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- Annex A: Pezzo et al., 2014 - Lunigiana (Submitted)
References[Bovenga et al., 2010] F. Bovenga, L. Candela, F. Casu, G. Fornaro, F. Guzzetti, R. Lanari, D.O. Nitti, R. Nutricato, D. Reale, “The COSMO SkyMed constellation turn on the L'Aquila earthquake: DinSAR results of the MORFEO project”, In Proc. IGARSS'10, Jul. 25-30 Honolulu (USA), 2010. [D'Agostino et al., 2012] D'Agostino, N., D. Cheloni, G. Fornaro, R. Giuliani, and D. Reale. Space-time distribution of afterslip following the 2009 L'Aquila earthquake, J. Geophys. Res., 117, B02402, doi:10.1029/2011JB008523, 2012. [Dee et al., 2011] Dee, D. P., Uppala, S. M., Simmons, A. J., Berrisford, P., Poli, P., Kobayashi, S., Andrae, U., Balmaseda, M. A., Balsamo, G., Bauer, P., Bechtold, P., Beljaars, A. C. M., van de Berg, L., Bidlot, J., Bormann, N., Delsol, C., Dragani, R., Fuentes, M., Geer, A. J., Haimberger, L., Healy, S. B., Hersbach, H., Hólm, E. V., Isaksen, L., Kållberg, P., Köhler, M., Matricardi, M., McNally, A. P., Monge-Sanz, B. M., Morcrette, J.-J., Park, B.-K., Peubey, C., de Rosnay, P., Tavolato, C., Thépaut, J.-N. and Vitart, F. The ERA-Interim reanalysis: configuration and performance of the data assimilation system. Q.J.R. Meteorol. Soc., 137: 553–597. doi: 10.1002/qj.828., 2011. [Doin et al, 2009] M.-P. Doin, C. Lasserre, G. Peltzer, O. Cavalié, C. Doubre. Corrections of stratified tropospheric delays in SAR interferometry: Validation with global atmospheric models. J. App. Geophys., Vol. 69, pp. 33-50, 2009. [Hanssen, 1998] R. F. Hanssen. Atmospheric heterogeneities in ERS tandem SAR interferometry. Delft Univ. Press, Delft, The Netherlands, DEOS Rep. 98.1, 1998. [Hooper et al., 2007] A. Hooper, P. Segall, and H. Zebker, Persistent scatterer interferometric synthetic aperture radar for crustal deformation analysis, with application to Volca´n Alcedo, Gala´pagos, J. Geophys. Res., 112, B07407, doi:10.1029/2006JB004763, 2007 [Jolivet et al., 2011] R. Jolivet, Grandin, R., Lasserre, C., Doin,M.-P. and Peltzer,G., Systematic InSAR tropospheric phase delay corrections from global meteorological reanalysis data, Geophys. Res. Lett., 38, L17311, doi:10.1029/2011GL048757, 2011. [Lanari et al., 2010] R. Lanari, P. Berardino, M. Bonano, F. Casu, A. Manconi, M. Manunta, M. Manzo, A. Pepe, S. Pepe, E. Sansosti, G. Solaro, P. Tizzani, G. Zeni, Surface displacements associated with the L'Aquila 2009 Mw 6.3 earthquake (central Italy): new evidence from SBAS-DInSAR time series analysis, Geophysical Research Letters, Vol. 37, L20309, doi:10.1039/2010GL044780, 2010. [Pinel et al., 2011] V. Pinel, A. Hooper, S. De la Cruz-Reyna, G. Reyes-Davil, M.P. Doin. The challenging retrieval of displacement field from InSAR data for andesitic stratovolvanoes: case study of Popocatepetl and Colima Volcano, Mexico, J. of Volcanology and Geothermal Research, 200 (1–2), pp. 49–61, 2011. [Reale et al., 2011] D. Reale, D.O. Nitti, D. Peduto, R. Nutricato, F. Bovenga and G. Fornaro (2011). Postseismic deformation monitoring with the COSMO/SkYMED Constellation. IEEE Geosc. Rem. Sens. Lett., 8, pp. 696-700, doi:10.1109/LGRS.2010.2100364, 2011. [Rüeger, 2002] J. Rüeger, Refractive index formulae for electronic distance measurements with radio and millimetre waves, Unisurv Rep. 109, pp. 758–766, Univ. of N. S. W., Sydney, Australia, 2002. [Werner et al., 2001] C. Werner, U. WegmÏller, T. Strozzi and A. Wiesmann, Gamma SAR and interferometric processing software, In Proc. ERS-ENVISAT Symposium, 15-20 October 2000, Gothenburg, (Sweden), 2001. Annex A: Pezzo et al., 2014 - Lunigiana (Submitted)Title: The 2013 Lunigiana (Central Italy) earthquake: seismic source analysis from DInSAR and seismological data, and geodynamical implications for the northern Apennines. Running title: 2013 Lunigiana (Central Italy) earthquake Authors: Giuseppe Pezzo* (1), John Peter Merryman Boncori (1), Simone Atzori (1), Davide Piccinini (2), Andrea Antonioli (1), Stefano Salvi (1) *Corresponding author (1) Istituto Nazionale di Geofisica e Vulcanologia, Centro Nazionale Terremoti, Via di Vigna Murata, 605, 00143 Rome, Italy (2) Istituto Nazionale di Geofisica e Vulcanologia, Sede di Pisa, Via della Faggiola, 32, 56126 Pisa, Italy Accepted Received in original form
In this study we use Synthetic Aperture Radar Differential Interferometry (DInSAR) and seismological data to constrain the source of the mainshock of the 2013 Lunigiana (North-western Italy) seismic sequence, namely an Mw 5.1 event occurred on 2013 June 21. The sequence took place in a transfer zone, characterized by right-lateral strike-slip faulting, located between the Lunigiana (North) and Garfagnana (South) graben. As the destructive Mw 6.2 earthquake occurred in 1920 demonstrated, this area is seismically active and is considered the most hazardous area of the Northern Apennine. Hypocentre relocations of the Lunigiana sequence aftershocks well-identify a ~ 45° NNW-dipping fault plane, whereas the focal mechanism solution highlights a dip-slip mechanism with a slight right lateral strike-slip component. Surface displacements estimated from ascending COSMO-SkyMed imagery acquired in the time-span of a single day around the mainshock were used to derive an elastic dislocation model. The estimated slip distributions computed on fixed and variable size meshes show peak values of 30 cm and 40 cm respectively. Static stress variation analysis was performed to analyze possible stress overloads on the closest seismogenic sources. Our results provide insight into the tectonics of the Northern Apennines, suggesting the fundamental role of transfer fault zones in intra-mountain basin origin and in the assessment of seismic hazard in an extensive tectonic regime.
Lunigiana earthquake; northern Apennines; extensional tectonic; InSAR measurements; Seismic source modelling; CFF variations; seismic sequence relocation Introduction On 2013 June 21, at 10:33 UTC, a Mw 5.1 earthquake struck the Lunigiana area (North-western Italy one of the most hazardous area of the Northern Apennine, well known due to the disastrous Mw 6.2 event on September 7, 1920. The focal solution for the mainshock indicates a dip-slip solution with a slight right lateral strike-slip component. Aftershock sequence lasted for weeks, producing a large number of earthquakes and occurred in a transfer fault zone, separating the northern Lunigiana graben and the southern Garfagnana graben (e.g. Elter et al., 1975). The study area is also characterized by active faulting along right-lateral strike-slip fault zones (Di Naccio et al., 2013; Brozzetti et al., 2007). At a larger scale the Lunigiana and Garfagnana graben are located in the western portion of the Northern Apennine, characterized by a NW-SE oriented system of extensive structures that crosscut and dissect the compressive structures of the chain (e.g. Elter et al., 1975). In this study, we measure the ground deformation due to the 2013 June 21 earthquake using Differential Synthetic Aperture Radar Interferometry (DInSAR). We define the source geometry based on seismological data and on the focal mechanism and subsequently carry out a linear inversion of the surface displacements to retrieve the slip distribution. Static stress transfer, studied through a Coulomb Failure Function (CFF) analysis, is used to understand and verify the possible interactions between the modeled source and other seismogenic sources located close the epicentral area. Finally, we discuss how our study contributes to the interpretation of the Northern Apennine tectonics and the implications for seismic hazard evaluation.
The study area is located in the south-eastern sector of the Lunigiana graben, extending for ~45 km along the upper Magra River valley, from the town of Pontremoli to the northern side of the Apuane Alps (Fig. 1). The northern Apennines are a NE verging thrust-and-fold belt formed in the Oligocene after the Corsica-Sardinia and Adria continental block collision (e.g. Elter et al., 1975). The tectonic units can be grouped from top to bottom in: a) the Ligurian allochton; b) the Subligurian unit; and c) the Toscan unit (Bortolotti et al., 2001; Carmignani et al., 2001; Castellarin, 2001; Vai and Martini, 2001, and references therein). The first one consists of an upper Cretaceus stratigraphic succession, folded and thrusted during the late Cretaceous-Eocene subduction of the Liguride-Piedmont oceanic lithosphere. The Subligurian unit is a Palecocene-Oligocene stratigraphic succession originated in an intermediate position between the Ligurian oceanic domain and the Tuscan continental domain. The latter consists of upper (non metamorphic) and lower (metamorphic) units varying from Triassic to Neogene (Vai and Martini, 2001, and references therein). The current tectonic setting of the Lunigiana area consists of a large NW-SE oriented graben delimited by NE-dipping (to the west) and SW-dipping (to the east) normal fault systems, as highlighted by many authors based on seismic reflection profiles (e.g. Argnani et al., 2003). These tectonic features represent the result of the extensional tectonics started during the early Pliocene, that dissected the compressive tectonic structures inherited from the previous contractional phase (Elter et al., 1975; Bartolini et al., 1982; Raggi, 1985; Carmignani and Kligfield, 1990; Bernini et al., 1991; Carmignani et al., 2001; Bernini and Papani, 2002; Argnani et al., 2003). Continental deposits cover the central part of the basin, recording the early Pliocene to Quaternary tectonic evolution (Federici et al., 1978; Bernini et al., 1990; Bertoldi, 1997; Bernini and Papani, 2002). In fact, these sediments are locally offset or strongly controlled in the deposition by normal faults (e.g. Bernini and Papani, 2002). Moreover the drainage network is now incising the continental deposits because of the present regional-scale Quaternary uplift that, according to Argnani et al. (2003), has accelerated during the last 1 Ma. NE-dipping normal faults generally show dip angles of 30-60° and cumulative displacements > 4 km; in contrast, SW-dipping fault dip angles vary between 50-70° and the relative cumulative displacements don't exceed 2.5 km, showing a significant asymmetry (Raggi, 1985; Bernini et al., 1991; Castaldini et al., 1998; Bernini and Papani, 2002). Moreover Argnani et al. (2003) show how the two NE- and SW-dipping fault systems are the shallower crustal splay of a major NE-dipping detachment fault plane, representing the northern termination of the NE-dipping normal fault system and characterized by low dip angles, extending along the entire northern Apennines (Barchi et al., 1998; Boncio et al., 1998; 2000). 
Figure : Geostructural map of the 21 June 2013 earthquake showing the Lunigiana and Garfagnana graben modified from Di Naccio et al. (2013). The map shows the normal fault systems, the main geologic units of the bedrock and the continental deposits, as detailed in the legend. The 2013 seismic sequence (depth-colored dots) and the 21 June 2013 focal mechanism (Scognamiglio et al. 2009) are also reported in the map. Fault names: a) Mt. Picchiara, b) Mt. Grosso, c) Mt. Carmuschia, d) Mulazzo, e) Olivola, f) Mocrone, g) Arzengio, h) Fivizzano, i) Groppodalosio, j) Compione–Comano, k) north Apuane transfer fault zone, l) Minucciano, m) Casciana–Sillicano–Mt. Perpoli, n) Bolognana–Gioviano, o) Verrucole–S. Romano, p) Corfino, q) Barga, r) Mt. Prato, s) Colle Uccelliera, t) Montefegatesi–Mt. Memoriante, u) Mt. Mosca. Continental deposits: a = alluvial deposits (latest Pleistocene–Holocene); ta = terraced alluvial deposits and fanglomerates (middle–late Pleistocene); PQ = clays, sands, and conglomerates of lacustrine and alluvial environment (early Pliocene (Ruscinian)–to early Pleistocene (late Villafranchian)). Sources: Carmignani et al., 2000; Bernini and Papani, 2002; Coltorti et al., 2008; 1:10,000 geologic maps of the Tuscany Regional Authority available at http://159.213.57.103/geoweb/listmet/lista_metadati_10k.htm; 1:50,000 Italian Geologic Map of the CARG project available at http://www.isprambiente.gov.it/MEDIA/carg/toscana.html; Brozzetti et al., 2007. Historical seismicity from CPTI 11 catalog (Rovida et al., 2011). At the southern termination the Lunigiana Graben is delimitated by a nearly E-W strike-slip fault, characterized by N-dipping and normal-oblique right-lateral kinematics (Fig. 1). Some authors (Di Naccio et al., 2013 and Brozzetti et al., 2007) interpreted this fault as a presently active transfer fault between the Lunigiana and the southern Garfagnana extensional graben. Seismicity The Lunigiana area falls within the second most severe seismic zone (zone 2) of the Italian seismic classification, and is considered one of the most seismically active district of north-western Italy (DPC 2012, http://www.protezionecivile.gov.it/jcms/it/classificazione.wp). Moreover this area is characterized by medium-to-high historical seismicity, as shown by the earthquakes occurred in the last millennium (Rovida et al., 2011). The 2013 epicentral area was affected by 23 historical earthquakes from 1481 to 1995 with 4 < equivalent Mw < 6.48, of which 8 with an equivalent Mw > 5, as listed in Table 1. The spatial distribution of historical seismicity highlights how the majority of the earthquakes are clustered close the right-lateral strike-slip fault zone that separates the Lunigiana graben from the Garfagnana one (Fig. 1). The seismic activity of the study area seems to be related to two major tectonic features: the NE- and SW-dipping normal faults and the ~E-W oriented right-lateral strike-slip fault system, the activity of which is documented by numerous works on the quaternary continental deposits, as discussed in the previous paragraph.
Table : Equivalent Mw > 5 historical earthquakes in the 2013 seismic sequence area from CPTI11 (Rovida et al., 2011) The Lunigiana 2013 earthquake sequence started on June 21, at 10:33 UTC, with an ML 5.1 (MW 5.09) mainshock occurred in the area between Fivizzano and Equi Terme (Equi label in Fig. 1). This area corresponds approximately to the epicentral area of the 1837 historical earthquake. In the following two months an intense sequence of aftershocks composed by more than 2000 earthquakes occurred. Earthquake sequence was felt by the population and recorded by high dynamic permanent stations and a temporary seismic network deployed soon after the mainshock by INGV and Università di Genova. The focal solution of the mainshock obtained using Time Domain Moment Tensor technique (Scognamiglio et al. 2009) indicates a dip-slip solution with a slight right-lateral strike-slip component. In the 2 hours following the mainshock an Ml 4.0 aftershock took place three kilometres northward respect to the mainshock, while the most energetic earthquakes occurred eastward between June 23 and 30 (Ml 4.4 and Ml 4.5 respectively) (see Table 2). The earthquake distribution presented here is obtained locating the automatic phase picking routinely obtained by the INGV earthquake monitoring system by using the double-difference algorithm (Waldhauser, 2000). Location results (see Fig. 1) show a NE elongation of the seismic sequence with respect to the epicentre of the Mw 5.1 mainshock. Although the horizontal formal errors obtained after the location process are relatively small (< 1 km), the lateral geological and lithological heterogeneities present in the area affect the constraint at depth of seismicity. Nevertheless the located seismicity shows an overall deepening towards NW, in good agreement with one of the planes suggested by the focal solution.
Table : Main events of the 2013 Lunigiana seismic sequence. DInSAR data The SAR data listed in Table 3 were processed with the SUSIE processing chain (Merryman Boncori et al. 2010) to study the surface deformation due to the mainshock.
Table : SAR image pairs available on the area of interest in the week following the mainshock. Only the I3 pair was used for source parameter inversion. The remaining pairs were temporally decorrelated. Inc. angle is the incidence angle at scene centre (counted from the surface normal). Btemp and Bperp represent the temporal and perpendicular baselines respectively, the latter calculated at the scene centre. Dfdc represents the difference in Doppler centroid values. Two-pass DInSAR (Massonnet and Feigl 1995) was applied to measure surface displacement along the radar Line-of-Sight (LoS), i.e. the direction of the shortest path between a point on ground and the SAR antenna phase centre. Only the I3 pair, with an exceptionally short temporal baseline of 1 day, retained sufficient coherence for the measurements to be successful (Fig. 2A). Using the same image pair we were instead unsuccessful in retrieving the motion component in the azimuth direction, i.e. the ground projection of the satellite flight-path, using the Spectral-Diversity (Scheiber and Moreira, 2000) or Multi Aperture Interferometry technique (Bechor and Zebker 2006). This implies that any underlying north-south displacement component is below the measurement noise floor of about 5 cm for this pair. 
Figure : A) and B) DInSAR wrapped interferogram and displacement map derived from the I3 interferometric pair, as detailed in Table 4. DInSAR deformations are positive from the ground towards the satellite. The LoS arrow shows the ground projection of the directions of positive displacement. The black rectangle represents the surface projection of the modelled fault plane, with a solid line indicating the fault trace. DInSAR interferograms were generated on a 10 m posting, using the SRTM-X DEM (©DLR/ASI 2010) (Farr et al., 2007) to remove the topographic phase contribution. The latter was found to provide a significantly improved topographic phase flattening with respect to the SRTM-C and ASTER GDEM, also available for this area. DEM co-registration was refined through the cross-correlation of a simulated intensity image based on the DEM (Eineder 2003) and the true radar intensity, a processing step which further improved the quality of the phase flattening. Finally the filtering approach of (Goldstein and Werner 1998) was applied with a 700 m x 700 m spectral density estimation window and a constant parameter of 0.8.
Table : Fault parameters of the June 21 2013 earthquake. Lat and Lon are referred to the centre of the fault trace. Given the apparent correlation of our displacement map with topographic features in non deforming areas, which could suggest the presence of tropospheric propagation delay artefacts, we attempted two corrective approaches: the first, based on SAR data alone, consisted in estimating a linear delay/elevation coefficient in a least-squares sense, from non deforming areas (Doin et al., 2009); the second was to compute differential delay maps from the atmospheric parameters provided by the European Centre for Medium range Weather Forecasts (ECMWF) ERA Interim reanalysis data, through an independent implementation of the method described in (Jolivet et al., 2011). Both procedures however had no noticeable impact on the measurements. This might be due to the stronger horizontal gradients in the relevant atmospheric parameters, which could not be resolved by the meteorological data (available at an 80 km x 80 km posting), and even less by any image-wide correction coefficient. It should be noted that the same correction approach applied using ECMWF analysis data, available at a 20 km x 20 km posting, did not yield significantly better results. The displacement map derived from I3 is shown in Fig 2B and shows a main SW-NE oriented elliptic dislocation pattern reaching a maximum value of ~ -3 cm in LoS, indicating a subsidence and/or eastward motion. It is located close to the the June 21 mainshock, near the town of Equi Terme (cfr. Fig. 1). Based on the error estimation procedure of Mohr and Merryman, 2008 and on a mean-latitude atmospheric turbulence model (Merryman and Mohr, 2008), the expected LoS displacement uncertainty (1) in the epicentral area is less than 1 cm (Fig. S1). It should also be noted that the ellispoidal dislocation pattern does not follow the local isohypses, but crosses several valleys and ridges, which would not be the case if it were mainly due to tropospheric propagation artefacts.
We solved for the slip distribution of the source associated to the mainshock through an inversion of the DInSAR coseismic field, based on the elastic half-space dislocation equations (Okada, 1985). The computational load was firstly reduced by sub-sampling the displacement map at a regular 1000 m x 1000 m posting far from the mainshock, and at 150 m x 150 m posting in its proximity. This choice is justified by the decreasing spatial resolution of the model at increasing depth, as described in Atzori and Antonioli (2011) and discussed later in this section. To constrain the fault geometry and mechanism we then considered the focal mechanism proposed by Scognamiglio et al. (2009), indicating a normal fault with a right-lateral component, and the relocated aftershocks described above. Strike angle, dip angle and fault location were obtained by fitting the aftershock cloud (Fig. 3), yielding values of 256° and 47° respectively, in excellent agreement with the focal mechanism. The rake angle was fixed to -116° based on the focal mechanism. All selected parameters are reported in Table 4. 
Figure : Top (A) view of the modelled faults in relation to the June 21 mainshock of the 2013 Lunigiana seismic sequence (magenta star) and aftershocks with Mw > 3.0 (gray dots). (B) view from NW; (C) view from SSW (D) view from SW. Slip distributions were calculated using two different approaches: one with a fixed square patch size of 2 km and one with a variable patch size, ranging from 1.75 to 10 km, obtained with the adaptive algorithm proposed by Atzori and Antonioli (2011). A damped, non-negative, least square solution was calculated for both cases, yielding the slip distributions shown in Fig. 4A and 4B and Fig. 4C and 4D respectively. 
Figure : A and B) Slip distribution for the 2 km × 2 km inversion mesh for the modelled seismic source of the 2013 June 21 event. (A) 3D view from NW. (B) View perpendicular to the fault. (C and D) Slip distribution for the variable-mesh model, obtained with the algorithm of Atzori and Antonioli (2011). (C) 3D view from NW. (D) View perpendicular to the fault. The intersections between the faults and topography are marked with black lines. The associated model resolution values and uncertainties are shown in Fig. 5 and Fig. 6 respectively. The latter are shown only for the variable patch model, since in this case the variance-covariance matrix is nearly diagonal and values are representative of the real slip uncertainty for each element. In the fixed patch solution, covariance values are important and diagonal values are not fully representative of the real uncertainty, since they neglect the correlation between adjacent elements (Atzori and Antonioli, 2011). 
Figure : Model resolution values for the 2 km × 2 km inversion mesh (A and B) and for the variable-mesh model (C and D). Elements nearly perfectly resolved have values close to 1, while 0 is for those completely undetermined. (A and C) 3D view from NW. (B and D) Views perpendicular to the fault. The intersections between faults and topography are marked with black lines. 
Figure : Uncertainty (1) relative to the variable-mesh slip distribution of Fig. 3C and 3D. Values are obtained with the standard rules for error propagation in linear systems, as described in Appendix B of Atzori et al. (2008). (A) 3D view from NW. (B) View perpendicular to the fault. The intersection between fault and topography is marked with a black line. This fixed-patch slip distribution in Fig. 4B shows a peak of about 40 cm, at a depth of ~6 km, whose location is confirmed in the variable slip distribution of Fig. 4D, although in this case the peak value is only 30 cm. As seen from Fig. 5D however, at a 6 km depth, a full model resolution can be obtained only for patches wider than 3 km. Patches of 2 km have a model resolution value of ~0.3, which means that our linear system is not able to resolve this level of detail (see Menke 1989 and Atzori and Antonioli 2011 for a review of the model resolution matrix). We therefore consider the variable patch size slip distribution more reliable. The distribution of relocated aftershocks around the slipped area is in good agreement with the retrieved slip distribution (Fig. 4), with most of the events clustered around the peak slip value and a second patch with an average slip dislocation of 20 cm. In Fig. 7 we compare the observed (Fig. 7a) and the modelled (Fig. 7b) displacement fields, which also show a good agreement, as highlighted in the residual map (Fig. 7c). The latter has an RMS of 0.43 cm, comparable to the expected measurement noise (Fig. S1).
To investigate the possibility of an interaction between the seismic structures of the Lunigiana region and the variation in their seismogenic potential after the 2013 June 21 mainshock, we carried out an analysis of the Coulomb stress variation. We focused on the two faults closest to the mainshock, mapped in the Database of Individual Seismogenic Sources catalogue (DISS Working Group, 2010). Coulomb stress analysis helps to understand whether a fault has moved towards or away from failure in its seismic cycle. We do not show here the stress variations on structure which are further away from the mainshock location, because their magnitudes would be negligible with respect to the stress drop involved in this kind of events. The Coulomb failure stress (CFF) is defined as:
If the medium is considered to be homogenous and isotropic, we introduce the apparent friction coefficient μ’, where also the pressure effects are taken into account (e.g. Harris 1998). Eq. (1) then becomes: CFF'S (2) μ’ values usually fall between 0 and 0.6 (Deng and Skyes, 1997). Here μ’ is kept fixed at a value of 0.4 (see King and Cocco, 2000). Due to our lack of knowledge of the background absolute stress values (Harris, 1998), only the Coulomb stress variation is computed and not its absolute value. Since, as a first order approximation, S can be considered constant, the ΔCFF becomes: ΔCFF='(3) Positive ΔCFF due to induced stress variations induced by the mainshock should advance loaded structures towards failure, whereas negative ΔCFF represents stress release and therefore a delayed fault-failure time. The two closest structures that bound the Lunigiana fault are the Aulla fault on the West, a pure normal fault with approximate dimensions 9 by 7 km, strike 320° and dip 40° and the Garfagnana North on the East, a pure normal fault (18 by 11 km) with strike 305° and dip 40°. All the information on these structures is taken from the DISS database and comes in turn from geological and geomorphological data. All the other structures of the region are too far for an effective interaction with the 2013 sequence. In our analysis (Fig. 8) we notice that the smaller Aulla fault, West of the Lunigiana fault, which is believed could produce up to a Mw=5.8 event (DISS Working Group, 2010), experiences positive CFF values throughout the entire fault area, but its values are mainly so small that an interaction is not expected, even if the existence of a stress a threshold value is still widely debated (e. g. Harris 1998). Only the south eastern edge of the fault plane is loaded with values of up to 0.03 Pa, still very small compared with the stress drop of an earthquake (tyipically a few MPa), but higher than 0.01 MPa, considered as a possible threshold for an actual interaction by some authors (e.g. Haredebeck et al., 1998).
We can conclude that the 2013 Lunigiana earthquake has changed the stress loading on the surrounding faults mapped in the area but, while the Garfagnana North fault could have been advanced in its seismic cycle because of the high positive CFF variation, the stress variation of the Aulla fault is mostly negligible. Discussion and conclusions DInSAR surface displacements and hypocentre relocation allowed to identify a ~45° NNE dipping fault plane activated during the 2013 June 21 earthquake. The focal mechanism solution highlights a main dip-slip mechanism with a slight right-lateral strike-slip component (Scognamiglio et al., 2009). This fault geometry provides a good fit to the deformation pattern, with a small underestimation of the main lobe, with the majority of residuals in the order of the measurement uncertainties, (RMS value of 0.43 cm). Our most reliable dislocation model provides a slip distribution showing a peak value of about 30 cm, located near the mainshock hypocentre, and an overall very good correspondence with the hypocentres of the aftershocks relocated in this work (Fig. 3). Although the activity of the right-lateral strike-slip fault zone (Minucciano fault – L, in Fig. 1) between the Lunigiana and Garfagnana faults was already documented in previously works (Di Naccio et al., 2013; Brozzetti et al., 2007), as well as the quaternary activity of the main NE-dipping and the secondary SW-dipping normal fault systems forming the two extensional basins (Argnani et al., 2003 and references therein), our results provide further insight into the fault geometry and kinematics. In fact, our modelled fault plane shows a dip angle of 45° and a strike angle of 256° with an oblique mechanism (main dip-slip with a slight right lateral strike-slip component). This is a "low" dip angle, with respect to that expected for a strike-slip transfer fault. Argnani et al., 2003 support the suggestion that the Lunigiana and Garfagnana NE-dipping normal faults form a unique large extensional normal fault system passing from the transfer Minucciano fault (Fig. 1) that, under this hypothesis, should accommodate a main vertical subsiding movement of the NE-located sector. Our results seem to be in agreement with this hypothesis, even though a more comprehensive study would be necessary to confirm it. Furthermore, as previously mentioned, the Database of Individual Seismogenic Sources catalogue (DISS Working Group, 2010) reports two seismogenic sources for this area: the Aulla and the Garfagnana normal faults. These tectonic structures belong to the extensional NW-SE normal fault systems that dissect the compressive thrust and folds structure of the northern Apennines. These faults are considered the seismic sources of the largest historical earthquakes of the 1481 (Aulla fault) and 1920 (North Garfagnana fault) reported in the CPTI 04 catalog (DISS Working Group, 2010). However, the 2013 Lunigiana earthquake and the findings of this work clearly demonstrate the seismic activity of the Minucciano fault system, located in the transfer zone between the Lunigana and Garfagnana graben. This fault is not mentioned in the Seismogenic Source catalogue, as for several of the "transfer faults" along the Apennines. In the light of this result, a re-evaluation of the seismogenic potential of this transfer fault zones seems to be a fundamental step for a correct seismic hazard evaluation of the Italian peninsula. Acknowledgement G. Pezzo and J.P. Merryman Boncori were funded by the INGV-ASI MUSA project. The COSMO-SkyMed data are copyrighted by the Italian Space Agency. The authors thank D. Di Naccio for the fruitful discussions. 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