Shallow earthquakes in a viscoelastic shear zone with depth���dependent friction and rheology (original) (raw)
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Influence of friction and fault geometry on earthquake rupture
Journal of Geophysical Research, 2000
We investigate the impact of variations in the friction and geometry on models of fault dynamics. We focus primarily on a three-dimensional continuum model with scalar displacements. Slip occurs on an embedded two-dimensional planar interface. Friction is characterized by a two-parameter rate and state law, incorporating a characteristic length for weakening, a characteristic time for healing, and a velocity-weakening steady state. As the friction parameters are varied, there is a crossover from narrow, self-healing slip pulses to crack-like solutions that heal in response to edge effects. For repeated ruptures the crack-like regime exhibits periodic or aperiodic systemwide events. The self-healing regime exhibits dynamical complexity and a broad distribution of rupture areas. The behavior can also change from periodicity or quasi-periodicity to dynamical complexity as the total fault size or the length-to-width ratio is increased. Our results for the continuum model agree qualitatively with analogous results obtained for a one-dimensional Burridõe-Knopoff model in which radiation effects are approximated by viscous dissipation. context of a three-dimensional continuum model and a one-dimensional Burridge-Knopoff model. In our studies, dynamical complexity refers to observations of a
Dynamic Shear Cracks with Friction as Models for Shallow Focus Earthquakes
Geophysical Journal International, 1971
In this paper a dynamical model of an earthquake source is investigated. This necessarily idealized model consists of a uniform elastic half space under a shearing pre-stress which tends to produce strike slip on a vertical fault plane. The fault plane is a plane of weakness across which the material is not welded but is initially inhibited from moving by a static frictional resistance which increases with depth. At a certain instant in time and depth in the half space a region of relative slip across the fault plane is initiated which spreads upwards and downwards so as to occupy at all times an infinite strip. Thus we shall be concerned only with two-dimensional SH motion (anti-plane strain). Once slipping occurs only reduced tractions act across the region of slip and it is the resulting stress drop which drives the mechanism. This model is almost the same as that considered by Berg and Weertman but goes further in that the dynamical problem is solved. We here extend previous work by Burridge and Burridge & Willis, in that we now find as part of the solution how the zone of slip spreads as well as the relative displacements, how the increasing friction prevents the crack (zone of slip) from penetrating very deeply, and eventually how it brings the whole mechanism to rest. Finally we calculate the pulse shapes in the far-field radiation and the residual displacements and stresses on the fault plane.
Ductile creep, compaction, and rate and state dependent friction within major fault zones
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.
Seismic Fault Rheology and Earthquake Dynamics
2006
As preparation for a workshop on "The Dynamics of Fault Zones" (95 th Dahlem Workshop, Berlin, January 2005), specifically on the sub-topic "Rheology of Fault Rocks and Their Surroundings", we addressed critical research issues for understanding the seismic response of fault zones in terms of the constitutive response of fault materials. That requires new concepts and a host of new observations and experiments to document material response, to understand the shear localization process and the inception of earthquake instability, and especially to understand the mechanisms of fault weakening and dynamics of rupture tip propagation and arrest during rapid, possibly large, slip in natural events. We examine in turn the geological structure of fault zones and its relation to earthquake dynamics, the description of rate and state friction at slow rates appropriate to the interseismic period and earthquake nucleation, and the dynamics of fault weakening during rapid sl...
Viscous roots of active seismogenic faults revealed by geologic slip rate variations
2013
Viscous flow in the deep crust and uppermost mantle can contribute to the accumulation of strain along seismogenic faults in the shallower crust 1 . It is difficult to evaluate this contribution to fault loading because it is unclear whether the viscous deformation occurs in localized shear zones or is more broadly distributed 2 . Furthermore, the rate of strain accumulation by viscous flow has a power law dependence on the stress applied, yet there are few direct estimates of what the power law exponent is, over the long term, for active faults. Here we measure topography and the offset along fault surfaces created during successive episodes of slip on seismically active extensional faults in the Italian Apennines during the Holocene epoch. We show that these data can be used to derive a relationship between the stress driving deformation and the fault strain rate, averaged over about 15 thousand years (kyr). We find that this relationship follows a well-defined power law with an exponent in the range of 3.0-3.3 (1σ). This exponent is consistent with nonlinear viscous deformation in the deep crust and, crucially, strain localization promoted by seismogenic faulting at shallower depths. Although we cannot rule out some distributed deformation, we suggest that fault strain and thus earthquake recurrence in the Apennines is largely controlled by viscous flow in deep, localized shear zones, over many earthquake cycles.
Temperature fields generated by the elastodynamic propagation of shear cracks in the Earth
Journal of Geophysical Research, 2004
Thermal perturbations associated with seismic slip on faults may significantly affect the dynamic friction and the mechanical energy release during earthquakes. This paper investigates details of the coseismic temperature increases associated with the elastodynamic propagation of shear cracks and effects of fault heating on the dynamic fault strength. Self-similar solutions are presented for the temperature evolution on a surface of a mode II shear crack and a self-healing pulse rupturing at a constant velocity. The along-crack temperature distribution is controlled by a single parameter, the ratio of the crack thickness to the width of the conductive thermal boundary layer, " w. For ''thick'' cracks, or at early stages of rupture (" w > 1), the local temperature on the crack surface is directly proportional to the amount of slip. For ''thin'' cracks, or at later times (" w < 1), the temperature maximum shifts toward the crack tip. For faults having slip zone thickness of the order of centimeters or less, the onset of thermally induced phenomena (e.g., frictional melting, thermal pressurization, etc.) may occur at any point along the rupture, depending on the degree of slip localization and rupture duration. In the absence of significant increases in the pore fluid pressure, localized fault slip may raise temperature by several hundred degrees, sufficient to cause melting. The onset of frictional melting may give rise to substantial increases in the effective fault strength due to an increase in the effective fault contact area, and high viscosity of silicate melts near solidus. The inferred transient increases in the dynamic friction (''viscous braking'') are consistent with results of high-speed rock sliding experiments and might explain field observations of the fault wall rip-out structures associated with pseudotachylites. Possible effects of viscous braking on the earthquake rupture dynamics include (1) delocalization of slip and increases in the effective fracture energy, (2) transition from a crack-like to a pulse-like rupture propagation, or (3) ultimate rupture arrest. Assuming that the pulse-like ruptures heal by incipient fusion, the seismologic observations can be used to place a lower bound on the dynamic fault friction. This bound is found to be of the order of several megapascals, essentially independent of the earthquake size. Further experimental and theoretical studies of melt rheology at high strain rates are needed to quantify the effects of melting on the dynamic fault strength.
Elastodynamic analysis of earthquake sequences on slowly loaded faults with rate and state friction
2000
Lapusta et al., 2000(2) have developed an efficient and rigorous numerical pro- cedure for elastodynamic analysis of earthquake sequences on slowly loaded faults. This is done for a general class of rate and state friction laws with positive direct velocity effect. We use the procedure to study the response of a 2-D strike-slip fault model with depth-variable properties. We find the fol- lowing (as partially reported by Lapusta et al., 2000(2)): Small events appear in increasing numbers for decreasing values of the characteristic slip distance of the friction law. The nucleation phase of small and large events is very similar. For a large event that is preceded by a small event (and hence hetero- geneous stress distribution), moment acceleration in the beginning of dynamic propagation exhibits "slow-downs" and subsequent "speed-ups", consistently with some observations. Insufficient time and space discretization qualita- tively changes the results. Incorporating ...
Journal of Structural Geology, 2018
Whatever the processes involved in the natural fracture development in the subsurface, fracture patterns are often affected by the local stress field during propagation. This homogeneous or heterogeneous local stress field can be of mechanical and/or tectonic origin. In this contribution, we focus on the fracture-pattern development where active faults perturb the stress field, and are affected by fluid pressure and sliding friction along the faults. We analyse and geomechanically model two fractured outcrops in UK (Nash Point) and in France (Les Matelles). We demonstrate that the observed local radial joint pattern is best explained by local fluid pressure along the faults and that observed fracture pattern can only be reproduced when fault friction is very low (µ < 0.2). Additionally, in the case of sub-vertical faults, we emphasize that the far field horizontal stress ratio does not affect stress trajectories, or fracture patterns, unless fault normal displacement (dilation or contraction) is relatively large.
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.