Is complex fault zone behaviour a reflection of rheological heterogeneity?

Abstract

Fault slip speeds range from steady plate boundary creep through to earthquake slip. Geological descriptions of faults range from localized displacement on one or more discrete planes, through to distributed shearing flow in tabular zones of finite thickness, indicating a large range of possible strain rates in natural faults. We review geological observations and analyse numerical models of two-phase shear zones to discuss the degree and distribution of fault zone heterogeneity and effects on active fault slip style. There must be certain conditions that produce earthquakes, creep and slip at intermediate velocities. Because intermediate slip styles occur over large ranges in temperature, the controlling conditions must be effects of fault properties and/or other dynamic variables. We suggest that the ratio of bulk driving stress to frictional yield strength, and viscosity contrasts within the fault zone, are critical factors. While earthquake nucleation requires the frictional yield to be reached, steady viscous flow requires conditions far from the frictional yield. Intermediate slip speeds may arise when driving stress is sufficient to nucleate local frictional failure by stress amplification, or local frictional yield is lowered by fluid pressure, but such failure is spatially limited by surrounding shear zone stress heterogeneity. This article is part of a discussion meeting issue ‘Understanding earthquakes using the geological record’.

Linking earthquake magnitude-frequency statistics and stress in visco-frictional fault zone models

“…Depth-dependent stress is introduced to the reference model-set, simply by scaling the viscosity distribution by a depth-dependent factor that mimics the Arrhenius temperature-dependence of typical rheology. This simplified viscosity distribution may represent a transition from pressure-solution creep of only some lithologies within much of the seismogenic zone (37), to bulk weakening of most lithologies at higher temperatures (27). We ignore the variation of frictional properties and normal stress with depth, in order to isolate the influence of viscous creep on the frictional-viscous transition.…”

Insights from the geological record of deformation along the subduction interface at depths of seismogenesis

“…Deformation fabrics within the mélange reflect rheological heterogeneity between mudstones and isolated blocks of altered basalt, chert, and sandstone , and this heterogeneity could play an important role in the deformation behavior of rocks along the interface (Fagereng and Beall, 2021). In the mudstones, there is a pervasive, scaly fabric of anastomosing slip surfaces that, combined with the preferred orientation of blocks, defines the mélange foliation (Figs.…”

Understanding earthquakes using the geological record: an introduction

“…Nonetheless, significant progress has been made in linking such diverse observations and models. For example, the recognition of transient and mostly aseismic slip in some parts of subduction zones being coupled to field-based analysis of mixed-lithology fault zones has resulted in a new understanding of the mechanisms for generating transient fault slip at velocities intermediate between earthquakes and plate motions [1][2][3][4][5]. Similarly, observations of earthquake faulting in the lower continental crust made both petrologically (preserved as pseudotachylytes) and seismologically has resulted in a new appreciation of the diversity of rheology in the continental crust, and the importance of trace amounts of water in controlling the deformation [6][7][8][9][10].…”

“…Behr & Bürgmann [3] and Fagereng & Beall [4] discuss a second prominent theme of the meeting, which was the link between observations of transient fault motion and the behaviour of mixed-rheology fault zones. The continuing development of geodetic and seismological monitoring systems and analysis methods is revealing an increasingly rich diversity of rates, lengthscales and timescales of fault motion, some of it occurring transiently at rates that are intermediate between earthquake slip and plate motion rates [23][24][25].…”

“…Because much of the transient motion is aseismic, and because the spatial resolving power of seismological and geodetic methods decreases with the depth of the deformation or structure to be imaged, field observations of exhumed fault zones have proved crucial in understanding the mechanisms of transient fault slip. Prominent among the proposed mechanisms is motion in lithologically heterogeneous fault zones, with strong clasts embedded in a weaker shear zone giving rise to mixed frictional-viscous behaviour, possibly also requiring the presence of transient fluctuations in fluid pressure (as described, modelled and reviewed elsewhere in this volume [3,4]). Considerable attention is now focused on exploring the link between the different types of transient fault motions and how they are generated and expressed geologically, and their implications for the large-scale strength and behaviour of major fault systems.…”