No abstract

Determination of the frictional properties of rocks is crucial for an understanding of earthquake mechanics, because most earthquakes are caused by frictional sliding along faults. Prior studies using rotary shear apparatus revealed a marked decrease in frictional strength, which can cause a large stress drop and strong shaking, with increasing slip rate and increasing work rate. (The mechanical work rate per unit area equals the product of the shear stress and the slip rate.) However, those important findings were obtained in experiments using rock specimens with dimensions of only several centimetres, which are much smaller than the dimensions of a natural fault (of the order of 1,000 metres). Here we use a large-scale biaxial friction apparatus with metre-sized rock specimens to investigate scale-dependent rock friction. The experiments show that rock friction in metre-sized rock specimens starts to decrease at a work rate that is one order of magnitude smaller than that in centimetre-sized rock specimens. Mechanical, visual and material observations suggest that slip-evolved stress heterogeneity on the fault accounts for the difference. On the basis of these observations, we propose that stress-concentrated areas exist in which frictional slip produces more wear materials (gouge) than in areas outside, resulting in further stress concentrations at these areas. Shear stress on the fault is primarily sustained by stress-concentrated areas that undergo a high work rate, so those areas should weaken rapidly and cause the macroscopic frictional strength to decrease abruptly. To verify this idea, we conducted numerical simulations assuming that local friction follows the frictional properties observed on centimetre-sized rock specimens. The simulations reproduced the macroscopic frictional properties observed on the metre-sized rock specimens. Given that localized stress concentrations commonly occur naturally, our results suggest that a natural fault may lose its strength faster than would be expected from the properties estimated from centimetre-sized rock samples.

A pseudotachylyte bounded by a carbonate-matrix implosion breccia was found at a fossilized out-of-sequence thrust in the Shimanto accretionary complex, Japan. This occurrence resulted from the following events: first implosion of host rock due to interstitial fluid pressure increase and asymmetric fracturing; second, Ca-Fe-Mg carbonate precipitation; and third, frictional melting. The rock-record suggests that these events took place in a single seismogenic slip event. Resulting from abrupt drop in fluid pressure after implosion, hydro-fracturing and fluid escape, recovered high effective friction promoted melting during fault movement. Coexistence of fluid implosion breccia and pseudotachylyte has never been reported from continental pseudotachylytes, but might be characteristic from hydrous seismogenic faults in subduction zones.

Scaly fabrics in the décollement at the toe of the northern Barbados accretionary prism occur in centimeter-thick zones interpreted as the horizons where tectonic displacement is concentrated. Detailed microstructural investigations of the scaly fabrics have been carried out, using optical, scanning (secondary and backscattered modes), and transmission electron microscopy. These observations show that the scaly fabrics essentially comprise three types of microstructures, which arise from a combination of shear and flattening. In these microstructures, strain is concentrated in micrometer-to millimeter-thick deformation bands, caused by clay-particle rotation associated with porosity collapse (compactional plastic strain), resulting in the formation of domains with marked preferred orientation of clay particles. However, this preferred orientation affects only a minor part of the sediment involved in the scaly-fabric zones and only a small proportion of the total décollement thickness. On the basis of the mode of microstructure associations, we propose a model for the kinematic evolution of the scaly-fabric zones. In these, deformation initiates by the formation of a spaced foliation corresponding to flattening band arrays, then continues by concentration of shear strain in S-C (schistosité-cisaillement [schistosity-shear]) bands geometrically analogous to the S-C tectonites common in metamorphic shear zones. Partitioning of deformation results in the late formation of fracture networks at the periphery of the S-C bands, the fractures networks being possible precursors of S-C band widening. Compactional strain in the deformation bands is typical of normally or undercompacted sediments and implies expulsion of pore fluid. Preferred orientation of clay particles makes the deformation bands potential pathways for fluid circulation in deformation zones, but compactional strain requires the bands to be dilated by excess pore pressure to have significant permeability. We infer cyclic variations of stress state in the scaly-fabric zones, related to pore-pressure variations. Formation of scaly fabrics by compactional plastic shear strain would be achieved under relatively low pore pressure and significant shear stress, whereas high pore pressure would inhibit further compactional strain while increasing permeability. Tectonic displacement is likely to be favored at the sharp boundaries of the scaly-fabric zones during high pore-pressure episodes; formation of the scaly fabrics thus would account for only part of the cumulative displacement.

The preparation process of natural earthquakes is still difficult to quantify and remains a subject of debate even with modern observational techniques. Here, we show that foreshock activity can shed light on understanding the earthquake preparation process based on results of meter-scale rock friction experiments. Experiments were conducted under two different fault surface conditions before each run: less heterogeneous fault without pre-existing gouge and more heterogeneous fault with pre-existing gouge. The results show that fewer foreshocks occurred along the less heterogeneous fault and were driven by preslip; in contrast, more foreshocks with a lower b value occurred along the more heterogeneous fault and showed features of cascade-up. We suggest that the fault surface condition and the stress redistribution caused by the ongoing fault slip mode control the earthquake preparation process, including the behavior of foreshock activity. Our findings imply that foreshock activity can be a key indicator for probing the fault conditions at present and in the future, and therefore useful for assessing earthquake hazard.