Subduction zone earthquakes: A case study on the application of lab data to understand earthquake processes
Professor Greg Hirth, Brown University, USA
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 100oC up to at least 600oC 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.
Superplasticity in rocks
Professor Takehiro Hiraga, University of Tokyo, Japan
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.
Can grain size sensitive flow lubricate faults during the initial stages of earthquake propagation?
Dr Nicola de Paola, Durham University, UK
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.
Phase-Transformation-Induced Nanometric Lubrication of Earthquake Sliding
Professor Harry W. Green, University of California, Riverside, USA
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.