Subsurface fluid distribution and possible seismic precursory signal at the Salse di Nirano mud volcanic field, Italy

Mud volcanoes are geological systems often characterized by elevated fluid pressures at
depth deviating from hydrostatic conditions. This near-critical state makes mud volcanoes
particularly sensitive to external forcing induced by natural or man-made perturbations. We
used the Nirano mud volcanic field as a natural laboratory to test pre- and post-seismic effects
generated by distant earthquakes.We first characterized the subsurface structure of the Nirano
mud volcanic field with a geoelectrical study. Next, we deployed a broad-band seismic station
in the area to understand the typical seismic signal generated by the mud volcano. Seismic
records show a background noise below 2 s, sometimes interrupted by pulses of drumbeatlike
high-frequency signals lasting from several minutes to hours. To date this is the first
observation of drumbeat signal observed in mud volcanoes.
In 2013 June we recorded a M4.7 earthquake, that occurred approximately 60 km far
from our seismic station. According to empirical estimations the Nirano mud volcanic field
should not have been affected by the M4.7 earthquake. Yet, before the seismic event we
recorded an increasing amplitude of the signal in the 10–20 Hz frequency band. The signal
emerged approximately two hours before the earthquake and lasted for about three hours. Our
statistical analysis suggests the presence of a possible precursory signal about 10 min before
the earthquake.

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