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State of stress in slabs as a function of large-scale plate kinematics: an approach with 2D and 3D numerical models
No full textControl of geometry and kinematics on the state of stress of subduction zones: an application to the Mediterranean region
Full text linkKnowing the stress field at subduction zones is fundamental as here is released most of the seismic energy in the Earth. In particular, most M W > 8.0 earthquakes originate at shallow depths along the frictional interface between subducting and overriding plates. This observation emphasizes the crucial role played by the geologic-time scale dynamics of convergent margins over the short-time scale seismogenic processes. Despite an obvious relevance to seismic hazard, knowing the driving forces generating the stress field at subduction zones is a long-standing problem. In this thesis, by means of 2D and 3D numerical viscoelastic models, I simulated the stress field in convergent plate margins to evaluate which properties control subduction dynamics. Models are built to evaluate the contribution of plate kinematics, geometry of the system, rheology and gravitational forces to the definition of the present-day stress field at different subduction zones. This has been achieved with the development of several sets of generic (i.e., not simulating specific subduction zones) 2D and 3D
models. The aim is to analyze the interaction between the subducted slabs and the geodynamic forces (e.g., slab pull, mantle flow, plate convergence) that stress the system, to reproduce the observed stress fields measured in different subduction zones worldwide for both the upper and lower plates at crustal depths and for intermediate and deep subducted lithosphere. The interaction between subducting slabs and the viscosity jump at upper-lower mantle transition has been also investigated. Although generic, model geometries are consistent with natural geometries observed in real subduction zones worldwide. Modelling results are compared with stress data available in the world stress map database for different convergent
margins. To define the stress field affecting the subducting plates, special attention must be paid to the choice of
the righteous initial parameters, since from them depend the delicate balance between the applied tectonic
forces and the geometric characteristics of the whole system. For this reason and to validate or reject the observations made for the general cases, the central Mediterranean subduction system was chosen to model a natural subduction zone.
Building and constraining a model requires the knowledge of the real system. Subduction zones are primarily described by their geometry, and today the slab interfaces in the Mediterranean are still uncertain. I defined them reviewing and integrating literature data from various disciplines, collecting geometries into a specifically designed database. Unlike similar databases already available in the literature, in the database that I contributed to build the subduction interfaces are fully-parametrized, i.e., characterized by geometric (strike, dip, depth), kinematics and dynamic (rake, slip rate, seismic coupling, maximum earthquake magnitude) parameters. The database so designed, and its on-line publication makes it a valuable tool for the geometric description of active subductions in the Mediterranean area and provide the basis to investigate their seismic hazard
State of stress in slabs as a function of large-scale plate kinematics
No full textThe state of stress of slabs subducting worldwide, as revealed by seismicity, is extremely variable both with depth and between different subduction zones. Although in principle, slab pull should enhance down-dip extension in the slab, the reconstructed stress fields for intermediate depths (between 100 and 300 km) range from downdip compression to downdip extension. Using 2-D viscoelastic plane strain models we investigate the dependency of the stress field of slabs on geometry (dip of the slab) and kinematics (velocity of convergence between upper and lower plates and their absolute velocity with respect to the underlying mantle) of subduction zones. We conclude that although the state of stress in slabs is also controlled by other processes, downdip compression in the subducting slab is enhanced by mantle flow opposing the direction of the dip of the slab, whereas downdip extension is favored by mantle flow in the same direction of the slab dip (i.e., sustaining it). These predictions are in agreement with available geophysical observations, although exceptions to this simple pattern are observed worldwide. In addition, if the slab is decoupled from the upper plate, convergence between upper and lower plates induces a downdip compressional component of stress within the slab, decreasing the magnitude of extension in models characterized by mantle flow sustaining the slab and increasing compression in models with mantle flow opposing subduction. However, these are second-order variations when compared to the control exerted by absolute plate kinematics and by the magnitude of slab pull. Sensitivity analysis of rheological parameters allows us to conclude that these results are generally consistent, although low values of viscosity of the lithospheric mantle render this prediction less stable
Normal fault earthquakes or graviquakes
Full text linkEarthquakes are dissipation of energy throughout elastic waves. Canonically is the elastic energy
accumulated during the interseismic period. However, in crustal extensional settings, gravity is
the main energy source for hangingwall fault collapsing. Gravitational potential is about 100 times
larger than the observed magnitude, far more than enough to explain the earthquake. Therefore,
normal faults have a different mechanism of energy accumulation and dissipation (graviquakes) with
respect to other tectonic settings (strike-slip and contractional), where elastic energy allows motion
even against gravity. The bigger the involved volume, the larger is their magnitude. The steeper the
normal fault, the larger is the vertical displacement and the larger is the seismic energy released.
Normal faults activate preferentially at about 60° but they can be shallower in low friction rocks. In
low static friction rocks, the fault may partly creep dissipating gravitational energy without releasing
great amount of seismic energy. The maximum volume involved by graviquakes is smaller than the
other tectonic settings, being the activated fault at most about three times the hypocentre depth,
explaining their higher b-value and the lower magnitude of the largest recorded events. Having
different phenomenology, graviquakes show peculiar precursor
The south-Tyrrhenian seismically-active compressional belt: preliminary results from numerical modeling
No full textTo the north of Sicily, Italy, the south-Tyrrhenian region includes the transition from the Vavilov
and Marsili oceanic backarc basins, to the north, to the inner Maghrebian-Calabrian thrust-fold belt,
to the south. Since about mid-Pleistocene time, the tectonic activity along the frontal thrusts of this
south-verging belt have come to a substantial end or pause and tectonic compression has resumed at
the back of the orogenic wedge in the south-Tyrrhenian region. This recently-resumed compression
is well revealed by an E-W-oriented compressional seismic belt. Several features, which are the
prerequisite to interpret the future tectonic evolution of this belt, remain, however, to be better
constrained. For instance, it is unclear whether the south-Tyrrhenian compression is presently
activating north-verging thrusts, which may represent the southward subduction onset for the
Tyrrhenian backarc basin, or it is simply reactivating south-verging thrusts at the rear of the
orogenic wedge. Based on previous tectonic studies of the south-Tyrrhenian region, we have made
several assumptions on the present tectonic architecture, and have run a series of numerical tests to
envisage possible future scenarios of this segment of the Africa-Eurasia boundary. We have
computed the present-day deformation in the southern Tyrrhenian area, integrating recent GPS
velocities, and global relative plate motions, with the contribution of the main tectonic features,
such as active faults and deformation zones. We have used finite element methods on a well-
adapted grid, built with appropriate dimension to avoid boundary effects. With a set of numerical
experiments, we have obtained preliminary results, being useful to characterise a complex setting in
the south-Tyrrhenian region, where different tectonic activities probably produce coexisting effects,
in the context of the Africa-Eurasia plate boundary
Local, regional, and plate scale sources for the stress field in the Adriatic and Periadriatic region
No full textThe stress field at a specific location is the sum of regional stress, controlled by plate-scale tectonic processes, with local sources. Here we evaluate and discuss the different sources (from geodynamic to local scale) and the controlling factors (including the Jurassic paleogeography of the Adriatic passive margin) of present-day stress in the Adriatic Sea and Periadriatic regions of Italy using two kinds of numerical models: 3D mantle scale viscoelastic models and 2.5D lithosphere scale thin-shell models.The subcrustal stress field of the slabs in the Dinarides-Hellenic and Apennines-Calabrian subduction zones are characterized by downdip extension and compression, respectively. This difference is explained by the velocity of the Adriatic slab with respect to the upper mantle in the hotspot reference frame. The slab in the Calabrian subduction zone is encroached and down-pushed, whereas the slab in the Hellenic subduction zone is sustained by the mantle flow. At the plate scale, the upper crustal stress field at the Apennines front is governed by the different nature of the lithosphere (probably oceanic in the Ionian Sea and continental elsewhere), which in turn determines first-order rotations of the stress axes. Second order rotations of the stress axes depend on the Jurassic paleogeography and consequent differential advancement of the thrust fronts, with recesses and salient occurring in correspondence with Jurassic structural highs and lows respectively. The eastward relative motion of the mantle with respect of the lithosphere enhances the onset of NW-SE oriented compression within the Adriatic plate, consistent with observations. Finally, 2.5D thin-shell models demonstrate that present-day stress does not simply follow the long wavelength pattern (500-1000 km) due to plate tectonics and mantle interaction, but is greatly affected by the presence of second and third order sources, like crustal structure, faults and topography. Most importantly, these factors give an observable effect in data. © 2012 Elsevier Ltd
Developing seismogenic source models based on geologic fault data in the Euro- Mediterranean area: SHARE mission accomplished?
No full textWe present our latest achievements in the making of a seismogenic source model for the Euro-Mediterranean area to be used in P SHA. Data incorporated into the model are stored in a database that is being made available to the public through a web-based GIS application. This effort is being driven by the EU P roject SHARE (http://www.share-eu.org/) with a partnership of 18 institutions, nine of which actively contribute geologic fault data. The aid and collaborative support of a large number of elicited experts was fundamental in gaining insight into active faulting at regional and local scales. In the process of collecting active fault data, we adopted different strategies in different regions of the Euro-Mediterranean area. This approach allowed us to account for the variety of geologic signatures and tectonic environments, and also to give the proper credit to each local scientific legacy. Homogeneity of data was accomplished by using common standards and definitions cross-checked with existing similar models from around the world. As of May 2010, the updated database includes over 400 records of fully parameterized seismogenic sources for a total of about 30 thousand kilometers of faults. These seismogenic sources cover the Euro-Mediterranean area in length and breadth, from Iberia to Greece and from central Europe to North Africa. Mapping of faults in Turkey is in progress with close collaboration with geologists involved in the companion project EMME (http://www.emme-gem.org/). Our collaborative effort is aimed at contributing to a worldwide model that will be hosted by project GEM (http://www.globalquakemodel.org/)
Investigating fault reactivation through mechanical and numerical modelling: an application to the Central- Northern Apennines of Italy
No full textInversion tectonics refers to regions that experience a switch in the stress regime. In these contexts, the reactivation of pre-existing discontinuities depends upon a wide number of structural, stratigraphic and mechanical factors. Among these, a fundamental aspect is represented by the orientation and attitude of pre-existing discontinuities with respect to the new stress field. Starting from some field example of positive and negative inversion tectonics from the Central-Northern Apennines fold-and-thrust belt (Italy), mechanical (slip tendency analysis) and numerical (using COULOMB 3.3 software) models have been developed with the aim to investigate how the orientation of fault planes influences their tendency to reactivation. Both field data and modelling results point to a strong influence exerted by the orientation and attitude of pre-existing discontinuities in inversion tectonic processes. During the positive inversion tectonic event, NNE-SSW/N-S high-angle faults, oblique to the shortening direction, were preferentially reactivated as thrusts (rather than NW-SE orthogonal planes) because they can take advantage of their orientation that allows a strike-slip component of movement. Also for the negative inversion tectonic phase, model results support our field observations highlighting, in this case, the major tendency to reactivation as normal faults of the NW-SE planes rather than of the oblique N-S discontinuities
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