W. Ashley Griffith

Experimental constraints on dynamic fragmentation as a dissipative process during seismic slip

Various fault damage fabrics, from gouge in the principal slip zone to fragmented and pulverized rocks in the fault damage zone, have been attributed to brittle deformation at high strain rates during earthquake rupture. Past experimental work has shown that there exists a critical threshold in stress–strain rate space through which rock failure transitions from failure along a few discrete fracture planes to intense fragmentation. We present new experimental results on Arkansas Novaculite (AN) and Westerly Granite (WG) in which we quantify fracture surface area produced by dynamic fragmentation under uniaxial compressive loading and examine the controls of pre-existing mineral anisotropy on dissipative processes at the microscale. Tests on AN produced substantially greater new fracture surface area (approx. 6.0 m2 g−1) than those on WG (0.07 m2 g−1). Estimates of the portion of energy dissipated into brittle fracture were significant for WG (approx. 5%), but appeared substantial in AN (10% to as much as 40%). The results have important implications for the partitioning of dissipated energy under extreme loading conditions expected during earthquakes and the scaling of high-speed laboratory rock mechanics experiments to natural fault zones. This article is part of the themed issue ‘Faulting, friction and weakening: from slow to fast motion’.

Integrating field observations and fracture mechanics models to constrain seismic source parameters for ancient earthquakes

Most of our understanding of earthquake rupture comes from interpretation of strong-ground-motion seismograms; however, near-rupture-tip fields of stress and particle motions are difficult to resolve. In particular, the decay of frictional resistance from a peak value at the leading tip of the rupture to a residual kinetic value and subsequent healing characterizes the earthquake process, yet the nature of this evolution in situ is still unclear. Knowledge of this coseismic frictional constitutive behavior has been supplemented by laboratory experiments, yet scaling laboratory experiments to natural faults is non-trivial because these experiments do not exactly reproduce the boundary conditions governing natural earthquakes. Field investigations of exhumed faults provide the spatial resolution needed to integrate remote seismological observations, laboratory experiments, and numerical models of the rupture process for natural ruptures. Here we build on previous work that showed that the orientation of pseudotachylyte injection veins found along the Gole Larghe fault zone in granitoid rocks of the Italian Alps can be used to infer rupture directivity and velocity. We demonstrate that the length of these veins can be used to further constrain the rupture size, slip weakening distance, stress drop, and fracture energy. The results are consistent with seismological observations and recent friction experiments incorporating rapid accelerations, placing constraints on coseismic frictional evolution in granitoid rocks.

Influence of mechanical stratigraphy on clastic injectite growth at Sheep Mountain anticline, Wyoming: A case study of natural hydraulic fracture containment

Sandstone injectites ranging from 1 km in outcrop length intrude the Cretaceous Mowry Formation in the vicinity of Sheep Mountain anticline (Bighorn Basin, Wyoming, USA). These injectites were sourced from the Peay Sandstone Member of the overlying Cretaceous Frontier Formation and represent a significant possible fluid pathway through impermeable shales. Sand injection occurred along dikes and sills, interacting with bedding discontinuities and preexisting joints in the Mowry Formation during the early folding of Sheep Mountain anticline. We argue that, in contrast to the passive sweeping of sediments into fissures characteristic of Neptunian dike formation, downward intrusion of the Peay sand was forceful and made possible by a highly stratified horizontal stress field resulting from the deposition, burial, and lithification history of the rock units in the area. The internal structure of the injectites is dominated by two sets of mutually offsetting deformation bands. The deformation bands have shear and compaction components, exhibiting significant porosity loss, as well as cataclasis and minor pressure solution. After formation of the deformation bands, subsequent faulting was localized along the margins of deformation bands, evidenced in the field by slickensided surfaces. A detailed kinematic analysis of slickenline lineations yields shortening and extension axes consistent with deformation band formation during early Laramide–oriented shortening, and continuing through the folding of Sheep Mountain anticline. Beyond the formation and deformation of these sandstone injectites, this study highlights the importance of mechanical stratigraphy in the containment of hydraulic fractures.