Seismic monitoring of gas emissions at mud volcanoes: The case of Nirano (northern Italy)

Seismic signals generated at the Nirano mud volcanoes in Northern Italy have been monitored by deploying a set of small dimensions seismic arrays of vertical geophones and thee-directional sensors. During two seismic surveys campaigns, seismic signals characterized by sequences of short impulsive signals (lasting 0.1 s-0.2 s) were identified above the background seismic noise. The respective seismic sources have been identified at shallow depths (<30 m) and results sparsely distributed over a wide area. Estimated propagation velocities and polarization analysis indicate that detected pulses also include a significant S waves contribution. These findings have been interpreted as the effect of a stick-slip mechanism due to the interaction between exsolved gas bubbles, mud plugs and the vent walls. On the basis of this model, an estimate of the gas outflow was attempted and results in line with independent measurements of CH4 and CO(2 )emissions carried out in the area.