R.M. Bustin

Bio: R.M. Bustin is an academic researcher from University of British Columbia. The author has contributed to research in topics: Thrust fault & Shear zone. The author has an hindex of 1, co-authored 1 publications receiving 80 citations.

Thickness of the Seismic Slip Zone

Abstract: This article reviews geologic and other evidence constraining the thickness of the principal slip zone (PSZ) that accommodates the bulk of coseismic shear displacement during an individual rupture event. Surface deformation from rupturing may occupy swaths tens of meters or more in width, but trenches across active faults generally reveal that incremental slip is accommodated by a PSZ that is tens of centimeters or less in thickness. Geomorphic evidence, coupled with the observations from trenching, suggest a PSZ may stay well localized for distances of several kilometers through many rupture episodes. Mine exposures and exhumed fault zones demonstrate that PSZs separating different lithologies within the “fault core,” although contained within “damage zones” of variably fractured rock ranging up to hundreds of meters in thickness, often comprise just a few centimeters of gouge/ultracataclasite that have accommodated large finite displacements (>1 km). Microstructural studies demonstrate incremental slip localized still further down to 1–10 mm, as do other fault-rock assemblages (slickensides and slickenfibers, fault-veins of pseudotachylyte friction-melt, intravein septa in hydrothermal fault infills). The accumulated evidence indicates that localization of coseismic shearing to less than 10 cm on planar faults is widespread throughout the crustal seismogenic zone, with extreme localization to less than 1 cm not uncommon. However, some distributed coseismic shear may also develop, especially at rupture irregularities. Coseismic reduction of shear resistance from friction-melting (Δ T ∼ 1000°C) or from transient thermal pressurization of aqueous fluids (Δ T ∼ 100°C) requires slip during moderate-to-large earthquakes ( u > 1 m) to be restricted to narrow zones, respectively a few centimeters or tens of centimeters in thickness. Given the evidence for slip localization, the apparent scarcity of pseudotachylyte suggests either that seismic friction-melting is a rare phenomenon, or that pseudotachylyte is only rarely preserved in recognizable form within mature hydrated fault zones.

Earthquakes and rock deformation in crustal fault zones

Abstract: The physical origin of earthquakes lies ultimately in the geological structure of fault zones and the deformation processes that occur therein in response to tectonic stress. Although the possibility now exists that the nucleation regions for large earthquakes at depths of,..., 10 km may shortly become directly accessible by deep drilling, our present knowledge of fault structure and the shallow earthquake source is derived largely from a variety of indirect sources. These include seismological studies, surface studies of fault zones and earthquake ruptures, geodetic information on modes of fault slip, ·geophysical constraints on fault zone structure and rheology, and information garnered from materials science and experi­ mental rock deformation. Studies of fault zone structure and the rock products of faulting provide complementary information on deformation processes at depth in fault zones. Although descriptions of fault rocks are widespread in the geological literature (e.g. Spry 1969, Higgins 1971), it is only in the past decade that they have begun to be interpreted in the context of the physical conditions and processes prevalent in seismically active fault zones at different crustal depths (Sibs on 1977, Watts & Williams 1979, Anderson et a11983, Wise et aI 1984). Such interpretations are still at an early stage, but the deformation textures and structural associations of fault rocks have already been shown to have the potential to yield information on such diverse topics as shear stress levels, power dissipation, and seismic efficiency during earthquake faulting; on fluid pressure levels and episodic fluid flow accompanying

Effects of frictional heating on the thermal, hydrologic and mechanical response of a fault

Abstract: The mechanical response of a fault zone during an earthquake may be controlled by the diffusion of excess heat and fluid pressures generated by frictional heating. In this study we formulate a fault model which incorporates the effects of frictional heating on the thermal, hydrologic, and mechanical response of a small patch of the failure surface. This model is used to examine the parameters that control the fault response and to determine their critical range of values where thermal pressurization is significant. The problem has two time scales: a characteristic slip duration and a characteristic time for thermal pressurization. The slip duration is set by the fault geometry. The characteristic time for thermal pressurization is set by the slip rate, the friction coefficient, and the thermal and hydraulic characteristics of the medium. The response of the fault depends on the relative magnitude of these two times. Results suggest that the fault width and hydraulic characteristics of the fault zone and adjacent medium are the primary parameters controlling the mechanical response. For earthquakes occurring across zones of low porous medium compressibility (< 10−9 Pa−1) and permeability (< 10−18 m2) the characteristic time for thermal pressurization is small. In this case, frictional heating can cause fluid pressures to approach lithostatic values, the shear strength to approach zero, and the temperature rise to stabilize at a maximum value dependent on the pore dilatational and transport properties of the porous medium. Whether the patch acts as a barrier to slip or exhibits substantial strain weakening is dependent on the shear strain across the fault. Moderate slip events where shear strains exceed two cause substantial strain weakening and, consequently, large stress drops, accelerations, and displacements. Thus it is possible for the patch to act as a barrier for small earthquakes but not for large ones. Both the dynamic stress drop and total displacement decrease for zones with larger compressibility, permeability, or width. If the compressibility or permeability exceeds 10−8 Pa−1 or 10−14 m2 or the shear strain is less than one, then the effects of frictional heating may be negligible and the fault will exhibit no strain-weakening characteristics. Consequently, the patch acts as barrier that halts or resists further fault motion. Extrapolation of these results suggests that spatial variations in fault width and hydraulic characteristics will cause a heterogeneous stress drop and fault slip over the failure surface, explaining many of the features of active faulting (e.g., barriers, nonuniform slip, rupture stoppage, random ground accelerations, strong motions, and frequency-magnitude relations).