Kazuo Mizoguchi

Scale dependence of rock friction at high work rate

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.

Fault Heals Rapidly after Dynamic Weakening

Abstract How rapidly fault strength recovers after an earthquake is an important question for understanding the earthquake generation mechanism in seismic cycles. Here we show in laboratory experiments where a fault weakened dynamically at subseismic slip rates (∼85 mm/sec) recovers its frictional strength logarithmically with time and the healing rate of 0.2–0.3, one order of magnitude greater than those in previous studies. The fault can completely recover its frictional strength to preslip level within one day. We suggest that immediately after an earthquake a slipped fault surface can regain its potential to trigger the next earthquake, which might have important implications for forecasting future large earthquakes.

Transient water adsorption on newly formed fault gouge and its relation to frictional heating

Determination of the amount of frictional heating on faults during slips provides insight into the mechanics of faulting. Fault slips cause frictional heating as well as production of gouge materials. Newly formed gouge is mechanochemically stimulated to gain a transient water adsorption ability. We showed that the synthetic gouge from friction tests on rocks at a 1 mm/s slip rate without frictional heating adsorbed a large amount of water, comparable to that adsorbed by clay minerals. The amount of adsorbed water decreased significantly with increasing slip rate to 100 mm/s, where the fault temperature increased above 400 °C. We confirmed there was no additional adsorption on the heated gouges after termination of the slip. We conclude that the adsorption state can be considered a new indicator of a temperature rise of < 400 °C by frictional heating associated with recent, near-surface faulting.