Abstract
The Western Ionian Sea is characterised by an active and diffuse seismicity, directly related to the convergence of the European and African Plates and by gravitational sinking and rollback of the oceanic lithosphere. In this area, the location of earthquakes is characterised by considerable uncertainties due to large azimuthal gaps, resulting in notable location errors. This problem was partially overcome with the use of data recorded by NEMO-SN1 seafloor observatory (October 2002 - February 2003; June 2012 - May 2013). We relocated 1130 crustal and sub-crustal earthquakes using land network and NEMO-SN1 data. As most events occurred on Mt. Etna, we focused on 358 earthquakes in the offshore area and near the coasts of Sicily and Calabria. The use of the combined land-marine networks has improved the earthquake locations in terms of azimuthal GAP, as well as in horizontal and vertical errors. The comparison between locations performed with and without NEMO-SN1 data shows that differences in latitude, longitude and depths are more evident in the Western Ionian Sea and in the coast of Sicily, where values of the differences over 5 km correspond to structural heterogeneities. The increased number of seismic stations deployed on land from 2003 to 2012 did not influence the location of events occurring offshore, where NEMO-SN1 continued to be the distinctive tool in the location process. Moreover, the new 73 focal mechanisms computed with P-wave polarities from NEMO-SN1 and land stations are in agreement with the regional structural model, showing a prevalent normal, normal/oblique, and strike-slip kinematics. The similarity of two new focal solutions with the mechanisms of the main shock and aftershock of the 1990 earthquake demonstrates that the seismic structures are still active and potentially dangerous. The P-wave travel time residual analysis confirms the activity along the main structural alignments. A single point of observation in the Ionian Sea can significantly improve the quality of locations, giving an opportunity to focus on the seismogenic structures responsible for the occurrence of medium-to-high magnitude earthquakes.
Highlights
The location of seismic events in the offshore areas is uncertain using only land-based seismic networks due to the large azimuthal gaps
The two couples of events in central Sicily are characterized by similar locations, while remarkable differences of the azimuthal gap (180° for events 7-8 and 147° for events 9-10) are computed. These results indicate that the increasing seismic stations coverage has significantly improved the location of earthquakes occurring on land, without any influence in the offshore where NEMO-SN1 continued to be the distinctive tool in the location process
There having been no additional coverage in the Western Ionian Sea area, significant improvements in earthquake locations were not observed for events taking place offshore: the GAP remained substantially unchanged for couples of earthquakes 1-2, 3-4, and 5-6 (Figure 2; Table 1), all located with data recorded by the seafloor observatory, and their similar good quality is due to the use of NEMO-SN1 travel times in the location process
Summary
The location of seismic events in the offshore areas is uncertain using only land-based seismic networks due to the large azimuthal gaps. In Italy, the difficulty in precisely locating the offshore seismicity by land-based seismic networks prevalently concerns earthquakes occurring in marine areas, such as the Ionian Sea, one of the most seismically active areas in the Central Mediterranean. In this region, the highest magnitude earthquakes ever measured in Italy during historical and recent times occurred [1169, Mw 6.6; 1693, Mw 7.4; 1908, Mw 7.2; 1990, Mw 5.7; Boschi et al, 1997], often followed by severe tsunamis [Tinti et al, 2004)] Despite many studies in the Ionian Sea recognising the offshore seismogenic structures [Bianca et al, 1999; Gutscher et al, 2016], their locations are questionable or entirely unknown
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