Search

A new analytical methodology for computing synthetic seismograms in 3-D anelastic media is applied to the modelling of the local records of the 2003 Bam Mw 6.6 earthquake, Southeastern Iran. The method is based on the combination of the modal summation technique with the asymptotic ray theory. The 3-D models are determined by a set of vertically heterogeneous sections (1-D structures) that are juxtaposed on a regular grid. The distribution of these sections in the grid is done in such a way to satisfy the condition of applicability of the WKBJ approximation, that is, the lateral variation of all the elastic parameters has to be small with respect to the prevailing wavelength. The procedure, described very briefly, has been applied for the area of the destructive 2003 December 26 Bam earthquake Mw 6.6, with a maximum cut-off frequency of 6 Hz. Synthetic seismograms show fine agreement with recorded signals when comparing shapes, amplitudes and spectra and evidence the relevance of 3-D effects.

Recent advances in the physical knowledge of seismic waves generation and propagation processes, along with the improving computational tools, make it feasible the realistic modeling of the ground shaking caused by an earthquake, taking into due consideration the complexities of the source and of the propagation path. A neo- deterministic scenario based approach to seismic hazard assessment (NDSHA) has been developed that naturally supplies realistic time series of ground shaking, including reliable estimates of ground displacement readily applicable to seismic isolation techniques. The NDSHA procedure permits incorporating, as they become available, new geophysical and geological data, as well as the information from the different pattern recognition techniques developed for the space-time identification of strong earthquakes. All this leads to the natural definition of a set of scenarios of expected ground shaking at the bedrock. At the local scale, further investigations can be performed taking into account the local soil conditions, in order to compute the seismic input (realistic synthetic seismograms) for engineering analysis of relevant structures, such as historical and strategic buildings. The NDSHA approach has been already applied in several regions worldwide, including a number of local scale studies accounting for two-dimensional and three-dimensional lateral heterogeneities in inelastic media. A pilot application of the approach, including the detailed evaluation of the expected ground motion accounting for site effects and seismic engineering analysis, has been carried out at a site located in the Friuli Venezia Giulia Region (NE Italy). Further some applications worldwide of a new, highly efficient analytical methodology, developed for modeling the propagation of the seismic wavefield in three-dimensional inelastic media, are presented. This procedure, based on computer codes developed from a detailed knowledge of the seismic source process and the propagation of seismic waves in heterogeneous media, allows not only the detailed study of instrumental and macroseismic data but also the realistic estimate of the seismic hazard, in those areas for which scarce (or no) historical or instrumental information is available, and the relevant parametric analyses: different source and structural models can be taken into account to create a wide range of possible groundshaking scenarios from which to extract essential information, including uncertainty ranges, for decision making.

Lessons learnt from the destructive earthquakes occurred during the new millennium provide new opportunities to take action, revise, and improve the procedure for seismic hazard assessment (SHA). A single hazard map cannot meet the requirements from different end-users; the mapping of the expected earthquake ground motion that accounts for events’ recurrence may be suitable for insurances. When dealing with cultural heritage and critical structures (e.g., nuclear power plants), where it is necessary to consider extremely long time intervals, the standard hazard estimates are by far unsuitable, due to their basic heuristic limitations. While time-dependent SHA may be suitable to increase earthquake preparedness, by planning adequate mitigation actions, for critical structures (i.e., those for which the consequences of failure are intolerable) the maximum possible seismic input is relevant. Therefore the need for an appropriate estimate of the seismic hazard, aimed not only at the seismic classification of the national territory, but also at the capability of properly accounting for the local ampli␣cations of ground shaking, as well as for the fault properties, is a pressing concern for seismic engineers. A viable alternative to traditional SHA is represented by the use of the scenario earthquakes, characterized at least in terms of magnitude, distance, and faulting style, and by the treatment of complex source processes. The relevance of the realistic modeling, which permits the generalization of empirical observations by means of physically sound theoretical considerations, is evident, as it allows the optimization of the structural design with respect to the site of interest. The time information associated with the scenarios of ground motion, given by the intermediate-term middle-range earthquake predictions, can be useful to public authorities in assigning priorities for timely mitigation actions. Therefore, the approach we have developed naturally supplies realistic time series of ground motion useful to preserve urban settings, historical monuments, and relevant man-made structures.

A new methodology for computing synthetic seismograms, complete of the main direct, refracted, converted phases and surface waves, in three – dimensional anelastic lateral heterogeneous media is applied to the modeling of the local records of the 2003 Bam Mw 6.6 Earthquake, South-Eastern Iran. The method is based on the combination of the Modal Summation (MS) technique with the Asymptotic Ray Theory (ART). The three – dimensional models are determined by a set of vertically heterogeneous sections (1D structures) that are juxtaposed on a regular grid. The distribution of these sections in the grid is done in such a way to satisfy the condition of applicability of the WKBJ approximation, i.e. the lateral variation of all the elastic parameters has to be small with respect to the prevailing wavelength. The procedure, described very briefly, has been applied for the area of destructive 26 December 2003 Bam earthquake Mw= 6.6.

The systematic observations of seismic activity on the territory of the Republic of Moldova the influence of strong seismic events generated in Vrancea zone (south-eastern Carpathian Mts.). This fact is used for hazard and risk estimation, which involves show (probabilistic approach, at local and regional scales). The probabilistic approach uses attenuation law as an approximation of the seismic field, and does not consider all source and site effects, therefore, a detailed deterministic analysis is required to fix corrections and gaps for more realistic seismic risk assessment of the territory of the Republic of Moldova. With this purpose as a starting point, we perform a full parametric study for the city of Cahul, located in the southern part of the country, and its surrounding area. The analysis is done in the framework of 1D approximation, i.e. the analyzed region is modeled with a stack of isotropic, homogeneous and inelastic layers. The parametric test shows that the local structure has primary influence on local seismic impact, and geological, tectonic and geophysical data are the key factors, which distinguish the seismic field from site to site. At the regional scale, the seismic wave-field is different from site to site due to the directivity effects of the source. This aspect can be extended in order to perform a more realistic scenario for the entire territory of the Republic of Moldova.

The study presents and solves the 2-D elastodynamic model for seismic in-plane wave propagation in laterally inhomogeneous geological profiles imbedded in a vertically inhomogeneous half-space in which an earthquake source is buried. To this end, an efficient hybrid modal summation- boundary integral equation method (MSM-BIEM) is developed and applied. The MSM is used as a tool for simulating wave propagation from the source position to the multilayered laterally inhomogeneous geological profile where the BIEM is applied. The proposed model and the hybrid tool are used to investigate the phenomena of site effects. In fact, such a methodology has the potential to investigate the combined effects of different physical phenomena like surface topography, lateral inhomogeneity and the existence of water saturation in soils on the estimation of site effects. The model and hybrid computational tool developed are applied to contribute to the seismic risk analysis of the Bulgarian capital Sofia.

2007/2008 In this thesis a new methodology for computing synthetic seismograms, complete of the main direct, refracted, converted phases and surface waves, in three – dimensional anelastic lateral heterogeneous media is presented. It is based on the combination of the Modal Summation technique with the Asymptotic Ray Theory. The three – dimensional models are determined by a set of vertically heterogeneous sections (1D structures) that are juxtaposed on a regular grid. The distribution of these sections in the grid is done in such a way to satisfy the condition of applicability of the WKBJ – approximation, i.e. the lateral variation of all the elastic parameters has to be small with respect to the prevailing wavelength. In each knot of the grid a vertically heterogeneous section is located, hence, the values of the phase velocities, of the phase attenuation and of the group velocities are assigned once and for all. Inside the grid the source and the receiver are located, assigning their coordinates by means of a Cartesian reference system introduced in the grid itself. By this way a vertically heterogeneous structure, hence one-dimensional structure, is associated to the source and another to the receiver. The eigenfunctions of these two structures do contribute to the seismogram. The computational scheme is based, besides on the WKBJ - approximation for weak lateral heterogeneities, on the two point ray – tracing algorithm, by means of the bi - dimensional shooting method. It can be summarized as follows: at first the ray connecting two points, the source and the receiver, is computed solving the Cauchy problem for the system of ordinary differential equations governing the phenomenon of the evolution of the ray itself; the system is solved employing the numerical fourth – order Runge – Kutta method. Once the ray is determined, the attenuation is computed along it, solving, once again using the fourth – order Runge – Kutta method, the Cauchy problem for a system of ordinary differential equations that is made up of four equations: three equations for the ray and one equation governing the evolution of the attenuation along the ray itself. Finally, the geometrical spreading is computed considering two more rays that leave the source with an azimuth that is determined increasing and decreasing the azimuth of the characteristic curve of the ray – tracing system (the true ray) by a fixed quantity. The thesis is divided in two main parts, the first contains a theoretical treatment of the above mentioned arguments, so it opens with a brief summary about the generation of synthetic seismograms in one-dimensional structures by mean of the Modal Summation technique and goes on with the introduction of the WKBJ – approximation for treating the lateral heterogeneities. Then, there is the presentation of the numerical procedure used in this work. The second part is devoted to the validation of the new method, so the simulations executed to this aim are shown. It is very important to stress that the computational codes used in this work are still under development. They will be used for verifying and optimizing the results up to now obtained, both in terms of seismic sources and in terms of structural models, in region of the Scotia Arc. In questa tesi si presenta una nuova metodologia per il calcolo di sismogrammi sintetici completi delle principali fasi dirette, rifratte, convertite ed onde superficiali in mezzi tridimensionali anelastici lateralmente eterogenei, basata sulla Somma Modale (SM) combinata con la Teoria Asintotica dei Raggi (TAR). I modelli tridimensionali sono determinati da un insieme di sezioni verticalmente eterogenee (strutture 1D) che vengono affiancate su una griglia regolare. La distribuzione di dette sezioni nella griglia e’ tale da soddisfare la condizione di applicabilità della approssimazione WKBJ (acronimo dei nomi dei quattro elaboratori della metodologia: Wentzel, Kramers, Brillouin and Jeffreys), cioè la variazione laterale di tutti i parametri elastici deve essere piccola rispetto alle lunghezze d’onda prevalenti. In ogni nodo della griglia e’ collocata una sezione verticalmente eterogenea, sono, quindi, assegnati una volta per tutte i valori della velocità di fase, dell’attenuazione di fase e della velocità di gruppo. All’interno della griglia si collocano la sorgente ed il ricevitore, assegnando le loro coordinate attraverso un sistema cartesiano di riferimento introdotto nella griglia stessa. In questo modo si associa una struttura verticalmente eterogenea, quindi unidimensionale, alla sorgente ed una al ricevitore. Le autofunzioni di queste due strutture contribuiscono al sismogramma. Lo schema computazionale è basato, oltre che sull’approssimazione WKBJ per eterogeneità laterali deboli, sull’algoritmo per il ray-tracing tra due punti, mediante lo shooting-method bidimensionale. Esso può essere riassunto come segue: dapprima si calcola il raggio che unisce i due punti, la sorgente ed il ricevitore, risolvendo il problema di Cauchy per il sistema di equazioni differenziali alle derivate ordinarie che governa il fenomeno dell’evoluzione del raggio stesso; il sistema è risolto per via numerica mediante il metodo di Runge-Kutta del quarto ordine. Una volta che il raggio è determinato, si calcola lungo esso l’attenuazione, risolvendo, ancora una volta mediante il metodo di Runge-Kutta del quarto ordine, il problema di Cauchy per un sistema di equazioni differenziali alle derivate ordinarie che è costituito dal sistema che governa l’evoluzione del raggio più una quarta equazione che governa l’evoluzione dell’attenuazione lungo il raggio stesso. Infine, il geometrical spreading è calcolato considerando due ulteriori raggi che partono dalla sorgente con un azimuth 5 che è determinato aumentando e diminuendo l’azimuth della curva caratteristica del sistema (raggio vero) di un valore fissato. La tesi è divisa in due parti principali, la prima parte contiene una trattazione teorica degli argomenti precedentemente menzionati, si apre quindi con un breve riassunto sulla generazione di sismogrammi sintetici in strutture unidimensionali mediante la tecnica della Somma Modale e prosegue con l’introduzione dell’approssimazione WKBJ per la trattazione delle eterogeneità laterali. Si passa poi alla presentazione della procedura numerica utilizzata. La seconda parte è dedicata alla validazione del nuovo metodo, dunque sono presentate le simulazioni eseguite a questo scopo. E’ da sottolineare che i codici di calcolo utilizzati, attentamente testati e ripetutamente validati, sono in continuo sviluppo. Essi verranno utilizzati per la verifica e l’ottimizzazione dei risultati fin qui conseguiti, sia in termini di sorgenti sismiche che di modelli strutturali, nella regione dell’Arco di Scotia. XXI Ciclo 1977

Earthquake ground motion is a natural phenomenon associ- ated with a sudden release of energy due to a fault rupture. Strong-motion seismology deals with seismic waves radiated by large earthquakes. More precisely, it is concerned with measurement, interpretation and prediction of strong shaking generated by damaging earthquakes. Because of the engineering need for estimates of future strong-ground motion, the scope of strong-motion seismol- ogy encompasses seismic source theory and all other aspects of seismology that can provide insights helpful in making the best possible earthquake ground-shaking scenarios given the limited observational data.

Seismic hazard assessment can be performed following a neo-deterministic approach (NDSHA), which allows to give a realistic description of the seismic ground motion due to an earthquake of given distance and magnitude. The approach is based on modelling techniques that have been developed from a detailed knowledge of both the seismic source process and the propagation of seismic waves. This permits us to define a set of earthquake scenarios and to simulate the associated synthetic signals without having to wait for a strong event to occur. NDSHA can be applied at the regional scale, computing seismograms at the nodes of a grid with the desired spacing, or at the local scale, taking into account the source characteristics, the path and local geological and geotechnical conditions. Synthetic signals can be produced in a short time and at a very low cost/benefit ratio. They can be used as seismic input in subsequent engineering analyses aimed at the computation of the full non-linear seismic response of the structure or simply the earthquake damaging potential. Massive parametric tests, to explore the influence not only of deterministic source parameters and structural models but also of random properties of the same source model, enable realistic estimate of seismic hazard and their uncertainty. This is particular true in those areas for which scarce (or no) historical or instrumental information is available. Here we describe the implementation of the seismological codes and the results of some parametric tests performed using the EU-India Grid infrastructure.

The study solves 2D elastodynamic model for seismic in-plane wave propagation in laterally inhomogeneous geological profiles situated in a vertically inhomogeneous half-space with a seismic source. For the aim, an efficient hybrid Modal Summation-Boundary Integral Equation Method (MSM-BIEM) is applied. The MSM is used as a tool for the simulation of wave propagation from the source position to the local multilayered laterally inhomogeneous geological profile, where the BIEM is applied. The BIEM is based on the frequency-dependent fundamental solution of the governing equation in elastodynamics and the hybrid method works in the frequency domain. The inverse FFT solution is applied to obtain time histories. The hybrid tool is applied to several models for the investigation of local site effects due to: (a) the impedance contrasts between soil layers, (b) surface topography and lateral inhomogeneity, (c) the seismic source properties and (d) the existence of water saturation in soils. The application of the modeling tool is a contribution to the seismic risk analysis of Sofia city.