Pierdominici, Simona

Contemporary and concurrent extension and compression in Italy Paola Montone1, M. Teresa Mariucci1 and Simona Pierdominici2 1-Istituto Nazionale di Geofisica e Vulcanologia, Rome , Italy 2 – GFZ GeoForschungsZentrum, Potsdam, Germany We present the latest updating and the complete collection of data on the contemporary stress orientations in Italy. Data are relative to different stress indicators: borehole breakouts from deep drillings, crustal earthquake focal mechanisms and fault data. With respect to the previous compilation, performed in 2004, 206 new entries complete the definition of the horizontal stress orientation and tectonic regime in the most part of the territory, and bring new information mainly in Sicily and along the Apenninic belt. With an increase of 37% with respect to the previous compilation, now the global Italian dataset consists of 499 records with a reliable quality for stress maps. The total dataset includes the following active stress indicators: 56% borehole breakouts, 39% single earthquake focal mechanisms, and 5% represented by formal inversions of focal mechanisms, faults and overcoring data. A quality ranking between A and E is assigned to each stress data, with A being the highest quality and E the lowest. Only A-, B- and C-quality stress indicators are considered consistent for analyzing stress patterns. Depth interval of the entire dataset is between 0 to 40 km. The results in map are reported in terms of minimum horizontal stress (Shmin) because most of earthquakes present an extensional regime. Concerning breakouts, their orientations correspond to Shmin; since all the considered faults are normal faults, we assume the Shmin direction as perpendicular to the fault strike when no information on slip direction is available. The achieved results can be summarized in 3 main points: i) in some areas of Italy (Sicily, Friuli and Po Plain in the northern Italy), the alignment of horizontal stresses closely matches the ~N-S direction of ongoing crustal motions with respect to stable European plate. This result can be associated to the first-order stress field that drives the plate movement; ii) along the entire Apenninic belt – from north to south- a diffuse extensional stress regime is clearly showed by a large dataset indicating a NE-SW direction of extension, probably related to a second-order stress field; iii) the stress rotations observed in some areas (i.e., Po Plain minor arcs and Gela thrust front) reflect a complex interaction between first order stress field and local effects, revealing the importance of the inherited tectonic structure orientations. In particular in this work the simultaneous occurrence of different stress regimes is discussed. Finally, we underline that this kind of map is very useful to those many users that work on this topic and/or related ones such as, for instance, geophysical modeling, seismic hazard assessment, rock mechanics laboratory experiments, deep drillings but also on oil and gas well production and construction of nuclear waste deposits.

The deployment of a temporary seismic network in Southern Italy during 2001-2004 (the SAPTEX array, Southern APennine Tomography EXperiment) allowed us to relocate the hypocenters of Southern Apennines earthquakes with low uncertainty among the location parameters. The best array distribution of the SAPTEX network for the analysis of seismicity in the Lucanian Apennines and foredeep was reached in the first two years of recording. The SAPTEX data were merged with those of the Italian National Seismic Network (RSNC) operated by the Istituto Nazionale di Geofisica e Vulcanologia (INGV). For the hypocenters computation of events in the upper Agri Valley we also included P- and S- waves arrivals from the local Eni-Agip network. The seismicity for the Lucanian Apennines and foredeep in the analyzed period has magnitudes ranging from 2.0 to 4.1. A major finding is the identification of two different crustal domains: the westernmost characterizing the chain, mostly with shallow earthquakes (within about 20 km of depth), and the easternmost one belonging to the outer margin of the chain and to the foredeep, with deeper seismicity (mostly between 20-40 km of depth). Thirty fault-plane solutions were computed and used for stress inversion; most of them are related to earthquakes within the chain sector and indicate a generalized NE-SW extension. Moreover, the dense network allowed us to improve the location of events relative to two low magnitude sequences which occurred in the study period.

We present two examples of active stress field analysis conducted by integrating geological, geophysical and statistical data. The areas selected are Cittá di Castello-Sansepolcro in central-northern Apennines and the Val d'Agri-Melandro-Pergola in southern Apennines. Although low seismicity is recorded since the instrumental era (1980 to present), the areas have been repeatedly struck by moderate to large earthquakes in the historical time (since 461 B.C.). This suggesting that they may contain earthquake sources but are in a “quiescent” period. Our studies are focused on the characterization of the active stress field in these regions and on the definition of the spatio-temporal distribution of the earthquakes. Then, our data contribute to improve the knowledge of the “seismogenic behavior” of the areas and provide useful information for seismic hazard evaluation. In order to detect the pattern of the active stress field, we analyzed -the earthquakes recorded by national and local seismic networks in the period 2001-2002, with particular attention to the sequences; - ten deep wells (down to 5.5 max depth) for borehole breakouts analysis. At last, we have used two non-parametric statistical procedures (Tanner and Wong, 1984; Faenza et al., 2003) to characterize the spatio-temporal distribution of large historical earthquakes and to account tectonics-physics parameters that can influence the spatio-temporal variability. The results show that the areas are characterized by: i. tectonic structures favorably oriented with the active stress field oriented N44°+-18° in the southern Apennines and N50°+-17° in the central Apennines; ii. stress regime with a mainly extensional kinematics; iii. cluster distribution of seismicity; iv. the probability that an earthquake with the M≥5.5 will occur in the next 10 years is about 40%.

The occurrence of the Mw 6.3, April 6, 2009 earthquake has highlighted how critical is the knowledge of the location and of the characteristics of the active faults in a seismic region. This is true not only as a contribution to the seismic hazard assessment but also for the local planning of residential areas, plants and infrastructures. The 2009 earthquake occurred on the Paganica normal fault (PF hereinafter) and produced 3 km-long, maximum 0.1 m-high surface ruptures along its central section, as well as secondary slip along nearby tectonic structures and secondary effects such as liquefaction and landslides over a wide area.We will show the preliminary results from the analysis of a “special” site where an amazing “coseismic” trench, caved by the overpressure produced by the broken pipe of an aqueduct, provided the exposure of a 30-m wide fault zone of the PF, as well as from other cuts crossing the most recent scarp of the PF

A 350m deep boreholewas drilled in the Colli Albani volcanic district (Central Italy) in order to: understand the shallow crust structure beneath the volcanic complex; characterize the rock physical properties especially through in-situ measurements and, afterward, laboratory experiments; assess the local present-day stress field; install a broad-band seismometer at depth. The borehole is located adjacent to the western rim of the Tuscolano–Artemisio caldera, where several phenomena of unrest recently occurred. In 1989–90 a seismic swarm affected this area and a related uplift was recognized. In addition, high gas concentrations (mainly CO2 and H2S), in aquifers and soils, caused illnesses and casualties among inhabitants and animals in the past. We describe the investigations carried out at the drill site and the results achieved from data analysis.Wire-line drilling produced a complete stratigraphic record of the Quaternary volcanic units down to the Plio–Pleistocene sedimentary sequence and geophysical logs allowed a characterization of the rock physical properties. From a tectonic point of view, data provided by Dipmeter and Borehole Televiewer were used for investigations on the recent and present-day stress field and the results are compared to those available in the literature. In the volcanic units we recognized two main fracture systems, SW and NW dipping. Several faults intersecting the borehole showplaneswith oblique striae, indicating a prevalent strike–slip component of the movement. Finally, borehole breakout analysis defined an active stress field with a ∼E–Woriented minimum horizontal component. At the end of the drilling, a blow-out occurred, due to pressurized fluids trapped into the sandy unit drilled in the last fewmeters of the hole. Sampling these fluids gave an additional value to the borehole, providing information about the deep volcanic circulation and its possible connection to a deep-seated magma chamber. The main results show water with a Na–HCO3 chemistry and the highest salinity ever recognised in the area (Electrical Conductivity=10.12 mS/cm). Stable O and H isotopes reveal ameteoric origin ofwater and the absence of tritium points out a long residence time in the aquifer. Emitted gas is CO2-dominated, with N2 as second most important component. Helium isotopic composition of the gas allows us to estimate a magmatic component ranging in the interval 40–50%, one of the highest in the Colli Albani. Carbon isotopes of CO2 (−0.53‰ vs. PDB) suggest that it could derive partly froma magmatic source and partly by the thermal decarbonation of the carbonatic basement.

The southern Apennines are a NE-verging fold-and-thrust belt, which formed from late Oligocene to Pleistocene times in response to deformation processes induced by the convergence between the African and European plates. The post-collisional phase includes the early Pleistocene development of strike-slip faults, responsible of lateral variations and of the segmentation of the belt. The last tectonic phase that affected the belt is relative to an extensional regime characterized by NW-SE faults and is still acting. Present-day stress state can be assessed by different techniques, such as borehole breakouts, focal mechanism solutions, active faults, hydrofracturing, overcoring, crustal deformation and differential strain. Our goals are to compare the local versus regional active stress in Irpinia region and to identify active shear zones along a deep well using borehole breakout and downhole log data. The selected area is characterized by diffuse low magnitude seismicity, although in historical times it was repeatedly struck by moderate to large earthquakes. On 23rd November 1980 a strong earthquake (M=6.9) occurred in this area producing the first unequivocal historical surface faulting ever documented in Italy. The mainshock enucleated on a 38 km-long normal fault, 308° striking and 60°-70° northeast-dipping, named Irpinia fault. The surface trace of this fault is very close to the San Gregorio Magno 1 deep oil well which should cross it approximately within an interval depth of 1500m. To discriminate the presence of the Irpinia fault and other possible active shear zones and to define the present-day stress along San Gregorio Magno 1 well, we have analyzed in detail borehole breakout and downhole geophysical data. Our analysis of stress-induced wellbore breakouts shows a direction of minimum horizontal stress N18°±24°, quite consistent to the regional Shmin trend (N44°±20°). Although some breakout zones with a different trend from the regional one have been identified, these have been related to slip on nearby faults. Comparing the breakout rotations with the downhole logs we have defined two most probable intervals where the Irpinia fault crosses the borehole around the depth of 2300 and 3800m. We conclude by considering the more general implications of our data for this area considered one of the regions with the highest probability (25%) of occurrence of an earthquake (M>5.5) for the next 10 years.

This work underlines the importance of experimental data and integrates the hydrogeological mapping methods. The work has analyzed the lithological features of the outcropping rocks and their attitude to be crossed by the meteoric waters. Geologic-structural analysis has allowed to identify elements that can constitute the hydraulic barriers and the hydrogeological complexes that can contain the aquifers. A detailed analysis of base flow was carried out through hydrogeological survey directly performed in the river. The hydrogeological survey has allowed us: 1) to appraise the river's base flow; 2) to identify the punctual and linear springs; 3) to quantify the water resource on average drained; and 4) to determine the discharge regime of springs and rivers. The Conceptual Hydrogeological Model for each individual aquifer have been derived from geologic-structural analysis and hydrogeological studies. The Conceptual Hydrogeological Models allowed us to calculate the Mean Effective Infiltration (Ieff) of every aquifer. The values of Ieff are gathered in classes. All merged information has been used in the preparation of Hydrogeological Complexes and Natural Springs Map. The manifold hydrogeological information cannot be represented in one document only; so the Experimental Hydrogeological Mapping has been prepared as the overlap of different informative levels: Hydrogeological Complexes and Natural Springs Map (principal document), Surface Hydrology Map and Conceptual Hydrogeological Models of several recognized aquifers (complementary elements).

Active stress field in Italy is quite well-known but in detail it lacks a univocal interpretation in some areas of both orientations and magnitudes. We analyzed new deep boreholes and compared the results of stress analysis with the active faults crossed by or near the wells to estimate the influence of different structures on well data. In some cases, gently or abrupt changes in the stress directions have been observed along the well in the vicinity of a tectonic structure. In areas where tectonic structures are unknown, these stress changes observed in other boreholes can support other kind of evidence in identifying and characterizing active faults. We also present results on stress magnitudes inferred from leak-off tests in oil wells (more than two hundreds new data, kindly provided by ENI S.p.A). We calculated the values of the principal stress axis at depths ranging from about 200m to 5000m and analysed them considering the leak-off data uncertainties. Then we compared the results to the horizontal stress orientations from borehole breakout analysis and from other geophysical and geological stress indicators and interpreted them within the different tectonic framework. Finally, we analyzed the pictures of stress regime at different depths speculating about the reasons of regime changes that are observed in some areas. The active stress field depicted by the new analysis shows, at regional scale, a general agreement with strain data (geodesy and seismic anisotropy) although some interesting characteristics arise at local scale.

see Abstract Volume

We performed paleoseismological investigations at four sites across the normal Paganica fault (PF) (source of the 2009 Mw 6.3 L’Aquila earthquake), with the goal of reconstructing the rupture history and of contributing to the evaluation of the maximum event expected along the PF. We recognized five distinct surface faulting earthquakes (including the 2009) in the trenches. The age of the penultimate event is consistent with the 1461 earthquake; the third event back occurred around 1000 AD. The two oldest events have larger uncertainties and occurred in the interval 760 BC–670 AD and 2900–760 BC, respectively. The along‐strike vertical displacement for each paleoevent has a limited variability consistently with the fairly homogeneous slip observed in 2009 along the northern part of the rupture. Conversely, the throws change between distinct events and range between 0.15 m in 2009 (maximum estimate) and close to 0.4 (lower bound estimate) in earlier events. These paleorecords and the high fault escarpments imply that earthquakes larger than 2009 occurred on the PF, with implications for the level of hazard. Recurrence intervals also reflect a change with time, the average interval before ∼1000 AD is longer compared to that after this date. Two events occurred in the 2000– 4000 years preceding ∼1000 AD, while three events occurred since ∼1000 AD. The age uncertainties affecting the interpreted events prevent the evaluation of a unique value for interevent interval; the older events appear closely spaced in time or far apart depending on the upper or lower boundary of the age interval. We tentatively assign an average interevent time of ∼500 years for the three youngest events, whereas the time elapsed between the previous ones could be larger, in the order of 1000–2000 years. We calculate a late Pleistocene dip‐slip rate for the PF of 0.2–0.4 mm/yr, consistent with 0.25–0.5 mm/yr for the early Pleistocene. Using age and throw of individual events, we calculate a similar late Holocene average dip‐slip rate of ∼0.3–0.4 mm/yr. This suggests that the portion of the PF where the 2009 continuous surface faulting occurred has fairly a constant average slip release since late Pleistocene. Finally, we discuss different rupture scenarios and alternative models of occurrence compatible with our data and their variability.