Earthquake relocations, crustal rheology, and active deformation in the central–eastern Alps (N Italy)

Tectonophysics 661 (2015) 81–98

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Earthquake relocations, crustal rheology, and active deformation in the central–eastern Alps (N Italy) Alfio Viganò a,⁎, Davide Scafidi b, Giorgio Ranalli c, Silvana Martin d, Bruno Della Vedova e, Daniele Spallarossa b a

Sezione Centro di Ricerche Sismologiche, Istituto Nazionale di Oceanografia e di Geofisica Sperimentale, Via Treviso 55, 33100, Udine, Italy Dipartimento di Scienze della Terra, dell’Ambiente e della Vita, Università degli Studi di Genova, Viale Benedetto XV 5, 16132, Genova, Italy Department of Earth Sciences and Ottawa-Carleton Geoscience Centre, Carleton University, 1125 Colonel By Drive, Ottawa K1S 5B6, Canada d Dipartimento di Geoscienze, Università degli Studi di Padova, Via Gradenigo 6, 35131, Padova, Italy e Dipartimento di Ingegneria e Architettura, Università degli Studi di Trieste, P.le Europa 1, 34127, Trieste, Italy b c

a r t i c l e

i n f o

Article history: Received 21 December 2014 Received in revised form 23 June 2015 Accepted 11 August 2015 Available online 23 August 2015 Keywords: Earthquake relocations Faults Rheology Seismotectonic model Adriatic crust Italian Alps

a b s t r a c t A revised seismic catalogue (1994–2007) for the central–eastern Alps (N Italy) is presented. 396 earthquake relocations, for local magnitudes in the 1.2–5.3 range, are performed using a 3D crustal velocity structure and probabilistic locations. The location procedure is validated by computing a set of 41 quarry shot solutions and all the results, both about shots and seismic events, are compared with those obtained using the routine location procedure. Results are shown for five contiguous seismotectonic domains, as supported by geological and geophysical evidence (e.g., fault systems, crustal tomography, focal mechanisms types). Earthquake hypocentres are mostly located in the upper crust (0–15 km of depth), in good agreement with thermo-rheological models about the brittle–ductile transitions (8–9 km of depth) and total crustal strengths (1.0–2.0 TN m−1). Epicentres are clustered and/or aligned along present-day active geological structures. The proposed seismotectonic model shows dominant compression along the Giudicarie and Belluno–Bassano–Montello thrusts, with strain partitioning along the dominant right-lateral strike-slip faults of the Schio–Vicenza domain. The present-day deformation of the Southern Alps and the internal Alpine chain is compatible with Adria indentation and the related crustal stress distribution. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Reliable earthquake locations are required for robust and exhaustive seismotectonic studies in active areas worldwide (e.g., Bressan et al., 2003; Hardebeck, 2010; Husen et al., 2003). The identification of seismogenic branches of geologically known fault structures is possible only if precise and accurate hypocentre solutions are computed. Although in recent years, a great effort has taken place in improving instrumental recording networks (e.g., Chiarabba et al., 2015), the accurate location of seismicity often remains an ill-conditioned problem, especially at regional or local scales (Husen et al., 2003). However, it has been demonstrated that more realistic velocity models (i.e., regionally calibrated seismic velocities within a three-dimensional crust), possibly combined with probabilistic solution approaches, can greatly improve earthquake solutions and their accuracy (Hardebeck, 2010; Husen et al., 2003). The central–eastern Alps (Northern Italy) are one of the most active areas of the Alpine belt where seismicity is related to deformation along the western margin of the Adriatic indentation (Slejko et al., 1989; Viganò et al., 2008). Deep seismic soundings show a complex ⁎ Corresponding author. Tel.: +39 0461 492585; fax: +39 0461 492525. E-mail address: alfi[email protected] (A. Viganò).

http://dx.doi.org/10.1016/j.tecto.2015.08.017 0040-1951/© 2015 Elsevier B.V. All rights reserved.

structure of the Adriatic crust with local fragmentation and lower crust wedges (Scarascia and Cassinis, 1997), as similarly observed for upper crustal strong heterogeneities by local earthquake tomography (Viganò et al., 2013). At the lithospheric scale, tomography suggests an abrupt change of polarity of subduction along the Giudicarie realm (Europe subduction in the Western Alps vs. Adria subduction in the eastern Alps; Kissling et al., 2006 and references therein) while the presence of steep lithospheric strength gradients across the eastern Alps is due to contrasting thermo-mechanical behaviours (Marotta and Splendore, 2014). The complex lithological and structural patterns of the study area are shown in Fig. 1. Stratigraphy extends from pre-Permian metamorphic basements of the Austroalpine and Southalpine domains to PlioQuaternary rocks in the Po and Venetian plains. Sedimentary covers of mainly carbonate sequences of Middle Triassic–Jurassic age extensively outcrop south of the Insubric Line, with variable lateral thicknesses between the so-called reduced Trento platform and the Lombardian basin to the West and the Belluno basin to the East (Castellarin et al., 2006; Doglioni and Bosellini, 1987; Picotti et al., 1995). Locally large magmatic bodies of Permian, Triassic, and Tertiary age intrude the upper crust (e.g., Bigi et al., 1990; Dal Piaz and Martin, 1998; De Vecchi et al., 1976; Macera et al., 2008). The intricate fault pattern of the Southern Alps (Fig. 1) is the result of a polyphase deformation history since the

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Fig. 1. Geological map of the central–eastern Alps. Modified after Bigi et al. (1990) and Rogledi (unpublished data for the Po plain).

Late Permian (Castellarin et al., 2006; Doglioni and Bosellini, 1987; Pola et al., 2014) resulting in three main tectonic systems, the ENE-WSW compressive Valsugana, the NNE-SSW transpressive Giudicarie, and the NW-SE strike-slip Schio–Vicenza fault systems (sketch in Fig. 1). In particular, two main compressive tectonic events accommodated deformation (i) along the Giudicarie fold-and-thrust belt and the Valsugana system (Middle–Late Miocene event), and (ii) along the Bassano del Grappa–Montello thrusts (Late Miocene–Pliocene event; Castellarin et al., 2006). Due to this geological complexity, despite the occurrence of frequent and important instrumental seismicity (Slejko et al., 1989; Viganò et al., 2008) and a recognised considerable seismic potential (Basili et al., 2008), a comprehensive and detailed analysis of the central–eastern Alps seismicity and its relation with tectonic structures is not yet available. In this study, we present a revised earthquake catalogue (period 1994–2007) obtained by an earthquake location procedure based on a 3D seismic velocity model of the crust, which allows accurate hypocentre solutions for the central–eastern Alps. The revised earthquake catalogue, the identification of present-day active faults and the interpretation of crustal rheological models for different seismotectonic domains allow the formulation of a new regional seismotectonic model in view of a better seismic hazard assessment. 2. Data We use data for earthquakes occurring in the period 1 January 1994–31 December 2007 from different seismic networks operating in the study area. Fig. 2a shows the distribution of seismic stations, subdivided according to their respective network (Provincia Autonoma di Trento, PAT; Istituto Nazionale di Oceanografia e di Geofisica

Sperimentale, OGS; Istituto Nazionale di Geofisica e Vulcanologia, INGV; others such as Swiss and Austrian networks), active in the period under consideration and normally used for earthquake locations (i.e., seismic bulletins). Since PAT and OGS are regional networks, their stations are located in delimited areas, while INGV is the national Italian network and its stations are more homogeneously distributed even if less concentrated. Most of original recordings are acquired by PAT which has been managing the Trentino seismic network since 1981 (“ST” permanent network code assigned in 2013 by the International Federation of Digital Seismograph Networks, FDSN; dismissed and operating permanent stations registered on the International Seismological Centre, ISC). Other original seismograms are obtained from nearby networks operated by OGS and INGV, and others available via the European Integrated Data Archive (EIDA). Most of seismic sensors are vertical-component short period (1 s and 5 s) or broad-band velocimeters (≥40 s), with several three-component instruments. Sampling rates are 125 Hz (PAT stations) or 100 Hz. An initial dataset comprising about 2000 earthquake locations from PAT network (with stations approximately located at the centre of the study area) is used to manually check OGS and INGV bulletins, both for computed locations and phase readings (for non-located events). About 18,000 digital waveforms (from OGS, INGV, and others via EIDA) are then collected and merged with PAT recordings into a single database. Finally, the most representative seismic events (396 earthquakes), in terms both of quantity and quality of integrated data, are selected for relocations. Fig. 2 shows the distribution of epicentres computed by PAT, OGS, and INGV, based mostly on stations belonging separately to each network. In fact, while PAT used only its own stations, sometimes OGS and INGV used stations also from other networks. Clear mismatching

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Fig. 2. Comparison of seismicity distribution from regional seismic bulletins: Provincia Autonoma di Trento (PAT), Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), Istituto Nazionale di Geofisica e Vulcanologia (INGV). (a) Distribution of seismic stations, subdivided according to their network, active in the period 1994–2007 and routinely used to calculate earthquake locations reported into the seismic bulletins. Some stations outside the study area and used for earthquake relocations are not shown in the map. (b, c, d) Distribution of PAT vs. OGS, PAT vs. INGV, and OGS vs. INGV earthquake locations.

of epicentral solutions, which is often much larger if considering also hypocentral depths, is primarily due to the highly variable and possibly poor station coverage for each network, in relation to the epicentral area. Moreover, the different number of events within catalogues (Figs. 2b, c, and d) is most likely due to different procedures in data elaboration (e.g., phase readings, velocity models, seismic codes) and accepted solution qualities for seismic bulletins. Error estimates associated to computed solutions from each seismic bulletin cannot be directly compared. In fact, while OGS and INGV catalogues (INGV until 2002) list the standard errors of the epicentre and the focal depth (HYPO-71 code; Lee and Lahr, 1975), PAT and INGV (after 2002) errors are expressed as the minor and major semi-axis lengths of the 68% jointconfidence error ellipsoid (HYPOELLIPSE code; Lahr, 1999). Regarding PAT locations, the errors associated to earthquakes located inside the network geometry are generally within about 4 km (minor semi-axis) and 10 km (major semi-axis), while the errors associated to the great number of seismic events located outside the network are totally

unreliable. The higher number of stations of the OGS and INGV networks permitted to obtain errors normally less than 3 km (horizontally) and 4 km (in depth), especially for solutions better constrained by a sufficient number of phases, in agreement with values computed on the overall catalogues by Gentili et al. (2011) and Chiarabba et al. (2005), respectively. In addition, OGS earthquake locations appear better constrained for the eastern portion of the study area where this seismic network is mainly deployed. The epicentral locations shown in Fig. 2, which differ by up to several kilometres between them, clearly justify a complete revision of the seismic database and relocations for the area. In fact, as earthquake locations reported in seismic bulletins are mainly computed for seismic alert purposes at national/regional scale, robust and detailed relocations are necessary to constrain further seismotectonic interpretations. All available seismic P- and S-phases underwent manual re-picking and quality assessment, in order to obtain a complete and homogeneous phase arrival time dataset. Reading accuracy and associated standard

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Fig. 3. Map of regional seismotectonic domains (D1–D5; see text for explanation), with fault patterns and distribution of seismic stations used in this work for relocations (cf. Fig. 2a). Diagrams compare seismic velocities (expressed as VP and VP/VS ratio) of the 1D standard regional model (grey colour) to average values from the 3D crustal velocity structure (after Viganò et al., 2013) within each domain (black colour).

error for each 0–3 quality code (from 0 best quality to 3 worst quality; cf. Lahr, 1999) were initially evaluated and calibrated on this specific waveform dataset. The dataset of arrival times includes 4164 P-wave arrivals and 3862 S-wave arrivals of 396 events variably recorded by 88 stations. The dataset contains 2130 P-wave arrivals with reading accuracy of ± 0.05 s (quality code 0) and 1254 with reading accuracy of ±0.10 s (quality code 1). The dataset includes 926 S-wave arrivals with reading accuracy of ±0.05 s (quality code 0) and 1501 with reading accuracy of ± 0.10 s (quality code 1). The remaining P- and S-wave arrivals have reading accuracies of ± 0.25 s (quality code 2) or ± 0.50 s (quality code 3). The total average reading accuracy is 0.12 s (P-phases) and 0.18 s (S-phases). 3. Methods 3.1. Relocation procedure and seismotectonic zonation The aim is to accurately relocate earthquakes in the central–eastern Alps and improve solutions over catalogues in the study area. High-

quality earthquake locations are here obtained (i) using all the available data from the improved phase arrival dataset, and (ii) modelling the effects of the velocity structure. We use a regional 3D seismic velocity model calibrated from the local earthquake tomography by Viganò et al. (2013), which shows lateral heterogeneities due to lithological and structural variations, especially in the upper crust for depths within about 10 km. The 3D velocity structure used for the relocation procedure is obtained by assigning P- and S-wave velocities on a cubic gridded volume (0.75 km spacing, value selected after specific calibrations; see below) that is exactly centred on the tomographic model (centre at 45.95°N latitude and 11.05°E longitude). The starting tomographic model has 7.5 km grid spacing in horizontal directions and variable grid spacing in vertical direction (layers at 0, 5, 10, 22, 40 km of depth). To obtain the finer and cubic grid required by the Eikonal finite-difference algorithm (Podvin and Lecomte, 1991) and adopted by the location code to calculate travel times (see below), velocity values are B-spline interpolated every 0.75 km in the 3D space from computed velocities. Since the considered seismic stations are generally well above the sea level, seismic velocities for topography (0–3000 m a.s.l.) are B-spline interpolated

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Fig. 4. (a) Map and cross-sections of quarry shot relocations, with solutions obtained using 1D (HYPOELLIPSE code, circles) and 3D velocity models (NonLinLoc code, stars). (b, c) Example of single quarry shot locations (23rd October 2012 in b; 25th January 2013 in c). True shot points (squares) are compared to HYPOELLIPSE locations (circles, with related uncertainties) and to NonLinLoc locations (stars). The clouds of small grey dots represent the scatter points from NonLinLoc. In the right lower corners, sketches mark the seismic station distribution used to obtain each solution.

between assigned values (at 3000 m a.s.l. VP = 3.6 km s−1 and VS = 2.03 km s−1; at 1000 m a.s.l. VP = 4.0 km s−1 and VS = 2.26 km s−1) and the velocity values at the top of the tomographic model. These values are assigned taking into considerations also the results from on-site velocity measurements at shallow depths (Vertical Seismic Profiling for rocky sedimentary formations; e.g., Poletto et al., 2015). To relocate earthquakes, we use NonLinLoc, a computer program for probabilistic, non-linear earthquake location in 3D media (Lomax et al., 2000). NonLinLoc follows the inversion approach of Tarantola and Valette (1982) and the earthquake location methods by Moser et al. (1992), Tarantola and Valette (1982), and Wittlinger et al. (1993). We apply the Oct-Tree importance sampling algorithm which gives

accurate, efficient, and complete mapping of earthquake location posterior probability density functions (PDFs), complete probabilistic solutions to the earthquake location problem (Lomax et al., 2000). NonLinLoc uses the 3D cubic gridded velocity model described above, within a volume of 700 × 700 × 203 km (x × y× z) centred at 45.95°N latitude and 11.05°E longitude. For earthquake relocations, we use stations with maximum epicentral distance of about 150 km (cf. Figs. 2a and 3 for location of seismic stations). Final hypocentre locations are given by the maximum likelihood values from the density plot, which draw samples from PDFs with the number of samples proportional to probability (i.e., 1000 scatter samples). Using the Oct-Tree algorithm location uncertainties are directly derived from the density scatter plots included in the probabilistic solution (cf. Fig. 4) or

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Fig. 5. Differences in longitude, latitude, and depth for earthquakes (black circles) and quarry shots (empty circles; cf. Fig. 4) between solutions obtained using 1D (HYPOELLIPSE code) and 3D velocity models (NonLinLoc code), classified according to the number of observations (nobs) used for each solution (vertical lines). The dotted vertical lines in each case delimit the area with number of observations ≤10 for quarry shots. For earthquakes, average values (with standard deviation) are 0.2 ± 2.6 km (longitude), 0.2 ± 3.0 km (latitude), and 2.6 ± 4.3 km (depth).

calculated according to the traditional Gaussian 68% confidence ellipsoid (Lomax et al., 2000). These Gaussian estimates can be interpreted as the results obtained using linearised location algorithms such as HYPO-71 (Lee and Lahr, 1975) and HYPOELLIPSE (Lahr, 1999). Contiguous principal seismotectonic domains (D1–D5) are delimited in Fig. 3 according to geology, fault systems, and seismicity distribution (see next Sections): D1, Giudicarie and central Southern Alps; D2, Valsugana and eastern Southern Alps; D3, Schio–Vicenza and Lessini; D4, Po plain; D5, internal Alpine chain. 1D velocity profiles extracted from the 3D velocity structure of the upper crust (0–22 km of depth) are shown in Fig. 3, where average VP and VP/VS vertical distributions (only velocities from well-solved nodes of the original tomographic model are used) are compared to the standard 1D layered velocity model, in order to show their major differences. This standard 1D velocity model is normally used by PAT and OGS networks for earthquake locations in the central–eastern Alps (i.e., seismic bulletins) (Gentili et al., 2011). This velocity model is mainly obtained from the crustal models of Scarascia and Cassinis (1997) and

is composed of two layers (z = 0–22 km, VP = 5.85 km s−1; z = 22–39.5 km, VP = 6.80 km s−1; VP/VS = 1.78) and a half-space at the base (z ≥ 39.5 km, VP = 8.00 km s−1; VP/VS = 1.78). Similarly, INGV also uses an optimised 1D velocity model to locate earthquakes in Italy (i.e., seismic bulletins) (Chiarabba et al., 2005, 2015). Average P-wave velocities from the 3D model are higher than 1D model values, with discrepancies generally increasing with depth, except for D4 where velocities at shallow depth are lower due to the soft sediments of the Po plain. VP/VS distributions (3D model) are quite constant over depth in D4 (very similar to 1D model) and D5 (lowest values) and more variable along the Southern Alps front (D1–D3). VP/VS shallower values (down to about 7 km of depth) are close to 1.78 for the central and eastern Southern Alps (D1, D2), but they are significantly lower (~ 1.71) for the Schio–Vicenza domain (D3). According to the interpretation proposed by Viganò et al. (2013), dominant thrust faults in the uppermost crust in domains D1–D2 are more pervasively fractured and/or fluid-enriched than strike-slip faults in domain D3.

Fig. 6. (a) Relocation error estimates (horizontal ERH, vertical ERZ) and (b) RMS values with calculated P- and S-phase time residuals for NonLinLoc relocations.

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Fig. 7. Uni-dimensional simplified lithology, geotherm, rheological profile (with values indicating the total strength of the upper crust), depth distribution of seismicity and seismic moment for each seismotectonic domain D1–5 (cf. Fig. 3). NC, non-carbonate sedimentary cover; C, carbonate sedimentary cover; UC, upper crust. The conversion between local magnitude and seismic moment is made using the formula by Franceschina et al. (2006).

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Fig. 8. Relocated seismicity distribution in the period 1994–2007, with horizontal error estimates from Fig. 6a (grey crosses). Legend for geology and faults as in Fig. 1. In the lower left corner, hypocentral depth distribution of relocated seismicity.

3.2. Testing the relocation procedure A set of 41 selected quarry shots occurring in the period February 2011–January 2013 is used to validate the relocation procedure applied in this study (NonLinLoc code with 3D velocity model, NLL-3D hereafter), in order to permit an independent test on the computed hypocentral solutions of tectonic earthquakes discussed in the next sections. The results of this procedure are compared with those obtained by the routine location procedure operated by the PAT network (HYPOELLIPSE code with 1D velocity model, HE-1D hereafter). This latter procedure is almost coincident with that used by OGS and INGV to calculate earthquake solutions reported in their seismic bulletins, with the only possible exception of the location code (HYPO71 instead of HYPOELLIPSE). Each quarry shot is precisely and unambiguously located using quarry alerts made available by the PAT Mining Survey. In Fig. 4, we plot NLL-3D and HE-1D solutions, both obtained starting from exactly the same number, spatial distribution (i.e., network geometry) and weights of available phase readings. The average number of phase arrivals is 8, with prevalence of P-phases, with a nearest station epicentral distance of about 16 km (average value from NLL-3D locations). The difference between locations by the two procedures is minimum in latitude (≤2 km), medium in longitude (≤7 km), and maximum in depth (≤13 km; Fig. 5, grey circles); only two shots show greater differences

(Figs. 4a and 5). For HE-1D locations, the mean difference between computed locations and quarry shots is 3.3 km horizontally and 11.8 km as the real minimum distance. For NLL-3D locations, the mean difference between computed locations and quarry shots is 1.0 km horizontally and 1.3 km as the real minimum distance (Fig. 4a). This means an average true location improvement of 10.5 km (2.4 km horizontally and 10.3 km vertically). HE-1D computes a maximum average location error of 3.4 km (from lengths of the major ellipsoid semi-axes; Lahr, 1999). For this reason, HE-1D locations are not only imprecise (see above) but also falsely accurate, because computed errors do not permit to include the true shot locations. Conversely, NLL-3D solutions are much more precise and have mean errors of 7.9 km horizontally and 7.8 km vertically. Median values are slightly smaller (4.2 km horizontally and 6.6 km vertically) because of the presence of few less-constrained solutions. In any case, considering computed solutions with the two different location procedures, the two groups of solutions are clearly distinct and the NLL-3D procedure significantly improves quarry shot locations. Figs. 4b-c show two examples for shots, where NLL-3D uncertainties of probabilistic locations are shown by scatter plots. Maximum likelihood solutions are very close to quarries. Moreover, relocating quarry shots provides important constraints on the absolute earthquake location uncertainty. In general, earthquakes are better located than shots

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Table 1 Relocated earthquakes in the period 1994–2007, with local magnitudes ML ≥ 3.2. Depth is expressed in km; ERH and ERZ are the maximum horizontal and vertical errors (in km); phases is the number of P- and S-phases used for relocations; RMS (root mean square travel-time residual) is expressed in seconds. ID

Date

Time

Latitude

Longitude

Depth

ERH

ERZ

ML

Phases

RMS

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

23 Jan 1994 24 Oct 1994 29 Oct 1995 03 Aug 1996 12 Apr 1997 11 May 1997 29 Sep 1997 03 Jun 1998 22 Oct 1998 10 Dec 1998 24 Dec 1998 26 Dec 1998 05 Jan 1999 26 Apr 1999 16 Jul 1999 28 Oct 1999 31 Oct 1999 03 Apr 2000 06 Apr 2000 16 Jun 2000 08 Sep 2000 17 Jul 2001 01 Oct 2001 07 Oct 2001 28 Oct 2001 13 Nov 2002 17 Jul 2003 03 Aug 2003 24 Nov 2004 04 Dec 2004 18 May 2005 12 Apr 2006 20 Jul 2006 17 Oct 2006 20 Oct 2006 28 Dec 2006

15:24:59 23:22:48 13:00:26 15:35:41 23:00:00 16:49:09 21:01:35 18:00:43 00:58:50 09:57:53 16:41:59 19:30:57 03:22:14 02:53:46 05:24:43 10:16:14 12:03:58 00:28:05 17:40:36 11:57:22 05:49:26 15:06:15 06:36:23 06:13:17 00:09:30 10:48:05 02:27:16 21:44:45 22:59:39 22:20:50 21:41:09 22:24:53 22:47:55 05:41:35 00:11:59 14:10:29

45.213 45.938 45.617 44.991 46.569 44.993 46.742 45.784 45.866 45.714 45.551 45.744 45.709 45.916 45.605 45.453 45.637 46.602 46.572 45.989 45.688 46.676 46.524 45.754 45.803 45.650 46.793 45.215 45.653 45.912 45.569 46.608 45.610 46.495 45.683 46.127

10.927 11.188 9.954 11.462 10.422 11.221 12.185 10.903 11.262 10.627 10.675 11.450 10.825 11.096 10.628 11.451 10.298 10.369 10.333 10.927 10.858 11.096 10.331 10.802 10.622 10.132 9.819 10.115 10.523 11.996 11.385 10.258 11.839 10.497 10.347 12.224

5.2 7.6 4.8 6.0 5.6 8.2 7.5 10.8 12.4 3.4 5.0 9.2 5.5 1.5 5.8 7.6 6.4 2.2 12.5 5.3 5.3 4.8 6.0 2.2 6.5 10.4 4.5 12.7 6.6 7.4 3.6 2.7 5.3 7.2 5.5 3.6

2.0 0.6 1.5 1.7 2.0 2.2 1.2 1.1 0.9 1.3 1.9 1.9 3.3 0.7 1.9 1.9 1.4 2.9 1.9 0.6 1.6 1.3 1.5 0.9 1.4 1.5 3.7 2.0 1.0 1.1 0.7 0.6 2.0 1.0 0.7 0.6

3.2 1.2 2.4 2.7 2.6 2.9 3.4 1.5 0.7 2.0 1.9 2.1 3.4 1.1 2.1 2.5 2.1 2.7 3.9 1.6 1.6 2.5 1.1 2.7 2.8 2.0 3.9 2.3 1.4 1.7 1.6 0.9 1.6 2.8 1.1 2.0

3.2 3.2 3.4 3.6 3.5 3.2 3.6 3.4 3.2 3.2 3.5 3.4 3.5 3.4 3.4 3.2 3.6 3.2 3.6 3.2 3.4 5.3 3.5 3.2 3.2 4.0 3.2 3.4 5.2 3.6 3.6 3.2 3.2 3.5 3.6 3.4

23 24 19 23 21 15 37 25 33 17 18 21 13 20 15 36 19 18 16 17 16 34 17 12 10 21 13 22 37 51 41 53 26 43 47 51

0.26 0.15 0.19 0.22 0.27 0.23 0.22 0.25 0.17 0.31 0.31 0.33 0.30 0.18 0.28 0.31 0.29 0.23 0.24 0.13 0.25 0.24 0.20 0.09 0.22 0.21 0.30 0.30 0.27 0.30 0.29 0.27 0.30 0.36 0.25 0.32

because the velocity model is more constrained at depth than near the surface where we assume strong heterogeneities and unfavourable ratio of focal depth to closest station distance. In fact, a better station coverage improves solutions also for shots (compare Figs. 4b and c). 3.3. Local magnitudes In the period 1994–2007, the different networks operating in the region have provided duration or amplitude magnitudes, from manual or automatic calculations, of variable quality depending on different instrumentations and data processing. Here we recalculate all earthquake magnitudes to obtain a revised dataset of local magnitudes (ML). ML values are mostly based on revised duration magnitudes (MD) from PAT and OGS, by applying the method of Rebez and Renner (1991). In very few cases, MD are retrieved from INGV seismic catalogues. For MD b 3.5, MD is converted to ML using the empirical formula proposed by Gentili et al. (2011) (ML = 1.508 MD – 1.743). For MD ≥ 3.5, local magnitude is taken equal to duration magnitude according to Bragato and Tento (2005). In some cases, when MD is not available due to the absence of readable signal durations, ML is retrieved from INGV or ISC revised seismic bulletins. 3.4. Thermo-rheological profiles The modelling methodology and the parameters to compute thermo-rheological profiles are the same as described in Viganò et al. (2012). For each seismotectonic domain of Fig. 3, a representative lithological profile is presented, by simplifying local geology (data

from Fantoni and Franciosi, 2010; Picotti et al., 1995; Zampieri and Massironi, 2007) and without considering local geological peculiarities (both the effects of crustal magmatic intrusions and the local variability of the sedimentary cover thickness are discussed in Section 4.2). Three crustal geotherms for the Po plain (D4 domain), the Southalpine (D1–3 domains), and the internal Alpine chain (D5 domain) are selected. The Po plain geotherm is taken from Viganò et al. (2012). For domains D1–3 and D5, temperature is fixed at 10 °C at the model top, which is variable according to the average elevation by a 10-m-resolution digital elevation model. The Southalpine and Alpine geotherms are obtained by acquiring representative 1D temperature profiles from 2D lithospheric thermal models along the TRANSALP transect in the eastern Alps (Vosteen et al., 2006) and the NFP-20-EAST TRAVERSE in the central Alps (Viganò and Martin, 2007). Temperatures at the bottom of the models (15 km of depth) are taken as the average temperature among the values computed for the 2D lithospheric models of the eastern and central Alps. In particular for the TRANSALP, Vosteen et al. (2006) proposed two end-member solutions, for well- or poorly differentiated middle crust. In this case, we use the average values between the two models, which show a maximum temperature difference of ~ 10 °C at 8 km of depth for the Southalpine (D1–3 domains) and ~ 25 °C at 15 km of depth for the Alpine chain (D5 domain). In the rheological model, the present-day tectonic regime in the frictional field is assumed compressional, strike-slip and extensional (see Ranalli, 1995 for formulas) for domains D1–2,4, D3 and D5, respectively. The strain rate is taken as 1.0 × 10−15 s−1 for all the domains (Caporali et al., 2011). Other parameters and the Po plain rheological profile are from Viganò et al. (2012).

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4. Results and discussion 4.1. Earthquake relocations The final earthquake catalogue contains 396 seismic events relocated with the NLL-3D procedure and does not include any possible quarry shots. In fact, the NLL-3D relocation procedure enables the identification of events at the surface (cf. Section 3.2). PAT, OGS, and INGV seismic catalogues have commonly reported event swarms NE of the city of Trento as earthquakes of tectonic origin, due to the absence of more detailed information and computed locations incompatible with shots at the surface (e.g., Chiarabba et al., 2005; Gentili et al., 2011). Only PAT quarterly bulletins have indications about the non-tectonic origin of these events, which are classified as uncertain, probable, or certain shots on the base of origin times and the vicinity to quarries. However, relocated quarry shots (Fig. 4) strongly suggest that the majority of all the events occurring in this area are not tectonic earthquakes but shots, with seismotectonic implications regarding, for example, the possible linking between the Schio–Vicenza fault system and the Trento–Cles fault (Fig. 1; see Massironi et al., 2006 and Zampieri et al., 2003 about the tectonic structures and the geological evolution of this alpine sector). Fig. 5 shows the differences in longitude, latitude, and depth between NLL-3D and HE-1D earthquake relocations (computed from the same data input), classified according to number of phases. However, it should be noted that these HE-1D relocations cannot be directly compared with locations reported in original PAT, OGS, and INGV seismic bulletins, because HE-1D relocations are performed using a different data input of seismic phases (cf. Section 2). There is no systematic trend of average horizontal (b 0.2 km) and vertical offsets (2.6 ± 4.3 km). A positive vertical shift for most earthquakes means that HE-1D locations are deeper relative to NLL-3D, as similarly observed for quarry shot relocations and for locations obtained before and after the tomographic inversions by Viganò et al. (2013). The differences are not associated with the number of observations, but large shifts are observed both for low and high numbers of phases. Larger differences in depth are interpreted as due to the effect of the new 3D velocity model and especially of the S-wave velocity distributions, highly heterogeneous in the different domains (cf. Fig. 3). Local magnitudes of relocated earthquakes are comprised between 1.2 and 5.3, with 66 events with ML ≥ 3.0 and 17 events with ML ≤ 1.5. The average azimuthal GAP in horizontal projection (largest azimuthal separation in degrees between nearby stations as seen from the epicentre) is 145°, a value largely reduced by the integration of all the available seismic stations from different networks (cf. Section 2). 292 relocations have GAP = 120°, 21 total phases and distance of the nearest station equal to 19 km, on average. 104 earthquake relocations have GAP N 180° and are carefully checked considering the total number of phases (83% of these events have number of phases ≥12) and distance of the nearest station (50% of these events have a distance ≤20 km) to ensure well-established event locations slightly outside the available network geometry (cf. Viganò et al., 2013 and references therein). The average error with standard deviation of NLL-3D earthquake relocations is 1.4 ± 0.7 km horizontally and 2.2 ± 0.9 km vertically (Fig. 6a). Estimated errors for earthquakes are significantly lower than for quarry shots (cf. Section 3.2) because the number of available stations for earthquakes is higher and the station distribution is more uniform. The average root mean square travel-time residual (RMS) with standard deviation is 0.20 ± 0.07 s (Fig. 6b). The few larger RMS values (i.e., ≥0.30 s) are associated with relocations with a lot of stations and/

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or related to earthquakes relocated where the velocity model is less constrained (e.g., the border of the study area). Histograms of traveltime residuals in Fig. 6b include only P- and S-wave residuals for phases with non-null computed weight during NonLinLoc relocations. Average phase residuals with standard deviation are 0.06 ± 0.33 s (P-waves) and −0.04 ± 0.57 s (S-waves). 4.2. Thermo-rheological profiles The geotherms in Fig. 7 show the hottest temperatures in the internal chain (D5 domain; 350 ± 20 °C at 15 km of depth) due to the predominant advection effects of exhumation (Vosteen et al., 2006). The anomalous temperature measured in the Merano well (maximum temperature of ~ 92–94 °C at the borehole bottom, about 2.3 km of depth from the surface; Della Vedova and Piller, 2010) within D5 is probably related to vertical fluid circulation within a known fault and is unlikely to affect the regional geotherm. Conversely, lower temperatures (~320 ± 20 °C at 15 km of depth) characterise both the alpine foreland (D4 domain) and the Southalpine (D1–3 domains). Along the exposed sector of the Southern Alps, deep meteoric water infiltrations explain lower thermal gradients within the uppermost fractured sedimentary sequences (Della Vedova et al., 2001). Moreover, the maximum temperature difference of about 10 °C at shallower levels (~4 km of depth) between domains D1–3 and D4 could be related to thrusting effects and fluid circulation in the tectonically active sector of the exposed Southern Alpine chain. In any case, if we consider the uncertainties of deep temperature estimates (typically in the order of ±10 °C at shallow depths; Della Vedova et al., 2001; Viganò and Martin, 2007; Vosteen et al., 2006), we can identify on average a significantly different temperature upper crustal distribution for the internal chain only. Average thermal gradients of the 1D models, computed between the surface and 4 km b.s.l. of depth, are all comprised between 20 and 25 °C km−1. Heat flow estimates in the same depth interval are 45–50 mW m−2 in D4, 55–60 mW m−2 in D1–3, and 65–70 mW m−2 in D5, in good agreement with undisturbed conductive surface heat flow estimates from well data and 1D thermal modelling (40–50 mW m− 2 in the Po plain and 60–70 mW m− 2 in the Alpine chain; Della Vedova et al., 2001). The rheology of the sedimentary covers is entirely within the brittle regime (frictional or high-pressure, with the first being clearly predominant), with the exception of the carbonate layer in domain D4 where temperatures at about 9 km of depth are high enough to allow for ductile calcite creep (Fig. 7). Local variations in the sedimentary cover thickness are expected to result in only very limited modifications in the rheological structure (cf. Viganò et al., 2012). Similarly, the addition of some magmatic complexes that intrude the upper crust in some portions of the seismotectonic domains (i.e., Adamello pluton in D1, Vicenza volcanites in D3; cf. Fig. 1) can affect rheological profiles at the local scale only. In any case, this effect would be mostly limited to the absolute ductile strengths because of the application of plagioclase rather than wet granite creep parameters. The modelled brittle– ductile transition is always located in the 8–9 km depth range, with minimum and maximum values in the D5 and D3 domains, respectively. However, according to Viganò et al.'s (2012) evaluations, a possible variation of ~ 2 km in depth for the brittle–ductile transition is to be expected considering uncertainties in thermal models. The brittle– ductile transition of domains D1–2 is located very close to important geological décollement levels (cf. Section 4.3), where the largest earthquakes occurred and the most important rheological contrasts are envisaged. At these depths, frictional strengths reach ~180 MPa (D3) or ~ 210 MPa (D1–2), while high-pressure fracture strengths exceed

Fig. 9. Cross-sections with relocated seismicity (classified by local magnitude), focal mechanisms (numbers and references as in Fig. 11b and Table 1), relevant geology and faults, and crustal tomography (from Viganò et al., 2013). Simplified geological sections are also shown (1.5× vertical exaggeration). Traces of cross-sections are shown on the structural map. Coloured circles identify earthquake clusters plotted on each cross-section. Geological sections (in grey colours) of cross-sections 2 and 3 are redrawn after Picotti et al. (1995) and Fantoni and Franciosi (2010), respectively.

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270 MPa. In the Southalpine, this implies that, for the same amount of applied stress difference, on average is easier to activate faults within the brittle level for D3 than for contiguous domains D1–2. The calculated strengths can also be reduced by other processes not considered here, such as the occurrence of diffusion creep (Ranalli, 1995) or pore fluid pressure variations in space and time (e.g., Chiarabba et al., 2015). The total strength of the upper crust (from the surface to 15 km in depth) shows higher values in D1–2,4 (1.8–2.1 TN m−1), and minimum values in D3 (1.2 TN m−1) and D5 (0.8 TN m−1) (Fig. 7). Most relocated seismicity for each seismotectonic domain is in agreement with rheological results, because earthquakes are largely concentrated in the levels predicted to be brittle under hydrostatic pore pressure conditions (an increase of pore pressures results in a thicker brittle crust) (Fig. 7). Seismicity in the ductile domains can be explained invoking the plastic instability hypothesis (e.g., Roberts and Turcotte, 2000). For domain D3, less frequent and larger earthquakes occur in the shallower crust (0–6 km depth), while a great amount of smaller events occur at deeper levels. A uniform distribution of cumulative seismic moment for the whole upper crust, also in agreement with the almost constant VP/VS depth distribution (Fig. 3), suggests seismic faulting along the whole depth extent of the vertical strike-slip faults. 4.3. Seismotectonic model Although any extrapolation to long-term seismotectonics is conditioned by the short time span of the database, some preliminary observations can be made on the basis of the revised earthquake location. The map of relocated seismicity shows a non-uniform earthquake distribution within the studied region (Fig. 8). In the internal Alpine chain (D5 domain), most of seismicity is located along the Insubric Line or in the Austroalpine zone, close to the Italy–Switzerland boundary (Bormio and Zebrù area; cf. Fig. 1; Marschall et al., 2013). Very low seismicity is observed in the Po plain sector (D4 domain), with the exception of a cluster most probably related to the northern Apennines frontal thrust. In the Southalpine (D1–3 domains), earthquakes are clustered and/or aligned near and/or along fault branches belonging to the Giudicarie, Schio–Vicenza, and Bassano–Belluno fault systems. At present, the Valsugana fault does not appear seismically active (cf. Fig. 1). Southern Alps epicentres are located almost completely in the sedimentary cover and associated magmatic rocks (yellow in Fig. 8). In the following, a detailed discussion of relocated seismicity is presented in order to interpret these earthquake clusters and alignments to identify active faults and propose a preliminary updated regional seismotectonic model. Computed horizontal errors are generally small enough to directly compare relocated seismicity and geological structures (Figs. 6a and 8) and relocated epicentres are closer to active faults than previous estimates. Hypocentral depths are almost completely within the upper crust (0–15 km depth range), with no relocated seismicity in the upper mantle or the lower crust (only two deeper events at ~20 km depth; sketch in Fig. 8). Table 1 lists the obtained relocations for all the ML ≥ 3.2 earthquakes. Three seismic events have ML ≥ 4.0: (i) 17 July 2001, ML = 5.3, Merano; (ii) 13 November 2002, ML = 4.0, Trompia valley; (iii) 24 November 2004, ML = 5.2, Salò (cf. Fig. 1). The 2001 Merano earthquake occurred in a region of low seismicity in the internal Alpine chain (Caporali et al., 2005; Viganò et al., 2008). The relocation (Table 1, no. 22) is almost coincident with the OGS location reported by Caporali et al. (2005). The epicentre is located on the north-western block of the Forst fault (formerly named South Passiria Line), a NE-SW high-angle sinistral fault nearly parallel to the Insubric Line (i.e., North Giudicarie fault termination near Merano) (Bargossi

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et al., 2010; Caporali et al., 2005). The focal mechanism from P-wave first polarities and the moment tensor solution show strike-slip kinematics, where the identified fault plane (strike N213°, dip 80°) is parallel to the local fault structures (Viganò et al., 2008; cf. also Fig. 11b, no. 22, discussed more fully below) and its sinistral kinematics is also constrained by geodetic observations (Caporali et al., 2005). Orientation, geometry and kinematics of this fault plane, supported by a relocated 4.8 ± 2.5 km hypocentre depth in relation to the average fault dip, define a projected trace at the surface optimally compatible with the mapped Forst fault (Bargossi et al., 2010) which is definitely considered the causative structure of the Merano event in agreement with the Caporali et al. (2005) interpretation. The 2002 Trompia valley earthquake testifies active deformation in the westernmost sector of the Giudicarie belt, here characterised by the thick sedimentary cover sequences of the Lombardian Basin (Fig. 1; Picotti et al., 1995). The computed depth (10.4 ± 2.0 km; Table 1, no. 26) and the predominantly compressive focal mechanism (Viganò et al., 2008; cf. Fig. 11b, no. 26) are optimally compatible with the activation of the décollement at the base of Middle Triassic units described by Picotti et al. (1995). The 2004 Salò earthquake represents an important reference in the definition of the seismogenic potential of the southern Giudicarie realm, especially if compared with historical data (Pessina et al., 2013 and references therein). Despite the moderate seismic hazard of the region, with expected maximum moment magnitude lower than 6.0 for the Giudicarie seismogenic source (Basili et al., 2008; DISS Working Group, 2010), high risk is associated with very high social and economic vulnerability. The epicentre (Table 1, no. 29) is relocated close to other available instrumental solutions (~4 km more to the south and southwest than Augliera et al., 2006 and Viganò et al., 2008, respectively). Considering the 1.0 km horizontal uncertainty, it is within the fault plane schematic representation provided by the seismogenic source database (DISS Working Group, 2010). The 6.6 ± 1.4 km relocated depth is compatible with the activation of a low-angle thrust fault (Viganò et al., 2008, 2013) at the contact between the sedimentary cover (hangingwall) and the underlying crystalline basement (footwall) (see the seismotectonic interpretation of the Giudicarie southern belt given below). This hypocentral solution is comprised within the depth interval estimated by 3D RMS grid-search analysis (3–9 km; Pessina et al., 2007) and by extended-source simulations (5–10 km; Franceschina et al., 2009). The deeper earthquake depth of about 13 km computed by Viganò et al. (2008) is most probably due to the location procedure and especially the use of a 1D velocity model (i.e., HE-1D type), which in the Lake Garda area shows important differences with respect to the 3D velocity model (Viganò et al., 2013). Moreover, Franceschina et al. (2009) showed that a directivity effect is able to explain highest macroseismic intensity values south-west of the epicentre, due to the modelled nucleation point located NNE on the fault plane with respect to its approximate centre. According to this evidence, the revised epicentre is located at the northern termination of the aftershock swarm located by a dense temporary network installed after the mainshock (Pessina et al., 2007). The most noticeable result made possible by the earthquake relocations in this study is the seismotectonic interpretation by combining seismicity and geological structures, as illustrated in Figs. 9 and 10. The Giudicarie fault system is seismically active mainly along its southern portion (South Giudicarie fault and fold-and-thrust belt in the sedimentary cover) where the relevant VP/VS crustal anomaly (about in the 0–10 km depth range) discussed in Viganò et al. (2013) is located. In fact, seismicity is generally observed where relatively high VP/VS values occur, suggesting a role of pre-existing fractures and fluids for

Fig. 10. Cross-sections with relocated seismicity (classified by local magnitude) and focal mechanisms (numbers and references as in Fig. 11b and Table 1). Simplified geological sections are also shown (1.5× vertical exaggeration). Traces of cross-sections are shown on the structural map. Coloured circles identify earthquake clusters plotted on each cross-section. The kinematics inferred from some strike-slip focal mechanisms in cross-sections 4 and 5 is only apparently discordant with the right-lateral movement shown on upper geological sections, since one nodal plane (i.e., the auxiliary plane) is almost parallel to the cross-section trace (cf. focal mechanisms and faults in map of Fig. 11b).

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earthquake nucleation (e.g., Chiarabba et al., 2015). At the northern termination of this area, we observe three seismicity clusters (cross-section 1 of Fig. 9. Hypocentres are compatible with the high-angle South Giudicarie fault (black circles) or border the western Adamello pluton (blue circles). A shallower seismicity (grey circles) is possibly associated with the presence of two artificial lakes (Bissina and Boazzo dams), as similarly observed in the Southern Apennines (Valoroso et al., 2009). The southernmost Giudicarie belt shows diffuse seismicity located along low-angle thrusts (western sector), with deeper hypocentres near the junction with the Schio–Vicenza fault system (eastern sector; cross-section 2 of Fig. 9). The largest seismic events (Salò earthquake; 28th August 2014 ML = 4.2 Garda earthquake, solution by the PAT seismic network not shown here) are located at the base of the Middle Triassic units, which is considered an important décollement level in the whole Giudicarie system (Picotti et al., 1995). This layer matches the base of the crustal volume with VP/VS N 1.78 where high fracturing and/or presence of fluids occur (Viganò et al., 2013). Moreover, focal mechanisms of these two events are also fully consistent with the deep fault orientation and geometry (events no. 29, 49 in cross-section 2 of Fig. 9 and in Fig. 11b). The seismicity of the eastern Southern Alps near Belluno appears concentrated within the sedimentary cover (cross-section 3 of Fig. 9), as similarly observed for the Giudicarie. The Valsugana thrust to the north is not seismic in the observed time-frame, which, however, is short, and consequently we cannot exclude activity with a longer recurrence time. Earthquakes are diffuse and located along the low-angle Belluno and Bassano thrusts. The high seismic potential of the Bassano thrust has been recently described by Anselmi et al. (2011) and Barba et al. (2013). The only evident cluster of events of this area is located at the western tip of a fault belonging to the Bassano system (red circles in cross-section 3 of Fig. 9). The southernmost Montello blind thrust does not show relocated seismicity in the investigated time-frame. However, Danesi et al. (2015) recently installed a local temporary seismic network and showed the seismic activity of the basal, sub-horizontal, portion of the Montello structure, as confirmed by compressive focal mechanisms (events no. 44–45, 50–51 in cross-section 3 of Fig. 9 and in Fig. 11b). The crustal sector comprised between the Giudicarie and the active Belluno and Bassano thrusts is characterised by dominant vertical faults with strike-slip kinematics (Castellarin et al., 2006; Massironi et al., 2006; Zampieri et al., 2003; Fig. 1). The seismicity is located between the surface and 15 km of depth, and it forms clear vertical alignments of earthquakes (Fig. 10). The NW-trending Schio–Vicenza, Priabona– Trambileno, and Campofontana faults are the most active structures (cross-section 4 of Fig. 10). Strike-slip kinematics is constrained by the most representative focal mechanisms (Viganò et al., 2008; Figs. 10 and 11b). The more easterly seismicity appears to activate different types of faults, with some clear evidence of vertical strike-slip faults (i.e., Montagna Nuova) cross-cutting the Bassano frontal thrust (crosssection 5 of Fig. 10). In the internal Southalpine chain, local thrusts are also seismically active (e.g., Vigolana earthquakes, green cluster in cross-section 4 of Fig. 10, no. 2 in Table 1 and Fig. 11b). In the internal Alpine chain seismicity clusters on limited sectors of the North Giudicarie fault (cross-section 6 of Fig. 10) and in the Austroalpine domain along the Zebrù fault systems (cross-section 7 of Fig. 10). The map of the cumulative seismic moment highlights areas with high seismic energy release in the considered period of time, allowing some initial considerations (Fig. 11a). In the internal Alpine domain,

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seismic moment concentrates along or north of the Insubric Line (Austroalpine). The Southalpine basement south of the Insubric Line and most of the Dolomites appear relatively aseismic (see the lowest GPS velocities near Bolzano in Caporali et al., 2013), except for sparse and low-energy earthquakes. Similarly, few earthquakes are located within the Po and Venetian plains, while the Northern Apennine buried thrust front is active very close to the faults causative of the May 2012 Emilia destructive sequence (Scognamiglio et al., 2012). In the Southern Alps, we observe seismic moment release along the Giudicarie southern belt (in coincidence with the anomaly discussed by Viganò et al., 2013) and a narrow band of activity along the eastern Alpine front. In the Schio–Vicenza and Lessini domain maximum moment release seems limited to the northern portion, mainly in the southern Trentino (Fig. 1). Even if strike-slip faults of the Schio–Vicenza system extend southward to Verona and Vicenza, the Lessini block appears much less seismic. This northward displacement of seismicity with respect to its easterly continuation along the Bassano thrust could be related to the presence of the deep-rooted so-called Verona–Vicenza gravity high (Zanolla et al., 2006; Fig. 11b). Relatively high-density magmatic rocks and intrusions as seen by tomography (Viganò et al., 2013) and a shallow Moho depth (e.g., Scarascia and Cassinis, 1997) can explain the observed gravity anomaly (Zanolla et al., 2006) and significantly contribute to a stronger and more competent lithosphere. A seismotectonic model of the study area, based on the updated data and consequent interpretations, is shown in Fig. 12. Faults identified by bold lines are considered (potentially) active in the short time-frame. Their identification is based mainly on relocated seismicity, with some additional considerations from historical seismicity and regional structural settings. With respect to previous seismotectonic models (e.g., Slejko et al., 1989), we can associate earthquake foci with active fault branches with higher precision especially in the Southern Alps, and interpret their kinematics (if focal mechanisms of larger seismic events are available) in relation with observed geological evidence. The analysis of all identified active faults within each seismotectonic domain allows to outline a model at the regional scale, consistent with the crustal deformation produced by large-scale geodynamic processes. The internal Alpine chain is characterised by dominant normal faulting (cf. cross-section 7 of Fig. 10 and Fig. 11b), with an average seismic moment release of ~ 1.8 × 1015 N m a− 1 (Table 2). The recent kinematics of the Austroalpine realm at the Brenner fault hangingwall, supported by geological evidence (Agliardi et al., 2009; Fellin et al., 2002; Massironi et al., 2006) and by results from numerical and analogue modelling (Caporali et al., 2013; Luth et al., 2013), is consistent with Adria indentation (Luth et al., 2013; Massironi et al., 2006), which causes lateral extrusion of the eastern Alps (e.g., Robl and Stüwe, 2005), although some authors invoke a dominant gravitational collapse at the lithospheric scale (e.g., Marotta and Splendore, 2014). In the Southern Alps, the NNW-ward direction of Adria–Europe convergence (e.g., Danesi et al., 2015 and references therein; Fig. 12a) produces shortening along the Giudicarie and the eastern Alps (Montello area; e.g., Cheloni et al., 2014; Danesi et al., 2015), with deformation along prevalent strike-slip faults in the central portion (Fig. 12). Quaternary geological faulting and related deformation confirm this behaviour for each domain (grey and white diamonds in Fig. 12). Only local thrust reactivations are observed within domain D3 (events no. 2, 46 in Fig. 11b). The Giudicarie belt is a rigid block with thick crust (N35–40 km; Scarascia and Cassinis, 1997) where the NW-oriented

Fig. 11. (a) Distribution of relocated seismicity and cumulative seismic moment release (10 × 10 km squares), with seismotectonic domains as in Fig. 3. (b) Focal mechanisms of the central–eastern Alps (in blue colour, Slejko et al., 1989; in red colour, Chiaraluce et al., 2009; Danesi et al., 2015; INGV, http://cnt.rm.ingv.it; Marschall et al., 2013; Saraò and CRS staff, 2013; Viganò et al., 2008). The magnitude type of pre-1989 seismic events is not specified by Slejko et al. (1989). Focal mechanisms of events in the period 1994–2007 are numbered as in Table 1; others are numbered in chronological order (no. 37–51). The Bouguer gravity anomaly contours (green with values in mGal; modified after APAT, 2005) and maximum horizontal compressive stress (Bressan et al., 2003; Marschall et al., 2013; Viganò et al., 2008; calculation method after Lund and Townend, 2007) are also shown. Other symbols as in (a).

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Fig. 12. Seismotectonic model for the central–eastern Alps. Grey stars are most relevant historical earthquakes after the Parametric Catalogue of Italian Earthquakes (CPTI11; Rovida et al., 2011), where Roman numbers are calculated epicentral intensities. Diamonds show the locations of geological sites where Quaternary fault activity along the Southalpine boundary is observed (grey diamonds for compressional tectonics: Benedetti et al., 2000; Livio et al., 2009; white diamonds for strike-slip and normal tectonics: Galadini et al., 2001; Sauro and Zampieri, 2001; Scardia et al., 2014). The N-S oriented structure of the Orsara graben (N of the city of Verona) is parallel to the maximum horizontal compressive stress orientation inferred by Viganò et al. (2008). Fault abbreviations are: BA, Bassano; BG, Ballino–Garda; BL, Belluno; BO, Baldo; CF, Campofontana; CV, Cerro Veronese; FO, Forst; MB, Montebelluna; MN, Montagna Nuova; MO, Montello; NG, North Giudicarie; PT, Priabona–Trambileno; SG, South Giudicarie; SV, Schio–Vicenza; TR, Tremosine; VI, Vigolana; ZE, Zebrù. The sketch (a) is the geodynamic framework of the central–eastern Alps, where the Insubric Line (IL) separates a strong and competent lithosphere to the south (Adria) from a decoupled lithosphere to the north (Europe) (according to Marotta and Splendore, 2014). Convergence rate in the eastern Alps after D’Agostino et al. (2008). Direction of the Adria plate after Danesi et al. (2015) and references therein.

present-day stress field implies mainly thrust fault activations (Viganò et al., 2008). This anticlockwise rotation of the crustal stress field with respect to the NNW-SSE contraction in the eastern Southern Alps can be attributed to the low-angle thrust geometry of the weak Giudicarie Table 2 Average annual cumulative seismic moment release (period 1994–2007) within the crust for each seismotectonic domain (cf. Fig. 3). Domain

Name

Seismic moment release (N m a−1) 0–15 km depth

1 2 3 4 5

Giudicarie and central Southern Alps Valsugana and eastern Southern Alps Schio–Vicenza and Lessini Po plain Internal Alpine chain

15

1.46 × 10 8.29 × 1013 1.20 × 1014 7.79 × 1013 1.77 × 1015

Crust 1.46 × 1015 8.29 × 1013 1.20 × 1014 8.21 × 1013 1.78 × 1015

southern belt, characterised by its peculiar crustal structure, as discussed above. East of the Giudicarie belt, a predominant simple-shear dextral reactivation of NW-SE strike-slip faults (i.e., Schio–Vicenza system) can be inferred (cf. Fig. 11b, events no. 14, 31, 37, 40, 41, 43), as would be expected given the NNW-SSE motion of Adria (Fig. 12a). The dextral kinematics probably characterises only the very recent activity of some of these faults and/or their dominant sense of movement only along the Southern Alps frontal zone (cf. Caporali et al., 2013; Pola et al., 2014). Left-lateral strike-slip reactivations along fault segments placed at high angles with respect to the principal fault system can be locally observed (e.g., Grezzana earthquake, 24th January 2012 ML = 4.2; fault plane identified by Convertito and Emolo, 2012; cf. event no. 47 in cross-section 2 of Fig. 9 and in Fig. 11b, and Fig. 12). Minor strike-slip faults are also active east of the Schio–Vicenza fault system, such as the Montagna Nuova fault and the Montebelluna fault (this latter can

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be interpreted as the lateral ramp of the Bassano–Montello thrusts, according to Anselmi et al., 2011 and Danesi et al., 2015; cf. Fig. 12). In this view, the 1695 Asolo historical earthquake, located at the western termination of the Montello structure near the Montebelluna fault (Fig. 12), has been recently interpreted as compatible with a strikeslip mechanism type (Sirovich et al., 2013). In addition, strike-slip faults locally cross cut thrusts, both along the Giudicarie and the eastern fronts (e.g., CV and MN in Fig. 12). The junction between the Giudicarie southern belt and the Schio– Vicenza fault system (the broad “Lessini block”) suggests a strong contrast in terms of stress concentration and potential deformation (i.e., seismicity). The so-called Verona earthquake (1117 AD), one of the most important historical earthquake in northern Italy (Guidoboni et al., 2005; Fig. 12), is probably related to this source region. The average seismic moment release along the Southalpine front is estimated as ~1.7 × 1015 N m a−1 (Table 2) in the considered period of time. 5. Conclusions A revised earthquake catalogue of the central–eastern Alps (period 1994–2007) has been presented. Earthquake solutions are obtained using a location procedure based on a 3D seismic velocity model and probabilistic locations (NLL-3D). Results can be summarised as follows: • The integration of seismological data from different seismic networks, a completely manual revision of phase readings, and the use of the NLL-3D location procedure permit to obtain more reliable earthquake solutions. The NLL-3D procedure here proposed, validated using selected quarry shots, is a useful tool for earthquake locations within the study area. • The 396 relocated seismic events (1.2 ≤ ML ≤ 5.3) have average errors of 1.4 ± 0.7 km horizontally and 2.2 ± 0.9 km vertically, and average RMS of 0.20 ± 0.07 s. Almost all computed depths are within the 0–15 km depth range (upper crust), with a frequency peak at about 7–8 km of depth, close to the brittle–ductile transition. • Different seismicity features can be more sharply imaged and earthquake foci are more accurately associated with tectonic structures; estimated hypocentres are much closer to active faults than previous locations. In particular, hypocentres (i) are located within the sedimentary cover along the low-angle thrust faults of the Giudicarie belt and the Belluno–Bassano fault system and (ii) are vertically aligned along the Schio–Vicenza fault system and other nearby minor faults. In addition, specific deep portions of the South and North Giudicarie faults, as well as structures of the internal Alpine domain (e.g., Zebrù faults), are seismically active. Seismicity is rare elsewhere in the investigated time-frame. • The three largest relocated earthquakes (ML ≥ 4.0) are clearly related to known geological structures: (i) the 2001 Merano earthquake nucleated along the Forst strike-slip fault; (ii) the 2002 Trompia valley and 2004 Salò earthquakes were located at the base of the Middle Triassic units, an important dècollement level (and thus rheological contrast level) in the whole Giudicarie system. • The thermo-rheological 1D models show temperatures at the model base (15 km of depth) in the 300–350 °C range and predict brittle–ductile transitions in the 8–9 km range of depth for each seismotectonic domain. The total strength of the upper crust is comprised between 1.0 and 2.0 TN m−1. • The proposed preliminary seismotectonic model, based on earthquake relocations, focal mechanisms, and other geological constraints, shows (dominant) compression along the Giudicarie and Belluno– Bassano–Montello fronts, with strain partitioning along dominant right-lateral strike-slip faults in the middle zone. The present-day deformation of the Southern Alps and the internal Alpine chain is compatible with Adria indentation, with observed and expected strain accumulations (i.e., seismicity) concentrated along the most favourable tectonic structures.

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Acknowledgements The authors gratefully acknowledge the Geological Survey and the Mining Survey of the Autonomous Province of Trento. Thanks are also due to A. Lomax for help with the NonLinLoc code, D. Andreis for computer programming, S. Gentili for regression analysis of magnitude conversion, and M. Anselmi for the 2004 “Feltre” earthquake focal mechanism parameters. We also thank two anonymous reviewers for their helpful comments and suggestions. Most figures were prepared using the GMT software (Wessel and Smith, 1998). This research was supported by the Geological Survey of the Autonomous Province of Trento (www.protezionecivile.tn.it). References Agliardi, F., Zanchi, A., Crosta, G.B., 2009. Tectonic vs. gravitational morphostructures in the central Eastern Alps (Italy): constraints on the recent evolution of the mountain range. Tectonophysics 474, 250–270. Anselmi, M., Govoni, A., De Gori, P., Chiarabba, C., 2011. 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