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We investigate a large geodetic data set of interferometric synthetic aperture radar (InSAR) and GPS measurements to determine the source parameters for the three main shocks of the 2016 Central Italy earthquake sequence on 24 August and 26 and 30 October (M-w 6.1, 5.9, and 6.5, respectively). Our preferred model is consistent with the activation of four main coseismic asperities belonging to the SW dipping normal fault system associated with the Mount Gorzano-Mount VettoreMount Bove alignment. Additional slip, equivalent to a Mw similar to 6.1-6.2 earthquake, on a secondary (1) NE dipping antithetic fault and/or (2) on a WNW dipping low-angle fault in the hanging wall of the main system is required to better reproduce the complex deformation pattern associated with the greatest seismic event (the Mw 6.5 earthquake). The recognition of ancillary faults involved in the sequence suggests a complex interaction in the activated crustal volume between the main normal faults and the secondary structures and a partitioning of strain release.

Campi Flegrei is a nested caldera in Italy, whose structure includes submerged and continental parts at the western edge of the Bay of Naples. Together with Vesuvius and Etna, it is one of the Italian GeoHazard Supersites. The last eruption took place in 1538 A.D. and since then intense degassing, seismic swarms and several episodes of ground uplift have been observed. The area is characterized by one of the highest volcanic hazard in the world, due to the very high density of inhabitants. A major unrest episode took place in 1982-84, when the town of Pozzuoli, located at the caldera center, was uplifted by 1.80 m (~1 m/yr). During the following decades the area has been generally subsiding but minor uplift episodes of the order of few cm, seismic swarms and degassing episodes took place in 1989, 2000-01 and 2004-06, showing that the caldera is in a critical state on the verge of instability. Since March 1970 leveling surveys were regularly carried out to monitor the elevation changes. In the following decades many efforts have been done to monitor the different aspects of the activity of the area, and nowadays Campi Flegrei is subjected to intense geodetic, geophysical and geochemical monitoring. In the last 30 years a number of geophysical investigations has provided important constraints to the description of the subsurface structure and the historical volcanic activity. Surface deformation, microgravity changes and geochemical anomalies at Campi Flegrei have been interpreted either in terms of instabilities of the hydrothermal system or variations in the magmatic source. In particular, discerning between magmatic vs hydrothermal origin of the source responsible of the large uplift episode during 1982-84 (most probably due to deep magmatic source) and of the mini-uplifts (e.g. 2000 and 2004-06, most probably due to pressure variations in the shallow aquifer) may have important implications in terms of civil protection. In the last two decades, the precise and large coverage InSAR technique results to be one of the most reliable tool for mapping the surface deformations and therefore for understanding the volcano dynamics. In addition to optimal data, characterizing the origin of deformation is of major importance for assessing the Campi Flegrei volcanic hazard, since the more realistic is the model employed (with parameters constrained by multidisciplinary data), the more reliable are the results and the ensuing interpretations. In the present work we show the contribution from different monitoring techniques, either in situ or from remote, in order to constrain the source of the activity at Campi Flegrei. In particular, we examine more deeply the contribution from geodesy. We consider leveling and EDM data for the large 1982-84 uplift and InSAR time series for the recent activity (2004-06 and ongoing uplifting detected by COSMO-SkyMed satellite). The use of modern satellite and ground-based geodetic techniques is a formidable tool to understand the state of activity of this volcanic area, providing a great contribution to the mitigation of risk.

Deformation data collected at Mount Etna from 1993 to 1997 show that the inflation of the volcano edifice is accompanied by instability of the eastern flank. We propose a 3D finite element model including topography and lateral variations of elastic constants. Source parameters of the inflating source are constrained by a direct search followed by an appraisal stage of the sampled solutions. The instability of the eastern flank is addressed using a kinematic approach, consisting of a rigid-body translation over a prescribed area. The aim is to evaluate how source parameters are affected by sliding of the eastern flank. When sliding is accounted for, the inferred source location shifts ∼ 1 km SE and its strength decreases by ∼ 20%.

Inversions of geodetic data are usually performed by using analytical forward models. Very few analytical solutions exist, and they are characterized by fixed source geometry (e.g. planar dislocation, sphere, dike) and simplified medium properties (e.g. homogeneous, flat and elastic halfspace). These assumptions can potentially bias the estimation of realistic source parameters. We develop a general tool to perform inversions of geodetic data taking into account lateral variations of mechanical properties of the medium and effects due to the topographic relief. The forward models are realized by Finite Element technique. The deformation source is a combination of simple point source mechanisms, i.e. dipoles and double couples. The FE forward technique is based on the equivalence, under specific conditions, of the element-source and the deformation of a 3D ellipsoid dilating under a constant pressure. This procedure is applied to study the inflation process on Mount Etna from 1993 to 1997, as evidenced by data recorded by GPS stations, EDM measurements and analysis of InSAR images. We build a matrix of displacement solutions at data points for each potential elementsource. We consider forward models characterized by heterogeneous medium and topographic free surface. A direct search is performed in the parameters space using the neighbourhood algorithm followed by an appraisal of the sampled solutions. From the inversions we retrieve a source located below the NW sector of Etna, at 6-7 km b.s.l. Our results suggest that while the effect of topography can be negligible, elastic heterogeneities in the medium can significantly alter the position of the inferred source. Furthermore, since the data show a significant signal in the SE sector due to flank instability, we also include in our study some simple sliding mechanisms.

The 1982-84 unrest episode at Campi Flegrei was characterized by huge deformation (about 1.8 m uplift) located inside the caldera and significant gravity variations correlated with the elevation changes (about -213 μGal/m). Due to the bell shape of the uplift, the source is usually interpreted to have a fixed spherical shape. In the present study, we combine simple point source mechanisms (dipoles and double couples) to represent arbitrary sources such as sphere, ellipsoid or sill. The models are realized by Finite Element and the medium may be characterized by elastic heterogeneities. We study the deformation detected by leveling and EDM techniques by coupling the FE forward models with an inversion procedure. The potential point sources are contained in a volume of 8×8×8 km3 located beneath Pozzuoli, the site of maximum displacement. We calculate the displacement field at each data point for each basic mechanism and we compare the result with the observed value. From the inversion of geodetic data we retrieve the best-fitting source parameters, without fixing the shape a priori. The best-fitting source is located beneath Pozzuoli at about 4.8 km b.s.l. and undergoes to horizontal compression and vertical dilatation.

We investigate the 2004-2006 uplift phase of Campi Flegrei caldera (Italy) by exploiting the archive of ascending and descending ENVISAT SAR data acquired from November 2002 to November 2006. The SBAS-DInSAR technique is applied to generate displacement mean velocity maps and time series. An appropriate post-processing step is subsequently applied to map the areas whose temporal deformation behavior is correlated with that of the maximum uplift zone. Our results show that the deformation also extends outside the volcanological limits of the Neapolitan Yellow Tuff caldera, without significant discontinuities. The DInSAR data are inverted by considering a finite spheroid and an isotropic point-source. The inversion results suggest that the new uplift is characterized by a source location similar to the previous small uplift event of 2000 and to the long term subsidence of the 1990's. In particular, the source is located at a depth of about 3.2 km and very close to the city of Pozzuoli (about 800 m offshore, to the SW); the associated volume variation is about 1.1 10(6) m(3)/year.

Uplift and subsidence alternate throughout the life cycle of a volcano mirroring magma accumulation, migration and/or gas pressurization, and magma cooling or depressurization, respectively. The unusual occurrence of coeval inflation and deflation is difficult to reconcile with the recharging or cooling of a single magma chamber, or hydrological processes. Here, we show a persistent flank uplift and central subsidence at Colli Albani volcano near Rome (Italy), by mapping about twenty years of deformation by InSAR data. The magmatic helium signature increases in correspondence with N-S faults along the western slope of Colli Albani, which are therefore interpreted as deep-seated structures directly or indirectly connected with an underlying magma reservoir. Deformation modelling shows that these faults are pathways for fresh magma intrusion, whereas a residual magma is cooling below the deflating caldera. Therefore, magma recharge at depth is controlled by major faults along which the vents of the most recent activity (< 200 ka ago) concentrate in the western side of the volcano and not by the caldera structures. We demonstrate that the Colli Albani magmatic system is slowly rejuvenating, posing a volcanic threat for Rome.

The 2011 Tohoku earthquake produced an unexpected large amount of shallow slip greatly contributing to the ensuing tsunami. How frequent are such events? How can they be efficiently modelled for tsunami hazard? Stochastic slip models, which can be computed rapidly, are used to explore the natural slip variability; however, they generally do not deal specifically with shallow slip features. We study the systematic depth-dependence of slip along a thrust fault with a number of 2D dynamic simulations using stochastic shear stress distributions and a geometry based on the cross section of the Tohoku fault. We obtain a probability density for the slip distribution, which varies both with depth, earthquake size and whether the rupture breaks the surface. We propose a method to modify stochastic slip distributions according to this dynamically-derived probability distribution. This method may be efficiently applied to produce large numbers of heterogeneous slip distributions for probabilistic tsunami hazard analysis. Using numerous M9 earthquake scenarios, we demonstrate that incorporating the dynamically-derived probability distribution does enhance the conditional probability of exceedance of maximum estimated tsunami wave heights along the Japanese coast. This technique for integrating dynamic features in stochastic models can be extended to any subduction zone and faulting style.

INGV-Gruppo Nazionale per la Vulcanologia; Ministero dell'Istruzione UniversitA e Ricerca- FIRB Project

Since 1993 geodetic data obtained by different techniques (GPS, EDM, SAR, leveling) detected a consistent inflation of the Mount Etna volcano. The inflation, culminated with the 1998-2001 strong explosive activity from summit craters and recent 2001 and 2002 flank eruptions, may be interpreted by magma ascent and re-filling of the volcanic plumbing system and reservoirs. Our purpose is to model the 1993-97 GPS and EDM data by pressurized sources simulating the magma reservoir using a 3D Finite Element modeling coupled to a Neighbourhood Algorithm inversion. The power of this technique, if compared with analytical inversions, is that sources can be placed in complex media (heterogeneous, with topography, inelastic, etc.) so that the inversion result is not influenced by the usual approximations of elastic, homogeneous half-space. We model the inflation by a deep volcanic source, considered as point source. The source intensity is given by its stress tensor, identifying an ellipsoidal pressure source. The FE model of Mt. Etna is characterized by a regular mesh below the volcanic edifice, and by arbitrarily distorted brick elements elsewhere. The potential point sources are contained in a cubic volume located below the summit craters, between 4 km and 8 km b.s.l.. For each potential source we compute the deformation and elongation at GPS and EDM observation points. The solution is a weighted combination of displacements (one for each stress component) computed for each of the potential sources. The best source parameters search is performed by the Neighbourhood Algorithm inversion technique. Good results from synthetic tests and previous inversions with only GPS data confirm the robustness of the method. We consider four classes of models characterized by: i) homogeneous medium and flat free surface; ii) homogeneous medium with topography of Mt. Etna; iii) heterogeneous medium with flat free surface; iv) heterogeneous medium with topography. Results from data inversion show evidence for a vertically elongated body. This result is a robust feature of the inversions, since it is common to models with/without topography and with/without rigidity contrasts. Differences in the location and dimension of the ellipsoid are due to the presence of the rigidity contrasts and topography. Solutions are compared also with those derived by inversions of analytical forward models in the homogeneous medium with flat free surface, showing good agreement.

SUMMARY Since 1993, geodetic data obtained by different techniques (GPS, EDM, SAR, levelling) have detected a consistent inflation of the Mt. Etna volcano. The inflation, culminating with the 1998–2001 strong explosive activity from summit craters and recent 2001 and 2002 flank eruptions, is interpreted in terms of magma ascent and refilling of the volcanic plumbing system and reservoirs. We have modelled the 1993–1997 EDM and GPS data by 3-D pressurized sources to infer the position and dimension of the magma reservoir. We have performed analytical inversions of the observed deformation using both spheroidal and ellipsoidal sources embedded in a homogeneous elastic half-space and by applying different inversion methods. Solutions for these types of sources show evidence of a vertically elongated magma reservoir located 6 km beneath the summit craters. The maximum elevation of topography is comparable to such depth and strong heterogeneities are inferred from seismic tomography; in order to assess their importance, further 3-D numerical models, employing source parameters extracted from analytical models, have been developed using the finite-element technique. The deformation predicted by all the models considered shows a general agreement with the 1993–1997 data, suggesting the primary role of a pressure source, while the complexities of the medium play a minor role under elastic conditions. However, major discrepancies between data and models are located in the SE sector, suggesting that sliding along potential detachment surfaces may contribute to amplify deformation during the inflation. For the first time realistic features of Mt. Etna are studied by a 3-D numerical model characterized by the topography and lateral variations of elastic structure, providing a framework for a deeper insight into the relationships between internal sources and tectonic structures.

Employing 3D finite element method, we develop an algorithm to calculate gravity changes due to pressurized sources of any shape in elastic and inelastic heterogeneous media. We consider different source models, such as sphere, spheroid and sill, dilating in elastic media (homogeneous and heterogeneous) and in elasto-plastic media. The models are oriented to reproduce the gravity changes and the surface deformation observed at Campi Flegrei caldera (Italy), during the 1982-84 unrest episode. The source shape and the characteristics of the medium have great influence in the calculated gravity changes, leading to very different values for the source densities. Indeed, the gravity residual strongly depends upon the shape of the source. Non negligible contributions also come from density and rigidity heterogeneities within the medium. Furthermore, if the caldera is elasto-plastic, the resulting gravity changes exhibit a pattern similar to that provided by a low effective rigidity. Even if the variation of the source volumes is quite similar for most of the models considered, the density inferred for the source ranges from ∼ 400 kg/m3 (supercritical water) to ∼ 3300 kg/m3 (higher than trachytic basalts), with drastically different implications for risk assessment.