Insights into the dynamics of the Nirano Mud Volcano through seismic characterization of drumbeat signals and V/H analysis

Mud volcanoes are rapidly-evolving geological phenomena characterized by the surface expulsion of sediments and fluids from over-pressurized underlying reservoirs. We investigate the Nirano Mud Volcano, Northern Italy, with seismic methods to better understand the dynamic evolution of the system and shed light on its subsurface structure. Our study allowed to detect and characterize three different types of high-frequency drumbeat signals that are present in the most active part of the mud volcano plumbing system. With a back-projection method based on the cross-correlation envelope of signals recorded at different station pairs, we can determine the source location of the drumbeats. These coincide with the location of V/H (vertical-to-horizontal) amplitude peaks obtained from an ambient vibration profile and resistivity anomalies identified in a previous study. We observe that the drumbeats are P-wave dominated signals, with characteristics similar to those found in magmatic settings, i.e. LPs (long-period signals). We suggest that such tremors originate from the migration of mud and gas inside the mud volcanic conduits. The source location, waveform and frequency content of the drumbeats evolve over time. We found that drumbeat occurrence is directly linked with morphological changes at surface.