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Sounds of geological plumbing systems - How transient fluid circulation processes in faults and volcanoes shape sources and patterns of microseismicity

Abstract : The activity of the smallest earthquakes that can be detected in the Earth is a symptom of minute changes that occur beneath our feet. Microseismic activity can in particular be used to monitor how fluid flows in geological plumbing systems like faults or volcanoes, from deep sources of fluid to the surface. In subduction zones for instance, the plate interface emits faint, intermittent, low-frequency seismic signals called tremor, composed of small individual low-frequency earthquakes (LFEs). In this manuscript, we try to understand how unsteady fluid circulation processes in the subduction fault zone can shape the source and spatio-temporal patterns of tremor, how tremor can therefore be used to probe dynamic fluid processes at depth. In a first part, we characterize the source of LFEs in Guerrero, Mexico, and find that their duration seems independent of their magnitude. This observation is inconsistent with the classical view that earthquakes should be generated by a rupture of the rock. It can however be explained by a model of LFE source that involves fluid pressure transients in the fault interface, fast and strong enough to generate seismic waves. The source of LFEs is not the sole indicator of strong, localized variations of fluid pressure in the fault zone. Geological observations of fossilized subductions indicate that the fault permeability opens and closes episodically, and that the fluid pressure drops and increases as a result. Such widespread unsteady hydraulic conditions in the fault could be a key factor shaping the complexity of spatio-temporal patterns of tremor activity. They are however seldom considered in attempts to describe the processes at the origin of the full spectrum of tremor patterns observed in subduction zones. In a second part, we therefore design a framework to describe the unsteady pro- cesses of fluid circulation in the subduction fault, and how they trigger elementary tremor events. We model fluid ascent from metamorphic dehydration sources through a permeable channel in the fault interface. Low-permeability segments of the channel impede the flux, locally producing strong pressure gradients. In an elementary valving mechanism, the permeability barriers break open and then heal shut in response to the strength of the local fluid pressure gradient. When valves break they produce a simulated LFE, the rapid fluid pressure transient potentially being the source of seismic waves. Valves can trigger each other through pressure transients. Their interaction lead to realistic, tremor-like patterns: cascades, synchronized bursts and migrations of activity, on both short and long time and space scales. We describe how the fluid input rate in the channel allows for activity, and more specifically which conditions best favor the emergence of realistic patterns of activity. We show that stronger interactions, and therefore more clustered, periodic activity can occur when valves are clustered in space, and when flux conditions make them more critical, and thus more susceptible to interactions. Based on our results, the emergence of large, complex, long-period activity transients could come from the collective behavior of valve like segments in the interface, shaping large scale valving behavior of the fault interface. This work provides a conceptual framework to understand how heterogeneous, intermittent fluid pressure conditions can emerge in the subduction fault. Our model of how the seismic source is triggered by the fluid pressure field and affects it in re- turn allows to conceptualize how elementary sources can interact in an hydraulic system, and build up patterns of seismicity. Beyond the subduction zone, this frame- work has a more general reach to interpret how fluid unsteadily flows in all geologic plumbing systems. The permeability valve mechanism is a powerful yet simple description that allows to think of fluid transport as a series of fluid pressure accumulation and release, that shape intermittent fluid transport, and intermittent seismicity as a symptom.
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https://hal-cnrs.archives-ouvertes.fr/tel-03816151
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Submitted on : Saturday, October 15, 2022 - 4:19:08 PM
Last modification on : Friday, October 21, 2022 - 3:34:06 PM

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  • HAL Id : tel-03816151, version 1

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Gaspard Farge. Sounds of geological plumbing systems - How transient fluid circulation processes in faults and volcanoes shape sources and patterns of microseismicity. Geophysics [physics.geo-ph]. Université Paris Cité; Institut de Physique du Globe de Paris (IPGP), 2022. English. ⟨NNT : ⟩. ⟨tel-03816151⟩

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