The fine structure of seismic emissions from the Nirano mud volcanoes (northern Apennines, Italy): a phenomenological study

Seismic signals generated by mud volcanoes could be used to monitor gas emission. To this end, a seismic array of vertical geophones and three-directional velocimeters have been deployed close to major emission cones in the Nirano mud volcano area, in the northern Apennines. A detailed analysis of the resulting time series has been performed both in time and spectral domains, revealing the existence of three distinct kinds of signals: background noise, paroxysmal phases (drumbeats), and regular sequences of identical pulses (drumrolls). Both drumbeats and drumrolls are characterised by higher frequencies than the background seismic noise. Drumbeats show relatively high amplitudes and long durations (in the order of several seconds) but an irregular time pattern, while drumrolls are constituted by very regular sequences of almost identical short (less than 1 s) monochromatic pulses. Both drumbeats and drumrolls appear to be constituted by S waves, originated locally in the same shallow subsoil area, close to the main vents. These seismic signals could be related to the mechanical interaction of shallow solid conduits with a two-phase (mud and gas) slug flow from depth, which is characterised by irregularly spaced long bubbles (drumbeats), alternated with trains of small bubbles (drumrolls).