy). Since the density of the network is not sufficient for the identification of the 3-D structure with a resolution of interest here, we restrict ourselves to a 1-D inversion using the well-known code VELEST. A main goal of the paper is the analysis of the stability of the inverted model parameters. For this purpose we carry out a series of tests concerning the initial guesses of the velocity structure and locations used in the inversion. We further assess the uncertainties which originate from the finiteness of the available data set carrying out resampling experiments. From these tests we conclude that the data catalogue is sufficient to constrain the inversion. We note that the uncertainties of the inverted velocities increases with depth. On the other hand the inverted velocity structure depends decisively on the initial guess as they tend to maintain the overall shape of the starting model. In order to obtain an improved starting model we derive a guess for the probable depth of the Moho. For this purpose, we exploit considerations of the depth distribution of earthquake foci and of the shear strength of rock depending on its rheological behaviour at depth. In a second step we derive a smooth starting model and repeated the inversion. Strong discontinuities tend to attract hypocentre locations which may introduce biases to the earthquake location. Using the smooth starting model we obtaine again a rather smooth model as final solution which gives the best traveltime residuals among all models discussed in this paper. This poses severe questions as to the significance of velocity discontinuities inferred from rather vague a priori information. Besides this, the use of those smooth models widely avoids the problems of hypocentre locations being affected by sudden velocity jumps, an effect which can be extremely disturbing in relative location procedures. The differences of the velocity structure obtained with different starting models is larger than those encountered during the bootstrap test. This underscores the importance of the choice of the initial guess. Fortunately the effects of the uncertainties discussed here on the final locations turned out as limited, that is, less than 1 km for the horizontal coordinates and less than 2 km for the depth.

The adaptive mesh double-difference tomography algorithm (tomoADD) was applied to absolute and differential P, S and S–P data to determine three dimensional VP, VS and VP/VS variations and event locations in southeastern Sicily (Italy). The obtained velocity images highlight vertical and lateral heterogeneities that can be associated with different geological units and main tectonic features. In particular, the sharp velocity contrasts are consistent with previously recognized active faults, allowing us to better determine their shapes and geometries at depth. Moreover, a striking correspondence between areas ruptured by earthquakes and velocity anomalies is observed. In fact, seismicity is mostly confined in the high velocity volumes and/or along the high–low velocity boundaries at mid-crustal depths, whereas it tends to avoid regions with lower than average VP and VS values and higher VP/VS ratios.

Instrumental seismic catalogues are an essential tool for the zonation of the territory and the production of seismic hazard maps. They are also a valuable instrument for detailed seismological studies regarding active volcanoes and, above all, for interpreting the magma dynamics and the evolution of eruptive phenomena. In this paper, we show the first instrumental earthquake catalogue of Mt. Etna, for the period 2000-2010, with the purpose of producing a homogeneous dataset of 10 years of seismological observations. During this period, 16,845 earthquakes have been recorded by the seismic network run by the Istituto Nazionale di Geofisica and Vulcanologia, Osservatorio Etneo, in Catania. A total of 6,330 events, corresponding to approximately 40% of all earthquakes recorded, were located by using a one-dimensional VP velocity model. The magnitude completeness of the catalogue is equal to about 1.5 for the whole period, except for some short periods in 2001 and 2002-2003 and at the end of 2009. The reliability of the data collected is supported by the good values of the main hypocentral parameters through the time. The spatial distribution of seismicity allowed the highlighting of several seismogenetic areas characterized by different seismic rates and focal depths. This seismic catalogue represents a fundamental tool for several research aiming to a better understanding of the behavior of an active volcano such as Mt. Etna.

Between the October 2011 and the July 2012, several seismic swarms occurred in the Hyblean foreland domain of SE Sicily (Italy) along the Cavagrande Canyon, one of the most impressive fluvial incisions of Sicily. Despite the low magnitude of the events (main shock with M ~ 3.7), they represent the biggest strain release of the Hyblean area over the last 10 years. A careful waveform analysis of the earthquakes revealed that most of them form a family of “multiplets”. These findings allow us to reconstruct the attitude of the accountable fault plane by interpolating their high-precision 3D location parameters into a GIS platform. A detailed morpho-structural analysis, performed at the ideal updip projection of the modeled plane, showed that during the Middle–Late Pleistocene the epicentral area has been deformed by a belt of extensional faults, a segment of which matches well with the computer-generated surface. Despite the field evidence, computed focal solutions support contrasting strike-slip kinematics on the same fault plane, clearly indicating a dextral shearing on this pre-existing normal fault. The seismic swarms nucleated on a small rupture area along a ~ 10 km long, NW-SE trending fault segment, that could be able to generate M ~ 6 earthquakes. Following our analysis and looking at seismicity distribution in the SE portion of Hyblean area, we assess that a stress pattern reorganization occurred all over the Hyblean foreland between the Late Pleistocene and present-day. Change in the trajectory of the max stress axes (from vertical to horizontal) seems to have involved a pre-existing large-scale fault configuration with considerable seismotectonic implications.

Geological studies and morphological analysis, compared with seismological and geodetic data, suggest that a compressive regime currently occurs at crustal depth in the western sector of Mt. Etna, accommodated by shallow thrusting and folding at the front of the chain, south of the volcanic edifice. In particular, a large WSW-ENE trending anticline, interpreted as detachment fold, is growing west and north of Catania city (the Catania anticline). Geological data suggest that during the last 6000 years the frontal fold has been characterized by uplift rates of ∼6 mm/yr along the hinge, consistent with the interferometric data (10 mm/yr) recorded in the last 20 years. Moreover, a NNW-SSE oriented axis of compression has been obtained by seismological data, consistent with GPS measurements over the last 20 years which have revealed a shortening rate of ∼5 mm/yr along the same direction. Besides the activity related to the volcanic feeding system, the seismic pattern under the Mt. Etna edifice can be certainly related to the regional tectonics. The compressive stress is converted into elastic accumulation and then in earthquakes along the ramps beneath the chain, whereas on the frontal area it is accommodated by aseismic deformation along an incipient detachment within the clayish foredeep deposits. The high rate of shortening at the aseismic front of the chain, suggests a greater “seismic efficiency” in correspondence of ramps at the rear.

SUMMARY The time delays and polarizations of shear wave splitting above small earthquakes show variations before the 2001 July 17‐August 9 2001 flank eruption on Mt Etna, Sicily. Normalized time delays, measured by singular value decomposition, show a systematic increase starting several days before the onset of the eruption. On several occasions before the eruption, the polarization directions of the shear waves at Station MNT, closest to the eruption, show 90 ◦ flips where the faster and slower split shear waves exchange polarizations. The last 90 ◦ -flip being 5 days before the onset of the eruption. The time delays also exhibit a sudden decrease shortly before the start of the eruption suggesting the possible occurrence of a ‘relaxation’ phenomena, due to crack coalescence. This behaviour has many similarities to that observed before a number of earthquakes elsewhere.

SUMMARY The Peloritani Mountains in northeastern Sicily make part of the Appennine-Maghrebian Chain, which forms the highly deformed southern margin of the European Continent. In this zone the NW–SE-striking ‘Aeolian-Tindari-Giardini’ System (ATG) separates two areas of seismicity. To the west of the ATG fault system, seismic activity below a depth of 40 km is essentially absent. To the east of this fault system, we note a significant presence of intermediate depth and deep events, which mark the subduction zone in the Calabrian Arc. Between 1994 and 2003 300 microearthquakes could be located with fair accuracy near the ATG fault system. Their depths range from less than 5 to 40 km, with greater depths occurring to the east of the ATG. We examined the resolution capability of the standard location by applying a grid search location for typical events. The distribution of the residuals shows trends recognizable in the standard locations that are in part an artefact of a non-ideal conditions of the standard locations, such as the station configuration, the use of an unsuitable velocity model and inconsistencies of arrival time pickings. By applying relative location techniques (the double-difference method and a master-event technique) we were able to reduce the scatter of hypocentres significantly. We focused in particular on earthquake families with similar waveforms and estimated the geometrical extent of hypocentre clusters. Compared to the standard location the dispersion of hypocentres decreased by an amount of over 90 per cent and the volume occupied by the foci contracted to ∼1 per cent. The significance of these geometries was tested with Monte Carlo experiments and by interchanging the master events. The cluster geometries are consistent with the dislocation patterns as inferred from fault-plane solutions but do not show a simple relation to the ATG. The role of fluid flow of plutonic origin may be invoked as a possible trigger mechanism. This hypothesis is supported by the presence of geothermal anomalies in the vicinity as well as by an upward migration trend in foci.

The spatial distribution of 414 earthquakes (1.0 M L 4.6), recorded from 1994 to 2002 in southeastern Sicily (Italy), has been analyzed. The seismicity generally coincides with mapped Plio-Quaternary faults, including the north-north- west-south-southeast striking offshore fault system, which is the most important tectonic structure of the area. For the best located events, we computed 70 focal mechanisms by combining P-wave polarities with S-wave polarizations. A predom- inance of strike slip and normal faults was observed. Focal mechanisms were then inverted for stress tensor parameters by using the algorithm of Gephart and Forsyth. The results highlighted a region governed mainly by a north-northwest-south-south- east to northwest-southeast compressional stress regime. Moreover, anisotropy anal- ysis of shear waves showed a polarization of fast S waves coherently aligned with this stress direction. A finer-scale analysis of the stress tensor evidenced three regions characterized by slightly differing orientation of the greatest principal stress axis, r1. The eastern sector displays a nearly horizontal r1 trending northwest-southeast; the central sector is affected by a low-dip north-northwest-south-southeast r1; whereas, in the western sector, a north-northwest-south-southeast-oriented r1 with a higher dip angle, was detected. Finally, the comparison of the spatial distribution of seismicity occurring during 1994-2002, with locations of previous instrumental earthquakes and larger (M 5.0) historical events showed that the seismicity patterns are persistent.

In November 1999 and January 2000, two microearthquake swarms occurred in southeastern Sicily (Italy). They were analytically located in the depth range 17-25 km, some kilometers northward from the buried front of a regional foredeep, below the active thrust zone of the Sicily mountain chain. Their hypocentral distribution showed two distinct clusters, and comparison of the waveforms revealed clearly that the two swarms formed two distinct families of multiplet events. This led us to (1) carry out a precise relocation relative to two chosen master events of the families and (2) better define the geometrical structure of the two clusters. The cross-spectral method was applied to obtain precise readings of the wave onsets. SH wave onsets were used instead of P waves, as they showed clearer onsets and a good signal-to-noise ratio. Residuals of the relative locations showed small values, no more than several meters on average. The vertical extent of the two relocated clusters was 500 and 250 m, respectively, while the horizontal extent was 250 m. Hypocenters of the first cluster clearly delineate a north-northwest-trending plane with almost vertical dip, matching one nodal plane of the focal mechanism obtained as a composite solution of all events of the cluster. Given the considerable gap angles, because of unfavorable network geometry with respect to the events, the stability of our results was tested carrying out a Monte Carlo experiment. Varying the onset times randomly in the range of ±5 msec, a dispersion of the locations less than 10 m in longitude and less than 50 m both in latitude and depth was found. Similar results were obtained when comparing relocations carried out with different master events. Thus, the overall geometrical characteristics of the clusters were not affected seriously by random errors. Considering the geostructural framework of the region, together with the location and time evolution of the two clusters, fluids of plutonic origin are suggested as the trigger mechanism. Manuscript received 26 June 2002.