Properties and Processes of Crustal Fault Zones: Volume I

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Rise time of proton cut-off energy in 2D and 3D PIC simulations

The Target Normal Sheath Acceleration (TNSA) regime for proton acceleration by laser pulses is experimentally consolidated and fairly well understood. However, uncertainties remain in the analysis of particle-in-cell (PIC) simulation results. The energy spectrum is exponential with a cut-off, but the maximum energy depends on the simulation time, following different laws in two and three dimensional (2D, 3D) PIC simulations, so that the determination of an asymptotic value has some arbitrariness. We propose two empirical laws for rise time of the cut-off energy in 2D and 3D PIC simulations, suggested by a model in which the proton acceleration is due to a surface charge distribution on the target rear side. The kinetic energy of the protons that we obtain follows two distinct laws, which appear to be nicely satisfied by PIC simulations. The laws depend on two parameters: the scaling time, at which the energy starts to rise, and the asymptotic cut-off energy. The values of the cut-off energy, obtained by fitting the 2D and 3D simulations for the same target and laser pulse, are comparable. This suggests that parametric scans can be performed with 2D simulations, since 3D ones are computationally very expensive. In this paper, the simulations are carried out for $a_0=3$ with the PIC code ALaDyn by changing the target thickness $L$ and the incidence angle $\alpha$. A monotonic dependence, on $L$ for normal incidence and on $\alpha$ for fixed $L$, is found, as in the experimental results for high temporal contrast pulses

Ground Motion Polarization in Fault Zones:Its Relation with Brittle Deformation Fields

Many recent studies indicate that ambient noise and seismic signals in fault zones tend to be polarized on the horizontal plane with a predominant orientation. Here we present a summary of past experiments as well as new study cases showing evidence of this effect. The approach combines the H/V technique in the frequency domain with the covariance matrix diagonalization method in the time domain. Common features are: i) a high stability of results at each site, independently of the nature and location of the source of seismic signals, ii) a predominant polarization characteristic for each fault, and iii) polarization is not parallel to the fault strike as it would be expected for fault-trapped wave generation. In previous papers, a role of fluid-filled microcracks in the damage zone was hypothesized. If this is true, a correlation is expected between seismic anisotropy and polarization. In the studied faults, when anisotropy results are available, the horizontal ground motion polarization is found to be perpendicular to the fast wave splitting component, confirming the role of fluid-filled microcracks in the damage zone. We have then checked this interpretation in terms of the fracture field orientation in the damage zone by applying the package FRAP3 (Salvini, 2002) to model the brittle deformation field expected in the damage zone of the studied faults. We have found a consistent orthogonal relation between the observed polarizations and the orientation of the predicted fracture systems. The quick and relatively inexpensive character of the method encourages to further tests for an extensive application to many fields of theoretical and applied geophysics

RAPPORTO ATTIVITA' UR-INGV

In questo rapporto di attività viene illustrata la seconda fase delle ricerche svolte dall’UR-INGV nei due comuni selezionati ai fini del progetto, Nocera Umbra e Cerreto di Spoleto. Per quanto riguarda Nocera Umbra, si sono analizzati i dati sismometrici registrati durante un esperimento mediante array sismico a piccola apertura, appositamente installato sulla collina di Nocera Umbra per lo studio dell'effetto topografico e la quantificazione del ruolo delle variazioni topografiche locali sull'input sismico. Va ricordato che la torre campanaria, pesantemente danneggiata durante le scosse più forti del Settembre e Ottobre 1997, come pure l’intero centro storico di Nocera Umbra, sono situati sulla sommità di una collina. E' apparso pertanto importante valutare gli effetti di amplificazione del moto del suolo con grande dettaglio spaziale nella zona dove sono localizzati gli edifici monumentali e la parte storica della città. Sono stati anche effettuati due profili geoelettrici a cavallo della faglia, probabilmente inattiva, che attraversa Nocera Umbra, e tramite inversione tomografica dei dati di resistività si è cercata una conferma dell’estensione laterale della zona di faglia, alla cui presenza è stata attribuita la causa principale dell’accelerazione di 0.6 g registrata nella stazione della rete accelerometrica nazionale ubicata nella cabina ENEL di Nocera Umbra, a circa 20 m dalla parete della faglia stessa. E' stato infine analizzato in dettaglio il ruolo giocato dalla zona di faglia nella propagazione delle onde sismiche, sia in termini di picchi di accelerazione e velocità che di amplificazioni spettrali, in campo lineare e non lineare. L’intervento su Cerreto di Spoleto ha visto l’installazione di stazioni sismiche nell’area urbana congiuntamente all’UR-ENEA, e l’esecuzione di misure geoelettriche per la definizione delle geometrie sepolte nella piana di Borgo Cerreto. Inoltre, si è studiato in dettaglio l’effetto di amplificazione in prossimità di una faglia che attraversa il centro storico di Cerreto di Spoleto, dove le registrazioni sismiche hanno evidenziato, analogamente al caso di Nocera Umbra, un forte effetto di canalizzazione dell’energia incidente sotto forma di "trapped waves", e dove precedentemente si era constatata la massima concentrazione dei danni durante le più forti scosse, localizzate nella zona di Sellano-Preci, dell’Ottobre 1997. Nei successivi paragrafi vengono mostrati i risultati di queste indagini

Indagine sismica a riflessione ad alta risoluzione con sorgente vibratoria Ivi-MiniVib svolta nel comune di Piedimonte Etneo (CT) in località Presa.

Il presente lavoro ha avuto come obiettivo lo studio dettagliato della porzione superficiale (0-500 metri di profondità) della faglia Pernicana attraverso l’acquisizione e l’elaborazione di dati sismici a riflessione e rifrazione ad alta risoluzione. E’ stato acquisito un profilo sismico di 715 metri, in località Presa nel comune di Piedimonte Etneo (CT), mediante l’impiego di una sorgente sismica vibratoria ad alta risoluzione. Questo sito è stato scelto in quanto è attraversato dalla rottura superficiale indotta dalla faglia Pernicana

GROUND MOTION POLARIZATION IN FAULT ZONES : RELATION WITH BRITTLE DEFORMATION FIELDS

Several recent studies indicate that ambient noise and seismic signals in fault zones tend to be polarized on the horizontal plane with a clear preferred orientation direction. Here we present a summary of past experiments as well as new study cases showing evidence of this effect: the Val d’Agri, the Pernicana and the Paganica faults in Italy, and the Hayward fault in California. We also analyze data recorded by the HRSN network at the Parkfield section of the San Andreas fault and find that stations MM and GH that are close to the fault damage zone show a similar persistent and marked polarization effect. The approach combines the H/V technique in the frequency domain with the covariance matrix diagonalization method in the time domain. Common features are: i) a high stability of results at each site, independently of the nature and location of the source of seismic signals, ii) a characteristic polarization for each fault, and iii) the preferred polarization is close to the fault-normal direction, rather than being fault parallel as would be expected for generation of fault zone trapped waves. In previous papers, the role of fluid-filled microcracks in the damage zone was hypothesized. We have then explored an hypothesis based on the fracture field orientation in the fault damage zone by applying the package FRAP3 (Salvini, 2002) to model the brittle deformation field expected for the studied faults. We have found a consistent orthogonal relation between the observed polarizations and the orientation of the predicted synthetic fracture systems. When anisotropy studies are available, the horizontal ground motion polarization is consistently found to be perpendicular to the fast wave splitting component. The results may reflect reduced stiffness in the fault-normal direction produced by the presence of damage fault zone rocks

Installation of a very broad band borehole seismic station in Ferrara (Emilia)

The Istituto Nazionale di Geofisica e Vulcanologia (INGV) is the Italian agency devoted to monitor in real time the seismicity on the Italian territory. The seismicity in Italy is of course variable in time and space, being also very much dependant on local noise conditions. Specifically, monitoring seismicity in an alluvial basin like the Po one is a challenge, due to consistent site effects induced by soft alluvial deposits and bad coupling with the deep bedrock (Steidl et al., 1996). This problem was tackled by INGV first with the Cavola experiment (Bordoni et al., 2007), where a landslide was seismically characterized using a seismic array and also down-hole logging of P- and S-wave travel times at a borehole drilled within the array; later, with an ad hoc project in 2000-2001, with the first installation of a broad band seismic station nearby Ferrara in a borehole of 135 meters depth. Comparison of recordings with a surface seismic station indicated a noise reduction of 2 decades in power spectral density at frequencies larger than 1.0 Hz (Cocco et al., 2001). The instrumentation in Ferrara has been working for several months but after that the seismic station was discontinued due to lack of maintenance manpower. The Centro di Ricerche Sismologiche (CRS, Seismological Research Center) of the Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS, Italian National Institute for Oceanography and Experimental Geophysics) in Udine (Italy) after the strong earthquake of magnitude M=6.4 occurred in 1976 in the Italian Friuli-Venezia Giulia region, started to operate the Northeastern Italy (NI) Seismic Network: it currently consists of 15 very sensitive broad band and 21 simpler short period seismic stations, all telemetered to and acquired in real time at the OGS-CRS data center in Udine (Fig. 1). Real time data exchange agreements in place with other Italian, Slovenian, Austrian and Swiss seismological institutes lead to a total number of about 100 seismic stations acquired in real time, which makes the OGS the reference institute for seismic monitoring of Northeastern Italy. Since 2002 OGS-CRS is using the Antelope software suite on several workstations plus a SUN cluster as the main tool for collecting, analyzing, archiving and exchanging seismic data, initially in the framework of the EU Interreg IIIA project “Trans-national seismological networks in the South-Eastern Alps”. SeisComP is also used as a real time data exchange server tool (Bragato et al., 2011). Among the various Italian institution with which OGS is cooperating for real time monitoring of local seismicity there is the Regione Veneto (Barnaba et al., 2012). The Southern part of the Veneto Region stands on the Po alluvial basin: earthquake localization and characterization is here again affected in this area by the presence of soft alluvial deposits. OGS ha already experience in running a local seismic network in difficult noise conditions making use of borehole installations (Priolo et al., 2012) in the case of the monitoring of a local storage site for the Italian national electricity company ENEL. Following the ML=5.9 earthquake that struck the Emilia region around Ferrara in Northern Italy on May 20, 2012 at 02:03:53 UTC, a cooperation of INGV, OGS, the Comune di Ferrara and the University of Ferrara lead to the reinstallation of the very broad band borehole seismic station in Ferrara. The aim of the OGS intervention was on one hand to extend its real time seismic monitoring capabilities toward South-East (Fig. 1), including Ferrara and its surroundings, and on the other hand to evaluate the seismic response at the site. As concerns the superficial geology of the area where the borehole seismic station has been installed, the outcropping materials are represented by alluvial deposits of different environments, like channel and proximal levee, inter-fluvial, meander and swamps deposits. As a consequence, the outcropping deposits are everywhere Holocene in age substantially loose or poorly compacted in the first meters-decameters and granulometrically could vary from clay to coarse sand. Two preliminary reports prepared by the Italian Department of Civil Defense (Dipartimento Nazionale di Protezione Civile) in collaboration with other institutions describe the data recorded by the national accelerometric network and complemented by additional data recorded by a number of temporary stations (Dolce et al., 2012a; Dolce et al., 2012b). These reports bear witness of strong ground motion values with an acceleration peak of about 0.9 g in the vertical component recorded during the ML=5.8 earthquake of May 29, 2012 by the Mirandola station, located at about 2 km from the epicentre. The analysis of the seismic noise recorded at some stations shows a quite pronounced peak of the horizontal-to-vertical spectral ratio (H/V) in the frequency range of 0.6 – 0.9 Hz common to all stations. Finally, strong evidence of liquefaction phenomena are reported at several sites (e.g.: S. Carlo, S. Agostino and Mirabello), most of which have been attributed to the occurrence of saturated sandy layer(s) at shallow depth deposited along an abandoned reach of the Reno River (Papathanassiou et al., 2012). Details of the station configuration and installation will be outlined, with first results

Quantum cosmology of scalar-tensor theories and self-adjointness

In this paper, the problem of the self-adjointness for the case of a quantum minisuperspace Hamiltonian retrieved from a Brans-Dicke (BD) action is investigated. Our matter content is presented in terms of a perfect fluid, onto which the Schutz's formalism will be applied. We use the von Neumann theorem and the similarity with the Laplacian operator in one of the variables to determine the cases where the Hamiltonian is self-adjoint and if it admits self-adjoint extensions. For the latter, we study which extension is physically more suitable.Comment: Latex file, 12 pages. Small changes made in the paper, and a a new appendix adde

MICROTREMOR MEASUREMENTS IN PALERMO, ITALY: A COMPARISON WITH MACROSEISMIC INTENSITY AND EARTHQUAKE GROUND MOTION

The city of Palermo is an appropriate test site where the efficiency of microtremors in predicting ground motion properties during earthquakes can be checked. Palermo is a densely populated city with important historical heritage and was object of previous studies. Areas of local amplification of damage were identified in downtown Palermo using historical macroseismic data. Moreover, aftershocks of the September 6, 2002, earthquake (Mw 5.9, 40 km offshore) provided a dataset of seismograms that quantify spatial variations of ground motion. The availability of more than 2000 boreholes in the city allowed a reconstruction of the 3D structure of surface geology, indicating that all the higher damage zones correspond to sediment-filled valleys. The high variability of the surface geology is mostly due to the presence of two filled river-beds of about 150 m width. In the framework of the SESAME project (Seismic EffectS assessment using Ambient Exctations, funded by the European Union), 90 microtremor measurements were performed across several profiles crossing the soft sediment bodies. The measurement points were intensified close to the valley edges (every 20 m), according to our geological reconstruction. H/V spectral ratio on ambient noise (HVSR) show significant variations along each profile: as soon as the transition stiff to soft is crossed, a typical spectral peak exceeding a factor of 3 in amplitude appears in the HVSR. The peak falls between 1 and 2 Hz and, along each profile, the peak disappears as soon as the other edge of the valley is crossed. These results indicate that microtremors are sensitive to the presence of large impedance contrasts of deep soft soil, at least in the Palermo area, with an important implication: the HVSR method seems to be able to recognize conditions potentially favourable to the occurrence of higher damage even when local geological characters are masked by the urban growth. However, we were not able to establish a quantitative correlation between microtremor properties and ground motion (or damage) amplification