The TYDE Experiment

ANNALS OF GEOPHYSICS, VOL. 49, N. 2/3, April/June 2006 An improved seismicity picture of the Southern Tyrrhenian area by the use of OBS and land-based networks: the TYDE experiment Tiziana Sgroi (1), Thomas Braun (2), Torsten Dahm (3) and Francesco Frugoni (1) (1) Istituto Nazionale di Geofisica e Vulcanologia, Roma, Italy (2) Istituto Nazionale di Geofisica e Vulcanologia, Sede di Arezzo, Italy (3) Institut für Geophysik, Universität Hamburg, Hamburg, Germany Abstract The problem of large location uncertainties for seismicity occurring in the Southern Tyrrhenian Sea have been par- tially overcame during the implementation of the long-term scientific mission of the TYrrhenian Deep-sea Exper- iment (TYDE), which allowed the installation of 14 wide-band Ocean Bottom Seismometers (OBS) and Hy- drophones (OBH) in the period December 2000-May 2001 on the seafloor around the Aeolian and Ustica Islands. Local events recorded at land-stations have also been observed on the seismograms of the Ocean Bottom Seismic Network (OBSN). Moreover, some hundreds of low magnitude events, undetected from the land networks, have been recorded. We combined the readings of body wave arrival times from OBS-OBH with those from land-stations to locate seismic events. We focused our study on three clusters of events representative of the seismic activity of the area: i) «deep» events, ii) Ustica, iii) NE-Sicily. The analysis of the integrated data set of the seismicity off-shore and on-shore, obtained from the combined land-OBS seismic network (Ustica sequence and Deep events), has improved locations in terms of RMS residuals, azimuthal gap, epicentral and hypocentral errors. More- over, further classes of events have been analysed: the first includes some local events that could be located only by integrating single trigger readings from the few available land-stations with the OBSN-data; the second com- prises local events that have been detected only by the OBS-OBH stations. In particular, the last cluster underlines the importance of an OBSN in the Tyrrhenian deep basin to reveal its unknown intense micro-seismicity, permit- ting to better understand both the tectonic and geodynamic picture of the area. Key words ocean bottom seismometer – integrated improve the locations of earthquakes occurring seismic networks – Southern Tyrrhenian Sea – Aeo- off-shore and in coastal areas and to complete da- lian Islands ta sets from the land-based networks. In the Northern Japan Trench hundreds of seismic events, recorded between July-August 1992, have 1. Introduction been relocated by Hino et al. (1996) using both OBS and land network data, improving focal Ocean Bottom Seismometer Networks (OB- depth errors to less than 5 km. Lawton et al. SN) are used in different regions of the world to (1982) studied the seismicity in the area of the Kodiak continental shelf (Alaska) and reported a mean dislocation of about 12 km of the hypocen- Mailing address: Dr. Tiziana Sgroi, Istituto Nazionale ters by integrating the OBS readings in their land di Geofisica e Vulcanologia, Via di Vigna Murata 605, network data set. Earthquake relocations, record- 00143 Roma, Italy; e-mail: [email protected] ed in the Central Juan de Fuca plate (west coast of 801 Tiziana Sgroi, Thomas Braun, Torsten Dahm and Francesco Frugoni North America) by a joint land and OBS network, a small OBS network operated for 11 days close provided a more accurate evaluation of the seis- to the Calabrian coast, recording microseismicity micity of the intraplate region (Bebel et al., 1992). not detected by permanent and temporary land- Also the use of OBSN allowed other aspects of based seismic networks (Soloviev, 1990); in seismological research to be developed, as an ex- 1996, a three week passive seismic campaign was ample in tomographic studies performed in the realised by a joint Italian-Japanese collaboration coastal region of Northern Chile, improving both around the Aeolian Islands and Ustica Island the hypocenter locations and three-dimensional vp (Aoyagi et al., 1997). The attempt to integrate and vp/vs structure (Husen et al., 2000). seismic data from land-based and OBS-stations The precise location of off-shore seismicity and to improve the epicentre determinations re- performed by land-based seismic stations is often vealed the following problems: difficult. Especially in the case of an insular or i) The use of short-period seismometers peninsular seismic network, the observations of (4.5 Hz), mainly used for seismic reflection sur- local earthquakes occurring in coastal areas show veys, turned out to be not appropriate for stud- large azimuthal gaps, resulting in a large location ies of local seismicity in the area. error. In Italy this problem particularly concerns ii) The three week long experiment took the Southern Tyrrhenian Basin, one of the most place in a seismic quiescent period. interesting regions of the Mediterranean Sea con- iii) The spacing of the OBSN (about 40 km) sidering its intense geodynamic activity linked to proved to be too large for the low magnitude lo- tectonic and volcanic processes. Up to now only cal seismicity location, therefore only a few seis- two short-term OBSs experiments have been per- mic events were simultaneously detected by more formed in the Southern Tyrrhenian Sea: in 1987, than 2 OBS-stations (Beranzoli et al., 1999). Fig. 1. Location of OBS-OBH (diamonds) and land-based seismic stations (triangles and squares) used in the present work. Each ocean bottom station was equipped with an OBH (1 channel), those equipped with an addi- tional 3-component seismic sensor (OBS) are represented by underlined numbers. Station no. 07 is not reported on the map as it was not recovered. The label 00 reports the position of the GEOSTAR multi-parametric observatory, hosting an OBH. 802 An improved seismicity picture of the Southern Tyrrhenian area by the use of OBS and land networks The implementation of the long-term scien- and RMS. Moreover, we show how only the tific mission of the multi-parameter Ocean Bot- presence of the OBSN placed in the Southern tom Observatory GEOSTAR (Beranzoli et al., Tyrrhenian Basin allows an intense micro-seis- 2000) in a depth around 1900 m near Ustica Is- micity linked to both volcanic and tectonic pro- land (station 00 in fig. 1) was a good opportuni- cesses to be recorded. ty to plan a new OBS experiment. Between December 2000 and May 2001 the Institute of Geophysics of the University of Ham- 2. Structural setting of the Southern burg (IGH), the Research Centre for Marine Sci- Tyrrhenian Sea ence at Kiel (GEOMAR) and the GEOSTAR- group of the Istituto Nazionale di Geofisica e Vul- In addition to high quality seismic data canologia (INGV) carried out a joint seismologi- recorded by a suitable network configuration, cal experiment in the Southern Tyrrhenian Sea, the accuracy of epicentre locations depends on the so called TYyrrhenian Deep-sea Experiment the reliability of the available crustal velocity (TYDE). A 14 stations OBSN was installed on model. In the past decades the main structural the seafloor around the Aeolian Islands in the characteristics of the crust under the Southern area comprised between latitude 38.4°N-39.6°N Tyrrhenian Sea have been the subject of numer- and longitude 14.0°N-15.4°N (stations 01-14 in ous studies, using mainly data from seismic re- fig. 1), at water depths ranging between 1500 m fraction experiments. To extrapolate velocity and 3500 m. Each station consisted of a wide- models also for depths or areas outside the stud- band hydrophone (OBH), six of them were addi- ied crustal sections, we briefly review the main tionally equipped with a 3-component seismic results of those former studies on the structural broadband sensor (OBS) (Dahm et al., 2002). All setting of the Southern Tyrrhenian area. the stations except OB07 were recovered. Addi- The tectonic evolution of the Tyrrhenian tionally, some temporary broad-band seismic sta- Basin must be considered in a more general geo- tions were installed in Sicily and on the Aeolian dynamic process of the collision between Euro- and Ustica Islands, to increase the density of the pean and African plates (Dewey, 1989). This col- permanent seismic network configuration (Braun lision caused the subduction of the ancient te- et al., 2001). The data set collected during the thyan lithosphere under the European plate, with TYDE-experiment overcomes the difficulties en- the fragmentation of the European plate margin countered in the past experiments, because: i) we and the Alpine orogenic belt and the opening of used broad-band instrumentation, able to record the Tyrrhenian Basin (e.g., Malinverno and Ryan, both local seismicity and even teleseismic events; 1986; Faccenna et al., 1997; Argnani, 2000), ii) the entire experiment lasted 6 months; iii) OB- which started in the Miocene (Kastens and Mas- SN was well-spaced (about 30 km) allowing the cle, 1990). simultaneous detection of local seismic events at The Southern Tyrrhenian Basin is charac- 3 or more OBSN-stations. terised by the presence of two areas floored by The present paper analyses the hypocentral oceanic crust: the Vavilov Basin of Pliocene age determinations of the local seismicity as deter- and the Marsili Basin, where opening started mined by the land-based network and the OB- less then 2 Ma ago (Marani and Trua, 2002). SN. We discuss the difficulties concerning the The Marsili Basin is surrounded to the south and choice of the crustal velocity model, demon- to the east by the Aeolian Arc, constituted by strating that the integration of observations from volcanic islands and submerged volcanoes of land-based stations and from the OBSN is use- Quaternary age (Barberi et al., 1974). Partly, ful to locate earthquakes in the Tyrrhenian Basin these volcanoes rest on a crystalline basement of more accurately.

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