Active stress map of Italy

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. Bll, PAGES 25,595-25,610, NOVEMBER 10, 1999 Active stress map of Italy Paola Montone, Alessandro Amato, and Silvia Pondrelli Istituto Nazionale di Geofisica, Rome Abstract. We present a new map of the present-day stress field in Italy obtained from all the available data. The map reports 200 horizontal stress directions inferred from 109 borehole breakout data, 44 centroid momentensor solutions, 34 other focal mechanisms, most of which are from polarity distributions, seven stress inversions of microearthquake data, two averages of T and P axes of earthquake focal mechanisms in zones of diffuse seismic activity, and four fault slip data. The integration of breakout data, which yield horizontal stress directions, with fault plane solutions, which reflecthe stress regime, allows us to obtain an improved map of the present-day stress in Italy. This stress field map can be used for a better comprehension of active tectonic processes, for seismic hazard assessment, and to foresee the behavior of faults recognized with other methods. Stress directions obtained from different data, although relative to different depth intervals (e.g., 0-7 km for breakouts and 0-20 km for most of the earthquakes) and to different tectonic units, are consistent. Since many regions in Italy are characterized by an extensional stress regime, we reporthe minimum horizontal stress ( hmin) orientations. The map shows that an extensional regime affects most of the Apenninic belt. Conversely, a compressional (or transpressional) regime characterizes the eastern Alps, the eastern side of the northern Apennines, and the southern Tyrrhenian to northern Sicily zone. An abrupt change in stress directions marks the transition betwee northern and southern Apennines, suggesting that the two arcs are characterized by a differentectonic setting and recent evolution. In this paper we report all the data analyzed to date, with their geographicoordinates and average stress directions, and we describe the main stress provinces in Italy in the framework of the tectonic evolution of the region. 1. Introduction In this paper we present a new map of the active stress of Italy (Plate 1) obtained from 200 data, 55% of which are horizontal Along the Italian peninsula the present-day geologic setting, related to the N-S convergence between Africa and Eurasia, is stress directions from breakouts, the remaining data come mainly particularly complex since different processes are acting at the from earthquakes. Compared to that of the World Stress Map, which contains mostly focal mechanisms' axes, our data set has a same time and in close proximity. These processes include the different composition. Our data were selected out of a data set northward indentation of the Adriatic microplate beneath the which includes more than 500 entries. The discarded data include southern Alps, the flexure of the continental Adriatic lithosphere both poorly constrained solutions (for both breakouts and focal below the Apennines, and the subduction/sinking of the Ionian plane solutions) and small earthquakes (magnitude < 4). The data lithosphere below the Calabrian Arc (Figure 1). This complex set used here includes 109 borehole breakout data (Figure 2 and geology is reflected in a strongly variable stress field. It is not clear, however, which is the relative contribution of the driving Table 1), 78 fault plane solutions of earthquakes with magnitude > 4 that occurred between 1908 and 1998 (Figure 3 and Table 2); forces which are deforming the Italian region. In recent ye, ars, these are 44 centroid moment tensor (CMT) solutions and 34 this problem has been faced with numeric dynamic modeling, additional focal mechanisms, most of which from polarity which used published stress data and maps to discriminate among distribution. Our data set includes also seven stress inversions of competing models [see, e.g., Bassi and Sabadini, 1994; Bassi et microearthquake data, two averages of T and P axes of al., 1997]. earthquake focal mechanisms in zones of diffuse seismic activity In the World Stress Map [Zoback, 1992] the active stress field (Figure 3 and Table 3) and four active fault data (Table 4). of Italy was poorly constrained, based on a few focal mechanisms Although focal mechanisms of earthquakes are indicative of of earthquakes. In the interpolated stress map of the strain and not directly of stress, we use here as a first Mediterranean region [Reba ' et al., 1992] the Italian peninsula was covered by only a few focal mechanisms for events of the approximation the projections of focal mechanisms' axes on the Apennine belt (as for the World Stress Map), while a lack of data horizontal plane to indicate Shmin, following the World Stress was clear for the Tyrrhenian coastal region and for the Adriatic Map criteria (see Table 5). Although we are aware that this foredeep and foreland. The result of the interpolation was assumption may have serious drawbacks in many circumstances, therefore unreliable in many regions of Italy. there are a few arguments which let us believe that this approximation is acceptable: (1) the general consistency with Copyright 1999 by the American Geophysical Union Paper number 1999JB900181. breakout-inferred stress directions; (2) the fact that in extensional and compressional stress regimes, as in most regions of Italy, the possible discrepancies of stress and focal plane solution axes are 0148-0227/99/1999JB900181 $09.00 on the horizontal plane (less critical than for strike-slip fault); and 25,595

25,596 MONTONE ET AL.' ACTIVE STRESS MAP OF ITALY I 9øE 113øE 117øE Friuli 1 2 3 45øN Umbria-Marche 1997 31 032 41 øn 33 0 34 35 036 39 040 37øN x, d 0 100 I I km Figure 1. Sketch map of main tectonic features of Italy simplified from Bigi et al. [1990]. CMTs for great earthquakes that occurred between 1976 and 1998 are shown. (a) thrust fault (pre-middle Pliocene)' (b) thrust fault (middle Pliocene-Recent)' (c) normal fault; (d) strike-slip fault; (e) undetermined fault. (3) we do not discuss here neither specific data points nor 10 ø- 20 ø rotations but only very general trends and the most important rotations. This map is meant to be an up-to-date and complete picture of the active stress field in Italy that can be used for a better comprehension of active tectonic processes, for seismic hazard assessment, and to possibly foresee the behavior of faults recognized with other methods (surface and subsurface geology, seismology, and geodetic data). This map can also be used for seismotectonic zoning and to identify and investigate local stress perturbations that may localize seismicity [Zoback, 1993]. An accurate knowledge of the stress field is particularly important for seismic zones located out of the main active fault belt of the peninsula and in regions where active faults have no clear surface expression. This is a very frequent case in Italy, either because repeat times of earthquakes are long (often thousands of years) or because many moderate but damaging earthquakes (M between 4 and 6) occur on blind f tults. 2. Data Description Here we do not describe the different stress measurement techniques (borehole breakout analysis, CMT computation, etc.) used to obtain the directions reported in the map, but we refer the reader to e.g., Bell and Gough [1983], Dziewonski et al. [1983], Miller et al. [ 1992], Zoback [ 1992], Amato and Morttone [ 1997], Frepoli and Amato [ 1997], and Morttone et al. [1997]. In this note, as in all the papers published on this topic [see Zoback, 1992, and references therein], the results and interpretations are based on the assumption that since no shear stresses can exist in the plane of the Earth's surface, one of the principal stresses in the upper crust should be oriented approximately perpendicular to it [Anderson, 1951; McGarr and Gay, 1978]. Since many regions in Italy are characterized by an extensional stress regime, the results are reported in terms of minimum horizontal stress orientations (Shmin, corresponding to either 02 or 03).

MONTONE ET AL.' ACTIVE STRESS MAP OF ITALY 25,597 45øN 7 e17 e4o o.52 Adriatic Sea 85e Tyrrhenian Sea 40øN Ionian Sea 106.108 10OE 15øE Figure 2. Location of 109 selected deep wells. Numbers correspond to data set of Table 1, where borehole breakouts are reported. We take into account mostly borehole breakouts and focal mechanisms of earthquakes. Also, we report a few active fault data determined from paleoseismological investigations, excluding the faults for which focal mechanisms are available and those for which the strike-slip component was not reported. The overall distribution of breakout data and fault plane solutions covers rather homogeneously the Italian peninsula, in most areas complementing each other (Figures 2 and 3). In general, the two distributions are almost anticorrelated, since oil and gas deep wells are often drilled in the Apenninic foredeep, whereas earthquakes occur mostly beneath the belt. There are a few regions, however, where both earthquakes and deep wells are located, such as in the southern Apenninic belt. 2.1. Borehole Breakout Data Most of the deep wells were analyzed thanks to the collaboration with the largest Italian oil company (Agip, today part of Eni, the National Oil Authority) that in the past 6 years provided the four-arm caliper data to analyze borehole breakouts. In addition, other companies have contributed to create this data set: Enel (the National Electricity Authority), Enterprise Oil, and Lasmo International. To date, 261 deep wells sampling the crust from 0 to 7 km depth were analyzed along the Italian peninsula and in Sicily [Cesaro, 1993; Amato et al., 1995; Montone et al., 1995, 1997; Mariucci et al., 1999; Montone and Mariucci, 1999; World Stress Map database], 109 of which gave valuable results in terms of Shmin directions (Plate 1). Unfortunately, the wells are not homogeneously distributed over Italy but are concentrated mostly along the Apenninic foredeep from Sicily through Calabria and as far north as the Po Plain, along a belt which is -10 to 50 km offset to the east of the seismically active zone (Figures 2 and 3). However, other wells are available along the southern Apennines belt, very close to the seismic zone, along the coastal Tyrrhenian volcanic belt, and a few sparse boreholes in other regions (Figure 2).

25,598 MONTONE ET AL.: ACTIVE STRESS MAP OF ITALY Table 1. Borehole Breakout Data Set Table 1. (continued) No. Latitude, Longitude, Shmin +s.d. Quality Source No. Latitude, Longitude, Shmin +s.d. Quality Source o N o E øn øe 1 44.882 10.944 76 2 44.880 11.049 290 3 44.843 11.173 288 4 44.820 10.771 275 5 44.819 10.608 307 6 44.786 10.703 289 7 44.767 11.306 57 8 44.727 10.428 277 9 44.138 11.730 39 10 43.834 13.691 320 11 43.762 13.062 336 12 43.590 13.724 322 13 43.514 13.128 74 14 43.388 13.924 302 15 43.334 14.021 80 16 43.269 13.741 319 17 43.263 14.357 324 18 43.232 12.283 77 19 43.201 10.842 22 20 43.196 10.747 27 21 43.197 10.748 117 22 43.189 13.590 332 23 43.182 11.036 89 24 43.169 13.601 309 25 43.167 13.494 75 26 43.159 11.063 76 27 43.085 14.018 180 28 43.045 13.888 40 29 42.930 13.860 284 30 42.904 13.879 11 31 42.863 14.419 23 32 42.857 11.561 47 33 42.843 11.685 16 34 42.841 14.159 14 35 42.839 14.159 327 36 42.839 13.929 26 37 42.830 11.572 279 38 42.809 11.646 51 39 42.784 13.798 30 40 42.661 14.583 34 41 42.656 14.154 1 42 42.646 11.876 58 43 42.568 11.783 88 44 42.549 11.839 66 45 42.546 11.903 60 46 42.529 13.865 54 47 42.500 13.881 48 48 42.421 13.915 51 49 42.420 14.091 75 50 42.417 14.452 26 51 42.416 13.855 53 52 42.346 14.904 48 53 42.237 14.281 70 54 42.154 14.564 39 55 42.133 14.695 37 56 42.115 14.651 70 57 42.093 14.561 320 58 41.972 14.940 19 20 ø B 9 20 ø B 9 23 ø C 9 10 ø B 9 15 ø B 9 14 ø B 9 10 ø B 9 10 ø B 9 10 ø B 1 23 ø C 6, 8 31 ø C 6,8 21 ø C 6,8 13 ø C 6,8 8 ø B 6, 8 20 ø C 6, 8 20 ø C 6, 8 24 ø C 6, 8 23 ø C 6, 8 22 ø C 11 11 ø C 11 19 ø C 11 20 ø C 8 16 ø B 11 23 ø C 11 25 ø C 8 21 ø C 11 15 ø B 8 20 ø B 8 12 ø B 8 3 ø C 8 19 ø C 8 8 ø A 4 10 o C 4 10 ø B 8 6 ø B 8 20 ø C 8 24 ø C 4 15 ø C 4 20 ø C 8 7 ø A 8 5 ø C 8 18 ø C 4 21 ø C 4 5 ø B 4 10 ø B 4 4 ø B 4 23 ø C 6, 8 18 ø B 6, 8 18 ø B 6, 8 24 ø C 6, 8 12 ø B 6, 8 4 ø B 6,8 16 ø B 6,8 11 o B 6,8 16 ø C 6,8 22 ø C 6, 8 19 ø C 6, 8 10 o C 6, 8 59 41.931 14.817 34 7 ø B 6,8 60 41.891 14.795 8 20 ø C 6,8 61 41.486 14.691 38 11 ø A 3,5 62 41.471 14.757 47 18 ø C 3,5 63 41.455 15.234 8 10 ø C 3,5 64 41.375 15.354 36 27 ø C 3,5 65 41.344 14.808 35 14 ø B 3,5 66 41.324 15.345 72 18 ø B 3,5 67 41.315 14.615 69 8 ø A 3,5 68 41.290 15.340 38 24 ø C 3,5 69 41.259 15.481 76 4 ø C 3,5 70 41.211 14.840 45 17 ø B 3,5 71 41.159 15.588 13 15 ø C 3,5 72 41.139 15.590 54 28 ø C 3,5 73 41.139 13.694 56 11 ø C 3,5 74 41.071 15.703 23 10 ø B 3,5 75 41.056 16.205 64 25 ø C 3,5 76 40.964 15.849 68 12 ø B 3,5 77 40.930 15.141 30 19 ø C 3,5 78 40.785 14.443 76 25 ø C 3,5 79 40.744 16.173 26 14 ø B 3,5 80 99.999 99.999 59 11 ø B 7 81 40.483 16.309 26 13 ø B 7 82 40.465 16.378 1 13 ø C 7 83 40.459 15.802 41 12 ø B 7 84 40.429 16.455 68 10 ø C 7 85 40.370 14.837 349 17 ø B 7 86 40.335 15.933 52 11 ø B 7 87 40.327 15.937 43 22 ø C 7 88 40.315 16.031 42 24 ø C 7 89 39.352 17.139 323 12 ø C 6 90 39.260 17.155 58 25 ø C 6 91 39.229 17.002 68 30 ø C 6 92 38.959 17.208 127 12 ø B 6 93 38.950 17.155 50 17 ø C 6 94 38.439 16.897 31 21 ø C 10 95 37.888 12.530 210 20 ø C 10 96 37.845 12.562 238 22 ø C 10 97 37.475 14.542 126 12 ø C 10 98 37.431 14.914 132 15 ø B 10 99 37.427 15.043 152 15 ø C 10 100 37.313 14.672 88 15 ø C 10 101 37.009 14.045 274 15 ø B 2 102 36.998 13.847 311 15 ø B 2 103 36.981 15.231 256 14 ø C 2 104 36.946 15.071 221 12 ø B 2 105 36.867 14.589 248 17 ø B 2 106 36.775 15.067 246 15 ø B 2 107 36.767 14.964 46 15 ø B 2 108 36.309 15.134 23 15 ø B 2 109 36.119 14.911 32 15 ø B 2 Geographic coordinates; minimum horizontal stress; s.d., standard deviation; for quality ranking see Table ;. Source: 1, Morttone et al. [1992]; 2, Cesaro [1993]; 3, Arnato et al. [1995]; 4, Morttone et al. [1995]; 5, Arnato and Morttone [1997]; 6, Montone et al. [1997]; 7, Mariucci et al. [1998]; 8, Mariucci et al. [1999]; 9, Montone and Mariucci [1999]; 10, World Stress Map database; 11, unpublishedata.

16 E 20'E 44'N S. m,. direction Data qua lity, breakout data / A ea hquakedata B fault data C Stress regime, nodal faulting: Sv>SHmax>Shmin thrust faulting; SHmax>Shmin>Sv strike-slip faulting: SHmax>Sv>Shm,n stress regime unknown 40' N 36 N CMT solutions: 44 Shmin directions 4.2<Mw<7 hypocentral depths: 0-37 km pedod: 1976-1998 B quality: 40 C quality: 4 Polarity solutions: 34 Shmin directions 4.0<M<7.0 hypocentral depths: 0-38 km period: 1908-1996 B quality: 3 C quality: 31 a Formal inversions and averages of FPS axes: 7 o 3 directions 2 mean Shmin directions Fault data: 4 C quality data Borehole breakouts data: 109 Shmin directions A quality: 4 B quality: 46 C quality: 59 depth of the wells: 0-7 km Well data courtesy of Eni, Enel and Enterprise Oil. // 500O - 2500 1500 800 400 loo o meters a.s.i. 0 50 100. km 36 N 8'E 12E 16'E Prc ec'l on: Lambea_Az/muthal_ EquaI-Area Plate 1. Active Stress Map of Italy, with minimum horizontal stress orientations

MONTONE ET AL.' ACTIVE STRESS MAP OF ITALY 25,601 45øN 23 Adriatic Sea 57 27 ß Tyrrhenian Sea 40øN Ionian 0 Sea 10OE 15øE Figure 3. Location of earthquake focal mechanisms. Numbers correspond to data set of Table 2; labels correspond to formal inversion and average focal mechanism data set reported in Table 3: CC, Caorso-Cortemaggiore; CF, Cesena-Forli; VU, Vulsini; GA, Gargano; CA, Colli Albani; UM, Umbria-Marche; WAH, western Alps, highest region; WAL, western Alps, lowest region; SIC, Sicily. To attribute a quality value to each well, we adopted the quality ranking system proposed by Zoback [1992] for the World Stress Map: from A (quality for the best data) to E (quality for discarded wells), depending on total breakout length and standard deviation of the mean direction (see Table 6). In the breakout data set the number of data belonging to category A is 4; to category B is 46; to category C is 59; to category D is 75, and, finally, 77 are the discarded wells. Categories D and E are not reported in Plate I or in Table 1. This "quality" estimate is a measure of the statistical distribution of the breakout data, not of the single breakout measurements. In most cases, we have found stress directions which do not vary significantly with depth and in different tectonic units. However, there are a few examples in which the breakouts rotate along the well, as where faults are crossed by the drilling, as described by Bell et al. [1992], Barton and Zoback [1994], and Mariucci et al. [1998], or when two different directions are detected in separate tectonic uaits. In such cases, even if the breakouts are well determined, the average stress directions are statistically poor (high standardeviation). 2.2. CMT Focal Solutions Assuming that the seismic deformation reflects the local stress state around a fault, the most reliable information on the active crustal stress distribution in Italy derives from the CMT focal solutions. Most of the data represented in Plate 1 are from the CMT catalog produced and implemented at Harvard University [Dziewonski et al., 1983] (all subsequent reports are in Physics of the Earth and Planetary Interiors]. The catalog includes all earthquakes that have occurred between 1976 and 1998, having a magnitude generally > 5.5. A recent modification of the CMT

25,602 MONTONE ET AL.' ACTIVE STRESS MAP OF ITALY Table 2. Earthquake Data Set Table 2. (continued) No. Latitude, Longitude, Shmil, Q SR Depth, M Date Source No. Latitude, Longitude, Shmi,, Q SR Depth, M Date Source øn øe km øn øe km 1 46.15 13.39 075 B TF 10 6.4 760506 1 45 38.125 15.600 281 B NF!0 7.0 081228 5 2 46.24 13.32 117 B TF 10 4.9 760507 1 46 41.986 13.648 225 C NF 8 6.8 150113 5,6 3 46.06 13.49 110 B TF 10 5.1 760509 1 47 41.054 15.359 002 C NF - 6.7 300723 7,13 4 46.00 13.04 075 B TF 10 5.0 760511 1 48 44.073 11.645 062 C NF 7 4.7 390211 5 5 46.40 13.50 071 B TF 10 5.3 760911 1 49 44.233 10.203 120 C NF 27 4.9 391015 5 6 46.15 13.34 079 B TF 10 5.6 760911 1 50 38.440 12.120 117 C NF 20 6.9 410316 5 7 46.18 13.27 250 B TF 10 5.9 760915 1 51 45.330 9.333 188 C SS 6 5.0 510515 5 8 46.20 13.23 069 B TF 10 6.0 760915 1 52 45.330 9.333 170 C SS 6 4.5 510516 5 9 46.18 12.82 086 B TF 12 5.2 770916 1 53 42.350 13.440 265 C NF 10 5.0 580624 5 10 38.10 16.03 167 B NF 33 5.2 780311 2 54 41.13 14.95 070 C NF 8 5.7 620821 8 11 38.39 15.07 093 B SS 14 6.0 780415 2 55 41.08 15.00 061 B NF 8 6.1 620821 8 12 46.40 13.16 279 B SS 10 4.7 790418 1 56 37.840 14.600 144 C SS 38 5.0 671031 5 13 42.81 13.06 079 B NF 16 5.8 790919 2 57 42.000 16.500 187 C NF 33 4.7 671209 5 14 38.28 11.74 252 B TF 33 5.3 791208 2 58 44.630 12.010 139 C TF 33 5.2 671230 5 15 38.48 14.25 257 B TF 14 5.7 800528 2 59 37.75 12.98 216 C NF 10 5.4 680115 9 16 40.91 15.37 039 B NF 10 6.9 801123 2 60 43.250 10.770 166 C SS 33 4.7 700819 5 17 40.65 15.40 020 B NF 10 5.4 801125 2 61 42.310 11.760 218 C NF 2 4.6 710206 5 18 40.95 15.37 027 B NF 15 5.2 810116 2 62 43.230 12.490 232 C NF 33 4.5 710212 5 19 44.69 10.32 054 B TS 37 5.0 831109 2 63 41.200 15.240 207 C SS 33 4.8 710506 5 20 43.27 12.57 039 B NF 14 5.6 840429 2 64 40.340 15.770 248 C SS 4 4.7 711129 5 21 41.77 13.89 057 B NF 10 5.9 840507 2 65 42.820 12.930 250 C NF 5 4.8 741202 5 22 41.83 13.95 056 B NF 13 5.5 840511 2 66 41.650 15.730 237 C SS 18 4.9 750619 5 23 43.25 13.94 122 B TF 12 5.1 870703 3 67 44.750 9.520 185 C NF 20 4.8 751116 5 24 42.37 16.57 085 B TF 4 5.4 880426 2 68 42.670 12.460 184 C TF 0.1 4.9 780730 5 25 40.75 15.85 228 B SS 26 5.8 900505 2 69 40.797 16.109 245 C NF 28 4.2 780924 5 26 37.32 15.25 227 B SS 10 5.6 901213 2 70 44.410 11.990 210 C NF 18 4.6 781205 5 27 41.90 15.97 225 B TF 10 5.2 950930 2 71 39.330 16.190 126 C NF 10 4.3 800220 5 28 44.79 10.78 246 B TF 10 5.4 961015 2 72 40.360 15.770 212 C NF 15 4.2 800514 5 29 43.01 12.90 046 B NF 10 4.5 970903 4 73 43.567 12.217 005 C NF 12 4.0 930117 10 30 43.02 12.89 237 B NF 10 5.7 970926 4 74 43.117 12.733 356 C SS 7 4.3 930605 10 31 43.03 12.85 048 B NF 10 6.0 970926 4 75 37.98 14.14 060 C TF 5 4.8 930626 11 32 43.01 12.97 056 B NF 10 4.5 970926 4 76 44.133 10.167 035 C SS 2 4.8 951010 10 33 43.09 12.81 238 C NF 6 4.3 970927 4 77 44.333 10.550 111 C SS 9 4.0 951231 10 34 43.03 12.84 040 B NF 10 5.2 971003 4 78 40.67 15.42 011 B NF 8 5.1 960403 12 35 42.94 12.93 221 B NF 10 4.7 971004 4 36 43.02 12.84 048 B NF 10 5.4 971006 4 SR, stress regime: NF, normal fault regime; NS, normal to strike- 37 42.99 12.82 045 C NF 12 4.2 971007 4 slip regime; SS, strike-slip regime; TS, thrusto strike-slip regime; 38 43.03 12.85 042 B NF 10 4.5 971007 4 TF, thrust regime; M, magnitude: M,,, (1 to 44); 45 to 78 see sources 39 42.91 12.94 238 B NF 10 5.2 971012 4 for different magnitude scales. Sources: 1, Pondrelli et al. [1999]; 2, 40 42.93 12.92 039 B NF 10 5.6 971014 4 Dziewonski et al. [1983]; 3, Pondrelli et al. [1998]; 4, Ekstrdirn et al. 41 43.04 12.89 242 C SS 10 4.3 971016 4 [1998]; 5, Gasparini et al. [1985]; 6, Ward and Valensise, [1989]; 7, 42 42.97 12.79 047 C NF 10 4.2 971019 4 Selvaggi et al. [1997]; 8, Westaway [1987]; 9, Anderson and Jackson, 43 43.16 12.70 054 B NF 10 5.1 980403 2 [1987]; 10, Frepoli andarnato, [1997]; 11, Azzara et al. [1993]; 12, 44 40.03 15.98 044 B NF 10 5.6 980909 2 Coccoetal. [1999]; 13, Boschietal. [1995]. standard method algorithm, based on inversion of intermediate first approximation, which is consistent with the World Stress period surface waves, has been developed and tested [Arvidsson Map, we assume that the projections onto the horizontal of the T- and Ekstr6m, 1998] and allows the determination of moment P focal mechanisms' axes correspond to Shmin and Shmax, tensors even for smaller events (down to Mw=4.2) using regional respectively. Figure 3 shows the location of the 44 CMT and teleseismic data. In this paper, solutions computed with this solutions, whose parameters are summarized in Table 2. new technique for the 1976 Friuli sequence [Pondrelli et al., To assign the quality value to each seismic event, we follow 1999], for the 1997 Umbria-Marche (Colfiorito) sequence the Zoback [1992] criteria. In the World Stress Map, most CMT [Ekstrdim et al., 1998], and for one moderatearthquake on the solutions fall in category B even though they are well constrained Adriatic coast [Pondrelli et al., 1998] are included. and with large magnitude. This is because the P or T axis for a For each CMT solution, P, T, and B axes are used to determine singl event can show significant differences from the orientation both the stress regime and the direction of the Shmin, applying of the actual stress that produces the slip [Zoback, 1992]. criteria derived from Zoback [1992] and shown in Table 5. As a According to the World Stress Map, only average P axis or

MONTONE ET AL.' ACTIVE STRESS MAP OF ITALY 25,603 Table 3. Average Focal Mechanisms and Formal Inversions of Stress Axes Site Latitude, Longitude, Shmin +s.d. N Depth, km Period øn øe CC 44.97 10.00 121 13 ø 5 0-8 1991-1995 CF 44.12 12.12 139 21 ø 32 0-15 1989-1995 M Quality SR Source 2.9-4.0 B TF 1 2.6-3.7 B TS 1 Site Latitude, Longitude, o. o 2 o N Mis. R Depth, øn øe km GA 41.70 15.67 130.73 310.17 040.00 17 5.9 0.5 14-24 SIC 38.00 15 50 143.73 026.08 293.15 20 4.6 ø 0.6 0-50 CA 41.70 12.70 321.49 131.40 225.05 45 8.3 ø 0.7 0-7 VU 42.80 11.80 143.30 293.57 045.14 67 5.9 ø 0.8 0-8 WAH 44.50 7.00 054.66 160.07 253.23 16 5.3 ø 0.5 0-25 WAL 44.70 7.70 282.05 180.22 023.67 14 4.7 ø 0.6 0-25 UM 43.25 12.75 342.86 129.02 219.01 19-0.4 0-15 Period M SR Source 1988-1995 2.8-4.4 NF 2 1968-1991 >2.5 NF 3 1989-1990 1.5-4.2 NS 4 1984-1990 1.5-4.2 SN 4 1959-1993 2.5-5.3 NF 5 1977-1993 2.7-4.8 TF 5 1987 0.8-2.1 NF 6 SR same as Table 2; N, number of data used for computation; Mis., mean misfit angle; R, stress ratio (o 2-03)/( o 2- ( )- Sources: 1, Frepoli and Amato [1997]' 2, A. Frepoli (personal communication; 1999); 3, Caccamo et al. [1996]; 4, Montonet al. [1995]' 5, Eva et al. [1997]; 6, Boncio et al. [1996]. formal inversion of four or more single-event solutions in close 2.4. Stress Inversions and Averages geographic proximity (at least one event with M=4.0, other As known, the best way to determine real stress axes from events M:3.0) belongs to category A. As a consequence, earthquake data would be to invert these data with one of the considering singularly all available CMT solutions, in our data techniques published in recent years [e.g., Gec, hart and Forsyth, set, 40 events fall in category B and four fall in category C due to 1984]. Unfortunately, this is not always possible for the Italian their smaller magnitude. region, due to the sparse distribution of seismicity. In our study 2.3. Polarity and Geodetic Fault Plane Solutions We carried out a careful investigation of all the focal mechanisms published in previous studies, considering only crustal earthquakes with magnitude > 4 which occurred in this century (Figure 3 and Table 2). Our primary source for the period 1908-1980 is the compilation by Gasparini et al. [1985]. Owing to the poor geometry of the networks operating in that period, many solutions present a large number of inconsistencies and have been discarded. When specific studies were carried out for some events, which increased the reliability of the solutions, these latter have replaced the polarity solutions in our database. For the 1908 Messina-Reggio Calabria earthquake (M=7.0) we kept Gasparini et al.'s solution instead of the geodetic solution [De Natale and Pingue, 1991], since there is a discrepancy of we considered all the data that were determined using stress inversion techniques in regions of diffuse seismicity (Figure 3 and Table 3). Some data were discarded if the inversion results were not satisfactory. For instance, Caccamo et al. [1996] published the results of stress inversions obtained from various combinations of fault plane solutions in Sicily. Their results for some of the regions are not well constrained, possibly due to heterogeneity of stress in the crustal volumes considered for the inversion. In this case, we took only the results that we consider more stable, and we refer to Caccamo et al. [1996] for further details. For northwestern Italy, we rely on the study by Eva et al. [1997], who collected all the relevant earthquakes of this region and carried out a stress inversion. Therefore our map reports only these results and has fewer data than the World Stress Map only a few degrees between the two in terms of Shmin trend. database and other published maps [e.g., Gr nthal and Conversely, we preferred to use the solution of Anderson and dackson [1987] for the 1968 western Sicily earthquakes, the solution of Westaway [ 1987] for the 1962 Irpinia earthquake, and the geodetic solution determined by Ward and Valensise [1989] for the 1915 earthquake in central Italy. For the period 1976-1998 Stromeyer, 1992; M ller et al., 1992; Eva et al., 1998], to which we refer fbr more detailed analyses. For the central Apennines we considered the inversion result obtained by Boncio et al. [1996], and by Morttone et al. [ 1995]. Table 3 also reports the parameters of two zones in which we we preferred the CMT solutions (see section 2.2) to the polarity ones. For the most recent years we used the fault plane solutions published by Frepoli and Amaro [1997], selecting earthquakes with magnitude > 4. Table 4. Fault Data No. Latitude, Longitude, Shmin Q SR Source o N o E 1 42.18 13.48 055 C NF Pantosti etal. [1996] 2 39.85 16.20 030 C NF Michetti et al. [1997] 3 39.82 16.25 065 C NF Cinti et al. [1997] 4 40.40 15.75 040 C NF Benedetti et al. [1998] See Table 2 footnotes. determined a stress direction by averaging axes of focal mechanisms obtained from polarities [Frepoli and Amaro, 1997] and for which stress inversion results were not available. These are not main shock-aftershocksequences but zones of rather continuous, weak seismicity. Although averaging focal mechanisms' axes has not a true physical meaning, in our experience the average directions of either T (in extensional stress regimes) or? axes (in compressional stress regimes) is parallel to ( 3 or ( determined with stress inversion procedures. 2.5. Fault Data Owing to both the relatively low magnitudes of crustal earthquakes (in this century, only a few exceeded magnitude 6.5) and the young age of many of these faults [see, e.g., Pantosti and

25,604 MONTONE ET AL.' ACTIVE STRESS MAP OF ITALY Table 5. Characterization of Stress Regime and Determination of Shmin Azimuth Plunge of Axes P/O1 Axis B/O2 Axis T/o Axis Stress Shmin Azimuth Regime pl> 52 ø 40 ø <_ pl < 52 ø pl < 40 ø <_ 35 ø NF T axis <_ 20 ø NS T axis _> 45 ø <_ 20 ø SS T axis pl <_ 20 ø _> 45 ø < 40 ø SS P axis + 90 ø pl _< 20 ø 40 ø to 52 ø TS P axis + 90 ø p1 <_ 35 ø _> 52 ø TF B axis In Plate 1 the solutions with NS and TS stress regime are reported as NF and TF, respectively. Modified after Zoback [ 1992]. Valensise, 1990; Hippolyte et al., 1994], in Italy there are not many well-documented active fault slip data. Several studies on active or recently active faults, identified with different techniques, have been published in the last few years. For more details we refer to the compilations by Vittori et al. [1997], F. Galadini et al. (1999, available at http://emidius.itim.mi.cnr.it /GNDT/P512/UR_CNR_IRTR.html), and Valensise and Pantosti [1999]. In our study we include only faults with evidence of coseismic activity, possibly with slip vectors determined in the field. This restricts our data set to only a few cases (Table 4). Faults for which seismic or geodetic focal mechanisms are also available were not included, such as the 1908 Messina (M=7.0) [Gasparini et al., 1985], the 1915 central Italy earthquake (M=6.8), and the 1980 Irpinia earthquake (M=6.9). Also here, we adopted the convention of the World Stress Map to determine the stress direction. As for focal mechanisms, we warn the readers that what is plotted in the stress map is the horizontal projection of the assumed slip vector, since for these faults the strike slip component is negligible. The fault strike has been taken as the average strike of the different segments composing the fault. The young age of the normal faults in Italy [Pantosti and Valensise, 1990' Hippolyte et al., 1994], however, suggests that the stress direction is well represented by the sense of motion on the faults. 3. Discussion The main tectonic sources of stress in the plates are those related to the plate driving forces (as ridge push, slab pull, trench suction, etc.) [Zoback, 1992]. The World Stress Map has shown that the horizontal stress orientations have regionally uniform patterns throughout many continental intraplate regions. Local variations in stress orientation and relative magnitude exist at a variety of scales [see also Rebai' et al., 1992]. These variations may be due to different forces acting on the lithosphere such as Table 6. Borehole Breakout Quality Ranking System Standard Deviation Length of <12 ø <20 ø <25 ø >25 ø Breakout Zone, m >300 A B C D 100-300 B B C D 30-100 C C C D 0-30 D D D D From Zoback [ 1992]. >40 ø the buoyancy and flexure forces on the bro:id wavelength end (100-5000 km) and thermal, topographic and other specific site effects on the very short wavelength end (<100 km) [Zoback, 1992]. Therefore a detailed map of active stress orientations allows us to identify both the orientation of the regional stress field and local stress rotations resulting from specific geologic and tectonic features. In this study, which is mainly devoted to the stress data compilation and dissemination in a region of complex tectonics, we briefly describe the past geologic evolution of the Italian region, then we review some of the studies carried out in the past few years for modeling the stress field (mostly based on previous compilations), and finally, we put forward some hypotheses to explain the first-order patterns evidenced in our improved map. Here, we do not discuss small rotations of the horizontal stress directions, both because these are generally within the errors of the measurement techniques and because for earthquake and fault data the plotted Shmin directions may differ significantly from the true stress axis. Italy is located in the central Mediterranean, within the broad deformation belt resulting from the interaction of two major plates, African and Eurasian. As already known [Miiller et al., 1992; Rebai' et al., 1992], and as confirmed by the data presented in this study, the stress field within this region is not directly related to the main plate motion, which is a slow convergence at a rate of < 1 cm/yr [De Mets et al., 1990]. The Neogene evolution of the western and central Mediterranean is characterized by the eastward propagation of a long subduction front which caused consumption of the Tethyan oceanic lithosphere [Dercourt et al., 1986; Dewey et al., 1989]. The migration of almost 1000 km of the subduction hinge and the associated surface tectonics have been explained in terms of slab pull and roll-back [see, e.g., Malinverno and Ryan, 1986; Kruse and Royden, 1994]. This may explain the controversial observation that the subduction front was roughly parallel to the convergence direction. Alternative explanations to the slab roll-back mechanism include lateral extrusion of crustal blocks [Mantovani et al., 1996] and eastward asthenospheric flow [Doglioni, 1991]. Backarc opening of the Algero-Provencal basin in the Miocene and of the Tyrrhenian basin from late Miocene to the Present led to the actual configuration. At present, only in the narrow (200 km) zone of the Calabrian arc there is still oceanic subduction, as well documented by both intermediate and deep earthquake distribution [Frepoli et al., 1996], seismic tomography [Amato et al., 1993; Lucente et al., 1999], and volcanic activity [Barberi et al., 1973]. However, also at both sides of this active arc (i.e., beneath the Apennines and beneath Sicily), there is evidence of past subduction, possibly of continentalithosphere, beneath the entire

MaNTONE ET AL.: ACTIVE STRESS MAP OF ITALY 25,605 Ty r eanian Shmin direction Stress regime e.:... breakout data ß ---<)---- earthquake data GG O formal inversions and :.,,,, /' unknown averages of FPS axes ----!::l--- fault data ß thrust faulting 0 normal faulting (} strike-slip faulting Data quality Tectonics 0 km 40 /,, thrust fault strike-slip fault (pre- middle Pliocene) (undifferentiated age) thrust fault undetermined fault (middle Pliocene-Recent) >x normal fault (undifferentiated age) (undifferentiated age) Figure 4. Comparison between Shmin directions and mapped tectonic structures [after Bigi et al., 1990]. (a) Eastern Alps; (b) northern Apennines (average focal mechanism labels for CC, Caorso-Cortemaggiore, and CF, Cesena- Forli); (c) central Apennines (formal inversion of fault plane solution axes, for CA, Colli Albani, and UM, Umbria- Marche); d) southern Apennines; (e) Sicily and Calabriarea (formal inversion of fault plane solution axes for SIC, Sicily). See Tables 1-4 for references. peninsula from seismic tomography [Amato et al., 1993; Spakman et al., 1993; Piromallo and Morelli, 1997; Lucente et al., 1999]. In this framework the Apennines fold and thrust belt has developed on top of the eastward migrating subduction hinge, associated with compression at the outer front and contemporaneous extension in the backarc region [Malinverno and Ryan, 1986; Patacca and Scandone, 1989]. According to Frepoli and Amato [1997] and Mariucci et al. [1999], this process is still active today in the northern Apennines. On the contrary, there is geologic and seismological evidence that the

25,606 MONTONE ET AL.: ACTIVE STRESS MAP OF ITALY 16øE ( Compression (SHmax direction) ( Extension (Shmin direction) ( Strike-slip 44øN,x,. Radial extension.-.. 40øN W 500! 00 ( AJ?ica foreland ( meters a.s.i. ß 0 50 100,. 36øN Figure 5. Summary of the stress data presented in Plate 1. The main stress provinces delineated by the data are sketched. The structural arcs with shaded triangles indicate the active compressional fronts; solid triangles show active oceanic subduction; open triangles delineate the location of Plio-Pleistocene thrust front, presently affected by prevalent extension.

MONTONE ET AL.: ACTIVE STRESS MAP OF ITALY 25,607 southern part of peninsula underwent a major stress change in the last 1 Myr [Patacca and Scandone, 1989; Pantosti and Valensise, 1990; Hippolyte et al., 1994; Westaway, 1993; Amato and Montone, 1997]. In the understanding of present-day tectonics and seismicity it is important to see how the active stress in the crust relates to the recent tectonic evolution (thus, indirectly, to the deep structures). This has been the subject of several studies carried out with different approaches in the last few years [see, e.g., Wortel and Spakman, 1992; Bassi and Sabadini, 1994; Faccenna et at, 1996; Bassi et al., 1997; Negredo et al., 1997]. Most of these studies have shown that a slab-pull mechanism is needed in order to explain the recent tectonics and the presentday stress. However, additional mechanisms have been proposed to explain differences in the tectonic setting and kinematics over the Italian region. These include the counterclockwise (CCW) rotation of the Adriatic microplate around a pole in northern Italy [Anderson and Jackson, 1987; Warc 1994; Bassi et al., 1997], horizontal motion of crustal blocks caused by the Africa-Eurasia convergence [Mantovani et al., 1996], lithospheric delamination [Mele et al., 1998], and the lateral migration of a detachment in the subducted lithospheric slabs [Wortel and Spakman, 1992; Carminati et al., 1998]. This large number of hypotheses, often conflicting each other, is due in our opinion to the lack of tight constraints which should allow us to discriminate among the proposed mechanisms. The objective of this paper is to provide one such constrainto the geologic and geophysical communities. There are several questions that must be answered if we want to fully understand the relationship between the stress in the crust and the active tectonic processes. The main points are (1) the kinematics of the plates involved in the deformation of the Italian region, particularly the motion of the Adriatic microplate relative to Africa and Eurasia, (2) the nature and geometry of the lithosphere subducted beneath the Apenninic belt, (3) the structure of the lower crust and its role in transmitting stresses from the upper mantle to the brittle crust, (4) the role played by pre-existing structural features in the stress rotations observed today; and (5) the influence of topography and sediment load. The quantification of the forces involved in these processes is complex and beyond the objective of this study. Here we summarize our findings, which in some cases confirm and strengthen previous hypotheses; in others, they are quite new. With this compilation we want to offer the geological and seismological communitiestudying the Meg2erranean region an improved tool to constrain geodynamic mo els. Our res:!ts show that there are large (102-103 km 2) regions in Italy within which the stress field is relatively uniform. 3.1. Northern Italy The eastern Alps are dominated by active compression, which shows thrust faulting earthquakes (Figure 4a and Plate 1), witr N-S direction of maximum compression. Here, the coexistence o; -E-W trending Alpine structures and NW-SE trending Dynarides structures determines a variability of the fault plane solutions, but the driving mechanism seems to be the northward push of the Adriatic plate below the Alps, consistent with the motion of Africa toward Eurasia. This mechanism can also explain the stress rotation observed in the western Alps, where the Adriatic plate is pushed to the west below the Alpine arc [see also Griinthal and Stromeyer, 1992; Miiller et al., 1992]. Eva et al [1997] have explained the seismicity of this region as due to backthrust faulting in the lowland regions and normal faulting in the most elevated area. Stress data north of the Alps reveal N-S compression [Miiller et at, 1992], but the discontinuous data distribution on the Italian side does not allow us to extrapolate the observed stress directions over the entire Alpine arc. 3.2. Apennines Active extensional tectonics is observed throughout the Apenninic belt (Figures 4c and 4d), in which the maximum principal stress is generally vertical and the stress regime is extensional, with NW trending fault planes. The largest normal faulting earthquakes (M between 6 and 7) occur in the most elevated regions of the Apennines, as indicated by the 1997 Umbria-Marche seismic sequence [Amato et al., 1998a; Ekstrom et al., 1998], the 1915 Fucino earthquake, the Val Comino earthquakes of 1984, the 1980 Irpinia events, and as far south as the 1998 earthquake at the Basilicata-Calabria border (latitude 40øN) (Figure 4d and Table 2). From the comparison of breakout, earthquake, and fault data the NE-SW extension seems to be continuous throughout the Apenninic belt. In southern Apennines the extension seems to be a general and widespread feature overprinting the previous compressional tectonics. The available stress indicators for the southern Apennines show a very uniform NE-SW,a hmin direction (Figure 4d), even i. the presence of a complex preexisting geology, including two different "plates" (the Apulian platform and the overlying tectonic stack). Where the normal faulting dominates, the Shmin trend is perpendicular to the thrust strike (Figures 4c and 4d), thus confirming the young age of extensional tectonics. This is possibly related to a recent stress reorientation, as proposed by previous studies based on different data [e.g., Patacca and Scandone, 1989; Pantosti and Valensise, 1990; Westaway, 1993; Amato and Montone, 1997]. However, despite the very consistent NE-SW orientation of Shmin, the stress regime off the belt (i.e., in the Adriatic foredeep and foreland) is not easily defined owing to the few focal mechanisms available. It seems that the normal faulting stress regime observed in the belt changes to a strike-slip regime in the foredeep and to a thrust regime in the Adriatic foreland, with a consistent direction of Shmin. It is importanto remark the absence of compressional stress perpendicular to the thrust front of the southern Apennines. A detailed discussion of the possible sources of tectonic stress in this region is given by Amato and Montone [1997]. On the contrary, there are several indications of active compression in the outer fronts of the northern Apenninic arc, including breakouts and seismicity. Also the analysis of background seismicity of this region, not included in the present compilation except for a few larger events, had led Frepoli and Amato [1997] to identify an outer narrow belt to the east characterized by thrust and strike-slip earthquakes and a broad area in the west where normal faulting earthquakes prevail. The stress inversion results obtained by Frepoli and Amato [ 1997] are consistent with this broad-scale differentiation. However, since these results are relative to two large regions within which we observe local rotations [see also Montone and Mariucci, 1999], they were affected by large misfit values and are not included in this map. An attempt of using data of background seismicity for the Italian region is in Frepoli and Amato [submitted to Tectonophysics, 1999]. In the backarc region of northern Apennines, where the paroxysmal phase of extension ended a few million years, we still observe predominantly normal faulting, in most cases parallel to the trend of the adjacent belt (Figure 5 and Plate 1, see also Montone et al., 1995]. Along the Tyrrhenian coast of central Italy we observe radial extension, particularly in proximity of large

25,608 MONTONE ET AL.: ACTIVE STRESS MAP OF ITALY Quaternary intrusions [Patacca and Scandone, 1989], although the low data density in these regions prevents a detailed identification of the stress regime (Figure 5). belt. This is similar to what we described for the northern Apennines (though rotated by 90 ø, as are the structural trends). More to the north, the Shmin directions rotate again to an almost The difference between the southern Apennines and the E-W to NE-SW trend. This stress direction is consistent with the northern Apenninic arc is quite evident in our map from the 90 ø rotation of the Shmin directions at latitude 42øN-43øN (see Plate 1). The persistence in time (from late Miocene to Present) of this northward motion of Africa toward Eurasia, although the two plates here do not really interact because of the presence of the Tyrrhenian basin north of Sicily. extension-compression pair in the northern Apennines has been interpreted by Mariucci et al. [1999] as the prolonged effect of the retreat of the Adriatic slab below the belt. This hypothesis is 4. Conclusion consistent with the presence of subcrustal seismicity (down to 90 km depth) described by Selvaggi and Amato [1992], who also pointed out that the subcrustal earthquakes in the Apennines occur only in the northern arc, north of latitude 42øN-43øN, in tight correspondence with the observed rotation of the stress directions. The occurrence of this subcrustal seismicity and its lateral extent match the presence of a deep lithospheric slab below the northern arc, as evidenced by seismic tomography [Amaro et al., 1993; Piromallo and Morelli, 1997; Amato et al., 1998b; Lucente et al., 1999]. A plausible mechanism to explain In this paper we have described an improvecl map of.the active stress in Italy, obtained from 109 borehole breakout data, 44 centroid moment tensor (CMT) solutions of earthquakes that occurred between 1976 and 1998, 34 polarity solutions, seven stress inversions from earthquake data, two averages of focal mechanisms' axes, and four fault slip data. A good agreement is found among stress data obtained with different methodologies, even if they are relative to different depth intervals and to different tectonic units. The map shows that some areas are the described correspondence between deep structures and stress dominated by compression in which the maximum principal at the surface is that of a still ongoing Adriatic slab sinking in the upper mantle and retreating, causing extension in the backarc and compression at the outer front. Differently from the northern arc, in the southern Apennines there is neither evidence of a continuous lithospheric slab from tomography nor of subcrustal seismicity. As seen before, this portion of the belt is characterized by widespread extension since -0.7 Ma [Patacca and Scandone, 1989; Pantosti and Valensise, 1990; Hippolyte et al., 1994; Amato and Montone, 1997]. This correspondence may suggest that here either the subducted lithosphere has detached in recent times, as proposed by Amato et al. [1993] or it has a different (less dense, possibly continentalstress is horizontal. This is the case of the eastern Alps, along the foredeep of the northern Apenninic arc, and in the southern Tyrrhenian-northern Sicily region. In all these regions the directions of Shrnin parallel the surface structural trends, suggesting that the present-day stress field has been persistent in the past few million years. Active compression in southern Italy (Sicily) probably reflects the northward motion of Africa relative to the Eurasian plate. North-south compression in the Alps is probably related to the relative motion of the Adriatic microplate, suggesting that this latter moves coherently with Africa. On the contrary, the stress field along the entire peninsula reflects different tectonic processesuch as the prolonged retreat of the like) composition. Adriatic-Ionian slab. Recently, it was proposed that the tectonic evolution of the entire Apenninic belt was mainly controlled by a N-S continuously propagating slab detachment at depth [Worte! and Spakman, 1992], which causes uplift in the regions above the detached lithosphere and subsidence where the slab is An abrupt-90 ø rotation of the stress directions clearly delineates the boundary between northern Apenninic and southern Apenninic arcs, indicating that their recent tectonic evolutions are different. A possible explanation is related to the t3ipe of lithosphere subducted between the two regions, which continuous. The stress pattern described in our map, with the could have induced slab detachment in the south. The observed abrupt 90 ø rotation which characterizes the two arcs, does not supporthis hypothesis. 3.3. Southern Italy (Calabria and Sicily) The region which lies on top of the active subduction of Ionian oceanic lithosphere is unfortunately characterized by low data density. Although this region (i.e., the Calabrian arc) is one of the most seismically active areas in Italy [Boschi et al., 1995], no recent earthquake data are available. The few breakout data are concentrated in the Ionian coastal region and show variable stress directions even at short distance. From geological data it was inferred that the sources of large historical earthquakes are normal faults trending mainly NE-SW [Bigi et al., 1990; stress directions strongly support the hypothesis that the Plio- Pleistocene outer thrusts of the southern Apennines are no longer active. Active extensional tectonics is observed throughout the Apenninic belt, where the maximum principal stress is generally vertical and the stress regime is extensional, with NW trending faults. The stress indicators for the southern Apennines show a very uniform NE-SW Shmin direction, within both the deep Apulian platform and the overlying tectonic stack, and notwithstanding the complex preexisting geology. In this region the active extension is superimposed on the preexisting structures (i.e., perpendicular to the thrust strike) and may be related to a recent stress reorientation, as proposed by previous studies. However, despite the consistent NE-SW orientation of Shmin, the Valensise and Pantosti, 1999], roughly parallel to the subduction stress regime in the Adriatic foredeep and foreland is not well hinge. The few focal mechanisms available seem to be consistent defined owing to the few focal mechanisms available. It seems with this trend (Plate 1), both in northern Calabria and in the Messina Straits, although no data are available in between. Stress directions in Sicily show a very consistent pattern. A NE-SW trend of Shmin, thus parallel to the main thrust front (see Figure 1), is evident in southeastern Sicily, which belongs to the Hyblean "African" foreland domain. A rotation to an almost orthogonal trend is detected in the adjacent foredeep, which could be due to the flexure of the "African" lithosphere beneath the that the extensional stress regime observed in the belt changes to a strike-slip regime in the foredeep and to a thrust regime in the Adriatic foreland. In Calabria and in Sicily the stress directions are more scattered, probably because of the interaction among different tectonic processes in close proximity, such as the subduction of the Ionian lithosphere beneath Calabria and possibly continental lithospheric subduction in the adjacent regions. In Sicily a remarkable stress rotation is revealed by our

MONTONE ET AL.: ACTIVE STRESS MAP OF ITALY 25,609 data, changing from active SE-NW compression in the foreland Valensise, Catalogo dei forti terremotin Italia dal 451 a.c. al 1980, region to a perpendicular trend in the foredeep, and -N-S 973 pp., Ist. Naz. di Geofis., Rome, 1995. Caccamo, D., G. Neri, A. Sarao, and M. Wyss, Estimates of stress compression in northern Sicily. directions by inversion of earthquake fault-plane solutions in Sicily, The first-order stress provinces outlined in the stress map Geophys. d. Int., 125, 857-868, 1996. appear to be in close relation with the history of oceanic and Carminati, E., M. J. R. Wortel, W. Spakman, and R. Sabadini, The role of continental subduction of the region. Modeling of these complex slab detachment processes in the opening of the western-central Mediterranean basins: Some geological and geophysical evidence, processes can now benefit from our improved compilation. Earth Planet. Sci. Lett., 160, 651-665, 1998. Cesaro, M., Plateau Ibleo: Campo di stress da studi di breakout, analisi e Acknowledgments. We thank M.T. Mariucci, A. Ciarlitti, A. Frepoli, modello interpretativo, nternal report Agenzia Italiana Petroll and M. Cesaro (Eni), who have contributed to the data analysis. We are (AGIP), San Donato Milanese, Italy, 1993. sincerely thankful to Eni, Enel that gave us the opportunity to analyze the Cinti, F. R., L. Cucci, D. Pantosti, G. D'Addezio, and M. Meghraoui, A breakout data. Enterprise Oil is acknowledged for permission to publish major seismogenic fault in a 'silent area': The Castrovillari fault the results of one deep well. Some of the breakout data in Sicily are taken (southern Apennines, Italy), Geophys. J. Inr, 130, 595-605, 1997. from World Stress Map Database. We are grateful to two anonymous Cocco, M., C. Chiarabba, M. Di Bona, G. Selvaggi, Lo Margheriti, A. reviewers for constructive criticism. We express our gratitude to Enzo Frepoli, F. P. Lucente, A. Basill, D. 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