Faulting, friction and weakening: from slow to fast motion

09:00-09:45 Subduction zone earthquakes: A case study on the application of lab data to understand earthquake processes

Professor Greg Hirth, Brown University, USA

Abstract

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.

09:45-10:30 Superplasticity in rocks

Professor Takehiro Hiraga, University of Tokyo, Japan

Abstract

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.    

10:30-11:00 Coffee

11:00-11:45 Can grain size sensitive flow lubricate faults during the initial stages of earthquake propagation?

Dr Nicola de Paola, Durham University, UK

Abstract

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.

11:45-12:30 Phase-Transformation-Induced Nanometric Lubrication of Earthquake Sliding

Professor Harry W. Green, University of California, Riverside, USA

Abstract

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.

13:30-14:15 Interpreting slow and fast earthquakes in terms of friction

Dr David Lockner, US Geological Survey, USA

Abstract

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.

14:15-15:00 Fault gouge deformation during seismic slip

Dr Steven Smith, University of Otago, New Zealand

Abstract

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.

15:00-15:30 Tea

15:30-16:16 Field geology observations on earthquake faults

Dr Christie Rowe, McGill University, Canada

16:15-17:00 On the effective stress law for frictional sliding, and fault slip triggered by means of fluid injection

Professor Ernest H. Rutter, University of Manchester, UK

Abstract

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.

17:00-17:45 Poster session

09:00-09:45 The mechanics of slow earthquakes and the spectrum of fault slip behaviors

Dr Chris Marone, The Pennsylvania State University, USA

Abstract

Slow earthquakes represent an important conundrum in earthquake science. Whereas normal earthquakes are understood as dynamic instabilities with rupture expansion dictated by elastic wave speeds, there is no such understanding for slow earthquakes with rupture durations of months or more and fault slip speeds of ~100 micron/s or less.  What determines the rupture propagation velocity in slow earthquakes and in other forms of quasi-dynamic rupture? What processes limit stress drop and fault slip speed in slow earthquakes? Existing lab studies provide some help, however these are mainly anecdotal and rarely include examples of repetitive slow slip that could isolate the underlying mechanisms. In this talk I summarize recent lab work documenting the complete spectrum of stick-slip behaviors, with systematic measurements of repetitive, stick-slip from transient slow slip to fast stick-slip. The work includes independent measurement of the stability parameter, k_c = (b-a)/D_c. Slow slip occurs near the threshold between stable and unstable failure, controlled by the interplay of fault zone frictional properties, including rate dependence of the friction rate parameter a-b, and elastic stiffness of the surrounding rock. Our results suggest that slow earthquakes and transient fault slip behaviors can arise from the same governing frictional dynamics as normal earthquakes. The mechanism we propose could act in concert with other proposed processes, including fluid pressure and dilatancy, and provides a generic mechanism for slow earthquakes, consistent with the wide range of conditions for which slow slip has been observed.

09:45-10:30 Dynamic fault weakening due to thermal pressurization of pore fluids

Professor David Goldsby, University of Pennsylvania, USA

Abstract

High-velocity friction experiments and geophysical observations suggest that mature faults weaken dramatically during seismic slip. However, it is still unclear which coseismic weakening mechanisms are most important. Thermal pressurization is one possible coseismic weakening mechanism driven by the thermal expansion of native pore fluids, which leads to elevated pore pressures and significant coseismic weakening. While thermal pressurization has been studied theoretically for many decades, and invoked in recent earthquake simulations, its activation in laboratory experiments has remained elusive. Here we show that friction experiments on confined, fluid-saturated, thermally-cracked, low-permeability (1.3*10‐18 to 6.1*10‐19 m2) diabase rocks, at constant normal stresses in the range 25-100 MPa and initial pore pressures in the range 10–25 MPa, yield a rapid decay of shear stress following a step‐change in sliding velocity from 10 μm/s to 4.8 mm/s. In one test, the decrease in shear stress of ~20% over the first 28 mm of slip at 4.8 mm/s agrees closely with the theoretical solution for slip on a plane [Rice, 2006], with an inferred weakening length parameter L* of ~500 mm, close to the value predicted by inserting laboratory-determined rock and fluid properties into Rice’s formula for L*. In another test, steps from 10 μm/s to three different velocities - 1.2 mm/s, 2.4 mm/s, and 4.8 mm/s - yield data that all fit the Rice solution with values of L* that vary systematically with velocity as predicted by the theory. To our knowledge, this is the best experimental validation of thermal pressurization to date.

10:30-11:00 Coffee

11:00-11:45 Effects of load path on rock damage at high strain rates: implications for earthquake rupture

Dr W. Ashley Griffith, University of Texas, Arlington, USA

Abstract

Various fault damage fabrics, from gouge and rock flower 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.  It has been proposed that these textures are indicative of deformation at high strain rates during catastrophic events such as earthquake rupture.  These fault zone fabrics are significant both in terms of the information they contain about the mechanisms by which they were formed and for how they influence future earthquake ruptures. Experimental studies of these textures have focused primarily on experiments using the Split Hopkinson Pressure Bar, with which samples are loaded by a uniaxial compressive pulse, and the results of these studies suggest overwhelmingly 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 pulverization.  We present new experimental results on Arkansas Novaculite and Westerly Granite in which we demonstrate that in addition to peak stress and strain rate, the fragmentation-pulverization transition is strongly dependent on the complete loading path as defined classically in stress-strain space.  We also present new data on fracture surface area produced during the experiments.  The results have important implications for the coseismic work budget and the scaling of SHPB experimental results to natural fault zones.

11:45-12:30 Professor Giulio Di Toro, University of Manchester, UK

Professor Giulio Di Toro, University of Manchester, UK

Abstract

Moderate to large earthquakes often propagate along faults cutting carbonate rocks (dolostones, limestones, etc.). Compared to silicate-bearing rocks, which melt, weaken and wear when sheared at seismic slip rates (ca. 1 m/s), carbonate rocks do not melt, the minimum friction coefficient is very low (down to 5% of static friction) and the wear rate negligible. In cohesive carbonate rocks, experiments simulating seismic deformation conditions and stopped at slip initiation (< 5 mm) suggest that initial frictional decay down to 50% of static friction is associated with CO2 emission plus formation of nanograins and amorphous carbon. Experiments stopped at larger slips (> 5 mm) show that the slipping zone consists of a foam-like sintered surface overlying a microporous fabric made of calcite and lime (plus periclase in dolostones) nanograins. Experiments with pressurized H2O show that the contribution of thermal pressurization is negligible. We propose that initial frictional weakening is due to the formation of patches of amorphous carbon (solid lubricant) at asperity contacts. With progressive slip and bulk temperature increase, nanograins accommodate large strain rates (ca. 104 s-1) by grain size dependent processes, as suggested by several authors. The presence of a microporous fabric boosts pore-controlled diffusive process propelled by CO2 gas exhaust due to decarbonation. Enhanced pore-controlled diffusive processes allow (1) efficient mass transfer during sliding and (2) sintering of the nanograins into a foam-like surface at the end of the experiment. The sintered surface is strikingly similar to the one found in natural faults cutting carbonate rocks.

13:30-14:15 Title TBC

Professor Raul Madariaga, Ecole Normale Superieure, France

14:15-15:00 Earthquake sequence simulations using measured mechanical properties for JFAST core samples and blueschist

Dr Hiroyuki Noda, Japan Agency for Marine Science and Technology, Japan

Abstract

Since the 2011 Tohoku earthquake, multi-disciplinary observational studies have promoted our understanding of both coseismic and long-term behaviour of the Japan Trench subduction zone. Investigation of tsunami deposits (Sawai et al., 2012 and 2015) inferred recurrence interval of great tsunamigenic earthquakes between 500 and 800 years, while there are more frequent M7 events which fail to propagate to the trench. Long-term heat generation along the fault was inferred to be comparable to the effective friction coefficient (dissipation per slip/overburden) of about 0.025 from heat-flow measurements (Gao and Wang, 2014). The Japan Trench Fast Drilling Project (JFAST) revealed coseismic frictional heating of about 27 MJ/m2 (Fulton et al., 2013) in the shallowest potion of the fault where about 50 m of coseismic slip was generated (e.g., Fujiwara et al., 2011). We also have suggestions for mechanical properties of the fault from experimental side. Fault materials obtained in JFAST was investigated in terms of hydraulic (Tanikawa et al., 2013) and frictional properties (Sawai, 2015, Doctor Thesis) under hydrothermal conditions. Frictional properties of blueschist, stable metamorphic rock in the deeper part of the ruptured area in the Tohoku earthquake, were also investigated under hydrothermal conditions (Sawai et al., submitted to GRL). The model-parameter space of earthquake sequence simulation is huge, and those experiments are useful in decreasing the degree of freedom. In the talk, numerical models of earthquake sequences (e.g,. Lapusta et al., 2000) will be presented which account for the experiments and are consistent with the observational studies raised here.

15:00-15:30 Tea

16:15-17:00 Heating, weakening and shear localization in earthquake rupture

Professor James R. Rice ForMemRS, Harvard University, USA

Abstract

Field and borehole observations of active earthquake fault zones show that shear is often localized to principal deforming zones of order 0.1 mm to 10 mm width.  The presentation addresses how frictional heating in rapid slip weakens faults dramatically, relative to their static frictional strength, and promotes such intense localization. Pronounced weakening occurs even on dry rock-on-rock surfaces, due to flash heating effects, at slip rates above ~0.1 m/s (it is noted that earthquake slip rates are of order 1 m/s).  But weakening in rapid shear is also predicted theoretically in thick fault gouge in the presence of fluids (whether native ground fluids or volatiles such as H₂O or CO₂ released by thermal decomposition reactions), and the predicted localization is compatible with such narrow shear zones as have been observed. The underlying concepts show how fault zone materials with high static friction coefficients, ~0.6 to 0.8, can undergo strongly localized shear at effective dynamic friction coefficients of order 0.1, thus fitting observational constraints, e.g., of earthquakes producing negligible surface heat out-flow and, for shallow events, rarely creating extensive melt. The results to be summarized include those of collaborative research published with Nicolas Brantut (Univ. Col. Lond.), Eric Dunham (Stanford Univ.), Nadia Lapusta (Caltech), Hiroyuki Noda (JAMSTEC, Japan), John D. Platt (Carnegie Inst. Sci.), Alan Rempel (Oregon State Univ.) and John W. Rudnicki (Northwestern Univ.).