Abstract
The problem of large location uncertainties for seismicity occurring in the Southern Tyrrhenian Sea have been partially exceeded during the implementation of the long-term scientific mission of the TYrrhenian Deep sea Experiment (TYDE), which allowed the installation of 14 wide-band Ocean Bottom Seismometers (OBS) and Hydrophones (OBH) in the period December 2000 May 2001 on the sea-bottom floor around the Aeolian and Ustica Islands. Local events recorded at landstations have been observed also 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 landstations to localise 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. Moreover, further classes of events have been analysed: the first one 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 one comprises 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, permitting to better understand both the tectonic and geodynamic picture of the area.
Highlights
Ocean Bottom Seismometer Networks (OBSN) are used in different regions of the world to Tiziana Sgroi, Thomas Braun, Torsten Dahm and Francesco FrugoniNorth America) by a joint land and OBS network, provided a more accurate evaluation of the seismicity of the intraplate region (Bebel et al, 1992)
Up to now only two short-term OBSs experiments have been performed in the Southern Tyrrhenian Sea: in 1987, a small OBS network operated for 11 days close to the Calabrian coast, recording microseismicity not detected by permanent and temporary landbased seismic networks (Soloviev, 1990); in 1996, a three week passive seismic campaign was realised by a joint Italian-Japanese collaboration around the Aeolian Islands and Ustica Island (Aoyagi et al, 1997)
Many low magnitude earthquakes occur along the tectonic and volcanic structures of the Southern Tyrrhenian Sea, but location of earthquakes recorded by the conventional land network continue to be less than satisfactory
Summary
North America) by a joint land and OBS network, provided a more accurate evaluation of the seismicity of the intraplate region (Bebel et al, 1992). Up to now only two short-term OBSs experiments have been performed in the Southern Tyrrhenian Sea: in 1987, a small OBS network operated for 11 days close to the Calabrian coast, recording microseismicity not detected by permanent and temporary landbased seismic networks (Soloviev, 1990); in 1996, a three week passive seismic campaign was realised by a joint Italian-Japanese collaboration around the Aeolian Islands and Ustica Island (Aoyagi et al, 1997). The attempt to integrate seismic data from land-based and OBS-stations and to improve the epicentre determinations revealed the following problems: i) The use of short-period seismometers (4.5 Hz), mainly used for seismic reflection surveys, turned out to be not appropriate for studies of local seismicity in the area. We discuss the difficulties concerning the choice of the crustal velocity model, demonstrating that the integration of observations from land-based stations and from the OBSN is useful to locate earthquakes in the Tyrrhenian Basin more accurately. We show how only the presence of the OBSN placed in the Southern Tyrrhenian Basin allows an intense micro-seismicity linked to both volcanic and tectonic processes to be recorded
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