Along-Strike Segmentation of Seismic Tremor and Its Relationship With the Hydraulic Structure of the Subduction Fault Zone

Along the strike of subduction zones, tectonic tremor episodicity is segmented on a geologic scale. Here, we study how this segmentation reflects large-scale variations of the structure and conditions of the fault interface where tremor is generated. We try to understand which properties of the hydraulic system of the fault allow elementary tremor sources to synchronize, leading to the emergence of long-period, large-scale episodic activity. We model tremor sources as being associated with rapid openings of low-permeability valves in the fault zone, which channels the upward flow of metamorphic fluids. Valve openings cause pressure transients that allow interaction between sources. In such a system, tremor activity is thus controlled by unsteady fluid circulation. Using numerical simulations of fluid flow, we explore the impact of valve spatial distribution and fluid flux on the emergence of large-scale patterns of tremor activity. We show that when valves are densely distributed and submitted to near-critical input flux, they synchronize and generate more episodic activity. Based on our model, the most periodic and spatially coherent tremor bursts should thus be emitted from segments densely populated with valves, and therefore of lower permeability than less synchronized segments. The collective activity of their valve population is responsible for fluid-pressure cycling at the subduction scale. In the tremor zone of Shikoku, Japan, the most temporally clustered segment coincides with a downgoing seamount chain, suggesting that the segmentation of the fault zone permeability, and hence of tremor activity, could be inherited from the topography of the subducting oceanic plate. In subduction zones, the fault zone controls plate convergence through friction, controlling if, when and where earthquakes occur. At depths larger than about 40 km, deformation in the fault vicinity transitions to a more stable, ductile regime. At those depths, no earthquakes are expected, and noisy, emergent tectonic tremor is detected instead. Geological and geophysical observations link tremor with the unsteady circulation of high-pressure fluid in the fault zone. Tremor could thus help understand how fluid flows along the subduction interface, where it acts to lower the fault strength and may therefore trigger seismic events. Tremor occurs intermittently, in bursts followed by quiet periods. In this study, we investigate the role of fluid circulation processes in generating tremor, and why its activity varies across different regions. In our model, the intermittence of tremor comes from the intermittence of fluid circulation in the fault. We describe how many small parts of the fault zone can interact, and open or close coherently, generating pulses of fluid flow and the observed bursts of tremor. This framework allows to interpret variations of tremor intermittence as a symptom of how strong the flow and how well fluid circulates in different parts of the subduction interface. In subduction zones, the intensity of temporal clustering and the periodicity of tectonic tremor are segmented along-strikeWe use a model of fluid circulation in the fault to show that segmentation of activity can be caused by variation of transport propertiesTremor segmentation aligns with subducting seamounts in Shikoku, Japan, suggestive of the influence of slab topography

Automated summary

This study investigates the segmentation of tectonic tremor activity along subduction zones and its relationship with the structure and conditions of the fault interface. The findings suggest that the density and distribution of valves in the fault zone play a crucial role in synchronizing tremor sources and generating large-scale episodic activity. The study also links the segmentation of tremor activity to the topography of the subducting oceanic plate.