The INGV's new OBS/H: Analysis of the signals recorded at the Marsili submarine volcano

The ocean bottom seismometer with hydrophone deployed on the flat top of the Marsili submarine volcano (790 m deep) by the Gibilmanna OBS Lab (CNT–INGV) from 12th to 21st July, 2006, recorded more than 1000 transient seismic signals. Nineteen of these signals were associated with tectonic earthquakes: 1 teleseismic, 8 regional (located by INGV) and 10 small local seismic events (non located earthquakes). The regional events were used to determine sensor orientation. By comparing the signals recorded with typical volcanic seismic activity, we were able to group all the other signals into three categories: 817 volcano–tectonic type B (VT-B) events, 159 occurrences of high frequency tremor (HFT) and 32 short duration events (SDE). Small-magnitude VT-B swarms, having a frequency band of 2–6 Hz and a mean length of about 30 s, were almost all recorded during the first 7 days. During the last 2 days, the OBS/H mainly recorded HFT events with frequencies of over 40 Hz and of a few minutes in length. Signals that have similar features in frequency and time domain are generally associated with hydrothermal activity. During the last two days a signal was recorded that had a frequency content similar to that of VT-B events was recorded. It will be referred to as continuous volcanic tremor (CVT). The SDE signals, characterized by a quasi-monochromatic waveform and having an exponential decaying envelope, may have been generated by oscillations of resonant bodies excited by magmatic or hydrothermal activity. By applying polarization and parametric spectral analyses, we inferred that the VT-B were probably multi P-phase events having shallow sources that were situated in narrow azimuthal windows in relation to the positions of the OBS/H. The parametric spectral analysis of the SDE signals allowed us to determine their dominant complex frequencies with high accuracy; these frequencies are distributed in two distinct clusters on the complex plane.