Faulting, friction and weakening: from slow to fast motion

Earthquakes within the oceanic lithosphere, and the slab-wedge interface, occur over an exceptionally wide range of conditions. Seismicity is observed at normal stresses ranging from ~200 to ~8000 MPa (even greater for deep focus events), temperatures from less than 100^oC up to at least 600^oC and under both dry and water saturated conditions. The range of conditions where these events occur provides the opportunity to explore the efficacy of using lab-derived mechanical properties to investigate processes responsible for seismicity. Furthermore, we know more about the rheological properties of olivine - the most abundant mineral in the oceanic lithosphere - than perhaps any other mineral that is stable where earthquakes occur. Thus, we can directly test models for how asperity-scale deformation processes are manifest in frictional behavior, at both laboratory and geologic conditions. Finally, the occurrence of earthquakes at conditions where well-calibrated phase transitions occur provides the opportunity to evaluate the role of reactions for promoting a wide range of slip behaviors observed in subduction zone settings.  In this presentation, I will first outline the conditions where earthquakes occur within the outer rise regions of subduction zones and compare these data with predictions based on extrapolation of lab data on the high-temperature frictional behavior of olivine. Using these observations as a constraint, I will then discuss mechanisms responsible for intermediate depth earthquakes, including the role of dehydration reactions in both peridotite and gabbroic lithologies.

Uniaxial constant-displacement-rate tensile tests were conducted on samples of polycrystalline forsterite + periclase, forsterite + diopside and forsterite + enstatite + diopside. At subsolidus temperatures of 1250-450 degree, a pressure of one atmosphere and a strain rate of 10^-5~10^-4 /sec, elongation up to 500% was achieved, exhibiting the superplastic behavior (Hiraga et al. 2010 Nature). Grains of the same phases aggregated perpendicular to the tensile direction which is identified in natural mylonite composed of fine-grained minerals (Hiraga et al. 2013 Geology). During superplastic flow, dynamic grain growth and constant relative grain size among the phases which is determined by Zener relationship are observed, the latter of which is also identified in nature (Tasaka et al. JGR 2014). Crystallographic preferred orientation develops during superplastic flow at a certain condition (Miyazaki et al. 2013 Nature) which we consider the result of significant grain rotation during superplasticity. To confirm this prediction, we conducted experiments to observe such grain rotation directly. The result indicates that grain-boundary-sliding is a primary source for grain rotation. We will also discuss some issues on extrapolating flow laws constructed at experimental conditions to nature that can vary a wide range from 10^-15 (mantle flow) to 10² /sec (seismic friction) in strain rate (!), for example.    

Recent friction experiments carried out under upper crustal P-T conditions have shown that microstructures typical of high temperature creep develop in the slip zone of experimental faults. These mechanisms are more commonly thought to control aseismic viscous flow and shear zone strength in the lower crust/upper mantle. In this study, displacement-controlled experiments have been performed on carbonate gouges at seismic slip rates (1 ms-1), to investigate whether they may also control the frictional strength of seismic faults at the higher strain rates attained in the brittle crust. At relatively low displacements (< 1cm) and temperatures (≤ 100 °C), brittle fracturing and cataclasis produce shear localisation and grain size reduction in a thin slip zone (150 μm). With increasing displacement (up to 15 cm) and temperatures (T up to 600 °C), due to frictional heating, intracrystalline plasticity mechanisms start to accommodate intragranular strain in the slip zone, and play a key role in producing nanoscale subgrains (≤ 100 nm). With further displacement and temperature rise, the onset of weakening coincides with the formation in the slip zone of equiaxial, nanograin aggregates exhibiting polygonal grain boundaries, no shape or crystal preferred orientation and low dislocation densities, possibly due to high temperature (> 900 °C) grain boundary sliding (GBS) deformation mechanisms. The observed micro-textures are strikingly similar to those predicted by theoretical studies, and those observed during experiments on metals and fine-grained carbonates, where superplastic behaviour has been inferred. To a first approximation, the measured drop in strength is in agreement with our flow stress calculations, suggesting that strain could be accommodated more efficiently by these mechanisms within the weaker bulk slip zone, rather than by frictional sliding along the main slip surfaces in the slip zone. Frictionally induced, grain size-sensitive GBS deformation mechanisms can thus account for the self-lubrication and dynamic weakening of carbonate faults during earthquake propagation in nature.

Frictional sliding is strongly dependent on normal stress, hence the increase of pressure with depth in Earth prohibits slip initiation deeper than ~50km and requires that deeper earthquakes initiate by a mechanism other than overcoming of static friction. However, there is no similar constraint on the sliding mechanisms. High-pressure faulting is initiated by phase transformation and yields a thin nanocrystalline oxide “gouge” exhibiting extraordinarily low friction, even in the absence of a fluid. The only known physical mechanism that can explain this lubrication is flow of the “gouge” by grain-boundary sliding (gbs). High-speed friction experiments on a wide variety of rock types show similar very low frictional resistance of topologically indistinguishable, fully-dense, randomly-oriented nanocrystalline gouge consisting of oxides not present initially, suggesting that lubrication occurs by the same mechanism as in high-pressure faulting – flow by gbs of a nanocrystalline “gouge” formed by phase transformation in the first second(s) of sliding. Microstructures within the Punchbowl Fault, a deeply-eroded ancestral branch of the San Andreas Fault, are consistent with this argument. This mechanism intrinsically resolves two major conflicts between earlier laboratory results and natural faulting -- lack of a thermal aureole around major faults (San Andreas Fault heat-flow paradox) and the rarity of pseudotachylytes (shear-heating-induced endothermic reactions prevent temperature rise to melting). Despite this strong indirect evidence of extremely weak rheology of nanocrystalline oxides, there is no direct experimental confirmation. Accordingly, we are investigating the rheology of fully-dense nanocrystalline MgO.  Preliminary results show that at moderate temperatures viscosity drops profoundly.

Dynamic stress drop events, or stick-slip, were identified as laboratory analogues of earthquakes nearly fifty years ago [Byerlee and Brace, 1968]. Developing scaling laws for relating stick-slip to earthquakes requires an understanding of both fault surface properties and loading frame characteristics (i.e., dynamic unloading response). Advances in high speed instrumentation reveal basic characteristics of stick-slip on bare granite-on-granite surfaces, including rapid stress drop, slip duration of 0.1-0.3 ms, average slip speed of 1-20 m/s, and at normal stresses above about 300 MPa, the common occurrence of surface melt and total stress drop. Dynamic friction systematically drops with increasing normal stress to µ ~0.2. Thermocouples embedded 2.5 mm from the fault surface provide total frictional heat production up to 120 kJ/m², while 1D thermal modeling predicts peak surface temperature approaching 2000°C (consistent with observed surface melt) in 400 MPa confining pressure experiments. For slower stick-slip at low normal stress, a spring and lumped-mass-slider model provides a good representation of rupture dynamics. However, for short-duration events, the length and stiffness of the loading piston may be important in determining stress drop and event duration. Increasing confining pressure leads to systematic changes in stick-slip properties such as increased surface temperature, stress drop, event duration and average slip rate, as well as decreased average dynamic friction. Understanding these dynamic rupture processes in the laboratory will provide constraints on source mechanics of earthquakes.

Improving our ability to recognize fault rock microstructures that are “diagnostic” of particular dynamic slip conditions (e.g. slip velocity, fluid pressure) is a major focus of current research in structural geology. As one example of recent progress, my talk will focus on new experiments designed to test the long-held idea that foliated fault rocks (i.e. foliated gouges and cataclasites) are the product of distributed (aseismic) fault creep. Foliated gouges and cataclasites are often found together with localized slip surfaces, the latter indicating potentially unstable (seismic) behavior. One possibility is that fault zones containing foliated fault rocks and discrete slip surfaces preserve the effects of both seismic slip and slower aseismic creep. An alternative possibility explored in our experiments is that some foliated fault rocks and localized slip surfaces develop contemporaneously during seismic slip. We studied the microstructural evolution of calcite-dolomite gouges deformed experimentally at slip velocities <1.13 m s^-1 and for total displacements of 0.03 - 1 m, in the range expected for the average coseismic slip during earthquakes of M_w 3-7. As strain progressively localized in the gouge layers at the onset of high-velocity shearing, an initial mixed assemblage of calcite and dolomite grains evolved quickly to an organized, foliated fabric. Significant fabric evolution and grain size reduction in the bulk gouge occurred before and during dynamic weakening. Strain was localized to a bounding slip surface by the end of dynamic weakening and thus microstructural evolution in the bulk gouge ceased. Our experiments suggest that certain types of foliated gouge and cataclasite can form by distributed “brittle flow” as strain localizes to a slip surface during coseismic shearing. If true, the microstructure of such foliated gouges and cataclasites in natural fault zones will provide a new opportunity to collect data on the coseismic shear zone (e.g. evolution of thickness and grain size distribution) that have been difficult to access by other means. Our experimental foliated cataclasites that formed at high slip velocity bear striking resemblance to natural examples of foliated cataclasite preserved along active normal faults in the central Apennines of Italy, and ongoing work is focused on quantitatively comparing natural and experimental examples.

Fluid injection into deep rocks is increasingly used in connection with energy extraction and for disposal of fluid wastes.  It is well known that small- to medium-scale seismicity can be induced or triggered by fluid injection. Fluctuations in pore fluid pressure may also be associated with natural seismicity. The energy release in anthropogenically-induced seismicity is strongly sensitive to the amount and pressure of fluid injected, as a result of the way in which seismic moment release is related to slipped area. It is also strongly affected by the hydraulic conductance of the faulted rock mass. Bearing in mind the scaling issues that apply, fluid injection driven fault motion can be studied on laboratory-sized samples. Here, we investigate both stable and unstable induced fault slip on pre-cut planar surfaces in two sandstones, Darley Dale and Pennant sandstones, with or without granular gouge. The rocks were chosen for their very different permeabilities to fluid flow, differing by a factor of 10⁶, whilst their mineralogies are broadly comparable. In permeable Darley Dale sandstone fluid can access the fault plane through the rock matrix and the effective stress law is followed precisely. Pore pressure change shifts the whole Mohr circle laterally. In tight Pennant sandstone fluid only injects into the fault plane itself, and the stress state in the rock matrix is not affected. Sudden access by overpressured fluid to the fault plane via hydrofracture causes seismogenic fault slips.