Microstructure and chemical composition of Roman orichalcum coins emitted after the monetary reform of Augustus (23 B.C.)

A 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

A rate-state model for aftershocks triggered by dislocation on a rectangular fault: a review and new insights

We 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

New insights into the application of the Coulomb model in real-time

The Coulomb model for stress change estimation is considered one of the most powerful physics-based forecasting tools, even though its calculations are affected by uncertainties due to the large number of a priori assumptions needed. The aim of this paper is to suggest a straightforward and reliable strategy to apply the Coulomb model for real-time forecasting. This is done by avoiding all dispensable assumptions, thus reducing the corresponding uncertainties. We demonstrate that the depth at which calculations are made is a parameter of utmost importance and apply the Coulomb model to three sequences in different tectonic regimes: Umbria-Marche (normal), Landers (strike-slip), and Chi-Chi (thrust). In each case the results confirm that when applying the Coulomb model: (i) the depth of calculation plays a fundamental role; (ii) depth uncertainties are not negligible; (iii) the best forecast at a given location is obtained by selecting the maximum stress change over the whole seismogenic depth rang

The effect of trivalent cations on the major lower mantle silicates

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2011.Cataloged from PDF version of thesis. Vita.Includes bibliographical references (p. 151-165).I have investigated the effect of composition, especially ferric iron and aluminum, on the equations of state and phase stability of perovskite and post-perovskite. The presence of trivalent cations decreases the bulk modulus of perovskite at pressures corresponding to the upper lower mantle. Ferric iron in perovskite undergoes a spin-pairing transition from the high spin state to low spin in the octahedral site. Ferric iron in the dodecahedral site remains high spin. In the absence of aluminum, the spin transition is gradual between 0 and 55 GPa, and bulk modulus increases at the completion of the spin transition. In the presence of aluminum, there is an abrupt increase in the amount of low spin ferric iron near 70 GPa, likely the result of site mixing. The high compressibility of the structure below 70 GPa results in the volume nearing that of magnesium endmember, MgSiO₃ , perovskite. Concurrent with the spin transition in aluminum-bearing perovskite, the structure stiffens. The increase in density and bulk modulus at -70 GPa results in an increase in bulk sound speed that may be related to heterogeneities in bulk sound speed observed seismically at 1200-2000 km depth in the Earth. The effect of composition on the perovskite to postperovskite phase transition was also investigated. No change in the spin state of ferric iron was found at the perovskite to post-perovskite phase transition: ferric iron is low spin in the octahedral site and high spin in the dodecahedral site. At the phase transition, ferric iron only slightly broadens the perovskite plus post-perovskite mixed phase region while ferrous iron and aluminum were each found to significantly broaden the mixed phase region to hundreds of kilometers thick. The effect of background mineral phases was assessed for a basaltic system, rich in aluminum. The coexisting minerals were found to significantly reduce the effect of the aluminum, producing a boundary that is potentially sharp enough for seismic detection in silicon-rich systems, such as basalt.by Krystle Carina Catalli.Ph.D

“News” e tasso di cambio €/$ in alta frequenza: stime econometriche Garch

Analizzando l’impatto che news scheduled ed unscheduled possono avere sul tasso di cambio €/$ a frequenza oraria per un arco temporale di sei anni, il presente lavoro indica la necessità di utilizzare queste due tipologie di news per spiegare al meglio l’andamento del cambio. Si rileva inoltre come l’ordine in cui gli indicatori macroeconomici scheduled vengono rilasciati all’interno del mese impatta sulla loro diversa significatività. Molto importante è anche la separazione degli eventi tra favorevoli e sfavorevoli per il cambio in esame: si conferma il noto risultato della letteratura finanziaria, secondo cui il mercato reagisce alle news in modo asimmetrico, dando un maggior peso alle bad news piuttosto che alle good news.

Sorgenti sismiche: rappresentazione matematica ad applicazione al calcolo degli spostamenti

Se 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))

A rate-state model for aftershocks triggered by dislocation on a rectangular fault: a review and new insights

We 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