Dynamic triggering of mud volcano eruptions during the 2016-2017 central Italy seismic sequence

On 24 August 2016 a seismic event (Mw 6.0) was the first of the long Central Italy sequence (ongoing at the end of 2017) of medium-to-high magnitude earthquakes, with nine Mw ≥5 up to October 2017, and with about 74.000 seismic events registered after one year. The largest was the Mw 6.5 30 October 2016 event near Norcia. After the major seismic events, seventeen mud volcanoes erupted around Monteleone di Fermo village (Marche region). Mud volcano eruptions generally occurred a few hours to a few days after the main earthquakes, suggesting a seismic triggering. We analyzed the peak ground velocities (PGV) and dynamic stresses during the three largest earthquakes. We also evaluated the static stress changes in order to assess the potential influence of normal stress changes on the feeder system of the activated mud volcanoes. We find a correlation with dynamic stresses, whereas static stress changes are negligible or negative (with values reaching -0.44 bar, clamping feeder dykes). We conclude that seismic shaking (up to ~3.9 bar during Norcia earthquake) is the dominant driver for these eruptions. Finally, we evaluated the response ratio as a function of the dynamic stress. It increases exponentially with peak dynamic stress varying from 0.4 bar, to >50% for peak dynamic stress >2 bar, indicating a link between earthquake shaking and mud volcano activity.