Velocity structures and kinematics in the Ionian Sea (Italy) from seismological data recorded by NEMO-SN1 seafloor observatory

Abstract

In the Western Ionian basin high magnitude earthquakes have occurred in the past and in recent times (e.g. 1193, M 6.6; 1693, M 7.4; 1908, M 7.2; 1990, M 5.7), sometimes followed by violent tsunamis [Boschi et al., 1997; Bianca et al., 1999]. The sources and mechanism related to the generation of these events are still either unknown or debated due to the inadequacy of the monitoring network to detect offshore medium low magnitude earthquakes and to the lack of a satisfactory velocity model to locate these events. In the periods October 2002February 2003 and June 2012May 2013 the NEMOSN1 seafloor observatory operated about 25 km from the eastern coast of Sicily at 2100 m of depth. During the two periods, NEMOSN1 recorded several hundreds of local events. Thanks to the good signal to noise ratio [Monna et al., 2005] of the seismological signals, the seafloor observatory recorded medium low magnitude earthquakes linked to the explosive eruption of Mt. Etna volcano occurred between October 2002 and January 2003, and, in addition, an intense microseismicity not recorded by any land station [Sgroi et al., 2007]. We integrated the travel times of about 1000 earthquakes recorded by NEMOSN1 and by the land stations with the aim of improving the location of these events. Moreover, thanks to the seismological data recorded by NEMOSN1 we were able to calculate a new 1D velocity model for the Western Ionian Sea. The first step was the preliminary relocation of the whole dataset (Figure 1a). From this dataset, we selected 108 best quality hypocentres (GAP ≤ 220°; rms ≤ 0.5 s; P and S phases number ≥ 8) and 33 seismic stations to ensure the best possible coverage of earthquakes and stations around NEMOSN1. Since the inversion involves the use of a starting velocity model [Kissling et al., 1994], we considered five initial reference models [Steinmetz et al., 1983; Hirn et al., 1991; De Voogdt et al., 1992; Continisio et al., 1997; Polonia et al., 2016] to better represent the structural heterogeneity and velocities in the offshore area of Sicily and south Calabria. Then, we computed the new 1D velocity model using the VELEST software [Kissling, 1995]. We performed many trials, adjusting the layer thickness of the initial model to better estimate the depth of the main discontinuities and the Moho. The minimum misfit of the traveltime residuals was achieved after many inversions and the output model was considered to be stable. The new 1D velocity model for the Ionian Sea consists of six layers above the Moho, located at a depth of 21 km (Figure 1b). The thickness of the layers and velocities are in agreement with the lithostratigraphic interpretation proposed by Polonia et al., [2016]. We used this new model to relocate the earthquakes of the whole dataset. The final relocations show a major concentration of earthquakes in the Mt. Etna volcano sector (in relation to its volcanic activity); a minor and more dispersed seismicity is evident in the Ionian Sea. Although events originated in the depth range 080 km, most of the earthquakes have hypocentral depths less than 30 km. An important achievement was the improvement in the location of earthquakes using the new velocity model, particularly in the Ionian offshore area, in terms of RMS, GAP, and horizontal and vertical errors. To infer the kinematics of earthquakes occurred in the Ionian basin, we computed 66 new fault plane solutions, by applying the standard FPFIT procedure [Reasenberg & Oppenheimer, 1985]. The events had a minimum number of eight clear polarities and most of the selected 66 faults plane solutions did not have discrepant polarities. The map of distribution of focal mechanisms and the EW and NS sections (Figure 2) show that earthquake kinematics are rather homogeneous. Normal, normal to strike-slip and strike-slip faulting mechanisms prevail, showing a depth distribution in good agreement with the regional structural model.