JpGU-AGU Joint Meeting 2020

Presentation information

[J] Oral

S (Solid Earth Sciences ) » S-SS Seismology

[S-SS15] Fault Rheology and Earthquake Physics

convener:Keisuke Yoshida(Tohoku University), Keishi Okazaki(Japan Agency for Marine-Earth Science and Technology), Shunya Kaneki(Disaster Prevention Research Institute, Kyoto University), Hiroyuki Noda(Kyoto University, Disaster Prevention Research Institute)

[SSS15-12] Weakening of quartz rocks at subseismic slip rates due to frictional heating

*Kyuichi Kanagawa1, Asuka Sugita2, Miki Takahashi3, Michiyo Sawai1 (1.School of Science, Chiba University, 2.Faculty of Science, Chiba University, 3.Research Institute of Earthquake and Volcano Geology, Geological Survey of Japan)

Keywords:quartz rocks, frictional heating, weakening, subseismic slip rates

Quartz rocks are known to show weakening commonly at subseismic slip rates of 0.1−10 cm/s, in contrast to other rocks in which weakening commonly occurs at seismic slip rates of ≧10 cm/s. We show by experiments and theoretical considerations that weakening of quartz rocks at subseismic slip rates occurs due to frictional heating.

We conducted rotary-shear friction experiments on intact agate and silica-gel gouge at a normal stress of 1.5 MPa and equivalent slip rates (Veq) of 0.1−10 cm/s monitoring temperature (T) adjacent to the slip surface or the gouge layer. It should be noted that the actual slip-surface or gouge temperature during the experiment was much higher than T. Steady-state friction coefficient μss of both intact agate and silica-gel gouge decreased with increasing Veq from 0.6−0.7 at Veq = 0.1 cm/s to 0.03−0.2 at Veq = 10 cm/s, while T increased with increasing Veq from ≈25°C at Veq = 0.1 cm/s to 88−105°C at Veq = 10 cm/s. Spikes of high friction followed by T maxima and subsequent weakening suggest that slip at strong asperity contacts induced frictional heat, which in turn resulted in weakening. These results indicate that the frictional strength of intact agate and silica-gel gouge at slip rates of 0.1−10 cm/s is controlled by temperature, which increases by frictional heating.

Based on the flash-heating model of Rice (2006), temperature increase ΔT (°C) by flash heating at an asperity contact can be described as follows:
ΔT = μp(4Si3V2σA2π3)1/4/ρcp
where μp is peak friction coefficient, Si is indentation strength, V is slip rate, σ is normal stress, A is slip surface area, ρ is density, cp is heat capacity, and α is thermal diffusivity. Because μp, ρ, cp and α are not much different among rocks, the above equation implies that at a given condition of V, σ and A, ΔT depends primarily on Si and is proportional to Si3/4. Although only limited Si data are available at present, indentation hardness Hi can be correlated with Si, and Hi value of quartz (12 GPa) is much larger than those values of other common rock-forming minerals, e.g., 6 GPa for feldspars, 3.4−5 GPa for amphiboles, 3.4−6.5 GPa for pyroxenes, 6.5−8.4 GPa for olivine, 1.5 GPa for calcite, and 1−2 GPa for micas (Spray, 2010). Thus at a given condition of V, σ and A, ΔT of quartz rocks would be much higher than those of other rocks so that much more amount of frictional heat would be induced at asperity contacts in quartz rocks than in other rocks, which must be responsible for weakening of quartz rocks at subseisimic slip rates.