Earthquake triggering of mud volcanoes and fluid seepage systems in fold-and-thrust belts and subduction zones

Abstract

<p>Various types of fluid expulsion features occur often at fold-and-thrust belts and subduction zones. The seepage features originate from the discharge and extrusion to the topographic surface of fluids, gases and possibly solid material, which are sourced from in-depth reservoirs. Earthquakes can occasionally trigger the eruption or increased activity of mud volcanoes and other seepage systems. The role of static and dynamic stress changes in the triggering will vary depending on the position of the seepage features with respect to the earthquake source fault. When the seepage system is controlled by faults that rupture and generate earthquakes, the role of static stress changes is likely to be influential. Subduction zones have the highest seismic potential on Earth, so large subduction earthquakes can stress massively the fault-controlled feeder systems of seepage features located above subduction thrusts. The potential role of coseismic static stress loading on fluid expulsion systems has been evaluated for accretionary and erosional subduction margins. The most significant effects occur in the epicentral area where subduction earthquakes can produce coseismic normal stress changes exceeding 20-40 bar, although these are generally restricted to relatively small regions. The magnitude of such stress changes may exceed the tensile strength of many rock anisotropies and increase crustal permeability by dilating fault-controlled conduits channeling fluids upwards. Also in fold-and-thrust belts seepage features may be associated with seismogenic faults. For instance, rupture of the Chihshang Fault (Taiwan) in 2003 produced the Mw6.8 Chengkung earthquake, which unclamped by 3 bar the feeder system of the nearby mud volcanoes that erupted shortly after the earthquake. A similar setting is also inferred for the seismogenic Pede-Apennine thrust system in northern Italy, which is also structurally controlling a number of mud volcanoes located on its hangingwall.</p><p>Seepage features can be often trigged off by dynamic stress changes created by earthquake faults located in the intermediate- to far-field. Peak dynamic stresses related to historical and recent earthquakes that produced a response of seepage systems in the Northern Apennines fold-and-thrust belt (Italy) are calculated through PGV (measured or evaluated through GMPEs). We document response of seepage systems to some historical and recent earthquakes. Some methane vents and springs showed paroxysmal activity that was influenced by peak dynamic stress of 0.3-0.4 bar, while mud volcanoes apparently showed lower sensitivity, being influenced by dynamic stresses with amplitude ranging between 0.5 and 3.5 bar. Recently, 17 mud volcanoes erupted shortly after the main seismic events of the 2016 Central Italy seismic sequence (Mwmax6.5), showing a clear correlation with peak dynamic stresses of the order of 2-4 bar (static stress changes are instead negligible or negative).</p><p>These results collectively suggest that seepage features may respond in different ways to dynamic and static stresses depending on earthquake magnitude and epicentral distances, and that they may show different sensitivity to stress changes. Dynamic stresses are likely to exert the dominant control on the triggering, even though static stress changes can also significantly influence seepage features in the near-field.</p>