Friction and faulting editor's note

Earthquakes have long been recognized as resulting from a stick–slip frictional instability. The development of a full constitutive law for rock friction now shows that the gamut of earthquake phenomena—seismogenesis and seismic coupling, pre-and post-seismic phenomena, and the insensitivity of earthquakes to stress transients—all appear as manifestations of the richness of this friction law.

Journal of Geophysical Research, 1996

The coefficient of friction and velocity dependence of friction of initially bare surfaces and 1-mm-thick simulated fault gouges (< 90 gm) of Westerly granite were determined as a function of displacement to >400 mm at 25øC and 25 MPa normal stress. Steady state negative friction velocity dependence and a steady state fault zone microstructure are achieved after-18 mm displacement, and an approximately constant strength is reached after a few tens of millimeters of sliding on initially bare surfaces. Simulated fault gouges show a large but systematic variation of friction, velocity dependence of friction, dilatancy, and degree of localization with displacement. At short displacement (<10 mm), simulated gouge is strong, velocity strengthening and changes in sliding velocity are accompanied by relatively large changes in dilatancy rate. With continued displacement, simulated gouges become progressively weaker and less velocity strengthening, the velocity dependence of dilatancy rate decreases, and deformation becomes localized into a narrow basal shear which at its most localized is observed to be velocity weakening. With subsequent displacement, the fault restrengthens, returns to velocity strengthening, or to velocity neutral, the velocity dependence of dilatancy rate becomes larger, and deformation becomes distributed. Correlation of friction, velocity dependence of friction and of dilatancy rate, and degree of localization at all displacements in simulated gouge suggest that all quantities are interrelated. The observations do not distinguish the independent variables but suggest that the degree of localization is controlled by the fault strength, not by the friction velocity dependence. The friction velocity dependence and velocity dependence of dilatancy rate can be used as qualitative measures of the degree of localization in simulated gouge, in agreement with previous studies. Theory equating the friction velocity dependence of simulated gouge to the sum of the friction velocity dependence of bare surfaces and the velocity dependence of dilatancy rate of simulated gouge fails to quantitatively account for the experimental observations. EXPERIMENTAL FAULTS of slip when the sample strength is lowest and the friction velocity dependence is the most negative, changes in dilation rate are systematically smaller (Figure 5c, inset c2), similar to the initially bare surface response (Figure 4c). These observations can be quantified by determining the net thickness changes described by tz = AL / AlnV [Marone and Kilgore, 1993] and the changes in dilation rate A(dL / d5)ss / AlnV Example 1

2011

Journal of Geophysical Research, 2008

Frictional properties of natural kaolinite-bearing gouge samples from the Median Tectonic Line (SW Japan) have been studied using a high-velocity rotary shear apparatus, and deformed samples have been observed with optical and electron (scanning and transmission) microscopy. For a slip velocity of 1 m s À1 and normal stresses from 0.3 to 1.3 MPa, a dramatic slip-weakening behavior was observed. X-ray diffraction analysis of deformed samples and additional high-velocity friction experiments on pure kaolinite indicate kaolinite dehydration during slip. The critical slip-weakening distance D c is of the order of 1 to 10 m. These values are extrapolated to higher normal stresses, assuming that D c is rather a thermal parameter than a parameter related to a true characteristic length. The calculation shows that dimensionally, D c / 1/s n 2 , where s n is the normal stress applied on the fault. The inferred D c values range from a few centimeters at 10 MPa normal stress to a few hundreds of microns at 100 MPa normal stress. Microscopic observations show partial amorphization and dramatic grain size reduction (down to the nanometer scale) localized in a narrow zone of about 1 to 10 mm thickness. Fracture energy G c is calculated from the mechanical curves and compared to surface energy due to grain size reduction, and energies of mineralogic transformations. We show that most of the fracture energy is either converted into heat or radiated energy. The geophysical consequences of thermal dehydration of bonded water during seismic slip are then commented in the light of mineralogical and poromechanical data of several fault zones, which tend to show that this phenomenon has to be taken into account in most of subsurface faults and in hydrous rocks of subducted oceanic crust.

2011

We explore experimentally and theoretically how fault edges may affect earthquake and slip dynamics, as faults are intrinsically heterogeneous with common occurrences of jogs, edges and steps. In the presented experiments and accompanying theoretical model, shear loads are applied to the edge of one of two flat blocks in frictional contact that form a fault analog. We show that slip occurs via a sequence of rapid rupture events that initiate from the loading edge and are arrested after propagating a finite distance. This event succession extends the slip size, transfers the applied shear across the block, and causes progressively larger changes of the contact area along the contact surface. This sequence of events dynamically forms a hard asperity near the loading edge and largely reduces the contact area beyond. These sequences of rapid events culminate in slow slip events that precede a major, unarrested slip event along the entire contact surface. We show that the 1998 M5.0 Sendai and 1995 Off-Etorofu Earthquake sequences may correspond to this scenario. Our work demonstrates, qualitatively, how a simple deviation from uniform shear loading can significantly affect both earthquake nucleation processes and how fault complexity develops.

Tectonophysics, 1995

Geophysical Research Letters, 1986

A "long" sequence of stick-slip cient) are approximately constant along the events generated along a laboratory fault, which fault. Stick-slip events generated in the model consists of eight spring-connected masses that are were found to show many similarities to earthelastically driven to slide on a frictional surface, has been examined to check whether the "large" events are predictable. The large events are found to recur at intervals of very different durations, although the elastic and frictional properties along the fault are quite uniform. The recurrence intervals are, however, approximately proportional to the displacements of the preceding quakes generated along a seismic fault; large events were found to generally occur at an approximately constant stress level and to have the maximum strain energy stored along the fault. masses may move twice in some cases, see Figure 3 in King [1975]), usually occur when most of the

Pure and Applied Geophysics, 2009

We use preseismic, coseismic, and postseismic GPS data of the 1999 Chi-Chi earthquake to infer spatio-temporal variation of fault slip and frictional behavior on the Chelungpu fault. The geodetic data shows that coseismic slip during the Chi-Chi earthquake occurred within a patch that was locked in the period preceding the earthquake, and that afterslip occurred dominantly downdip from the ruptured area. To first-order, the observed pattern and the temporal evolution of afterslip is consistent with models of the seismic cycle based on rate-and-state friction. Comparison with the distribution of temperature on the fault derived from thermokinematic modeling shows that aseismic slip becomes dominant where temperature is estimated to exceed 200°a t depth. This inference is consistent with the temperature induced transition from velocity-weakening to velocity-strengthening friction that is observed in laboratory experiments on quartzo-feldspathic rocks. The time evolution of afterslip is consistent with afterslip being governed by velocity-strengthening frictional sliding. The dependency of friction, l, on the sliding velocity, V, is estimated to be ol=o ln V ¼ 8 Â 10 À3 : We report an azimuthal difference of about 10-20°between preseismic and postseismic GPS velocities, which we interpret to reflect the very low shear stress on the creeping portion of the décollement beneath the Central Range, of the order of 1-3 MPa, implying a very low friction of about 0.01. This study highlights the importance of temperature and pore pressure in determining fault frictional sliding.

Journal of Geophysical Research, 1995

The shear traction on major strike-slip faults during earthquakes is much lower than that expected on a frictionally sliding surface in equilibrium with hydrostatic pressure. The low shear traction is explained if the fluid pressure at the time of the earthquake is much greater than hydrostatic pressure. Ductile creep within mostly sealed fault zones compacts the matrix and thus increases fluid pressure between earthquakes. Frictional dilatancy during earthquakes decreases fluid pressure below hydrostatic, and over the earthquake cycle, the fault zone is in long-term equilibrium with the country rock. This ductile mechanism is formally unified with rate and state theory for time-dependent friction when the difference between a critical porosity where the rock loses all strength and the actual porosity of cracks is used as a state variable. This choice is justified by percolation theory of mostly broken lattices. Timedependent behavior associated with changes in normal traction in the laboratory is explained by the formalism. Instability (earthquakes) sometimes occurs in the numerical experiments. However, fairly small amounts of frictional dilatancy during initial frictional creep decrease fluid pressure and preclude unstable sliding. Two coupled mechanisms for producing dilatancy on faults once an instability is well underway are evident. (1) Expansion of pore fluids associated with frictional heating increases fluid pressure offsetting the effects of increased pore volume during earthquakes. There is some tendency for pore volume increase to balance fluid expansion so that fluid pressure stays relatively constant. (2) Production of isolated voids that do not immediately decrease fluid pressure throughout the fault zone during earthquakes can occur to the extent that the fault zone is not significantly strengthened. Although the extent of both processes is constrained by energy considerations, the variation of fluid pressure during earthquakes is not yet well enough understood to predict stress drop from observable material properties.

Rendiconti Lincei-scienze Fisiche E Naturali, 2010

Despite considerable effort over the past several decades, the mechanics of earthquake rupture remains largely unknown. Moderate- to large-magnitude earthquakes nucleate at 7–15 km depth and most information is retrieved from seismology, but information related to the physico-chemical processes active during rupture propagation is below the resolution of this method. An alternative approach includes the investigation of exhumed faults, such as those described here from the Adamello Massif (Italian Alps), and the use of rock deformation apparatus capable of reproducing earthquake deformation conditions in the laboratory. The analysis of field and microstructural/mineralogical/geochemical data retrieved from the large glacier-polished exposures of the Adamello (Gole Larghe Fault) provides information on earthquake source parameters, including the coseismic slip, the rupture directivity and velocity, the dynamic friction and earthquake energy budgets. Some of this information (e.g., the evolution of the friction coefficient with slip) can be tested in the laboratory with the recently installed Slow to HIgh Velocity Apparatus (SHIVA). SHIVA uses two brushless engines (max power 280 kW) and an air actuator in a rotary shear configuration (nominally infinite displacement) to slide solid or hollow rock cylinders (40/50 mm int/ext diameter) at: (1) slip rates ranging from 10 μm s−1 up to 9 m s−1; (2) accelerations up to 80 m s−2; and (3) normal stresses up to 50 MPa. In comparison to existing high-speed friction machines, this apparatus extends the range of sliding velocities, normal stresses and sample size. In particular, SHIVA has been specifically designed to reproduce slip velocities and accelerations that occur during earthquakes. The characterization of rock frictional behavior under these conditions, plus the comparison with natural fault products, is expected to provide important insights into the mechanics of earthquakes.