1,720,982 research outputs found
Sorgenti sismiche: rappresentazione matematica ad applicazione al calcolo degli spostamenti
No full textSe si vuole trattare fisicamente il fenomeno del terremoto `e necessario prima di tutto conoscere
e poter rappresentare la sorgente dell’evento stesso e risalire in questo modo alla soluzione
analitica del problema fisico.
Una sorgente sismica pu`o essere identificata con una frattura nel mezzo (una faglia o parte
di essa) che sottoposta ad una sollecitazione di sforzo disloca, ovvero accade che i due lembi
della faglia si muovono l’uno rispetto all’altro. Questo tipo di sorgenti `e detta di taglio perch`e
la dislocazione `e provocata da sforzi di taglio. Se il movimento relativo dei due lembi di faglia
`e parallelo rispetto alla linea di rottura allora la faglia `e detta faglia trascorrente; se invece il
movimento `e parallelo alla profondit`a della faglia questa `e detta faglia a rigetto verticale. Una
dislocazione provoca nel mezzo circostante un campo degli spostamenti che pu`o essere studiato
nel contesto della teoria dell’elasticit`a, come vedremo nel capitolo (2).
In questa breve trattazione vogliamo rappresentare la sorgente sismica fisicamente e analiticamente
e trattare la teoria principale che riguarda le sorgenti dei terremoti (capitolo (2),
paragrafo (2.1), e capitoli (3) e (4)). Quindi vogliamo poter ricavare la descrizione esplicita
dei campi di spostamento, di deformazione e di sforzo che essa genera nello spazio circostante
(sezioni (2) e (2.5)). Nel capitolo (6) le conoscenze acquisite verranno tradotte in un codice di
simulazione del comportamento di una particolare sorgente sismica. Questo permetter`a di visualizzare,
attraverso delle mappe, il comportamento del mezzo circostante una faglia che disloca
in termini di spostamento, deformazione e sforzo. La comprensione di come reagisce il mezzo
circostante la sorgente sismica all’avvenire del terremoto `e importante per capire come diverse
faglie interagiscono fra loro, favorendo o inibendo vicendevolmente la propria rottura. E’ comunemente
noto nella letteratura specialistica che un terremoto generalmente ne favorisce altri
intorno a s´e (i suoi aftershocks) ed `e questo il cosiddetto fenomeno di triggering; negli ultimi
anni si fa pi`u attenzione anche al fenomeno opposto, ovvero al fatto che un terremoto ha anche
la capacit`a di inibire per un periodo di tempo altri fenomeni in una certa zona (scaricandone lo
sforzo accumulato). Quest’area si verr`a a trovare cos`ı in un periodo di cosiddetta quiescenza.
Capire quindi come reagisce il mezzo ad una dislocazione `e importante se si vogliono sviluppare
modelli di previsione a breve e lungo termine.
La struttura della trattazione `e pensata per dare al lettore inizialmente un approccio pi`u
concettuale di alcuni aspetti del problema proposto (in particolare della rappresentazione delle
sorgenti), pur portando avanti la sua soluzione analitica; solo in un secondo momento vengono
ripresi ed approfonditi, sia concettualmente che quantitativamente, alcuni degli argomenti proposti
nella parte iniziale. Nella prima parte (capitolo (2)) ci riferiamo in particolare al pi`u
semplice problema statico per un mezzo isotropo; in un secondo momento proponiamo alcuni
teoremi fondamentali in una veste pi`u generale, includendo anche la variabile temporale e
riferendoci ad un mezzo anisotropo (capitolo (4)). Vogliamo trattare, inoltre, le relazioni che
intercorrono tra i principali parametri di sorgente e l’energia sismica (capitolo (5)). Infine,
come gi`a accennato sopra, ci occupiamo di una specifica sorgente estesa di forma rettangolare
di cui calcoliamo e rappresentiamo i relativi tre campi di spostamento, sforzo e deformazione
su un piano, introducendola in un programma di simulazione (capitolo(6)).Submittedope
Combining stress transfer and source directivity: the case of the 2012 Emilia seismic sequence
No full textThe Emilia seismic sequence (Northern Italy) started on May 2012 and caused 17 casualties, severe damage to dwellings and forced the closure of several factories. The total number of events recorded in one month was about 2100, with local magnitude ranging between 1.0 and 5.9.
We investigate potential mechanisms (static and dynamic triggering) that may describe the evolution of the sequence. We consider rupture directivity in the dynamic strain field and observe that, for each main earthquake, its aftershocks and the subsequent large event occurred in an area characterized by higher dynamic strains and corresponding to the dominant rupture direction. We find that static stress redistribution alone is not capable of explaining the locations of subsequent events. We conclude that dynamic triggering played a significant role in driving the sequence. This triggering was also associated with a variation in permeability and a pore pressure increase in an area characterized by a massive presence of fluids
A rate-state model for aftershocks triggered by dislocation on a rectangular fault: a review and new insights
No full textWe compute the static displacement, stress, strain and the Coulomb failure stress produced in
an elastic medium by a finite size rectangular fault after its dislocation with uniform stress drop
but a non uniform dislocation on the source. The time-dependent rate of triggered earthquakes
is estimated by a rate-state model applied to a uniformly distributed population of faults whose
equilibrium is perturbated by a stress change caused only by the first dislocation. The rate of
triggered events in our simulations is exponentially proportional to the shear stress change, but
the time at which the maximum rate begins to decrease is variable from fractions of hour for
positive stress changes of the order of some MPa, up to more than a year for smaller stress
changes. As a consequence, the final number of triggered events is proportional to the shear
stress change. The model predicts that the total number of events triggered on a plane containing
the fault is proportional to the 2/3 power of the seismic moment. Indeed, the total
number of aftershocks produced on the fault plane scales in magnitude, M, as 10M. Including
the negative contribution of the stress drop inside the source, we observe that the number of
events inhibited on the fault is, at long term, nearly identical to the number of those induced
outside, representing a sort of conservative natural rule. Considering its behavior in time, our
model does not completely match the popular Omori law; in fact it has been shown that the
seismicity induced closely to the fault edges is intense but of short duration, while that expected
at large distances (up to some tens times the fault dimensions) exhibits a much slower decay.Published1259-1273JCR Journalope
Microstructure and chemical composition of Roman orichalcum coins emitted after the monetary reform of Augustus (23 B.C.)
Full text linkA collection of ancient Roman orichalcum coins, i.e., a copper-zinc alloy, minted under the reigns from Caesar to Domitianus, have been characterised using scanning electron microscopy (SEM-EDS) and electron microprobe analysis (EMPA). We studied, for the first time, coins emitted by Romans after the reforms of Augustus (23 B.C.) and Nero (63-64 A.D). These coins, consisting of asses, sestertii, dupondii and semisses, were analysed using non- and invasive analyses, aiming to explore microstructure, corrosive process and to acquire quantitative chemical analysis. The results revealed that the coins are characterized by porous external layers, which are affected by dezincification and decuprification processes. As pictured by the X-ray maps, the elemental distribution of Cu and Zn shows patterns of depletion that in some cases penetrate in deep up to 1 mm. The composition of the un-corroded nucleus is a Cu-Zn alloy containing up to 30% of Zn, typical of coins produced via cementation process
Archaeometric characterization of XIII century "Provisini" coin: a multi-analytical approach
No full text
A multi-analytical approach for the characterization of ancient Roman coins in orichalcum
No full textA selected number of Roman orichalcum coins, from private collections, have been studied. Numismatic analysis indicates that (Crawford, 1974; Sutherland, 1984) they are asses, sestertius and dupondium, minted by Julius Caesar, Augustus and Claudius. The aim of this study was to disclose the chemical composition of the orichalcum alloy, the nature of the patina and corrosion products; orichalcum is an ancient copper based alloy with a variable percentage of zinc (Craddock, 1978). With this aim a multi-analytical approach was involved, i.e. X-ray fluorescence (XRF), scanning electron microscope (SEM-EDS), electron micro probe analyser (EMPA). In particular, XRF analysis was performed on different spots of coins’ surfaces to have information about the chemical composition of the external layers. SEM-EDS analysis allowed to investigate alloy’s microtexture (e.g. segregation micro-domains) and to obtain morphological information. In addition, X-ray maps, acquired across section of coins, give information about elemental distribution inside the alloy; whereas EMPA analysis permits a quantitative chemical composition of major, minor and trace elements composing this ancient alloy. All these techniques confirmed that copper and zinc are the main components of the coins. Other metals, i.e. Fe, Pb, Sn, Co, Ni, As, Sn, Sb and Bi, were also found in the alloy. These elements can provide information about sourcing raw materials sites. Si, S, Cl, K and Ca were found on the surfaces, being contaminant from the soil. Quantitative analysis with microprobe permitted to evaluate the different percentage of Cu and Zn in all the samples. The use of invasive technique (EMPA) was necessary to quantify the abundances of each elements composing the original alloy, thus eliminating soil contaminants and alteration product of the layers developed through time on surfaces. These results, could contribute to fill the lack of knowledge about the orichalcum alloy
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Study of dezincification in orichalcum Roman coins
No full textA group of orichalcum Roman coins, from private collections, have been studied for this research. Numismatic examination indicated that they are asses, sestertium and dupondius, minted from Julius Caesar to Nero. Orichalcum is an ancient copper based alloy with a variable percentage of zinc.
The aim of this study was to investigate the process of dezincification from the external layers to the core of the samples. Furthermore, the research was aimed to disclose the real chemical composition of the orichalcum alloy.
In scientific literature is reported that the percentage of zinc in the alloy is related to the age in which the coin was minted.
The samples were firstly analysed by means of X-ray fluorescence spectroscopy to obtain information about the elemental composition (qualitative method) of the external layers. This non-invasive analysis allowed to discriminate coins made with zinc alloy from the bronze ones.
Because of the degradation of the external layers of orichalcum and the dezincification; process, other analysis were also carried out on cross section of some zinc based samples, such as scanning electron microscope (SEM-EDS) and electron micro probe analyser (EMPA). The first technique was useful to investigate the depth of corrosion and dezincification processes and to examine alloy’s microtexture (e.g. segregation micro-domains); the second one permitted a quantitative chemical analysis of major, minor and trace elements composing this ancient alloy, understanding the difference of chemical compositions between the unaltered core and the altered layers.
All the techniques performed both on the cross sections and on the external layers of the coins revealed that other metals, i.e. Fe, Sn, Pb, As, were also present in the alloy.
Finally, results were chronologically ordered and compared with literature’s data
Physical and stochastic models of earthquake clustering
No full textThe phenomenon of earthquake clustering, i.e., the increase of occurrence probability for seismic events close in space and time
to other previous earthquakes, has been modeled both by statistical and physical processes.
From a statistical viewpoint the so-called epidemic model (ETAS) introduced by Ogata in 1988 and its variations have become
fairly well known in the seismological community. Tests on real seismicity and comparison with a plain time-independent
Poissonian model through likelihood-based methods have reliably proved their validity.
On the other hand, in the last decade many papers have been published on the so-called Coulomb stress change principle, based
on the theory of elasticity, showing qualitatively that an increase of the Coulomb stress in a given area is usually associated with an
increase of seismic activity. More specifically, the rate-and-state theory developed by Dieterich in the ′90s has been able to give a
physical justification to the phenomenon known as Omori law. According to this law, a mainshock is followed by a series of
aftershocks whose frequency decreases in time as an inverse power law.
In this study we give an outline of the above-mentioned stochastic and physical models, and build up an approach by which
these models can be merged in a single algorithm and statistically tested. The application to the seismicity of Japan from 1970 to
2003 shows that the new model incorporating the physical concept of the rate-and-state theory performs not worse than the purely
stochastic model with two free parameters only. The numerical results obtained in these applications are related to physical
characters of the model as the stress change produced by an earthquake close to its edges and to the A and σ parameters of the rateand-
state constitutive law.Published141-153JCR Journalreserve
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