New observations of local seismicity by the SN-1 seafloor observatory in the Ionian Sea, off-shore Eastern Sicily (Italy)

Abstract

S U M M A R YSeismicity in Eastern Sicily as recorded by the Submarine Network-1 seafloor observatory (SN-1) in the period from 2002 October to 2003 May is examined with the aim of identifying the as yet poorly known seismogenic zones placed in the Ionian basin, where some of the strongest earthquakes have occurred. A comparison between the seismic recordings of land networks and the seafloor station has allowed us to focus on low-magnitude seismicity only recorded by SN-1. We have analysed 239 high-quality events from among a total of 485 seismic signals not included in the land-based network bulletins. The waveform features and the possible source zones for those events are investigated by means of polarization and particle motion techniques. Most of the 239 events (213) are characterized by high values of rectilinearity typical of P-and S-arrival particle motions, while the remaining 26 events have different polarization features, with an emergent first phase and prevalently planar polarization. We have interpreted the latter signals as being associated to submarine landslides. From particle motion analysis, we have determined the azimuthal distribution of the events and the incidence angles of P waves in respect to the Observatory with the aim of determining their distribution in relation to the active but scarcely known structural setting of the off-shore area. Moreover, the integrated locations of some earthquakes occurring in the study area and recorded by SN-1 and land stations was performed to determine the apparent P-wave velocity necessary to calculate source-station distances. As an additional result of the integration, we have obtained more accurate locations of earthquakes occurring in the coastal and off-shore areas of Eastern Sicily, associated with reduced horizontal and vertical errors and significantly lower values of azimuthal gaps. Lastly, a location distribution of the 213 analysed events was obtained by setting two conditions: (1) a maximum epicentral distance to a fixed depth coinciding with the depth of the seafloor station and (2) a minimum epicentral distance associated to the maximum depth of events. Accordingly, two patterns of seismicity were determined for the maximum and the minimum expected spread of the hypocentres. The main features of both patterns are a diffuse seismicity in the Western Ionian basin with a major epicentre density SE of SN-1 and a depth of most of the events within 60 km. Local magnitude determination was also performed, taking into account an attenuation law proposed for Southeastern Sicily. Despite the uncertainties in the location distribution using single-station recordings, the results show diffuse seismicity all around SN-1 and, in particular, in the off-shore area.

Seafloor Seismic Noise at Central Eastern Mediterranean Sites

“…In particular, in the western Ionian Sea the detection and location capability of the little-known offshore seismicity was improved by the use of seafloor seismometers, and Mt. Etna activity could be monitored and investigated at a deep-sea site (Sgroi et al, 2007). In the Tyrrhenian Sea, seafloor measurements have given us important information on little known but important structures such as the Marsili seamount (e.g., Beranzoli et al, 2014).…”

New insights on volcanic and tectonic structures of the southern Tyrrhenian (Italy) from marine and land seismic data

“…The only published tomography study based on data recorded by marine stations is by Montuori et al [2007], but it is at a much wider scale and is concentrated on the upper mantle of the southern Tyrrhenian. The ocean bottom seismometer (OBS) deployed during the long-term Tyrrhenian Deep Sea Experiment (TYDE EC project) [Dahm et al, 2002] and the SN-1 seafloor observatory [Monna et al, 2005;Sgroi et al, 2007] increased station coverage north (mostly) and east of Sicily, allowing the detection of previously unknown seismicity at sea. Thanks to these new long-term marine experiments, we could cover the gap between the available local and regional tomography models and obtain previously unknown information on this area.…”

NEMO-SN1 Abyssal Cabled Observatory in the Western Ionian Sea

“…SN1 recorded a significant number of events not recorded by the on-land seismic network, despite its very dense coverage. The analysis of these seismic signals shows two main categories of events, earthquakes, and submarine slumpings, which point to seafloor instabilities [14]. The quality of the SN1 seismic recordings is high, with good signal-to-noise ratio (SNR), mainly thanks to good coupling of the seismometer to the seabed [17].…”

“…In fact, the site of deployment of NEMO-SN1 observatory is one of the most seismically active areas of the Mediterranean [14]. Some of the strongest earthquakes (M7 ) shook the area in 1169 (M 6.6), 1693 (M 7.4), and 1908 (M 7.2) [15], also causing very intense tsunami waves [16].…”