What’s down there? The structures, materials and environment of deep-seated slow slip and tremor

Abstract

Deep-seated slow slip and tremor (SST), including slow slip events, episodic tremor and slip, and low-frequency earthquakes, occur downdip of the seismogenic zone of numerous subduction megathrusts and plate boundary strike-slip faults. These events represent a fascinating and perplexing mode of fault failure that has greatly broadened our view of earthquake dynamics. In this contribution, we review constraints on SST deformation processes from both geophysical observations of active subduction zones and geological observations of exhumed field analogues. We first provide an overview of what has been learned about the environment, kinematics and dynamics of SST from geodetic and seismologic data. We then describe the materials, deformation mechanisms, and metamorphic and fluid pressure conditions that characterize exhumed rocks from SST source depths. Both the geophysical and geological records strongly suggest the importance of a fluid-rich and high fluid pressure habitat for the SST source region. Additionally, transient deformation features preserved in the rock record, involving combined frictional-viscous shear in regions of mixed lithology and near-lithostatic fluid pressures, may scale with the tremor component of SST. While several open questions remain, it is clear that improved constraints on the materials, environment, structure, and conditions of the plate interface from geophysical imaging and geologic observations will enhance model representations of the boundary conditions and geometry of the SST deformation process. This article is part of a discussion meeting issue ‘Understanding earthquakes using the geological record’.

The stabilizing effect of high pore-fluid pressure along subduction megathrust faults: Evidence from friction experiments on accretionary sediments from the Nankai Trough

“…Our results therefore support the correlation between elevated pore-fluid pressures and the abundance of slow-slip observed in many subduction zones around the world (e.g. Behr and Bürgmann, 2021).…”

Episodicity and Migration of Low Frequency Earthquakes modeled with Fast Fluid Pressure Transients in the Permeable Subduction Interface

“…In a geological context, the tortuosity ratio is small and lies in a restricted range (see above) and permeability is expected to change by one order of magnitude [10,65,67]. Using equation (12) derived above for the threshold δp ratio, we conclude that the instability criterion should be met in geological ). In the first case, q red is too low to allow the valve to open at all, no activity is recorded.…”

“…The migration of LFE activity is highly suggestive of pore-pressure diffusion [26,35]. In exhumed subduction interfaces, one observes that the brittle-to-ductile transition is intrinsically coupled to transient pore-pressure cycling and fluid transport [4,12,73]. Episodes of fluid accumulation and drainage have been documented in shallow thrust faults and are recorded by metasomatic reactions in host rocks [3,82,93].…”

Tracking Deep Sediment Underplating in a Fossil Subduction Margin: Implications for Interface Rheology and Mass and Volatile Recycling

“…coherent tectonic slices similarly requires strain localization onto weak zones within the downgoing slab, where subsequent mechanical coupling to the overriding plate drives underplating (Agard et al, 2018), but mechanisms for generating the weak décollement, its relative location (e.g., in the sediment pile vs. in the downgoing slab mantle or crust), and controls on the thicknesses of underplated packages are much less well constrained. A better understanding of deep underplating processes is essential to making predictions about the rock types typically stranded on the deep interface, and their propensity to affect permeability and fluid flow, to control seismic velocities, and to host transient slip events (cf., Behr & Burgmann, 2020).…”