Present-day stress field vs faults: some examples in Italy...Shmin trend congruent with the NNW-fault orientation. The active stress field shows a prevalent NE-SW extension, perpendicular
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a b c d Sv>SHmax>Shmin SHmax>Sv>Shmin SHmax>Shmin>Sv Gela Gela Thrust Front Foredeep Hyblean Foreland Ionian Sea Aegadian Isla nds Malta Escarpment Etna Palermo Trapani Catania IBL1 IBL3 IBL2 Siracusa Messina Agrigento Caltanissetta Malta Pantelleria Ustica A eo l i a n A r c Fron t Ca la b r i a n A r c K a b yl i a n T h r ust 37N 38N 13E 15E 0 50 km Shmin orientation breakout data fault data overcoring data A B C D E earthquake data Data quality Stress regime normal faulting thrust faulting strike-slip faulting stress unknown normal faulting thrust faulting strike-slip faulting Calabrian-Peloritani Unit Mio-Pleistocene syntectonic deposits Pleistocene deposits Plio-Quaternary volcanic rocks Iblean Unit Platform Unit Miocene Flysch Sicilide Unit Legend Total data N N Deep interval breakout orientations Total data breakout zones 0 1000 2000 3000 4000 5000 6000 Alburno - Cervati Apulia Lagonegro II m APULIA FORELAND BRADANIC FOREDEEP POLLINO RANGE Vallo di Diano Sele plain Salerno IRPINIA FAULT Matera POTENZA IONIAN SEA TYRRHENIAN SEA 1990 1980 1998 2002 1996 1991 1561 1826 1857 1708 1836 1273 1694 1910 1930 1361 1731 989 1732 1456 1962 1702 1851 1853 1980 375 62 79 1836 951 1560 Earthquake Tectonic regime: Normal fault Thrust fault normal faulting strike slip faulting C D Shmin direction Stress data Instrumental Seismicity Historical Data quality A-B Fault Breakout M<3.0 Regional Shmin 3.0<M<4.5 4.5<M<5.5 M>.5.5 Focal mechanisms M>5.5 SGM1 well N 0 20 km 16.0 41.0 40.5 40.0 39.5 15.0 Valnerina thrust Olevano-Antrodoco Sibillini thrust Sangro- Volturno thrust Laga Fault Mt. Bove Gran Sasso thrust Paganica Fault Colfiorito Norcia L’Aquila Maiella Sulmona Basin Chieti Adriatic Sea Fucino Basin Main normal fault (Quaternary/active) Main transpressive/thrust front (Neogene) Shmin from earthquakes Shmin from borehole breakouts 13°E 43°N 42°N 14°E 0 km 20 N 19970926 09:40 Mw6.0 19970926 00:33 Mw5.7 19971014 Mw5.6 20090409 Mw5.4 19790919 Mw5.8 20090406 Mw6.3 20090407 Mw5.5 19150113 Mw6.9 V1 C1 Monte Civitello 1 well Perugia San Donato 1 well Tiber Sansepolcro Città di Castello Umbertide Gubbio Assisi Norcia Valley Tiber Valley Sibillini Mounts Gualdo Tadino 1 2 3 4 5 6 10Km a) b) d) c) LEGEND Shmin from breakouts from earthquakes N 04/03/1998 04/29/1984 Focal mechanism solutions of main recent events Ms = 5.3 Mw = 5.1 Mw = 6.0 Mw = 5.7 09/26/1997 (09.40h) 09/26/1997 (00.33h) 10/14/1997 09/19/1979 Mw = 5.6 Ms = 5.5 1 2 3 4 5 6 13.0° 12.0° 43.0° 43.5° # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # Pistoia Rimini Cesena Forlì Ravenna Bologna Modena Reggio Emilia Parma Cremona Mantova Rovigo Ferrara ( ( ( ( ( ( ( ( ( ( ( ( ( ! ! ! ! ! ! ! ! ! ! ! ! ! Po River Alps Apennines Po Plain Tyrrhenian Sea Adriatic Sea ® 12°0'0"E 11°0'0"E 44°0'0"N 12°0'0"E 11°0'0"E 45°0'0"N 44°0'0"N 0 20 40 km SHmax from 2012 sequence and main past earthquakes SHmax from breakouts scaled by quality Shmin from breakouts data groups 0 100 Projection: Lambert Azimuthal Equal Area 50 km Friuli 44°N Tyrrhenian Sea Adriatic Sea Ionian Sea Sardinia Rome Apulia Po Plain Sicily Calabria 40°N 12°E 16°E 36°N 8°E Dataset (499 Shmin) Borehole breakouts 186 Shmin orientations: 12 A quality 78 B quality 96 C quality (2 from World Stress Map) Earthquake focal mechanisms 278 C quality Shmin orientations M ≥4.0, hypocentral depth ≤40 km: 247 CMT-like solutions (1976-2010) 31 polarity solutions (1908-1995) Formal inversions 23 Shmin orientations 22 B quality 1 C quality Faults 11 C quality data Overcoring 1 C quality data (from World Stress Map) Thrust fault Tectonics Normal fault Strike-slip fault Shmin orientation breakout data fault data overcoring data A B C earthquake data Data quality Stress regime normal faulting: Sv>SHmax>Shmin thrust faulting: SHmax>Shmin>Sv strike-slip faulting: SHmax>Sv>Shmin stress regime unknown 1 2 3 4 5 The italian present-day crustal stress map includes 499 data of A–C quality relative to borehole breakouts, earthquake focal mechanisms and active faults. The alignment of horizontal stresses, closely matches the ~N–S first order stress field orientation of ongoing relative crustal motions between Eurasia and Africa plates. The Apenninic belt shows a diffuse extensional stress regime indicating a ~NE–SW direction of extension, related to a second-order stress field. The horizontal stress rotations observed in peculiar areas (see Emilia Ro- magna area) reflect a complex interaction between first-order stress field and local effects, revealing the importance of the tectonic structure orientations. The existence of first-order stress field (plate-scale) controlled by plate boundary forces, and second-order stress field (regional) con- trolled by major intraplate stress sources, has been clearly demonstrated. In areas where high data density is present, a third order stress field (local) can also be recognized linked to the presence of minor features (i.e. active faults, local inclusions, detachment horizons or density contrasts). Such features may explain stress rotations that can be found not only between two adjacent wells but also in the same well along depth, as observed when fractures, active faults or mechanically weak zones are crossed by, or close to, a borehole. The local stress orientations in some cases overrule the first- and second-order stress pattern: major discontinuities within a rock mass disturb the stress field causing localized increases in differential stress and an associated change in the orientation of the stress trajectory. The free surfaces of an open fracture in a rock body deflect stress trajectories in the closer surrounding area, so the smallest principal stress approaches the free surface at right angle. The stress field rotates in proximity to an active fault due to small slip increments on the fault, possibly induced by the tectonic movement or due to an increase of pore pressure on the fault when drilling through it. Stress rotations identified near active (or young) faults can be used to make assumptions about the strength of the fault zone compared with the surrounding rock mass. In fact shear zones usually show physical properties different from the nearby undamaged rock: these features can be recorded by downhole logs and then compared to stress data. On the contrary, if any stress perturbation occurs close to a fault, this latter can be considered a sealed fault with characteristics similar to the host rock body. Cartoons of stress induced borehole deformation with the corresponding breakout zones and drilling induced tensile fractures (green) that are perpendicular and parallel to the SHmax orientation, respectively. Examples of laboratory created breakouts from vertical boreholes: a) in a Westerly granite cube (Sh=50MPa, Sv=60MPa, SH=190 MPa); b) in an anisotropic Berea sandstone hollow cylinder subject to an external confining pressure of 75 MPa; c) in a porous Aztec sandstone (Sh=20MPa, Sv=30MPa, SH=45MPa), and d) a heated clear epoxy subject to a triaxial state of stress. Illustration of the three pure reference stress states with SHmax, Shmin, and Sv with respect to σ1, σ2 and σ3, according to the Andersonian stress model (Anderson, 1905). Example of breakout reorientation close to a fault zone evidenced by the comparison between structural analysis and breakout analysis based on dipmeter data. Along this well, some stress perturbations have been identified associated to open fractures and to active faults which have slipped recently. Active faults at depth are often identified also by downhole logs analysis because they show physical properties different from the nearby undamaged rock. The rose plot (blue one) - relative to the total breakout data - is clearly influenced by the presence of several shear zones; in the deepest part - where the shear zones are absent- the breakout orient ations are consistent to the regional stress field (red plot). In southern Apennines the stress field is well defined with an active extension NE-SW oriented. The stress analysis along the San Gregorio Magno 1 well (SGM1), indicates a quite consistency with the neighbouring regions. Different breakout orientations are explained by the influence of nearby active faults on the local stress field. The results, in terms of Shmin orientations, show a very complex pattern characterized by inhomogeneous trends. The structural setting of the area consists of arc shaped thrusts and the results partly reflect this geometry. In the inset, two main Shmin orientations are highlighted: the wells located near to the thrust front show directions parallel to it indicating an active compression; the ~N-S Shmin direction - confined farther from the thrust front - indicates the area where the active extension is prevalent. Moreover, these Shmin orientations have been found at different depths: NS breakout trend in the shallow part of the structures; whereas the E-W breeakout trend is predominant in the deeper part (below 2000 m). In Sicily, the alignment of horizontal stresses closely matches the ~N–S direction of ongoing crustal motions with respect to stable Europe. This result can be associated to the first-order stress field that drives the plate movement. In particular, in Sicily the data delineate a more complete tectonic picture highlighting adjacent areas characterized by distinct stress regimes. The small-scale changes in the stress orientations, and in some cases also in tectonic regime, over distances of a few tens of kilometres indicate complex tectonic processes and interactions amongst different stress field orders. The breakout orientations in this area show a slight difference with respect to the NE–SW apenninic regional trend. V1 Shmin orientation is controlled by the presence of ~N–S structures. The C1 well - placed on the hanging wall of the Laga fault - shows a Shmin trend congruent with the NNW-fault orientation. The active stress field shows a prevalent NE-SW extension, perpendicular to the main tectonic structures, in agreement with the breakout results. San Donato 1 well shows a Shmin orientation consistent with both the regional and local trend. The Shmin orientation inferred from Monte Civitello 1, ~N-S, slightly differs from the regional trend suggesting the influence of active structures, such as E-W normal faults or strike slip faults perpendicular to the belt or N-S transfer faults. Anderson E. M., 1905 - Geological Society, 8, 387- 402. Boncio P. and V. Bracone, 2009 - Tectonophysics, 476, 180–194, doi:10.1016/j.tecto.2008.09.018. Mariucci M. T., A. Amato, R. Gambini, M. Giorgioni, and P. Montone, 2002 - Tectonics, 21(4), 1021, doi:10.1029/ 2001TC001338. Barton C.A. and M.D. Zoback, 1994 - J. Geophys. Res., 99, 9373–9390, doi:10.1029/93JB03359. Bell J.S., G. Caillet, and J. Adams, 1992 - Geol. Soc. Lond. Spec. Publ., 65, 211–220, doi:10.1144/GSL.SP.1992.065.01.16. Emergeo WG, 2013 - Natural Hazards and Earth System Sciences, 13, 935–947, doi:10.5194/nhess-13-935-2013. Mariucci M.T., P. Montone, and S. Pierdominici, 2008 - Ann. Geophys., 51(2–3), 433–442. Mariucci M.T., P. Montone, and S. Pierdominici, 2010 - Geophys. J. Int., 182(2), 1096–1102, doi:10.1111/j.1365-246X.2010.04679.x. Montone P., and M.T. Mariucci, 1999 - Geodynamics, 28, 251–265, doi:10.1016/S0264- 3707(98)00041-6. Montone P., M.T. Mariucci and S. Pierdominici, 2012 - Geophys. J. Int., 189, 705–716, doi: 10.1111/j.1365-246X.2012.05391.x. Pierdominici S., M.T. Mariucci, P. Montone, and M. Cesaro, 2005- Ann. Geophys., 48(6), 867–881. Pierdominici S., M.T. Mariucci, and P. Montone, 2011- J. Geodyn., 52(3–4), 279–289, doi:10.1016/j.jog.2011.02.006. Schmitt D. R., C.A. Currie, and L. Zhang 2012 - Tectonophysics, 580, 1–26, doi:10.1016/j.tecto.2012.08.029. The italian present-day stress map 1 LOMBARDY 2 EMILIA-ROMAGNA 3 UMBRIA 4 ABRUZZO 5 SOUTHERN APENNINES 6 SICILY INTRODUCTION Stress rotations along the Padano-Adriatic belt Boncio and Bracone, 2008 Along the Ferrara- Romagna Arc, the SHmax is clearly N-S oriented following the first-order stress field. The local SHmax rotations are linked to the curvature of active thrust fronts structures deflecting the regional stress. Schmitt et al., 2012 Schmitt et al., 2012 Montone et al., 2012 Pierdominici et al., 2005 Montone and Mariucci, 1999; Emergeo WG, 2013 Mariucci et al., 2008 Mariucci et al., 2010 Montone et al., 2012 Mariucci et al., 2002 Pierdominici et al., 2011 References San Gregorio Magno Monte Foi Monte Foi well SGM1 well Pierdominici et al., 2011 40 th Workshop of the International School of Geophysics Properties and Processes of Crustal Fault Zones Erice, Sicily (IT), 18 - 24 May 2013 Present-day stress field vs faults: some examples in Italy Maria Teresa Mariucci 1 , Paola Montone 1 and Simona Pierdominici 2 1 Istituto Nazionale di Geofisica e Vulcanologia, Sezione Sismologia e Tettonofisica, Roma, Italy - 2 Helmholtz-Zentrum Potsdam, Deutsches GeoForschungsZentrum GFZ, Potsdam, Germany
