1,721,140 research outputs found
Influence of topography on coseismic displacement induced by the Friuli 1976 and the Irpinia 1980 earthquakes (Italy) analyzed through a two-dimensional hybrid model
No full textWe investigate the perturbations introduced by the local topography on the coseismic displacement components induced by crustal seismic faults. The approach, proposed by Tinti and Armigliato [2002], is applicable to homogeneous two-dimensional domains with arbitrary topography of the free surface. It uses both analytical results, valid for homogeneous spaces and half-spaces bounded by a flat free surface (FFS), and a numerical code solving the equations of equilibrium of linear elasticity through a finite element (FE) scheme. A major characteristic of our "hybrid" method is that it allows for the correct representation of the displacement components in correspondence with the fault, which does not need to be explicitly introduced in the FE mesh. We show that, in the case of homogeneous domains, this makes our approach preferable to pure FE schemes. Our hybrid approach is here applied to two different disastrous events of the recent Italian seismic history, for which topography is expected to play an important role, namely the 6 May 1976 (MS = 6.5) Friuli and the 23 November 1980 (MS = 6.9) Irpinia-Basilicata earthquakes. For each event, we selected hypotheses for the parent fault proposed in the literature and compared the coseismic displacements computed through the widely used analytical models dealing with FFS and through our hybrid approach, accounting for the effect of realistic topographies. The most relevant effects are observed in the case of the Irpinia earthquake: depending on the profile chosen, the computed misfits can be as high as 19 and 13% for the horizontal and vertical displacements, respectively, with absolute differences up to 9 and 10 cm. Copyright 2003 by the American Geophysical Union
Self-induced deformation on the fault plane during an earthquake part I: Continuous normal displacements
No full textThe problem of the coseismic deformation induced by an earthquake on its own fault plane is investigated here. The analysis concentrates on the on-fault displacement field accompanying the occurrence of an earthquake in response to a prescribed uniform shear slip and it is carried out on the basis of the classical analytical model by OKADA (1992) for a rectangular fault buried in an elastic homogeneous half-space delimited by a planar free surface. The analysis is subdivided into two separate papers: the first dealing with normal and the second with tangential on-fault displacements. In this first paper, concerning the study of the normal displacement component, the contributions of the source and of the correction introduced by the free surface are investigated separately and their dependence on the fault characteristics is thoroughly discussed. Particular attention is also devoted to the effects of the normal displacement on the fault surface geometry. It will be shown that the main effect is that of deforming the fault itself, with deformation consisting chiefly in a rotation of the plane and in a bending of the fault edges. The rotation angle is negligibly small (on the order of 1-10 μrad) for a single earthquake, although repeated seismic events occuring on the same fault might result in rotations of several degrees over geological time scales
Contribution of tsunami data analysis to constrain the seismic source: The case of the 1693 Eastern Sicily earthquake
No full textThe January 11, 1693 eastern Sicily earthquake is comparable only with the December 28, 1908 Messina Straits event in the Italian seismic history as regards magnitude (ML ≅ 7), level of destruction and number of victims. The shock generated a strong tsunami, which hit the entire eastern coast of Sicily and particularly the town of Augusta. The problem of which fault was responsible for the earthquake is still open. Several hypotheses have been formulated in the literature on the basis of different kinds of geological, macroseismic and tectonic data, but a general agreement has not been reached yet. An important contribution to the discussion may come from the analysis of the tsunami data. In two previous papers (Piatanesi et al., 1996; Piatanesi and Tinti, 1998), the hydro-dynamical study of the tsunami based on finite-element (FE) numerical simulations was carried out taking into consideration mostly theoretical faults, i.e. faults selected for their significance in tsunami generation irrespective of the corresponding geological evidence. This paper has to be considered the continuation of the mentioned works. We have studied new sources proposed in the literature, consistent with macroseismic data and/or geological observations, and simulated the corresponding tsunamis. We also built a new FE grid introducing significant improvements in the coastline representation, and developed and applied a new algorithm in order to account for the effect of the sea-bottom topography on the tsunami initial condition. Some of the examined faults are located in the Scordia-Lentini (SL) graben region and intercept the coastline, others are placed in correspondence with the Hyblaean-Malta (HM) escarpment and run parallel or sub-parallel to the coast. The conclusion of our work is that none of these faults respects fully the available observations on tsunami, and that the faults exhibiting the best fit are those placed in the SL region. It is worth stressing that our results are important for the assessment of seismic and tsunami hazard/risk in eastern Sicily, keeping also in mind that the 1693 earthquake has been selected as the reference event for an earthquake-scenario study in Catania and in south-eastern Sicily, called 'Catania Project', funded by CNR/GNDT (Consiglio Nazionale delle Ricerche/Gruppo Nazionale Difesa dai Terremoti)
Numerical simulation of the landslide-induced tsunami of 1988 on Vulcano Island, Italy
No full textOn 20 April 1988 a landslide of approximately 200,000 m3 occurred on the northeastern flank of the volcano La Fossa on the island of Vulcano. The landslide fell into the sea, producing a small tsunami in the bay between Punte Nere and Punta Luccia that was observed locally in the neighbouring harbour called Porto Levante. The slide occurred during a period of unrest at the volcano that was monitored very accurately. The study of this event is composed of two parts, the simulation of the landslide and the simulation of the ensuing tsunami; the former is studied by means of a Lagrangian-type numerical model in which the landslide is seen as a multibody system, an ensemble of material-deforming blocks interacting together during their motion; the latter is simulated according to the Eulerian view by solving the shallow-water approximation to Navier-Stokes equations of fluid dynamics, with the incorporation of a forcing term depending on the slide motion. Technically, the slide evolution is computed first, and this result is then used to evaluate the excitation term of the hydraulic equations and to calculate the tsunami propagation. Computed wave fronts radiate both toward the open sea, with rapid amplitude decay, and along the shore, in the form of edge waves that lose energy slowly. Comparison between model outputs and observations can be carried out only in a qualitative way owing to the absence of tide-gauge records, and results are satisfactory
The use of scenarios to evaluate the tsunami impact in southern Italy
No full textThis paper outlines the main contributions to the definition and evaluation of tsunami hazard and risk resulting from studies undertaken in Italy in recent years and emphasises that adopting characteristic cases or scenarios is a very useful and advantageous technique. Three main cases are given as valuable examples, that is the 1627 Gargano tsunami, the 1693 eastern Sicily tsunami and the 1908 Messina Straits tsunami, since: (1) they characterise three distinct tsunamigenic regions; (2) they are instances of destructive events; and (3) they have been extensively studied in the last decade. The paper elucidates the state-of-the-art of the research on these events, clarifies the chief points of agreement and disagreement among scientists, and illustrates the main issues that are to be addressed by future research to provide reliable assessment of tsunami risk and to implement efficient countermeasures to defend the life of people, coastal structures and the coastal environment against the attacks of tsunamis. © 2003 Elsevier B.V. All rights reserved
Displacements and stresses induced by a point source across a plane interface separating two elastic semi‐infinite spaces: An analytical solution
No full textThe problem of computing the static deformations and stresses produced by a point source in a homogeneous infinite medium was solved by Volterra [1907] in a closed analytical form at the beginning of this century. The similar problem of computing fields generated by point sources in a homogeneous half-space bounded by a free surface was later studied by Steketee [1958a,b] and several others [see Okada, 1985, 1992], who focused on point as well as on rectangular fault sources of interest in seismology. Here the model taken into account consists of two elastic half-spaces characterized by different elastic properties (rigidity modulus μ and Poisson coefficient ν) and separated by a planar interface: assuming that a point source is active in one half-space, static deformations and stresses generated by the source in the whole space are computed. The similar problem of two half-spaces welded together was solved by Heaton and Heaton [1989], but they imposed the simplifying constraint that both materials are Poissonian (i.e., both have the same Poisson coefficient ν=0.25). The present approach, which is based on the Galerkin vector method, is general and applicable to an arbitrary point source. In this paper the computations have been carried out explicitly only for the special case of a dislocation source having the form of a strike-slip double couple. The solution is provided in a closed analytical form by means of expressions involving the source descriptors (position and intensity) as well as the elastic parameters of the heterogeneous medium. The solutions have been illustrated and discussed with special attention given to the dependence of the displacement and stress components on the elastic parameters of the model. One interesting finding concerns the limiting case when the rigidity modulus of the half-space not containing the point source is equalized to zero. Although the solution in this half-space no longer makes sense, the solution in the other reduces exactly to the one computed for a half-space with a free surface, that is, to the solutions computed by Steketee [1958a] and Okada [1985] following an alternative approach
Determination of bulk mismatch values in transmission electron microscopy cross-sections of heterostructures by convergent-beam electron diffraction
No full textThe relaxation which occurs along the thinning direction of transmission electron microscopy (TEM) cross-sections of heterostructures is still poorly known. This has so far prevented the accurate determination of the corresponding bulk mismatch values from convergent-beam electron diffraction (CBED) patterns. In this paper it is demonstrated that, by using elasticity theory, it is possible to deduce a simple relationship for (001) heterostructures which relates the lattice mismatches along the different crystallographic directions of the TEM specimen, as deduced from a single CBED pattern, to the bulk value. Both [001] and [110] orientations of the TEM cross-sections are considered. However, to obtain accurate results: the validity of the kinematical approach used to deduce mismatches from high-order Laue zone line patterns must be critically checked; it depends on the crystallographic projection, on the beam voltage and, in the case of Si1-xGex heterostructures: on the Ge concentration. It is found that the best results are obtained at 100 kV, working in the [013] projection, and for Ge concentrations up to 20 at.%. The method has been applied to both uniform and graded Si1-xGex/Si heterostructures, as well as to a B+-implanted, epitaxially regrown Si wafer. Good agreement is found between the CBED results and the bulk values obtained from double-crystal X-ray diffractometry
Single-force point-source static fields: an exact solution for two elastic half-spaces
No full textStatic fields produced by point sources in elastic media are important for several disciplines and are of special interest in earth sciences, where distributions of displacements, deformations and stresses determined by sources inside the Earth are often needed: consider, for example, permanent deformations generated by earthquakes, or surface displacements produced by magma intrusions in volcanic areas, and consider, moreover, that extended sources can very frequently be treated by superposing fields associated with proper elementary point sources, since linear theory almost always holds in geophysical applications. This paper examines the problem of computing static fields caused by point sources consisting of single forces in an elastic medium comprising two half-spaces that are characterized by different elastic parameters and are separated by a plane interface. The problem is posed in the framework of Volterra's (1907) theory and is solved analytically in both half-spaces for the two canonic cases of a force normal and parallel to the interface. It is stressed that the solutions deduced here can be used as the basis for the calculation of fields produced by an arbitrary point source of geophysical interest: indeed, forces pointing in arbitrary directions can be dealth with through linear combinations of the fundamental solutions, whereas single-couple or double-couple point sources can be handled by means of appropriate differentiations and subsequent linear combinations of them. It is emphasized that the solution is provided in a closed analytical form. The solution formulas are discussed, paying special attention to the dependence of the resulting static fields on the elastic parameters of the model, namely the Poisson coefficients and the rigidity moduli of the two half-spaces. One interesting result concerns the limiting case when the rigidity modulus of the half-space not containing the point-source is equated to zero: although the solution in this half-space no longer makes sense, the solution in the half-space containing the source is identical to the solution for the case of a homogeneous half-space delimited by a free surface, that is a surface on which no external traction is acting, as is assumed in several geophysical applications for the surface of the Earth
Self-induced deformation on the fault plane during an earthquake part II: Continuous tangential displacements
No full textThis paper regards the on-fault displacement field generated by an earthquake on its own fault plane, with special attention to the tangential displacement vector. It is the continuation and the completion of a previous paper (ARMIGLIATO et al., 2003) concerning the analysis of the normal on-fault displacement component. It is here recognised that in addition to the discontinuous shear displacement, which is the main contribution to the seismic dislocation on the fault, there is a tangential displacement contribution that is continuous across the fault. The continuous tangential displacement is exclusively produced by the presence of the free earth's surface. The tangential displacement vector has non-vanishing components in both directions, parallel as well as perpendicular to the imposed uniform shear slip, the first being predominant on the second. We perform a set of computations aimed at evaluating the dependence of the tangential displacement magnitude on the relevant fault parameters for basic cases of rectangular faults embedded in a homogeneous half-space
A 2-D hybrid technique to model the effect of topography on coseismic displacements. Application to the Umbria-Marche (central Italy) 1997 earthquake sequence
No full textWe face the problem of modelling the influence of the irregular topography of the Earth's crust on coseismic displacements. The modelling technique we propose is valid for a homogeneous elastic 2-D domain and is based on a two-step approach that uses (1) the traditional analytical results valid for elastic spaces and elastic half-spaces with a flat with free surface (FFS) and (2) a finite-element (FE) numerical code to compute the disturbance induced by the local topography. Attention is particularly focused on: (1) the comparison between the results obtained through our method and the analytical formulae valid for FFS geometries, which are used extensively today in both forward and inverse modelling even in the presence of irregular topography and (2) the advantages of our hybrid approach over pure FE models. After discussing the theoretical experiment of a Gaussian topography, we present an application to modelling the displacement fields induced by selected events of the earthquake sequence that hit the Umbria-Marche region in the central Apennines (Italy) in 1997, devoting major attention to the effect of the Apennines chain topography in the source region. In all cases the topography is found to play an important role and to introduce non-negligible corrections to the coseismic surface displacement components computed through the FFS approximation. The main effect is that topography not only affects the magnitude of the surface signal, but it may also lead to highfrequency disturbances that tend to grow with the shallowness of the fault
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