HR: 15:25h
AN: T13E-08    [Abstracts]
TI: Intermediate-velocity Friction Barrier and its Implication for Earthquake Generation
AU: * Shimamoto, T
EM: shima@kueps.kyoto-u.ac.jp
AF: Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University, Kitashirakawa Oiwake-cho, Kyoto, 606-8502 Japan
AU: Hirose, T
EM: hirose@kueps.kyoto-u.ac.jp
AF: Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University, Kitashirakawa Oiwake-cho, Kyoto, 606-8502 Japan
AB: High-velocity friction (frictional properties of faults at high velocities and large displacements where frictional heating plays decisive role) has been receiving increasing attention with respect to fault motion during large earthquakes. High-velocity friction associated with frictional melting, thermal pressurization, high-velocity gouge behavior and high-velocity rock-on-rock friction is distinctly different from ordinary fault constitutive properties (rate-and-state constitutive law or slip weakening law) that have been determined by traditional friction experiments at slow slip rates and small displacements. Contrasting constitutive properties between the two regimes clearly bring about the significance of _intermediate-velocity frictional regime_ that links the two regimes. This regime is very important in understanding how precursory slip grows to a large earthquake, or how and when frictional properties changes to those of high-velocity friction associated with a large earthquake. Thus, the intermediate-velocity regime is likely to be the most important area for establishing physical basis for earthquake prediction, and yet no systematic studies have been carried. A new low to ultra-high velocity friction apparatus was developed in Kyoto to cover wide slip rates from 3 mm/yr to about 10 m/s, by using four sets of gear arrangements. An ultrahigh velocity arrangement using two sets of belts increases rate of revolution by nine times and allows to produce such a high slip rate. Pressure vessel made of stainless steel can hold water pressure up to about 50 MPa, by using a 6 sets of high-velocity seals across which confining pressure is reduced in a stepwise manner to keep pressure difference across each seal below about 10 MPa. Since hollow-cylindrical specimens are used without jackets, water pressure in the vessel acts as pore pressure and normal stress to fault is applied with a hydraulic press. An external furnace will produce temperature to around 400°C, thereby producing supercritical condition. This is probably the only machine in the earth science community that allows one to study frictional properties crossing completely the intermediate regime. Old laboratory data on halite shear zone (Shimamoto, 1986) indicate that velocity weakening behavior at slow slip rates changes to velocity strengthening at slip rates on the order of a few tenths of mm/s. Whereas Tsutsumi and Shimamoto (1997) have shown that steady-state friction of gabbro increases as slip rate increases from about 10 mm/s to about 100 mm/s (velocity strengthening), but it changes marked velocity weakening behavior at higher velocities. For the sake of convenience, we define the intermediate-velocity regime as the velocity-strengthening regime between low- and high-velocity regimes, both characterized by velocity weakening. Shibazaki (2004) showed by modeling earthquake rupture propagation that the velocity-strengthening property in the intermediate regime definitely stabilizes fault motion. Thus we call this velocity strengthening _gintermediate-velocity barrier_Eto fault motion. Our preliminary experiments on gabbro show that the barrier is 0.2 to 0.3 in terms of frictional coefficient. We will show more data on this at the meeting. The any portion of a fault must possess such a barrier as an intrinsic fault property, and an interesting question in rupture propagation analysis is how local slip overcomes this barrier to grow into large earthquakes.
DE: 7209 Earthquake dynamics (1242)
DE: 8004 Dynamics and mechanics of faulting (8118)
DE: 8034 Rheology and friction of fault zones (8163)
SC: Tectonophysics [T]
MN: Fall Meeting 2005