Polarization and high resolution parametric spectral analisys applied to the seismic signals recorded on the Marsili submarine volcano

Vienna

The Ocean Bottom Seismometer with Hydrophone deployed by the Gibilmanna OBS
Lab (CNT-INGV) from the 12th to the 21st July 2006 on the flat top of the Marsili
submarine volcano (790m of depth) recorded more than 1000 seismic events. By
comparing them with the ones recorded in other volcanic areas and described in literature
(Wassermann, 2002; McNutt, 2002; Díaz et al., 2007), we grouped these events
in three categories: 817 VTB (Volcanic-Tectonic type B) events, 159 HF (High Frequency)
events and 53 SDE’s (Short Duration Event). Small-magnitude VTB swarms,
with frequency band between 2 and 6 Hz and mean time length of about 30 seconds,
were almost all recorded in the first 7 days, while in the last 2 days, OBS recorded
HF events with frequency band over 40 Hz and few minutes of length. Signals with
similar frequency and time domain features are associated, to hydrothermal activity
(Ohminato, 2006). The SDE waveform, characterized by a monochromatic signal with
a slowly decaying envelope, is generated by oscillations of a resonant body excited
by magmatic or hydrothermal activity (Chouet, 1996). We applied, to all the signals
dataset, polarization and high resolution parametric spectral analysis. This kind of
study allowed to mark the VTB events as multi P-phase events with shallow sources
placed in a narrow azimuthal window as regards the OBS/H position. The seismogenetic
volume is probably located in the North-East sector of the Marsili building.
The high resolution parametric spectral analysis of the SDE signals allowed to find
with high accuracy their dominant complex frequencies (!=f+ig). Plotting them in the
complex frequencies plane we identified two distinct clusters with middle complex
frequencies f=7.8s^−1, g=-0.35s^−1 and f=7.5s^−1, g=-0.47s^−1 respectively. These two clusters are probably linked two different seismogenetic volumes.

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