Investigating earthquake triggering of fluid seepage systems by dynamic and static stresses

Earthquakes can occasionally trigger the eruption or increase the activity of mud volcanoes and other fluid seepage systems. One long pending question is how static and dynamic stress changes can trigger eruptions of fluid expulsion features and modulate their activity. This study addresses the potential role of static and dynamic stress changes by investigating some well documented examples of triggered historical and recent eruptions in the Northern Apennines (Italy) and partly Azerbaijan. Peak dynamic stress at triggered fluid expulsion features has been estimated using measured PGV, or PGV estimated from attenuation relationships. The results suggest that seepage features are often triggered by dynamic stress changes created by earthquake faults located in the intermediate- to far-field. Paroxysmal activity of the considered fluid expulsion systems was influenced by minimum dynamic stress thresholds ranging from approximately 30-50 kPa to 15 kPa. Regarding co-seismic static stress changes, their magnitude can be large enough to dilate fault-controlled fluid pathways located in the near-field of a ruptured fault. This may be the case of the Pede-Apennine thrust (in northern Italy), which ruptured in 1501 (Mw ~6) beneath many mud volcanoes. Apart from this case, the considered fluid expulsion systems that responded to the earthquakes were stressed by negligible or subordinate changes in static stress, with few exceptions. Earthquake-related stresses have the ability to influence the eruptive activity of fluid seepage systems, but the recovery time between two consecutive eruptions is usually irregular. This variability in the repose time suggests that the achievement of a critical or trigger-able state is governed by a complex interplay among independent factors (e.g., production rate of driving gases, plumbing system characteristics, frequency of triggering earthquakes).