ALBERT

Description: The anatomy of the Southern Apennines is a matter of compelling debate. Both thick- and thin-skinned tectonic models are proposed due to paucity of deep seismic constraints. We yield new and firmly constrained information on the velocity structure of the mountain range by using an innovative, nonlinear tomographic technique applied to refraction/wide-angle reflection data. Remarkably, the obtained velocity/interface model shows a very high velocity region, with values exceeding 7 km/s, at 8–9 km depth. Such large velocities cannot be related to sedimentary rocks, and imply the presence of shallow crystalline rocks, i.e., the basement involvement in the thrust belt. The integration of our results with other geophysical data (local earthquake tomography, Bouguer and magnetic anomalies) confirms this interpretation that strongly supports thick-skinned tectonics and has significant implications for the seismotectonics and hydrocarbon potential of the Southern Apennines. Our inversion approach can be effective in other orogenic wedges, when seismic reflection profiling fails to uniquely resolve deep complex structures.

Description: Published

Description: 941-944

Description: reserved

Description: An integrated multi-scale seismic imaging flow including first-arrival traveltime tomography and waveform inversion is applied to dense onshore wide-aperture seimsic data recorded in a complex geological setting (southern Apennines thrust belt, Italy)

Description: Published

Description: 625-651

Description: open

Description: In this paper we investigate the upper crustal structure of the Irpinia region, Southern Apennines thrust-belt, Italy, through analysis and joint interpretation of gravity data, seismic reflection lines and subsurface information from many deep wells. The investigated region includes the epicentral area of the 1980 (Ms=6.9) Irpinia earthquake and is one of the Italian regions with the highest seismic hazard. The upper crustal structure is imaged by modeling a series of 15 SW-trending gravity profiles, about 5 km spaced, plentifully constrained by seismic reflection lines and wells, thus reducing the inherent ambiguity of the gravity modeling. Despite of the complexity of the modeled Bouguer anomalies, the application of a calibrating procedure to constrain the range of variability of the density values, as well as the use of geometric constraints, result in a good level of stability in the final density cross-sections, which appear in fact coherent both in the density values and in the geometrical features. The inferred model shows important lateral density variations, which can be mostly related to NW-trending geologic structures. High density bodies delineate carbonate platform thrust-sheets and broad antiforms involving Mesozoic basinal rocks, while low-density shallow bodies are associated with Pliocene basins. In addition, important density (i.e. lithological) variations are evident along the strike of the range, the most relevant being an abrupt deepening of the Apulia Carbonate Platform in the southeastern part of the investigated region. In the epicentral region of the 1980 event, we find that the geometry of the high-density, high-velocity carbonates of the Apulia Platform appears correlated with the distribution of the aftershocks and with the P-wave velocity anomaly pattern as inferred from a previous local earthquake tomography. The structural highs of the Apulia Platform correspond to high-velocity regions, where aftershocks and coseismic slip of the mainshock concentrated. This correlation suggests that the Apulia Carbonate Platform geometry played an important role in the rupture propagation and in the aftershock distribution

Description: Published

Description: 139-169

Description: reserved

Description: This paper presents an interpretation of crustal seismic refraction data from the northern sector of the Southern Apennines thrust belt, a region that in historical times experienced large destructive earthquakes. The data were acquired in 1992 along a seismic line 75 km long and parallel to the Apenninic chain, in order to determine a detailed 2-D P-wave velocity model of the upper crust in an area that had not been deeply investigated by geophysical methods previously. We have used a 2-D ray tracing technique based on asymptotic ray theory to model travel times of first and reflected P-wave arrivals. Synthetic seismograms have been produced by finite difference simulations in order to check the reliability of the velocity model inferred by ray-tracing modelling. The interpretation of the velocity model is constrained by stratigraphic and sonic velocity logs from wells for oil exploration located close to the seismic line. Gravity data modelling allows to check the velocity model and to extend the structural interpretation in 3-D. In the shallow crust, up to a depth of 3–4 km, strong lateral variations of the modelled velocities are produced by the overlapping of thrust sheets formed by: (1) Cenozoic flyschoid cover and basinal successions that underlie the seismic profile with P-wave velocities in the 2.8–4.1 km/s range and thicknesses varying between 0.5 and 4.5 km; (2) Mesozoic basinal sequences with a velocity of 4.8 km/s and a depth of 1.5–2.1 km in the northern part of the profile; (3) Mesozoic limestones of the Western Carbonate Platform with a velocity of 6.0 km/s and a depth of 0.1–0.8 km in the southern part of the profile. At a greater depth, the model becomes more homogeneous. A continuous seismic interface 3.0–4.5 km deep with a velocity of 6.0 km/s is interpreted as the top of the Meso-Cenozoic Carbonate Multilayer of the Apulia Platform, characterized by an increase in seismic velocity from 6.2 to 6.6 km/s at depths of 6–7 km. A lower P-wave velocity (about 5.0 km/s) is hypothesized at depths ranging between 9.5 and 11 km. As inferred by commercial seismic lines and data from two deep wells located in the Apulia foreland and Bradano foredeep, this low-velocity layer can be related to Permo-Triassic clastic deposits drilled at the bottom of the Apulia Platform. Seismic data do not allow us to identify possible deeper seismic interfaces that could correspond to the top of the Paleozoic crystalline basement; this is probably due to the low-velocity layer at the bottom of the Carbonate Multilayer that reflects and attenuates a great part of the seismic energy. The joint interpretation of seismic refraction and well data, in accordance with gravity data, provides the first detailed P-wave velocity model of the upper crust of the northern sector of the Southern Apennines, which differs considerably from previous 1-D velocity models used to study the seismicity of the region, and reveals new information about the structure of the thrust belt

Description: Published

Description: 273-297

Description: reserved

Description: Local earthquakes (passive seismic) tomography (LET) is a well established tool for the imaging of the sub-surface structure. Alternative to active seismics, the main advantages of using natural sources are the better sounding in deeper portions of the upper crust, the relatively low cost, and the direct availability of S-waves. The main drawback is the achievable model resolution, which is limited by the density of the seismic network and the distribution of elastic sources, rather than the elastic wave frequency. Recently, 4D variations (in space and time) of velocity anomalies have been recognized in active volcanoes (Patanè et al., 2006) and normal faulting systems and ascribed to the medium response to transient geological processes, like dyke intrusions or fluid pressure increase on fault planes. In this paper we show how LET contributes to the imaging of the upper crust in a very attractive region like the Val d’Agri in southern Italy, which hosts both significant oil fields and seismogenic structures. We show that LET allows to improve the definition of the crust structure, at depths larger than those sampled by conventional seismic profiles, and detect the space-time dependency of elastic properties in response to local variations of fluid pressure

Description: Published

Description: Rome

Description: 1.1. TTC - Monitoraggio sismico del territorio nazionale

Description: open

Description: We apply a two-step seismic imaging flow by combined first-arrival traveltime and frequency-domain waveform tomographies to dense wide aperture data collected in the Val d’Agri basin (southern Italy). A large wavelength Vp model determined by first-arrival traveltime tomography is used as a starting model for waveform tomography. The multiscale waveform tomography consisting of successive inversion of increasing frequencies allows to progressively reconstruct the short wavelengths of the velocity model, providing valuable information on the Quaternary basin and on range-bounding normal-faulting systems.

Description: Published

Description: Rome

Description: 3.3. Geodinamica e struttura dell'interno della Terra

Description: open

Description: Shallow imaging of fault zones is a challenging task. We investigate if this issue can be successfully addressed by combining reflection seismics with non-standard multifold wide-aperture profiling, a strategy pertaining to crustal scale exploration so far. Near-offset reflection and multifold wide-aperture data are simultaneously recorded along two profiles in a small basin crossed by an active fault in Southern Italy. Imaging consists of multiscale seismic tomography complemented by CDP processing of near-offset reflections and of data spanning a large offset range. By combining smooth Vp and reflectivity images we pinpoint the fault location and yield a valuable picture of the basin. Performance of each exploration technique strongly varies in the two surveys depending on the local geologic conditions. Thus, the combined exploration strategy not only yields complementary images for enhanced interpretation, but also allows extending the range of applicability of shallow seismics for fault detection, reducing the risk of unsuccessful investigations.

Description: Published

Description: L20310

Description: 3.3. Geodinamica e struttura dell'interno della Terra

Description: JCR Journal

Description: reserved

Description: An edited version of this paper was published by AGU. Copyright (2010) American Geophysical Union.

Description: Pre-Stack Depth Migration (PSDM) is an effective tool to improve seismic reflection imaging of laterally heterogeneous media. However, migration performance strongly depends on the accuracy of the velocity model, therefore, PSDM is often ineffective for ultrashallow imaging (100 m and less) of complex structures. In this study, we applied non-linear multiscale refraction tomography and PSDM on dense wide-aperture data to image the causative fault of the 1980, M6.9, Irpinia normal faulting earthquake in a very complex geologic environment. Dense wide-aperture profiling allowed us to build accurate velocity models by multi-scale non linear refraction tomography and to record wide-angle reflections from steep reflectors. PSDM provided superior imaging with respect to conventional post-stack depth migration, and a better definition of fault geometry and apparent cumulative displacement. The estimated throw of Irpinia fault is only 29-38 m. This value, combined with the vertical slip rate estimated by nearby trenches, suggests a young age (97-127 kyr) of fault inception. Our outcomes are in agreement with paleoseismic data and indicate that this imaging strategy can be very effective for near-surface fault detection and characterization.

Description: Grant L.R. n. 5/2002 - Annuity 2003 - to Pier Paolo Bruno

Description: Published

Description: L19302

Description: 3.2. Tettonica attiva

Description: JCR Journal

Description: reserved

Description: An application of full-waveform tomography to dense onshore wide-aperture seismic data recorded in a complex geological setting (thrust belt) is presented. The waveform modelling and tomography are implemented in the frequency domain. The modelling part is solved with a finite-difference method applied to the visco-acoustic wave equation. The inversion is based on a local gradient method. Only the P-wave velocity is involved in the inversion. The inversion is applied iteratively to discrete frequency components by proceeding from low to high frequencies. This defines a multiscale imaging in the sense that high wavenumbers are progressively incorporated in images. The linearized waveform tomography requires an accurate starting velocity model that has been developed by first-arrival traveltime tomography. After specific pre-processing of the data, 16 frequency components ranging between 5.4 and 20 Hz were inverted. Ten iterations were computed per frequency component leading to 160 tomographic models. The waveform tomography has successfully imaged southwest dipping structures previously identified from other geophysical data as being associated with high-resistivity bodies. The relevance of the tomographic images is locally demonstrated by comparison of a velocity–depth function extracted from the waveform tomography models with a coincident vertical seismic profiling (VSP) log available on the profile. Moreover, comparison between observed and synthetic seismograms computed in the (starting) traveltime and waveform tomography models demonstrates unambiguously that the waveform tomography successfully predicts for wide-angle reflections from southwest-dipping geological structures. This study demonstrates that the combination of first-arrival traveltime and frequency domain full-waveform tomographies applied to dense wide-aperture seismic data is a promising approach to quantitative imaging of complex geological structures. Indeed, wide-aperture acquisition geometries offer the opportunity to develop an accurate background velocity model for the subsequent waveform tomography. This is critical, because the building of the macromodel remains an open question when only near-vertical reflection data are considered.

Description: Published

Description: 1032-1056

Description: open

Description: A dense wide-angle data set is used to test a two-step procedure for the separate inversion of first-arrival and reflection traveltimes. Data were collected in a complex thrust belt environment (southern Italy) along a 14 km line, with closely spaced sources (60 m) and receivers (90 m). A fully non-linear tomographic technique, that is specially designed to image complex structures, is applied to over 6400 first-arrival traveltimes in order to determine a detailed velocity model. A bi-cubic spline velocity model parameterization is used. The inversion strategy follows a multiscale approach and employs a non-linear velocity optimization scheme. The tomographic velocity model is adopted as the background reference medium for a subsequent interface inversion, which is aimed at imaging a target upper-crust reflector. The interface inversion method is also based on a multiscale approach and uses a non-linear technique for model parameters (interface position nodes) optimization. It is applied to over 1600 reflection traveltimes of a target event picked both in the near- and in the wide-angle offset range. The retrieved interface is well resolved in central part of the model, where ray coverage mainly includes clear post-critical reflections and the background velocity model is accurate in depth thanks to large offset deep turning rays. The determined velocity and interface models are consistent with VSP data and correlate well with the geometry of known geologic structures. This study shows that the used inversion approach is efficient for target-orientated investigations in complex geologic environments

Description: Published

Description: 264-278

Description: reserved