Fluidization and melting of fault gouge during seismic slip: Identification in the Nojima fault zone and implications for focal earthquake mechanisms (original) (raw)
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Journal of Structural Geology, 2010
Microstructures and grain size distribution from high velocity friction experiments are compared with those of slow deformation experiments of for the same material (Verzasca granitoid). The mechanical behavior of granitoid gouge in fast velocity friction experiments at slip rates of 0.65 and 1.28 m/s and normal stresses of 0.4-0.9 MPa is characterized by slip weakening in a typical exponential friction coefficient vs displacement relationship. The grain size distributions yield similar D-values (slope of frequency versus grain size curve ¼ 2.2-2.3) as those of slow deformation experiments (D ¼ 2.0-2.3) for grain sizes larger than 1 mm. These values are independent of the total displacement above a shear strain of about g ¼ 20. The D-values are also independent of the displacement rates in the range of w1 mm/s to w1.3 m/s and do not vary in the normal stress range between 0.5 MPa and 500 MPa.
Localized Slip and Associated Fluidized Structures Record Seismic Slip in Clay-Rich Fault Gouge
Journal of Geophysical Research: Solid Earth
Fault rocks can weaken dramatically with increasing slip rate, which results in localization of slip and earthquakes. Exhumed fault zones and fault rocks deformed at seismic rates in the laboratory both show that deformation can become extremely localized to zones less than or equal to millimeters thick. However, localization can occur during aseismic slip, so evidence of localization cannot necessarily be interpreted as having occurred coseismically. Dynamic weakening that occurs during earthquakes is the result of processes that are unique to seismic slip rates, and previous results from carbonates show that these processes produce unique microstructures. We evaluate whether coseismic deformation at low normal stress produces unique structures within the localized slip zones and adjacent gouge that develop in clay-rich gouge from the Central Deforming Zone of the San Andreas fault. We measured the thickness and orientations of localized slip zones and their internal lamina, particle orientations, and particle size distributions of gouge sheared from 0.35 to 1.3 m/s velocity, up to 25-m displacement, and 1-MPa normal stress, under water-wet and room-dry conditions. We find that the thicknesses of localized slip zones and their internal laminae are consistent with numerical formulations for thermal pressurization in wet gouge and both thermal decomposition and cataclastic deformation in dry gouge. Localized zones form coincident with a zone of fluidized gouge that accommodates at most 10% of the shear strain. We conclude that the combined occurrence of foliated localized shear zones with a zone of fluidized gouge may provide a record of seismicity in clay-rich gouges.
Geophysical Research Letters, 2010
We conducted high-velocity friction experiments on clay-rich fault gouge taken from the megasplay fault zone in the Nankai subduction zone under dry and wet conditions. In the dry tests, dehydration of clay minerals occurred by frictional heating, and slip weakening is related to thermal pressurization associated with water vaporization, resulting in a random distribution of clay-clast aggregates in the gouge matrix. In the wet tests, slip weakening is caused by pore-fluid pressurization via shear-enhanced compaction and frictional heating, and there is a very weak dependence of the steady-state shear stress on the normal stress. The resulting microstructure reflects the grain size segregation in a granular-fluid shear flow at high shear rates. These results suggest that earthquake rupture propagates easily through clay-rich fault gouge by high-velocity weakening, potentially leaving the microstructures resulting from the frictional heating or the flow sorting at high slip rates.
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.
Geochemistry Geophysics Geosystems, 2021
Seismically active faults pose a major threat to many communities worldwide. Therefore, it is vital to make appropriate predictions on the probability of large earthquakes and their associated effects, such as tsunamis and mass movements. Several factors contribute to the difficulties to estimate seismic hazard in the vicinity of such faults. Besides the vulnerability of structures and the societal impact, geological factors play an important role in seismic hazard assessment and the development of models that describe fault activity (Zöller & Hainzl, 2007). Current models for earthquake recurrence incorporate mathematical models of earthquake statistics (Gutenberg-Richter, Omori-Utsu-Aftershocks, Brownian-First-Passage-Time), numerical models of earthquakes and rupture processes (Rate-and-State-Friction), interseismic stress built-up and the interaction of multiple faults over a larger area via stress transfer (e.g.,
Laboratory gouge friction: Seismic-like slip weakening and secondary rate- and state-effects
Geophysical Research Letters, 2002
1] We investigate experimentally the frictional response of a thick sample of simulated fault gouge submitted to very high shear displacements (up to 40 m) in an annular simple shear apparatus (ACSA). The frictional strength of our granular material exhibits velocity-weakening consistent with classical rate-and statedependent friction laws. The length scale involved in the latter phenomenon is d c = 100 mm. However, the evolution of friction is largely dominated by a significant slip-weakening active over decimetric distances (L = 0.5 m). Interestingly, these decimetric frictional length scales are quantitatively compatible with those estimated for natural faults. During shearing, a thin and highlysheared layer emerges from the thick and slowly-deforming bulk of the sample. Because of the intermittent and non-local coupling observed between these two zones, we relate the large frictional length scales in our data to the slow structuring of the thick interface.
The effect of water on strain localization in calcite fault gouge sheared at seismic slip rates
Journal of Structural Geology
Strain localization during coseismic slip in fault gouges is a critical mechanical process that has implications for understanding frictional heating, the earthquake energy budget and the evolution of fault rock microstructure. To assess the nature of strain localization during shearing of calcite fault gouges, high-velocity (max 1 m s v =) rotary-shear experiments at normal stresses of 3-20 MPa were conducted under room-dry and wet conditions on synthetic calcite gouges containing dolomite gouge strain markers. When sheared at 1 m/s, the room-dry gouges showed a prolonged strengthening phase prior to dynamic weakening, whereas the wet gouges weakened nearly instantaneously. Microstructural analysis revealed that a thin (<600 µm) highstrain layer and through-going principal slip surface (PSS) developed after several centimeters of slip in both dry and wet gouges, and that strain localization at 1 m/s occurred progressively and rapidly. The strain accommodated in the bulk gouge layer did not change significantly with increasing displacement indicating that, once formed, the high-strain layer and PSS accommodated most of the displacement. Thus, a substantial strain gradient is present in the gouge layer. In water-dampened gouges, localization likely occurs during and after dynamic weakening. Our results suggest that natural fault zones in limestone are more prone to rapid dynamic weakening if water is present in the granular slipping zones.
Breaking Up: Comminution Mechanisms in Sheared Simulated Fault Gouge
Pure and Applied Geophysics, 2011
The microstructural state and evolution of fault gouge has important implications for the mechanical behaviour, and hence the seismic slip potential of faults. We use 3D discrete element (DEM) simulations to investigate the fragmentation processes operating in fault gouge during shear. Our granular fault gouge models consist of aggregate grains, each composed of several thousand spherical particles stuck together with breakable elastic bonds. The aggregate grains are confined between two blocks of solid material and sheared under a given normal stress. During shear, the grains can fragment in a somewhat realistic way leading to an evolution of grain size, grain shape and overall texture. The 'breaking up' of the fault gouge is driven by two distinct comminution mechanisms: grain abrasion and grain splitting. The relative importance of the two mechanisms depends on applied normal stress, boundary wall roughness and accumulated shear strain. If normal stress is sufficiently high, grain splitting contributes significantly to comminution, particularly in the initial stages of the simulations. In contrast, grain abrasion is the dominant mechanism operating in simulations carried out at lower normal stress and is also the main fragmentation mechanism during the later stages of all simulations. Rough boundaries promote relatively more grain splitting whereas smooth boundaries favor grain abrasion. Grain splitting (plus accompanying abrasion) appears to be an efficient mechanism for reducing the mean grain size of the gouge debris and leads rapidly to a power law size distribution with an exponent that increases with strain. Grain abrasion (acting alone) is an effective way to generate excess fine grains and leads to a bimodal distribution of grain sizes. We suggest that these two distinct mechanisms would operate at different stages of a fault's history. The resulting distributions in grain size and grain shape may significantly affect frictional strength and stability. Our results therefore have implications for the earthquake potential of seismically active faults with accumulations of gouge. They may also be relevant to the susceptibility of rockslides since non-cohesive basal shear zones will evolve in a similar way and potentially control the dynamics of the slide.
Fault slip controlled by gouge rheology: a model for slow earthquakes
Geophysical Journal International, 2004
During 1997 several slow earthquakes have been recorded by a geodetic interferometer located beneath Gran Sasso, central Italy. The strain rise times of the events range from tens to thousands of seconds and strain amplitudes are of the order of 10 −9. Amplitudes scale with the square root of the rise time and this suggests a diffusive behaviour of the slip propagation along the fault. In this work, we develop a model in which slip diffusion is the result of the presence of a gouge layer between fault faces, with a viscoplastic rheology. The fluid velocity field in the gouge layer diffuses in the directions of fault length and fault thickness, with different characteristic times. This model reproduces the relation between amplitude and rise time of measured strain signals. Synthetic straingrams, obtained for a horizontally layered, flat Earth and a source located a few kilometres from the instrument, are in agreement with observed signals.