GNGTS 2017 - 36° Convegno Nazionale

GNGTS 2017 S essione 2.1 259 Submarine landslide recognition and characterization near the coast of Genoa (“Genova Canyon”, Ligurian Sea). Towards a completely automatic processing in real time D. Scafidi 1 , M. Picozzi 2 , D. Spallarossa 1 , S. Barani 1 , G. Ferretti 1 , M. Pasta 1 1 DISTAV, Università degli Studi, Genoa, Italy 2 Università degli Studi Federico II, Neaples, Italy On August 2017 three unusual events of local magnitude (M L ) between 1.1 and 1.5 occurred offshore in front of the city of Genoa just few kilometres from the coast (Fig. 1). They were au- tomatically detected and located with high precision by the monitoring system of the Regional Seismic network of the Northwestern Italy (RSNI) belonging to the University of Genoa (DIS- TAV), where the accurate automatic software package RSNI-Picker (Spallarossa et al. , 2014) is working. Subsequent manual inspection on the registered waveforms lead us to question their tectonic origin. Indeed, the three events show waveforms with very peculiar shape (i.e., no clear S-wave arrival, signal length larger than that expected for the estimated M L ) and a limited spec- tral frequency content (Fig. 2). These characteristics are dis- tinctive more of rockslides than of earthquakes. As observed by Yama- da et al. (2012), in fact, rockslides generate seismic signals lasting several seconds (tens or more) in duration and show emergent onsets compared with small local earth- quakes, whose signals have usually shorter duration as well as different frequency content. In particular, the spectral attributes of rockslide sig- nals differ from those related to local earthquakes mainly in: i) landslide spectrograms are generally char- acterized by a triangular shape, ii) their amplitude and frequency peaks generally correspond, but higher frequencies decay more rapidly, and iii) the main energy content is usu- ally found within the 1–5 Hz range (Suriñach et al. , Dammeier et al. , 2011; Yamada et al. , 2012; Hibert et al. , 2014). In this work we explore the hy- pothesis that the recorded events are related to a submarine landslides. A first argument supporting the landslide hypothesis is provided by the locations of the three events. In- deed, they are shallow and locate in correspondence of the western flank of the northernmost sector of the “Polcevera” canyon, between about 300 (top) and 900 (bottom) m, in Fig. 1 - Map of the location of the three analysed events (red circles). Fig. 2 - Unfiltered waveforms of the nearest six seismic stations for one of the three events. For every waveform, the first phase arrival time is marked (green vertical lines).