Evidence of persistent seismo-volcanic activity at Marsili seamount

Abstract

The Marsili, the biggest European volcano, can be considered as being the key needed to understand the dynamics of spreading and back-arc lithosphere formation in the Tyrrhenian sector (Marani et al., 2004, and references therein). Despite of its dimensions, due to its geographical position, it is very difficult to monitor (D'Alessandro et al., 2011) and it still remains little known. In 2006 the INGV staff deployed a broadband OBS/H (Mangano et al., 2011) on the Marsili’s flat top at a depth of about 790 m. In only 9 days, the instrument recorded about 800 seismo-volcanic events (D'Alessandro et al., 2009). This experiment, for the first time, revealed an intense seismo-volcanic activity of the Marsili. However, the short duration of the experiment didn’t allow to characterize, in an exhaustively way, the seismo-volcanic activities currently in act on the seamount. For this reason, on February the 14th 2010 another OBS/H was deployed in the same point for a long time experiment (9 months). During the monitoring campaign, the submarine station recorded some thousands of local little magnitude events. The entire data set was classified, on the basis of the time and frequency domain appearances following Wasserman (2002); we recognized 589 Volcano-Tectonic type A (VT-A) events and 1952 Volcano-Tectonic type B (VT-B) events (Fig. 1), measuring their local magnitude following Havskov et al. (2003). The seismogram of a typical VT-A event is dominated by P and S phases and by a short coda (Fig. 1a). These events are characterized by P phases with impulsive and high-amplitude onsets. The spectrogram shows broadband body phases with very high frequency and energy content extending up to 80 Hz (Fig. 1c). The VT-A events recorded have local magnitude between 0.5 and 3 and time length between 40 and 70 s. For these earthquakes, separation of P and S waves is clear and TS-TP is between 0.35 and 0.55 s. Their average recurrence time is about 2-3 events a week, with moderate variation in the observed period. The VT-B events are characterized by P phases with emergent and low-amplitude onsets (Fig. 1b ). The waveforms don't show any clear S wave arrival and they are featured by long coda (Fig. 1b). The VT-B events have time length between 15 and 40 s and local magnitude between -0.5 and 1.5. Their spectrograms shows a narrow frequency content (Fig. 1d). The time distribution of VT-B occurrence shows periods of moderate activity (some events a week) alternating with periods of intense activity (70 events a day). From Fig. 1f is also clear a time cyclic process and an upward trend in the VT-B activity. An increase in VT-B activity is often reported in some active volcanoes before significant eruptions. The observation of VT swarms on the Marsili volcano strongly suggest that it is still active. Further, it should not be ignored that potential volcanic eruptions could efficiently generate tsunami along the nearby coastlines.