Abstract
We present the first investigation performed on the seismicity of Southern Tyrrhenian Sea, off-shore Sicily with the contribution of data from broad-band ocean bottom seismometers and hydrophones (OBS/H). Offshore data were recorded during the TYrrhenian Deep sea Experiment (TYDE) from December 2000 to May 2001 in the Southern Tyrrhenian Sea. Hypocenter locations of a cluster of 53 seismic events occurred in March 2001 in north-eastern Sicily were estimated by the integration of land (permanent network) and offshore (temporary network) data and compared with locations estimated from land data only. The scatter of the cluster was evaluated by dispersion parameters. The off-shore data significantly reduced the scatter of the swarm hypocenters also restricting the depth range of the cluster. Moreover, space trends of the event distribution originally shown by the land data were only partially confirmed by the land-sea joint data. In order to assess the efficiency in terms of hypocenter mislocations in the subject area, of a land-sea integrated network with respect to a land-based network, we performed simulations by assuming a grid distribution of earthquakes and a recent local 3D velocity model, computing synthetic arrival times of body waves to the stations of both network configurations (integrated and land-based) perturbing the computed times and relocating earthquakes by inversion. The results of the synthetic tests demonstrated that the presence of sea bottom stations in the Tyrrhenian basin can reduce the mislocations of large magnitude and/or superficial earthquakes in the southernmost Calabria and Messina Strait and of low magnitude and/or deep earthquakes in north-eastern Sicily. The major accuracy of synthetic earthquake locations obtained including OBS/H data provides an additional support to the interpretation of the cluster occurred in March 2001 and to the opportunity of long-term installation of an off-shore network like TYDE in the study region.
Highlights
The Southern Tyrrhenian region is characterised by a fairly rapid transition from a nearly oceanic crustal structure beneath the abyssal plain to a continental-like structure beneath Sicily and Calabria
Fault discontinuities have been mapped on grounds of geological and geophysical investigations: i) major NWtrending fault systems potentially capable to generate magnitude ≥6 earthquakes cross the lithosphere underneath the southernmost Tyr-Graziella Barberi, Laura Beranzoli, Paolo Favali, Giancarlo Neri and Tiziana Sgroi rhenian Sea (Finetti and Del Ben, 1986; Neri et al, 2003); ii) NNE- to N-trending normal faults are responsible for earthquakes of magnitude up to 7 in the Messina Strait and Southern Calabria (Boschi et al, 1997); iii) E-trending reverse faults produce events of magnitude up to 6 seismic in Central and Western Sicily (Ben-Avraham and Grasso, 1990, 1991; Neri et al, 2003)
The comparison between the D values computed from the two different network configurations reveals that, in spite of the uneven geometry of the sea network with respect to the area, the presence of seismological stations in the Tyrrhenian Basin reduces the horizontal mislocations at all depths, in north-eastern Sicily and in the southernmost Ionian Calabria, south-east of Messina Strait
Summary
The Southern Tyrrhenian region (fig. 1) is characterised by a fairly rapid transition from a nearly oceanic crustal structure beneath the abyssal plain to a continental-like structure beneath Sicily and Calabria. Rhenian Sea (Finetti and Del Ben, 1986; Neri et al, 2003); ii) NNE- to N-trending normal faults are responsible for earthquakes of magnitude up to 7 in the Messina Strait and Southern Calabria (Boschi et al, 1997); iii) E-trending reverse faults produce events of magnitude up to 6 seismic in Central and Western Sicily (Ben-Avraham and Grasso, 1990, 1991; Neri et al, 2003) These fault systems cross the crust underneath off-shore areas, and this emphasises the importance of off-shore seismometry in the region. Data recorded in the framework of TYDE allowed an unprecedented joint analysis of on-shore and offshore seismic data in Italy (see Sgroi et al, 2006) exploring the advantages of land-sea data integration with respect to local seismicity location
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