ALBERT

Beschreibung: Abstract. Teleseismic data recorded by temporary and permanent stations located in the Northern Victoria Land region are analysed in order to identify the presence and location of seismic anisotropy. We work on data recorded by 24 temporary seismographic stations deployed between 1993 and 2000 in different zones of the Northern Victoria Land, and by the permanent very broad-band stations TNV located near the Italian Base M. Zucchelli. The temporary networks monitored an area extending from Terra Nova Bay towards the South beyond the David Glacier and up to the Indian Ocean northward. To better constrain our study, we also provide an analysis of data recorded by TNV in the same period of time and we take into account also SKS shear wave splitting measurements performed by Barruol and Hoffman (1999) on data recorded by DRV. This study, to be considered as preliminary, reveals the presence of seismic anisotropy below the study region, with a mainly NW-SE fast velocity direction below the Terra Nova Bay area and rather large delay times, that mean a deep rooted anisotropic layer.

Beschreibung: Published

Beschreibung: 153-160

Beschreibung: open

Beschreibung: The project Retreating-trench, extension, and accretion tectonics, RETREAT, is a multidisciplinary study of the Northern Apennines (www.geology.yale.edu/RETREAT/index.htm), is funded by the United States National Science Foundation (NSF) in collaboration with the Italian Istituto Nazionale di Geofisica e Vulcanologia (INGV) and the Grant Agency of the Czech Academy of Sciences (GAAV). The main goal of RETREAT is to develop a self-consistent dynamic model of syn-convergent extension, using the Northern Apennines as a natural laboratory. In the context of this project a passive seismological experiment was deployed in the fall of 2003 for a period of three years. RETREAT seismologists aim to develop a comprehensive understanding of the deep structure beneath the Northern Apennines, with particular attention on inferring likely patterns of mantle flow. Specific objectives of the project are the crustal and lithospheric thicknesses, the location and geometry of the Adriatic slab, and the distribution of seismic anisotropy laterally and vertically in the lithosphere and asthenosphere. The project is collecting teleseismic and regional earthquake data for 3 years. This contribution describes the RETREAT seismic deployment and reports on key results from the first year of the deployment. We confirm some prior findings regarding the seismic structure of central Italy, but our observations also highlight the complexity of the Northern Apennines subduction system.

Beschreibung: Submitted

Beschreibung: open

Beschreibung: Shear-wave splitting estimates from recordings of 10 portable seismographic stations during the first year of the RETREAT seismic deployment, in combination with broadband data from the Italian national seismic network, are associated with seismic anisotropy within the upper mantle beneath the Northern Apennines. Anisotropic parameters derived from both shear-wave splitting and P travel-time residuals vary geographically and depend on event back-azimuth, reflecting complexity in the underlying mantle strain field. Variations of the splitting time delays and fast polarization seem to exclude a 2-D sublithosphere corner flow, associated with the Apennines subduction, as the main source of the inferred anisotropy. The anisotropic signal may be generated by a frozen-in fabric of the Adriatic and Tyrrhenian lithosphere domains, or by flow variations induced by episodic and fragmentary slab rollback.

Beschreibung: Published

Beschreibung: 157-170

Beschreibung: JCR Journal

Beschreibung: reserved

Beschreibung: The Montello–Conegliano Thrust is the most remarkable structure of the Southern Alpine fault belt in the Veneto-Friuli plain, as a result of the conspicuous morphological evidence of the Montello anticline, which is associated to uplifted and deformed river terraces, diversion of the course of the Piave River, as well as vertical relative motions registered by leveling lines (Galadini et al., 2005; Burrato et al., 2008). Many papers dealt with its geometry and evolution, and the presence of several orders of Middle and Upper Pleistocene warped river terraces (Benedetti et al., 2000) in the western sector strongly suggests that the Montello–Conegliano anticline is active and driven by the underlying thrust. However, in spite of the spectacular geomorphic and geologic evidence of activity of the Montello-Conegliano Thrust, there is only little evidence on how much contractional strain is released through discrete events (i.e. earthquakes) and how much goes aseismic. Benedetti et al. (2000) hypothesized that the western part of the thrust (Montello) may have slipped three times in the past 2000 years (during the Mw 5.8 778 A.D., Mw 5.4 1268 and Mw 5.0 1859 earthquakes), yielding a mean recurrence time of about 500 years, whereas, the eastern part of the thrust (Conegliano) would be silent. The Italian seismic catalogues have very poor-quality and incomplete data for these events associated with the Montello thrust, leaving room for different interpretations, as for example the possibility that these earthquakes were generated by nearby secondary structures. In this latter case, the whole Montello–Conegliano Thrust would represent a major “silent” structure, with a recurrence interval longer than 700 years, because none of the historical earthquakes reported in the Italian Catalogues of seismicity for the past seven centuries can be convincingly referred to the Montello Source. Given the uncertainties regarding the seismic potential of this segment of the Southern Alpine fault system, we designed and realized a new GPS network across the Montello region (Fig. 1), with the goal of detecting the present-day velocity gradient pattern and develop models of the inter-seismic deformation (i.e., geometry, kinematics and coupling of the seismogenic fault). In the 2009, we started realizing a new concept of GPS experiment, called “semi-continuous”. As the name suggests, the method involves moving a set of GPS receivers around a permanently installed network of monuments, such that each station is observed some fraction of the time. In practice, a set of GPS receivers can literally remain in the field for their entire life span, thus maximizing their usage. The monuments are designed with special mounts so that the GPS antenna is forced to the same physical location at each site. This has the advantage of mitigating errors (including possible blunders) in measuring the antenna height and in centering the antenna horizontally. This also has the advantage of reducing variation in multipath bias from one occupation session to another. The period of each “session” depends on the design of the operations. At one extreme, some stations might act essentially as permanent stations (though the equipment is still highly mobile), thus providing a level of reference frame stability, and some stations may only be occupied every year or two, in order to extend or increase the density of a network’s spatial coverage. In this work we will present the motivations and tools used to develop and implement the new GPS network. During the 2010 we will integrate the existing GPS network with 10 mobile seismic stations, belonging to the INGV mobile network, with the goal of illuminate local micro-seismicity patterns that would help constraining the locked fault geometry.

Beschreibung: Published

Beschreibung: trieste, italy

Beschreibung: 1.1. TTC - Monitoraggio sismico del territorio nazionale

Beschreibung: 1.9. Rete GPS nazionale

Beschreibung: 3.2. Tettonica attiva

Beschreibung: open

Beschreibung: We present here the new observations of seismic anisotropy obtained from SKS birefringence analysis. We studied 27 teleseismic earthquakes recorded by the temporary seismic network of RETREAT project in the Northern Apennines region. For each station–event couple we calculate the anisotropic parameters (delay time and fast-polarization direction) by minimizing the energy in the transverse component. Our measurements confirm the existence of two domains. The Tuscany domain, on the south-west with respect to the Apennines, shows mostly NW–SE fast axes directions, with a rotation toward E–W direction moving toward the Tyrrhenian Sea. The Adria domain, north-east of the Apennines orogen, shows more scattered measurements, with prevailing N–S to NNE–SSW directions; also with back-azimuthal dependence. The transition between the two domains is abrupt in the nothern part of the study region but more gradual in the southern part. Measured delay times (1.8 s on average) suggest that the detected anisotropy is located principally in the asthenosphere. Beneath the Adria domain, where the presence of a double-layer structure seems consistent, a lithospheric contribution is plausible. An interpretation in terms of ongoing mantle deformation suggests a differential evolution of the trench-retreat process along the Northern Apennines orogen. The orogen-parallel anisotropy in the study region is beneath the inner part of the belt instead of beneath its crest and no orogen-normal measurements are found in the Tuscany side. Compared to the anisotropy pattern of the typical slab retreat seen in southern part of the Northern Apennines, in the northernmost one the anisotropy suggests that an oblique trench-retreat has occurred, possibly linked to Northern Apennines retreat since 5 Ma.

Beschreibung: Published

Beschreibung: 68-82

Beschreibung: 3.3. Geodinamica e struttura dell'interno della Terra

Beschreibung: JCR Journal

Beschreibung: reserved

Beschreibung: We document quantitatively observations of quasi-Love waves obtained at permanent (Italian National Seismic Network) and temporary seismic stations deployed in Italy between 2003 and 2006 (Retreat, CAT/SCAN projects). We analyzed large earthquakes with source parameters that favor quasi-Love wave generation within this time-span, including the Sumatra–Andaman earthquake of 12/26/04. The presence or the absence of the quasi-Love phase is compared to the smoothed anisotropic pattern defined by the numerous SKS splitting measurements obtained in peninsular Italy, and to the Italian upper mantle structure as defined by seismic tomography. The large-scale anisotropic features, responsible for shear-wave splitting and documented also by Pn and surface-wave anisotropy, generally display the correct geometry to explain the scattered quasi-Love waves. Quasi-Love observations do not demand a tilted-axis anisotropic geometry. We argue instead for anisotropy with laterally-variable horizontal symmetry axis in the upper mantle below the Italian peninsula.

Beschreibung: Published

Beschreibung: 26-38

Beschreibung: 1T. Geodinamica e interno della Terra

Beschreibung: JCR Journal

Beschreibung: restricted

Beschreibung: The seismic moment tensor is the complete mathematical representation of the movement on a fault 10 during an earthquake, comprising of the couples of forces that produced it, the description of the fault 11 geometry, and its size by means of the scalar seismic moment M0. 12 The computation of seismic moment tensor has become a widely diffused activity because of the 13 relevance of this kind of data in seismotectonic and geodynamic studies and, in more recent times, 14 because it allows obtaining rapid information about a seismic event immediately after its occurrence. This 15 progress has been possible with the advent of modern standardized instruments since the early 1960s, 16 above all of the very broadband seismographic stations that started to record in the late 1970s. Further- 17 more, time after time, the easier availability of digital data impressed a strong incentive to improve the 18 procedures of source parameter computation.

Beschreibung: Unpublished

Beschreibung: 1-15

Beschreibung: 4IT. Banche dati

Beschreibung: restricted

Beschreibung: The seismic sequence that hit Umbria and Marche during 1997 and 1998 was particularly significant because it dramatically marked the evolution of analysis tools and the development of our geophysical knowledge of the region. Since September 1997, we have had a growing, coherent catalogue of source moment tensors that provides reliable information on earthquake sources in Italy and the surrounding regions. Together with borehole and other tectonic data, this has conspicuously improved our knowledge of the regional strain and stress fields. The main impact of these new data that include local information consists of the possibility to change the scale of the regional geodynamic frame. The simple description of extension tectonics that dominate the Apennines belt has evolved into the present-day maps of the strain and stress fields, where the active tectonics involve compression in the eastern Alps, extension and compression fronts in the northern Apennines, extension and strike-slip structures in the southern Apennines, and a compressional front along the southern Tyrrhenian Sea. This recent geodynamic evolution and the present-day seismotectonic sketch of the Italian peninsula are here described on the basis of the recognition of these tectonic features.

Beschreibung: Published

Beschreibung: 319-330

Beschreibung: 3.2. Tettonica attiva

Beschreibung: JCR Journal

Beschreibung: open

Beschreibung: Il progetto AlpArray (PI E. Kissling, Seismology and Geodynamics ETH) è un’iniziativa europea di collaborazione interdisciplinare sismologica e geodinamica, il cui obiettivo principale è quello di migliorare la comprensione della struttura profonda e della geodinamica delle Alpi (la catena montuosa più studiata al mondo) tramite l’acquisizione, l’analisi e l’interpretazione di dati sismologici di alta qualità. Per ottenere delle immagini di estremo dettaglio della crosta e del mantello, AlpArray propone la realizzazione di una rete sismica a maglia il più possibile omogenea (massima distanza inter- stazione 52 km, backbone network), tramite la condivisione dei dati delle reti permanenti esistenti e l’installazione, da parte di ciascun paese partecipante, di numerose stazioni sismiche temporanee a larga banda (BB). Il progetto prevede l'installazione di circa 250 stazioni sismiche in tutta Europa, in particolare in Italia, Francia, Svizzera, Germania, Austria, Croazia, Repubblica Ceca, Bosnia, Ungheria, Slovenia e Polonia. I dati confluiranno all’interno dell’archivio europeo denominato European Integrated Data Archive (EIDA). Considerata l’estensione geografica dell’area, i partecipanti combineranno le infrastrutture esistenti per l’acquisizione dei dati, il loro trattamento, l’applicazione delle tecniche più avanzate di imaging e l’interpretazione e modellazione dei risultati, in uno sforzo transnazionale ad una scala mai realizzata prima in Europa. Si tratta quindi di un’occasione fondamentale per lo scambio di competenze tecniche e scientifiche all’avanguardia. L’INGV, oltre a condividere i dati delle proprie stazioni permanenti nell’area di interesse, si occupa della installazione e della manutenzione sul territorio italiano di 20 nuove stazioni-BB temporanee i cui dati verranno trasmessi in tempo reale (partecipando così alla realizzazione del backbone) e coadiuva l’ETH nella ricerca dei siti italiani per altrettante stazioni svizzere e nella loro manutenzione ordinaria. L’acquisizione di una mole notevole di nuovi dati permetterà di raffinare le conoscenze sulla struttura e la composizione della litosfera e del mantello al di sotto dell’area alpina: Queste conoscenze sono anche utili ai fini della modellazione geodinamica. Il potenziamento del monitoraggio sismico aiuterà ad individuare e studiare in maggior dettaglio le aree sismogenetiche della regione alpina.

Beschreibung: Published

Beschreibung: Trieste

Beschreibung: 1T. Geodinamica e interno della Terra

Beschreibung: open

Beschreibung: We present shear-wave splitting results obtained from the analysis of core refracted teleseismic phases recorded by permanent and temporary seismographic stations located in the Victoria Land region (Antarctica). We use an eigenvalue technique to isolate the rotated and shifted shear-wave particle motion, in order to determine the best splitting parameters. Average values show clearly that dominant fast axis direction is NE-SW oriented, in accordance with previous measurements obtained around this zone. Only two stations, OHG and STAR show different orientations, with N-S and NNW-SSE main directions. On the basis of the periodicity of single shear-wave splitting measurements with respect to back-azimuths of events under study, we infer the presence of lateral and vertical changes in the deep anisotropy direction. To test this hypothesis we model waveforms using a cross-convolution technique for the cases of one and two anisotropic layers. We obtain a significant improvement on the misfit in the double layer case for the two stations. For stations where a multi-layer structure does not fit, we investigate lateral anisotropy changes at depth through Fresnel zone computation. We find that anisotropy beneath the Transantarctic Mountains (TAM) is considerably different from that beneath the Ross Sea. This feature influences the measurement distribution for the two permanent stations TNV and VNDA. Our results show a dominant NE-SW direction over the entire region, but other anisotropy directions are present and maybe interpreted in the context of regional tectonics.

Beschreibung: Published

Beschreibung: 421-432

Beschreibung: 3.3. Geodinamica e struttura dell'interno della Terra

Beschreibung: JCR Journal

Beschreibung: open