Structural complexities and tectonic barriers controlling recent 1 seismic activity in the Pollino area (Calabria-Lucania, Southern Italy) 2 - constraints from stress inversion and 3D fault model building

. We reconstruct the 3D Fault Model of the structures causative of the 2010-2014 Pollino seismic activity by 13 integrating structural-geological and high-resolution seismological data. We constrained the model at the surface with fault-14 slip data and at depth, by using the distributions of selected high-quality relocated hypocenters. Relocations were performed 15 through the non-linear Bayloc algorithm, followed by the double-difference relative location method HypoDD applied to a 3D 16 P-wave velocity model. Geological and seismological data highlight an asymmetric active extensional fault system 17 characterized by an E to NNE-dipping low-angle detachment, with high-angle synthetic splays, and SW-to WSW-dipping, 18 high-angle antithetic faults. 19 Hypocenter clustering and the time-space evolution of the seismicity suggest that two sub-parallel WSW-dipping seismogenic


Introduction
In recent years, the reconstruction of 3D fault models (hereinafter referred to as 3DFM), obtained by integrating surface ad subsurface data, has become an increasingly practiced methodology for seismotectonic studies (e.g., Lavecchia et al., 2017;Castaldo et al., 2018;Klin et al., 2019;Ross et al., 2020;Porreca et al., 2020;Barchi et al., 2021;Di Bucci et al., 2021;SCEC, 2021).Detailed structural-geological data are used to define the active faults geometry at the surface, whereas high-quality geophysical data are needed to constrain the shape of the sources at depth.The 3DFM building helps determine the spatial relationships and the interactions between adjacent sources and identify any barriers hampering the propagation of the coseismic rupture at depth.Moreover, such an approach leads to accurately estimating the area of the seismogenic fault, and therefore the expected magnitude.
Over time, detailed structural geological studies made it possible to recognize several seismogenic faults in the Apennine active extensional belt (Barchi et al., 1999;Galadini and Galli, 2000;Maschio et al., 2005;Brozzetti, 2011) and, in some cases, to document, through paleoseismological data, their reactivation during the Holocene (Galli et al., 2020).Furthermore, the increasing availability of high-resolution imagery allows fault mapping at the sub-meter scale (e.g., Westoby et al., 2012;Johnson et al., 2014;Cirillo, 2020;Bello et al., 2021bBello et al., , 2022)), while accurate geophysical prospections (e.g., ground-penetrating radar) allows the fault surface to be investigated at shallow depths (a few meters or tens of meters; e.g., Gafarov et al., 2018;Ercoli et al., 2013Ercoli et al., , 2021)).Conversely, the geometries of the faults at depth are rarely available since high-resolution deep geological and geophysical constraints are often lacking (i.e., deep wells and/or seismic profiles).In fact, in recent decades, seismic reflection prospecting and deep-well exploitation for hydrocarbon research avoided the area affected by active extension and focused on the eastern front of the chain and on the Adriatic-Bradanic foreland basin system (ViDEPI: https://www.videpi.com/videpi/videpi.asp,last access: 19 April 2021).
This lack can be compensated with well relocated highresolution seismological datasets, to be integrated with geological ones.In Italy, datasets of highly precise re-located hypocenters were collected during recent seismic sequences (Chiaraluce et al., 2004(Chiaraluce et al., , 2005(Chiaraluce et al., , 2011(Chiaraluce et al., , 2017;;Totaro et al., 2013Totaro et al., , 2015)).These sequences include thousands of earthquakes (in confined volumes of rock) which appear to roughly connect with the fault traces at the surface.Therefore, such distributions of earthquakes are generally referred to as ongoing rupture processes affecting an entire seismogenic fault or wide portions of them.
The study area of this work includes the northern sector of the so-called "Pollino seismic gap" (Fig. 1), in which paleo-earthquakes up to M = 7 are documented (Michetti et al., 1997;Cinti et al., 1997Cinti et al., , 2002)), whereas the location and size of seismogenic sources are a matter of debate (Michetti et al., 2000;Cinti et al., 2002;Papanikolaou and Roberts, 2007;Brozzetti et al., 2009Brozzetti et al., , 2017a)).Brozzetti et al. (2017a) mapped a set of active faults in the sector between the Mercure, Campotenese, and Morano Calabro Quaternary basins (Fig. 1a).During 2010-2014, this area was affected by a low to moderate instrumental seismicity (Pollino seismic activity), climaxing with the 25 October 2012 M w 5.2 Mormanno earthquake, and characterized by thousands of recorded events (Totaro et al., 2013(Totaro et al., , 2015)).During the sequence, two other moderate events occurred close to the village of Morano Calabro: on 28 May 2012 (M w 4.3) and on 6 June 2014 (M w 4.0; Fig. 1b).According to Brozzetti et al. (2017a), the whole seismicity was arranged in two major clusters and a minor one.Each major cluster was associated with one moderate event and was generated by an independent seismogenic structure.The pre-existence of a seismic network, that was implemented after the beginning of the sequence, provided a high-quality database of relocated hypocenters (Totaro et al., 2013(Totaro et al., , 2015;;Brozzetti et al., 2017a).
In such context, we reconstruct the 3DFM involved in the 2010-2014 seismic activity to investigate, at depth, the crosscut relationships between the faults with different attitudes and timing of activation.Furthermore, we provide the geometric parameters of the sources to estimate the expected magnitudes.Finally, we discuss some 3D-seismotectonic methodological aspects, which dwell on the improvements that the proposed procedure provides to the definition of the source model and on its limits.
During the uppermost Miocene and Pliocene times, the folds and thrusts pile was displaced by WNW-ESE-striking left-lateral wrench faults (Grandjacquet, 1962;Ghisetti and Vezzani, 1982;Van Dijk et al., 2000).Subsequently, regional-scale extensional fault systems, consisting of E-and W-dipping conjugate normal faults, dissected the Tyrrhenian side and the core of the orogen, which assumed a typical basin and range relief.This Quaternary phase caused the reactivation of the previous strike-slip structures such as the Pollino fault (POL), whose normal to normal-oblique kinematics has been documented since the Early-Middle Pleistocene (Ghisetti andVezzani, 1982, 1983;Brozzetti et al., 2017a).
The other two fault sets strike ∼ NW-SE and dip ∼ SW (blue lines in Figs.1b, 2).The western one, developing from Rotonda to Campotenese villages, consists of two main right-stepping en echelon segments.They are referred to as the ROCS system and include the Rotonda-Sambucoso (RSB) and Fosso della Valle-Campotenese (VCT; Fig. 2).The eastern set, including the en echelon Castello Seluci-Piana Perretti-Timpa della Manca (CSPT), the Viggianello-Piani del Pollino (VPP), and the Castrovillari (CAS) faults, represents the break-away zone of the Quaternary extensional belt.In the area between these two W-dipping sets, the W-to NW-dipping Morano Calabro-Piano di Ruggio (MPR) and Gaudolino (GDN) faults show evidence of Late Quaternary activity (Brozzetti et al., 2017a;Fig. 2).
GPS and DInSAR analysis demonstrated that the Pollino area was affected by important deformation rates during the 2010-2014 seismic activity, with increasing and decreasing slip values due to the temporal and spatial variation of the recorded seismicity (Passarelli et al., 2015).

Seismotectonic setting
According to Michetti et al. (1997Michetti et al. ( , 2000) ) and Cinti et al. (1997Cinti et al. ( , 2002)), POL and the adjacent CAS faults were associated with at least two strong earthquakes (M6.5 and M7.0), occurring in the period 2000-410 B.C. and 500-900 A.D., respectively.The epicenter of the 8 January 1693 earthquake (M5.3,CPTI15, Rovida et al., 2020Rovida et al., , 2021;;Tertulliani et al., 2014;Figs. 1b) is also located within the hanging wall of the CAS and at the footwall of the MPR fault, some kilometers eastward of the strongest 2012 and 2014 Morano Calabro events.The epicenter locations of the 1708 M w 5.5 and 1894 M w 5.1 earthquakes (Tertulliani et al., 2014;Rovida et al., 2021), close to the northern termination of the RSB and within its hanging wall, allow the latter fault to be hypothesized as the possible seismogenic source.
The main instrumental event recorded in the Pollino area is the M w 5.6 Mercure earthquake (9 September 1998; Fig. 1b), which was followed by some hundred aftershocks and that was associated by Brozzetti et al. (2009) with the SWdipping CSPT (Figs. 1b,2), located some kilometers to the NE of the Mercure basin.
All the associated WSW-ENE-oriented T axes are also quite parallel to the geological and seismological least compressional axis, as provided by the tensorial analysis in the neighboring Mercure area (Brozzetti et al., 2009;Ferranti et al., 2017) or derived from borehole breakouts (Montone and Mariucci, 2016;Mariucci and Montone, 2020) and GPS data (Devoti et al., 2011;D'Agostino et al., 2014;Cambiotti et al., 2020).As discussed by Totaro et al. (2015Totaro et al. ( , 2016) ) and Brozzetti et al. (2017a), the available focal solutions well correlate with the Quaternary normal faults recognized in the epicentral area, represented by N-S-to NNW-SSE-striking (W-dipping) seismogenic sources.
Correlating the hypocenter distribution with the active faults at the surface, the seismogenic source of the 25 October 2012 Mormanno Earthquake (M w 5.2) is identifiable in both the segments of the WSW-dipping ROCS system (RSB and VCT in Figs.1b, 2).These faults dip 70-75 • , at the surface, and would reach a dip of ∼ 55 • at depth (Brozzetti et al., 2017a).Through similar reasonings, the WSW-dipping MPR fault was suggested to be the causative fault of the eastern Morano Calabro cluster (Fig. 1b) and of its two major events (M w 4.3 on 28 May 2012 and M w 4.0 on 6 June 2014).The fault extends for ∼ 7 km in a N170 direction and is co-axial with the W-dipping nodal planes of the two main events of the sequence (Fig. 1b).The partial reactivation of the CAS could be invoked to explain the minor cluster of seismicity recorded at the eastern side of the study area, although some of the events seem to be located at its footwall.

Structural survey and fault kinematic analysis
We performed a series of fieldwork campaigns, at 1 : 25 000 scale, in the study area and surrounding sectors, to collect fault-slip data to be integrated with the geological-structural observations reported in Brozzetti et al. (2017a).In addition to traditional survey methods, we used the Fieldmove App (PetEx Ltd., version 2019.1)installed on a tablet computer to acquire the data in the field (e.g., Allmendinger et al., 2017;  Novakova and Pavlis, 2017;Testa et al., 2019;Brozzetti et al., 2020;Cirillo, 2020), and we managed them in ArcGIS v.10.8 (ArcMap©).Figure 2 shows the location of the survey sites, considered structurally homogeneous outcrops falling within a maximum distance of 500 m (see also Fig. S2 in the Supplement).The overall fault-slip dataset was first subdivided into minor and local homogenous kinematic subsets, the latter represented as pseudo-focal mechanisms using FaultKin 8 software (Marrett and Allmendinger, 1990;Allmendinger et al., 2012;Fig. 3).The fault-slip data were subsequently inverted (see following Sect.4.3).

Hypocenter location
To better characterize the 3D features of the tectonic structures located in the study area, we performed a high-quality hypocenter location.We enlarged, with respect to previous works by Totaro et al. (2013Totaro et al. ( , 2015) ) and Brozzetti et al. (2017a) , 2).For each fault system, the density contour of the T axis computed for each focal mechanism is reported (lower hemisphere projection).A.s.v.: average striae value, A.f.p.: average fault plane, n: number of fault-plane measurements.Numbers in the rectangles (top left of each focal mechanism) refer to the group of fault-slip data belonging to or neighboring a single site (location in Fig. S2).
including both temporary and permanent stations managed by the University of Calabria and INGV (D'Alessandro et al., 2013;Margheriti et al., 2013), consisted of 61 stations with a maximum epicentral distance of 150 km (Fig. S1 in the Supplement).We computed accurate absolute hypocenter locations by applying first the non-linear Bayloc earthquake location algorithm (Presti et al., 2004(Presti et al., , 2008) ) and subsequently the double-difference relative location method HypoDD (v.2; Waldhauser, 2001) and using the 3D velocity model by Orecchio et al. (2011).The Bayloc algorithm gives for each earthquake a probability density cloud with shape and size related to the main factors involved in the location process (e.g., network geometry, picking errors) and allows a generally more accurate estimate of hypocenter parameters and location uncertainties with respect to the more commonly used linearized location methods (e.g., Lomax et al., 2000;Husen and Smith, 2004;Presti et al., 2008).The application of the Bayloc algorithm provides, on average, horizontal and vertical errors of the order of 1.0 and 1.5 km, respectively, allowing us to obtain a well-constrained database.As the second step, we apply the HypoDD algorithm, which minimizes phase delay-time residuals between pairs of events recorded at common stations (Waldhauser and Ellsworth, 2000).We compute the delay times from each event to its 30 nearest neighbors within 10 km distance, and to further ensure the robustness of the double-difference inversion, only event pairs with at least eight phases observed at common stations were used.The final relocated dataset consists of 3109 events (Figs. 4 and S1).During the decade before the 2010-2014 Pollino sequence, the instrumental data, available within a range of nearly 75 km from the Mercure basin, referred to background seismic activity (Frepoli et al., 2005;Castello et al., 2006;Brozzetti et al., 2009).A significant seismic activity which affected the region was the moderate-magnitude 1998-1999 Mercure sequence that developed in the northern part of the homonym Quaternary basin (Fig. S1; Guerra et al., 2005;Arrigo et al., 2006;Brozzetti et al., 2009) and showed some similarities to the recent Mercure-Pollino sequence (e.g., prevalent kinematics of focal mechanisms and hypocentral depth range).We explored the data available for this seismic activity to compute a high-quality earthquake location, following the procedure described above for the 2010-2018 earthquake dataset.Since the recording network operating during the 1998-1999 seismic phase was significantly different from today, in terms of the number of stations deployed in the region and their spatial distribution, the available data do not allow the high level of constraint needed to perform the 3D structural model reconstruction to be reached.

Geological and seismological stress tensor inversion
To investigate the coherence between the geological and the seismological stress fields, we applied stress tensor inversions to the available fault-slip data (Figs.2, 3) and focal mechanisms (Fig. 4).We used the "TENSOR" program and the inversion procedure proposed in Delvaux and Sperner (2003).We applied it separately on the different datasets.
The procedure computes the orientation of the three principal axes of the stress ellipsoid (σ 1, σ 2, σ 3) and the stress ratio = (σ 2−σ 3)/(σ 1−σ 3) that optimize the misfit function (i.e., F5 in the "TENSOR" program, described as f3 in Delvaux and Sperner, 2003).The latter is built (i) to minimize the slip deviation between the observed slip line and resolved shear stress (30 • misfit value is not expected to be exceeded) and (ii) to favor higher shear stress magnitudes and lower normal stress to promote slip on the plane.The inversion procedure provides for the preliminary (kinematic) analysis of data using an improved version of the Right Dihedron method (Angelier and Mechler, 1977) to determine the starting model parameters (e.g., the reduced stress tensor).The stress ellipsoid is then computed through a 4D grid-search inversion involving several runs during which the reduced tensor is rotated around each stress axis with a decreasing range of variability (from ±45 to ±5 • ), and the full range of values (0-1) is checked.Each step attempts to find the parameters that minimize the misfit function and that are used as a starting point for the next run (see for details Delvaux and Sperner, 2003).
The geological data input consists of 268 qualitycontrolled fault-slip data measured in the study area (Figs. 2,  3).During the formal inversion, the same weight value was assigned to each fault.The seismological data input is represented (initially) by both nodal planes of each focal mechanism; afterwards, the plane that is best explained by the stress tensor in terms of the smallest misfit function is considered as the actual fault plane (Delvaux and Barth, 2010).The inverted seismological data are represented by focal mechanisms from Totaro et al. (2015Totaro et al. ( , 2016) ) and reported in Fig. 4.An exponential weighting factor (corresponding to the earthquake magnitudes) has been assigned to account for the prevailing kinematics of the most energetic events.The final inversion (Fig. 5) includes only the fault and focal planes that are best fitted by a uniform stress field (Gephart and Forsyth, 1984).
In particular, we created several sets of closely spaced transects (distance: 2 km) to cross and sample the seismogenic fault zones in different directions (Fig. 6).The set orihttps://doi.org/10.5194/se-13-205-2022Solid Earth, 13, 205-228, 2022 ented SW-NE is approximately perpendicular (e.g., Fig. 6a  and b) to the ROCS (VCT and RSB).Differently, the set oriented NW-SE is approximately sub-parallel to MPR active faults (e.g., Fig. 6f).A further NNE-SSW-striking set of transects was traced approximately perpendicular to the active fault alignment bounding the study area to the east, which includes the CSPT and VPP faults (Fig. 6g and h).The 3DFM building was carried out following three steps graphically depicted in Fig. 7 and synthetically described below.
Step 1 -Extrusion of fault traces to shallow depth The traces of the Quaternary faults are "extruded" to a pre-set depth of 2 km b.s.l., according to the fault-plane dip measured in the field.In the absence of measured dip angles, we assumed a fixed value of 60 • .The obtained so-called "fault ribbons" are rimmed upward by the topographic surface (a 10 m resolution DEM; Tarquini et al., 2012).
Step 2 -Down-dip extrapolation of the faults along seismological sections Starting from the analysis of the seismological transects (Fig. 6), we traced the deep geometries by connecting the fault ribbons with the seismicity clusters at depth (Fig. 7b, c) downward to the base of the seismogenic layer.
Step 3 -Building of 3D fault surfaces This step allows the final 3D reconstruction (Fig. 7c,  d) to be reached by interpolating, through the Delaunay triangulation method (Delaunay, 1934), all the fault lines as interpreted along the seismological cross sections (Step 2).The result is the fault plane surface that best approximates and connects the clusters of seismicity and the surface geology (represented by the fault traces extruded).

Geological and seismological stress tensors
The computed geological stress tensor (Fig. 5) shows a relevant percentage of fault-slip vector pairs (∼ 53 %) consistent with a uniform extensional stress field which is characterized by a N244-trending and sub-horizontal σ 3. The stress ratio = 0.22 ± 0.13, and the rank quality is QRw = A (ranking as in Sperner et al., 2003).Nearly all the kinematic axes related to the inverted data belong to a normal-fault regime, as also pointed out by the triangle in Fig. 5 (Frohlich, 2001).
In both the inversions, a normal-fault regime with subhorizontal and collinear (∼ SW-NE-trending) σ 3 axis has been obtained.This result points out the coherence between the geological (long-term) and the present-day stress field and the persistence of this extensional regime at least since the Middle Pleistocene (Brozzetti et al., 2017a).
In addition, it is worth noting that 76 % of the successfully inverted fault-slip vector pairs are related to the active fault planes belonging to the E-and W-dipping domains (Fig. 5a), while the remaining 24 % include data related to the S-dipping system (CVN and POL).The evidence together with the similarity between the computed stress tensors is consistent with the prevalent activation, in the Late Quaternary, of the E-and W-dipping fault systems.

Time-space evolution of the Pollino sequence
The 2010-2018 seismic activity in the Pollino-Mercure area followed a peculiar evolution over time (Fig. 4) with epicenters concentrated in a series of neighboring clusters, numhttps://doi.org/10.5194/se-13-205-2022Solid Earth, 13, 205-228, 2022  bered as Cluster 0, 1, 2, and 3, from west to east, according to their activation time.Such clusters, independent and unconnected to each other, are related to fault segments that are not in an along-strike continuity.Cluster 0 (30 January 2010-31 July 2011) includes low magnitude (1.0 ≤ M L ≤ 2.9) activity located in an NNE-SSW-oriented sector at the western boundary of the epicentral area.It is delimited westward by the more external segment of the E-dipping CRFS.
Cluster 1 started after 5 October 2011 and lasted for the entire 2011-2014 seismic activity.It extended continuously, both northward and southward, reaching a NW-SE length of ∼ 12 km (Fig. 4a-c).It comprehends the higher number of earthquakes and is largely the major cluster as regards the wideness (∼ 60 km 2 ) and energy release.It includes 30 events with M L ≥ 3.0 besides the strongest event of all the Pollino seismic activity (25 October 2012).During the 2015-2018 interval, Cluster 1 area was affected by low seismic activity, mostly distributed in its northern and southern portions; conversely, its central part, where epicenters were particularly dense between 2011 and 2014, became less active.Overall, the surface extent of Cluster 1, which partly overlaps with Cluster 0, is limited eastward by the W-dipping RSB and VCT faults.Its southern boundary nearly coincides with the southeastern continuation of the AVN fault (PAC, Fig. 4c).
Cluster 2 started in May 2012 in the sector between the two WSW-dipping RSB and the MPR faults.It elongates in the N-S direction, for ∼ 7 km to the northwest of the Morano Calabro town.Afterward, it was nearly continuously active, particularly during the periods May 2012-October 2014 (Fig. 4b, c); in the period 2015-2018, significant seismicity also persisted (Fig. 4d).Cluster 2 includes mainly low-magnitude events besides the strongest ones of 28 May 2012 and 6 June 2014 and three other earthquakes with 3.0 ≤ M L ≤ 3.5.

3D fault model of the Pollino area fault system
The obtained 3DFM (Fig. 8), which includes the seismogenic fault system involved during and after the 2010-2014 Pollino seismic activity (CRFS, ROCS, and MPR), also encompasses those faults (GCG, PPS, AVN, BAT, CSPT, VPP, SDD, CAS) that, while showing no direct evidence of recent seismic activity, play a significant role in the seismotectonic frame of the area.
Overall, this style is characterized by an asymmetric extension driven by a low-angle (20 to 35 • ) E-dipping detachment fault, which represents the basal decollement of all the other extensional structures.In the model, all the faults are traced at the surface with their dip angle as measured in the outcrop and evolve downward with nearly listric geometries to join the detachment at increasing depth from west to east.The latter represents the structurally controlled base of the seismogenic layer.The GCG (Figs. 1b, 8), which crops out at a low angle and overcomes all the other east-dipping faults (in terms of both slip and associate extension), is the currently inactive break-away zone of such a detachment.The AVN and BAT (Figs. 2,8), which are the easternmost E-dipping splays, are suggested to be active and seismogenic, being possibly the causative structures of the Cluster 0 of hypocenters (Fig. 4a).Cluster 1 and Cluster 2, which are confined downward by the E-dipping detachment, confirm the activity of the WSW-dipping ROCS and MPR faults, so that we consider them the main geological structures involved during the 2010-2014 seismic activity (Figs. 4 and 8a,a1).Further east, the 3DFM has been widened to include the W-dipping CSPT and VPP faults, considered the outer seismogenic front of the extensional system.The along-strike continuity of POL and CVN is interrupted by the W-dipping ROCS and MPR faults (Fig. 8c, d), coherently with the cross-cut relationships observed in the field (Fig. 2).The deep geometry of POL and CVN is interrupted by the NNE-dipping AVN (Fig. 8d), which acts as the southern and basal boundary of the entire active fault system.Finally, the 3DFM shows that almost all the 2010-2018 seismicity correlates with the W-dipping structures but without affecting their southern termination zones.In other words, no or very few events are located south of the intersection with POL and CVN faults.This latter observation suggests that although the POL and CVN did not play an active role in causing the considered seismicity, they play a significant role in influencing its distribution.

From 3D fault model to expected earthquake magnitude
Coherently with what is observed in most Apennine chains (D'Agostino et al., 2001;Ferranti et al., 2014;Montone and Mariucci, 2016;Mariucci and Montone, 2020), the upper crustal Pollino seismicity develops in response to WSW-ENE-oriented extension.This is well constrained by the focal solutions of the strongest events (M w 5.2, 25 October 2012; M w 4.3, 28 May 2012; and M w 4.0, 6 June 2014 earthquakes) and of all the M w ≥ 3.5 earthquakes that occurred during 2010-2014, and with the results of the geological and seismological inversion (Fig. 5).Such consistency suggests that the present stress field is in continuity with the long-term one, which has been set up at least since the Early-Middle Pleistocene, as already suggested by previous works (Papanikolaou and Roberts, 2007;Brozzetti et al., 2009Brozzetti et al., , 2017a)).
Comparing the distribution of all 2010-2018 seismic activity with the Late Quaternary structures mapped at the surface, we maintain that the ROCS and the MPR faults are suitable as the seismogenic sources for the Mormanno (2012, M w 5.2) and Morano Calabro (2012, M w 4.3 and 2014, M w 4.0) earthquakes, respectively.In addition, our 3DFM allows a parameterization of the sources and their seismogenic potential assessment.The map view of the W-dipping faults (Fig. 9a) depicts irregularly shaped seismogenic boxes which are delimited to the east by the fault traces (at the surface) and to the west by the branch line of each fault with the base of the seismogenic layer.Some of these boxes include historical or instrumental earthquakes (Fig. 9b), while others are not associated with any significant event.
The performed 3D reconstruction allowed us to estimate the effective area extent of all the fault segments (Fig. 9c), that, when inserted in the appropriate scaling relationships, provide the expected magnitude possibly releasable in case of entire rupture (Fig. 9c).
We also computed the magnitude values obtained using the regressions as a function of the surface fault length (Fig. 9c).Using six different empirical relations (Wells and Coppersmith, 1994;Wesnousky, 2008;Leonard, 2010;Stirling et al., 2013), we compared the values determined for all the investigated active normal faults (Fig. 9d, e).
It is evident that, for each fault, the expected magnitude computed using fault area is lower than the one calculated by using fault length.The range of variation is narrower for the values calculated on the ground of fault-area regressions (yellow bars in Fig. 9d, e).
Given the significant difference in the magnitude values computed using area-or length-based scaling relationships, we suggest that (where possible) the reconstruction of a 3Dfault geometry should be pursued and preferred in order to derive more reliable parameters to be used (Table S1).This is even more essential in complex extensional systems such as the one we investigated along the Calabrian-Lucanian border.
In fact, the 3DFM highlights that the areal extension of the W-dipping faults depends on their position within the hanging wall of the detachment (see Sect. 5.3).This implies that faults with comparable length at the surface may have significantly different areas, depending on the depths reached.The CSPT, VPP, and CAS crop out at the greatest distance from the GCG break-away zone.Consequently, they intersect the basal detachment at the higher depth and have the maximum area extent among the W-dipping fault set (Fig. 9a, d).
By applying the aforementioned scaling laws (Fig. 9) to the W-dipping faults identified to be involved during the 2010-2014 seismic activity, we calculated the expected magnitude of ∼ M w = 6.1 for the VCT and the RSB, and of ∼ M w = 6.2 for the MPR.Since the two faults (RSB + VCT) of the W-dipping ROCS have been interpreted to join at hypocentral depth to form a single structure (thus a unique seismogenic patch was reconstructed -Fig.10a), a value of ∼ M w = 6.4 could be reached in the case of complete and concurrent ruptures on both the segments.The aforesaid values are sensibly higher than the magnitudes of the earthquakes recorded to date in the Mercure-Campotenese area (Figs. 1b,9b), thus suggesting that the considered faults may have only partially released their seismogenic potential during historical times.This inference also agrees with the distribution and evolution of the 2010-2018 seismic activity.The clusters of the relocated hypocenters concentrated in the deepest parts of the ROCS and MPR faults (Fig. 6) confirm that only a portion of such faults ruptured during the sequence, without the rupture reaching the surface.

Seismogenic patches activated during 2010-2014
The seismogenic patches activated on the ROCS and MPR faults during the 2010-2014 seismic sequence are considered as the reasonable approximation of the actual portion of the faults which broke during the mainshock and the sequence of the early aftershocks.We obtained them by projecting the relocated hypocenters on the reconstructed fault surface and depicting their distribution using the Kernel density geostatistical analyst, available as a tool of the ESRI ArcGIS software package.The delimitation of each seismogenic patch and its parameterization allowed us to verify the correlation between its dimensions and the magnitude released by each fault during the mainshocks.
The temporal analysis of the sequence shows that their overall extent was already well defined within the first 72 h after the major events.In any case, inside the surrounding volumes, some seismicity had started before the mainshock and continued to persist constantly throughout the development of the entire sequence so that they include ≥ 70 % of all hypocenter locations.The along-strike elongation and area extent of the patches obtained over the VCT and MPR fault surfaces can be assumed respectively as the effective subsurface rupture length and rupture area (RLD and RA in Fig. 10b, and c, respectively, according to Wells and Coppersmith, 1994) associated with the M w 5.2 Mormanno (on VCT fault) and M w 4.0 and 4.3 Morano Calabro (on MPR fault) earthquakes.
The parameters obtained for the VCT fault are RLD = 4.9 km and RA = 8.3 km 2 , while RLD = 1.2 km and RA = 3.6 km 2 are assessed for the MPR fault.Introducing the aforesaid parameters in the appropriate scale relationships (Fig. 10b, c), we observe a good agreement between the theoretical magnitudes based on the subsurface rupture length and the magnitudes of the mainshocks.The value obtained for the VCT fault (causative of the M w 5.2 Mormanno earthquake) is M w 5.3, whereas for the MPR fault (causative of the M w 4.0 and 4.3 Morano Calabro earthquakes) it is M w = 4.5.The magnitude calculated using the RA-based relationships provides values slightly lower than expected for the VCT (4.9<M w <5.0) and slightly higher for the MPR (4.5<M w <4.6).In both cases, however, the magnitude values obtained using the scale relationships differ from those observed by an amount <0.3.

Possible geometric restraints to coseismic rupture propagation
The seismological dataset we used demonstrates that the two main clusters of earthquakes of the 2010-2018 seismicity were generated by as many independent sources related to the sub-parallel, 10 to 15 km long, ROCS and MPR faults.Brozzetti et al. (2017a) highlighted that the above seismogenic style, characterized by a perpendicular-to-fault strike evolution of the seismic activity, is unlike from those which followed the major instrumental earthquakes recorded in the Apennine Extensional Belt of Italy in recent years, such as the Colfiorito 1997 (M w 6.0), L'Aquila 2009 (M w 6.3), and Norcia 2016 (M w 6.5) events (Chiaraluce et al., 2011(Chiaraluce et al., , 2017;;Lavecchia et al., 2011Lavecchia et al., , 2012aLavecchia et al., , 2016)).They also speculated that this peculiar behavior could have been controlled by the geometric fault pattern of the area, which is characterized by WSW-dipping faults bounded southward by nearly E-W pre-existing structures.These latter are genetically related to the regional-scale, long-lived, "Pollino lineament s.l." (Bousquet, 1969(Bousquet, , 1971;;Ghisetti andVezzani, 1982, 1983;Knott and Turco, 1991;Van Dijk et al., 2000) and determine the abrupt contact between the Apennine carbonate platform unit and the San Donato metamorphic core complex (Grandjaquet, 1962;Servizio Geologico Nazionale, 1970;Amodio Morelli, 1976).The cross-cut relationships detected in the field between the ROCS-MPR set and POL-CVN, highlighted in our 3D model, lead us to exclude the latter fault to have a present seismogenic role, as also supported by the distribution of the instrumental earthquakes which clusterized along with N-S-striking crustal volumes.However, this significant structural-geological boundary could exert an influence on the southward propagation of the currently active seismogenic faults, driving the eastward transfer of the active extensional deformation belt.This inference is confirmed by the spatial distribution of the hypocenters of all 2010-2018 relocated seismicity which is confined within the CVN footwall (Fig. 8d).(Wells and Coppersmith, 1994;Wesnousky, 2008;Leonard, 2010;Stirling et al., 2013)

Conclusions
We reconstructed in detail the 3D geometry and kinematics of the interconnected fault pattern responsible for the moderate-magnitude earthquakes which recently affected the Pollino area (Calabrian-Lucanian boundary).
The main original outcomes are summarized as follows: -The geological and seismological stress tensors computed using geological and seismological data demonstrated that they are consistent with a uniform normal faulting regime characterized by an ENE-WSWtrending, sub-horizontal σ 3.This result confirms the coherence between the long-term and the present-day stress field and the persistence of this extensional regime at least since the Middle Pleistocene.
-The 2010-2018 seismic activity which affected the study area followed a peculiar evolution characterized by the concentration of epicenters in a series of subparallel ∼ NNW-SSE elongated clusters, independent and unconnected, which can be related to two major near-coaxial WSW-dipping faults possibly splaying from a common east-dipping basal detachment and concurrently releasing seismicity.
-The accurate hypocenter re-locations provided a seismological dataset that was correlated with the active faults mapped at the surface.The hypocenter spatial analysis allows the geometry (3DFM) of the seismogenic sources to be reconstructed, which released seismicity during 2010-2014 and through 2018.This reconstruction, extrapolated down to the depth of ∼ 10-12 km, was the interpretative key to obtain the over-all model of the Quaternary and active extension in the northern Calabria-Lucania Apennines.The 3DFM model includes all the faults playing a significant role, (either direct or indirect), on the seismogenesis of the study area.
-The western segment of the Pollino Fault (CVN), despite not being currently active, seems to maintain a significant seismotectonic role.In fact, juxtaposing crustal sectors with different structures and compositions (Apennine platform domain to the north and San Donato metamorphic core to the south) may act as a barrier to the southern propagation of the seismogenic faults of the Mercure-Campotenese sector (ROCS, MPR), limiting their dimensions and seismogenic potential.
-Based on the dimension and shape of all the active faults of the Pollino area, we estimated the expected magnitudes using appropriate scaling relationships.The complete rupture of individual W-dipping faults, which are recognized to have been causative of the 2010-2014 seismic activity, is expected to release a magnitude of ∼ M w = 6.1 for the VCT and the RSB, and of ∼ M w = 6.2 for the MPR.Higher values, up to M w = 6.4,could be reached in the case of the complete and concurrent rupture on both RSB and VCT.The estimated values exceed the magnitudes of the associated earthquakes which struck the area to date, leading to the hypothesis that the aforesaid faults only partially released their seismogenic potential.
-The delimitation of the fault patches involved during 2010-2014 and their geometrical parameterization support the consistence between the theoretical magnitudes based on the subsurface rupture length and the magnitudes of the mainshocks.
-The estimates provided M w = 5.3 for the VCT fault (which released the M w 5.2 Mormanno earthquake) and M w = 4.5 for the MPR fault (which released the M w 4.0 and 4.3 Morano Calabro earthquakes)The magnitudes calculated using the relationships based on the subsurface rupture area (M w ∼ 5.0 for the VCT and M w ∼ 4.6 for the MPR), show slightly greater deviation from the observed values.
This study pointed out that even in the case of low-tomoderate seismic activity, like the Pollino 2010-2014 one, the approach based on the three-dimensional reconstruction of the Quaternary fault surfaces (both directly involved and neighboring in the extensional system) represents a real breakthrough in the seismotectonic analysis and, ultimately, in the cognitive path that leads to a better assessment of the seismic hazard of a tectonically active area.Review statement.This paper was edited by Massimiliano Porreca and reviewed by Barreca Giovanni and one anonymous referee.

Figure 3 .
Figure3.Kinematic analysis and pseudo-focal mechanisms obtained from fault-slip data using the FaultKin 8 software(Allmendinger et al., 2012).Pseudo-focal mechanisms are boxed with different colors on the basis of the fault system to which they belong to (color key as in the map of Figs.1, 2).For each fault system, the density contour of the T axis computed for each focal mechanism is reported (lower hemisphere projection).A.s.v.: average striae value, A.f.p.: average fault plane, n: number of fault-plane measurements.Numbers in the rectangles (top left of each focal mechanism) refer to the group of fault-slip data belonging to or neighboring a single site (location in Fig.S2).

Figure 4 .
Figure 4. Time-space evolution of the 2010-2018 seismic activity in the Pollino area.Each panel shows the distribution of focal mechanisms(Totaro et al., 2015(Totaro et al., , 2016) ) and epicenters concentrated in a series of neighboring clusters numbered as Cl 0, 1, 2, and 3 from west to east, according to their activation time.See Sect.5.2 for the sequence description.The focal mechanisms are classified followingFrohlich (2001) kinematics classification (blue beach ball: normal kinematics; light blue: normal strike kinematics).Small red circles represent the epicenters of focal mechanism solutions.

Figure 5 .
Figure 5. Stress inversion results for the geological (a) and seismological (b) data.On the lower hemisphere Schmidt nets, the fault plane/slickenline pairs (a) and focal plane/kinematic indicators (rake) (b) are reported (great circles represent the fault planes; the dark and pink arrows indicate the measured slip directions (or rake) and resolved shear stress respectively).The histograms represent the corresponding misfit angles vs. the number of data points; nt: total number of fault data; n: number of successfully inverted fault data; σ 1, σ 2, σ 3: principal stress axes; : stress ratio = (σ 2-σ 3) / (σ 1-σ 3); quality ranking factors (QR) and the stress inversion parameters with associated uncertainties (1σ standard deviations) are listed in panel (c).On the small upper left nets, the computed stress field represented as a focal mechanism is also reported.The triangles reported on the lower right corner of each panel (a) and (b) show the kinematic classification of data according to Frohlich (2001).(c) Geological and seismological stress tensor parameters computed starting from slip-vector measurements collected along the investigated fault systems (Figs. 2, 3) and focal mechanisms, respectively (see.Sect. 3 and Fig.4).Key: nt: total number of data (e.g., plane/slickenline); n: inverted data; σ 1, σ 2, σ 3: principal stress axes; stress ratio = (σ 2-σ 3) / (σ 1-σ 3).QR: quality ranking; AQRw as inSperner et al. (2003) and A-QRfm as inHeidbach et al. (2010).

Figure 6 .
Figure 6.Epicentral map (upper-right panel) and hypocentral distributions (a-i) of the 2010-2018 seismic activity that occurred in the Pollino area.In the cross sections the earthquakes (grey dots) within a half-width of 1 km have been also reported as density contours computed using Kernel Density Estimation.The histograms related to each section show the depth distribution of the hypocenters.The traces of all the serial cross sections analyzed in this study are reported in map view (upper-right panel) as thin grey lines, while the bold lines relate to the sections (a-i) shown in this figure.

Figure 7 .
Figure 7. 3D fault model building, from the surface (10 m resolution DEM from Tarquini et al., 2012) to the base of the seismogenic layer.Fault acronyms as in Fig. 2. (a) "Fault ribbons" obtained by extruding the fault traces mapped at the surface down to 2 km depth and considering the fault dip angles measured in the field.(b) 3D fault model as in (a) with the relocated seismicity.(c) Fault extrapolation at (seismogenic) depth through the clusters of hypocenters; the modeled faults connect the ribbons with the zones at the depth where concentrations of hypocenters are higher.The density contours of the seismicity and the base of the seismogenic layer are also shown (see also d).(d) Final 3D fault model obtained integrating the detailed Quaternary fault pattern with the high-quality 2010-2018 seismicity dataset.

Figure 8 .
Figure 8. 3D fault model of the extensional system at the Calabrian-Lucanian boundary extrapolated down to ∼ 10-12 km.In panels (a, b, c) the geological-structural map (from Brozzetti et al., 2017a) is superimposed over a 10 m resolution DEM (from Tarquini et al., 2012).The reconstruction of the fault systems is discussed in the paper.In (a), the lower right inset (a1) shows the detail of the main faults involved during the 2010-2018 seismic activity.(d) 3DFM of all extensional fault realized through the Move software.For the acronyms, see Sect.S1.The faults belonging to the E-NE-dipping CRFS fault set are represented in red and violet, whereas the antithetic ROCS and MPR faults are shown as blue surfaces (fault acronyms as in Fig.2).The yellow surface is the three-dimensional surface of the POL and its westernmost segment (CVN) bounding, to the north, the Campotenese basin.

Figure 9 .
Figure 9. (a) Seismotectonic 3D fault model in map view.(b) Box representation of the W-dipping seismogenic faults belonging to the 3DFM with a detailed segmentation pattern.Fault traces are numbered according to the table of (c).The associated historical earthquakes from CPTI15 v3.0 (4.5<M w <6.0; Rovida et al., 2020, 2021) and the epicentral distribution of the 2010-2018 seismic activity that occurred in the Pollino area (1.0<M w <5.2) are also reported.(c) Expected magnitude according to scaling laws(Wells and Coppersmith, 1994;Wesnousky, 2008;Leonard, 2010;Stirling et al., 2013) and calculated based on fault area (A) and length (L).(d, e) Comparison of magnitude values calculated for all the investigated active faults, using fault-area-based (d) and fault-length-based (e) scaling relationships.
Figure 9. (a) Seismotectonic 3D fault model in map view.(b) Box representation of the W-dipping seismogenic faults belonging to the 3DFM with a detailed segmentation pattern.Fault traces are numbered according to the table of (c).The associated historical earthquakes from CPTI15 v3.0 (4.5<M w <6.0; Rovida et al., 2020, 2021) and the epicentral distribution of the 2010-2018 seismic activity that occurred in the Pollino area (1.0<M w <5.2) are also reported.(c) Expected magnitude according to scaling laws(Wells and Coppersmith, 1994;Wesnousky, 2008;Leonard, 2010;Stirling et al., 2013) and calculated based on fault area (A) and length (L).(d, e) Comparison of magnitude values calculated for all the investigated active faults, using fault-area-based (d) and fault-length-based (e) scaling relationships.

D
. Cirillo et al.: Structural complexities and tectonic barriers controlling recent seismic activity ful to the Editor Massimiliano Porreca, to Giovanni Barreca and to an anonymous reviewer for improving the paper with their review.Financial support.This research was supported by DPC-INGV PROJECTS-S1 2014-2015 UR-Unich funds to Francesco Brozzetti and from DiSPuTer Department research funds to Rita de Nardis.This research was also supported by PRIN 2017 (2017KT2MKE) funds from the Italian Ministry of Education, University and Research to principal investigator Giusy Lavecchia.