Seismic loading of fault-controlled fluid seepage systems by great subduction earthquakes

Marco Bonini

doi:10.1038/s41598-019-47686-4

Marco Bonini

Open Access

https://doi.org/10.1038/s41598-019-47686-4

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Abstract

Various types of fluid expulsion features (mud volcanoes, pockmarks, authigenic carbonate mounds and associated gas pipes, etc.) are often found above subduction zones, which have the highest seismic potential on Earth. Faults potentially control the liberation of deep-seated greenhouse gases into the feeder systems of seepage features located above subduction thrusts. These feeder systems could be stressed by large earthquakes, yet the mechanisms that can drive episodic mobilization of stored hydrocarbon gases remain poorly understood. Here I address the potential stress loading on fluid expulsion systems created by past earthquakes nucleated at both accretionary and erosive subduction margins. The most significant effects occur in the epicentral area where subduction earthquakes can produce normal stress changes as high as 20–100 bar, although these are generally restricted to relatively small regions. Coseismic normal stress changes and elastic strain relaxation upon a ruptured subduction thrust could increase crustal permeability by dilating fault-controlled conduits, and channelling fluids to the seafloor. Fluid pressure pulses released during subduction earthquakes can greatly contribute to the rupture of fluid pathways that have been brought closer to failure from coseismic static stress changes, although the inaccessible location of most submarine seepage systems has so far hampered probing these relationships.

Highlights

Methane and other greenhouse gases are released from submerged margins of subduction zones through a variety of fluid seepage systems including mud volcanoes, authigenic carbonate mounds, calderas, pockmarks and associated gas pipes/chimneys that may be controlled by directivity through faults[1,2,3,4,5]
I have evaluated above the effect of coseismic static stress changes produced by mega-earthquakes on thrust and normal faults serving as channels in the present-day methane seeps located above subduction thrusts
Subduction megathrust earthquakes produce multiple effects on mud volcano and other methane seepage systems, namely (1) coseismic shaking and pressurization/depressurization of fluid reservoirs by dynamic and static strains, (2) coseismic and early post-seismic opening/closing of fault-controlled fluid channels, and (3) fluid pulses released from ruptured overpressured compartments

Summary

Introduction

Methane and other greenhouse gases are released from submerged margins of subduction zones through a variety of fluid seepage systems including mud volcanoes, authigenic carbonate mounds, calderas, pockmarks and associated gas pipes/chimneys that may be controlled by directivity through faults[1,2,3,4,5]. Activity of submarine fluid expulsion features is likely to be episodic, similar to terrestrial mud volcanoes that release large amounts of greenhouse gases during eruptive events that may occasionally interrupt their quiescent activity[10,11]. Coseismic static stress changes are generally much smaller than dynamic stresses, but are permanent and may be exceptionally large at mud volcanoes located above subduction zones The reason for this behaviour is twofold: (1) the extremely large magnitude of megathrust earthquakes, and (2) the structural position of seepage structures and their deep plumbing systems, which are often controlled by faults rooted into the subduction thrust[8,9,18]. The consideration that fault-controlled conduits generally extend at depth near the subduction thrust (Fig. 1) has motivated the choice of a modelling that involves an elastic and isotropic rheology (see Methods)

Methods

Discussion

Conclusion