GNGTS 2008 SESSIONE 1.1 Fig. 1 – Variazione del centroide della sismicità. dio è stato poi fuso con quello del CMT italiano. Con il catalogo risultante sono state costruite una relazione magnitudo durata – momento sismico ed una magnitudo locale –momento sismico: entrambe sono state utilizzate per calcolare il valore di magnitudo momento per tutti gli eventi per i quali non esisteva una stima CMT. Il catalogo finale è risultato composto da 1720 eventi superfi- ciali con magnitudo momento M ≥ 3,5, valore questo che ne garantisce una quasi completezza. Per verificare se vi sia una coerenza spazio-temporale nella sismicità, si è scelto di analizzare le caratteristiche del comportamento del centroide. Poiché l’attività sismica presenta tassi di accadi- mento molto diversi da un’area all’altra é stata costruita una griglia con una spaziatura di 1° e il momento sismico all’interno di ciascuna cella é stato normalizzato rispetto al valore medio, consi- derando intervalli temporali di un mese. Il passo finale é consistito nell’analizzare l’andamento tem- porale del centroide utilizzando solo i modi a frequenza più bassa con una tecnica del tipo EMD. Il risultato ottenuto, mostrato in Fig. 1, indica che circa il 20% del momento totale normalizza- to presenta una variazione coerente del centroide, con la sismicità che si muove alternativamente nella direzione nord-sud con un periodo di circa 1,7 anni. Bibliografia Castello B., Selvaggi G., Chiarabba C., Amato A.; 2006: Catalogo della sismicità italiana 1981-2002., versione 1.1 INGV-CNT. Roma, http://www.igv.it/CSI/. International Seismological Centre; 2001: On line Bulletin. Internatl. Seis. Cent. Thatcham. United Kingdom, http://www.isc.ac.uk. Pondrelli S., Salimbeni S., Ekström G., Morelli A., Gasperini P., Vannucci G.; 2006: The italian CMT dataset (from 1977 to present). Phys. Earth Planet. Int., 4, 286-303. Postpischl D.; 1985: Catalogo dei terremoti italiani dall’anno 1000 al 1980. C.N.R., Bologna, pp 240. ASAIN: AN EXPANDING MULTIPURPOSE BROADBAND NETWORK FOR SEISMOLOGICAL RESEARCH IN WESTERN ANTARCTICA AND TIERRA DEL FUEGO M. Russi 1, P. Suhadolc 2 1 Istituto Nazionale di Oceanografia e di Geofisica Sperimentale – OGS, Trieste 2 Dipartimento di Scienze della Terra, Università degli Studi di Trieste Introduction. Scientific investigations of the Scotia Sea region are crucial to understand the his- tory of the Antarctic continent tectonic evolution and the influence of the aperture of the Drake pas- sage in establishing the Circumpolar Antarctic Current, as stressed by many authors (e.g. Lodolo, 2008). The Scotia Sea occupies a roughly rectangular area of about 900,000 km2. This area is lim- ited on three sides by the Scotia Arc, formed by islands and oceanic ridges, which is a remnant of the mountain chain that joined the South American Andes to the Antarctic Peninsula. The western border is represented by the about 1000 km wide Drake passage, that separates today the Tierra del 41 GNGTS 2008 SESSIONE 1.1 Fuego in South America from the Antarctic continent. A review of the tectonics and evolution of the Scotia Sea can be found in Barker, 2001.The start of the geophysical studies in this area dates back to several decades ago, but only after 1990 instrumental passive seismology started to be widely applied to investigate the crustal properties and the properties of seismic sources responsible for the strong seismicity level observed along the Scotia plate boundaries. ASAIN: past and present. The first Antarctic Seismological Argentinean-Italian Network (ASAIN) station became operational at the Argentinean base Esperanza during January 1992. This temporary station operated during three years, which allowed the Italian OGS and Trieste University researchers together with the Instituto Antartico Argentino (IAA) partner seismology group to obtain useful information on the regional seismicity. In 1995 Esperanza was upgraded to a permanent observatory equipped with a Guralp CMG-3T broad band sensor, a Reftek 72A-08 recorder and GPS time base. Within March 1997 two similar installations were put into operation at Ushuaia and La Pataia (near Ushuaia) and Base Orcadas on the Laurie Is. in the South Orkney Is. archipelago. This configuration was further expanded in 2002 when stations Estancia Despedida in Tierra del Fuego and Base Jubany in the South Shetland Is. archipelago started their operation. (Russi et al., 2004) The dataset obtained with the 5-station ASAIN configuration and the neighbouring GSN stations EFI (East Falkland Is.), HOPE (South Georgia Is.) and PMSA (Palmer Station) was then extensive- ly exploited. The dataset permitted to obtain smoothed local dispersion curves in correspondence of the main geological and tectonic features, and through their non-linear inversion, the average S- wave velocity versus depth profiles in the sub-Antarctic Scotia Sea region by means of surface- wave tomography techniques (Vuan et al., 2000). The results obtained both with synthetic experi- ments and observed data inversions encouraged the systematic application of this methodology, in the perspective of focusing the study on low-level seismicity detected by temporary arrays in Antarctic Peninsula and Tierra del Fuego. The year 2003 represents a benchmark in the development of the network. The availability of satellite links in the Argentinean Antarctic bases allowed the upgrade of the Antarctic ASAIN sta- tions to remote real-time data acquisition and the inclusion of stations Esperanza, Jubany, Orcadas and San Martin, the latter the most recently (February 2007) opened one, in the Virtual European Broad-band Seismic Network (VEBSN). Work in progress and planned actions. Further scientific results, including the determination of the focal mechanism, not published by other agencies, for some aftershocks of the August 4th, 2003 M7.6 earthquake with epicentre in the Scotia Sea, 70 km North East of the Orcadas Base, and the detection of a relevant microseismicity in Tierra del Fuego, convinced us to pursue a policy of further expansion of the ASAIN, both in the Antarctic and in Tierra del Fuego, during the years 2007-2009, in coincidence with the International Polar Year (Plasencia, 2008). Two main priorities have been identified on which to work: A further expansion of the ASAIN network even more to the south. After the opening of San Martin station on February 2007, the first ASAIN site located south of the polar circle, another sta- tion will be installed at Belgrano II Base during the 2008-2009 Antarctic summer by the OGS-IAA ASAIN team. Base Belgrano II (77° 52’ S, 34° 37’ W) is the southernmost Argentinean permanent base and is located on a rocky outcrop (Nunatak Bertrab) on the Filchner barrier about 1200 km from the South Pole. Improve the coverage of the ASAIN network in Tierra del Fuego both on the Argentinean and the Chilean sides. In fact, the whole area has a considerable seismic hazard, since a large (Ms 7.5) earthquake generated on the Magellanes-Fagnano strike-slip fault system affected the area in 1949 producing severe damage in Punta Arenas. As a preliminary step a Guralp CMG-40TD instrument was put in operation for a day on a bedrock outcrop (sandstones) in the proximity of Fuerte Bulnes during April 2007. The noise meas- 42 GNGTS 2008 SESSIONE 1.1 Fig. 1 - ASAIN Network location map including Belgrano II and Fuerte Bulnes planned stations. urements demonstrate that the site is ideal for the installation of a broadband instrument. The new station will be the westernmost of the Tierra del Fuego ASAIN Branch allowing an improved mon- itoring of the Magellanes-Fagnano strike-slip fault system also in its western section and a more precise determination of the local seismicity focal parameters. Considering that there is no perma- nent seismographic station south of Puerto Montt in Chile, in spite of the fact that the area to the south is extremely interesting from a seismological point of view, being close to the triple junction among the South American, Antarctic and Scotia plates, Fuerte Bulnes station will also provide valuable information to scientists interested in triple junction studies. Acknowledgments. The “Broad-band seismology, lithospheric structure and geodynamics in the Scotia Sea region” project is funded by the Programma Nazionale di Ricerche in Antartide (PNRA), research area 2: ”Geodesia ed Osservatori”. A grateful acknowledgment is directed to the Argentinean civil and military per- sonnel who operates the ASAIN stations in Antarctica and to the crews of the Argentinean vessels ARA “Almirante Irizar” and Puerto Deseado. We would like to remember also our friend José Febrer, who sudden- ly died on May 2, 2008. José participated together with Marino Russi, Daniel Nieto and Francesco Fanzutti to the installation of the first ASAIN station in Esperanza, and, since then, has been our main reference at the Instituto Antartico Argentino for all activities concerned with the ASAIN network. We will always miss him. 43 GNGTS 2008 SESSIONE 1.1 References Barker, P. F., 2001. Scotia Sea regional tectonic evolution: implications for mantle flow and palaeocirculation. Earth Science Reviews, 55, 1-39. Lodolo, E., 2008. Relazioni tra tettonica e clima: il caso dell’Arco di Scozia. Geoitalia, 23, 3-7. Plasencia, M., 2008. Lithospheric characteristics and seismic sources in the Scotia Arc through waveform inversion. Ph.D. Thesis, Trieste University, Italy. Russi, M., Plasencia, M., 2007. Observational seismology in the Scotia Sea Region. Fifteen years of continued operation and growth of the ASAIN Nertwork. Poster presentation at IUGG XXIV General Assembly, Perugia, Italy. Russi, M., Plasencia, M. Guidarelli, M., 2004. Further developments of the ASAIN network in Antarctica and Tierra del Fuego. Bollettino di Geofisica Teorica ed Applicata, Vol. 45 – n. 2 supplement, 92-95 . Vuan, A., Russi, M., Panza, G.F., 2000. Group velocity tomography in the Sub-Antarctic Scotia Sea region. Pure and Applied Geophysics, 157, 1337-1357.