HR: 09:00h
AN: T21D-04    [Abstracts]
TI: ``Partial Melting'' Of Fault Zones: A Mechanism Of Seismic Slip Termination
AU: * Otsuki, K
EM: otsuki@dges.tohoku.ac.jp
AF: Department of Geoenvironmental Sciences, Graduate School of Science, Tohoku University, Aramaki Aoba, Aoba-ku, Sendai, 980-8578 Japan
AU: Koizumi, Y
EM: scooter@dges.tohoku.ac.jp
AF: Department of Geoenvironmental Sciences, Graduate School of Science, Tohoku University, Aramaki Aoba, Aoba-ku, Sendai, 980-8578 Japan
AB: Our stick-slip experiments demonstrated that frictional melting terminates fault slips, and our numerical simulations demand that there should be a mechanism of enforced cooling (Koizumi and Otsuki, in this session). We present this mechanism referring the previous pin-on-disc experiments under severe conditions where friction is thermally controlled (Montgomery 1976; Ettles, 1986). Friction coefficient $\mu$ and wear rate W show a spectrum depending on the product of slip velocity V and normal stress $\sigma$. 1) Flash-melting: small $\mu$ (ca. 0.3) and W at small VP. Blobs of scratched debris play new asperities. The asperity contacts are easily flash-melted, but the temperature cannot rise up further, because the most of the heat flows to the counter face and the melt materials are immediately removed from the contacts. $\mu$ is self-adjusted by the equilibrium between the heat generation and cooling rates and formulated as, $\hspace*{10mm}$$\mu$ = 1.88 (Tm/Ph) (k$\rho$c/V)$^{1/2}$ (n*/$\sigma$)$^{1/4}$ ------- (1) where Tm: melting temperature, Ph: penetration hardness, k: thermal conductivity, $\rho$c: heat capacity of unit volume, n*: number density of asperities, and $\sigma$: normal stress. Eq. (1) represents velocity weakening and normal stress weakening. 2) Partial melting: abnormally large $\mu$ (up to 1.4) and W at moderate VP. The majority of the apparent contact surface remains cool in the flash-melting regime, but the temperature increases albeit slowly. Once it increases beyond a critical temperature, the penetration hardness decreases significantly and W increase abruptly. Wear debris produced at a high rate can cool the small amount of the melt materials, resulting in very high frictional resistance. If the external force is not sufficiently large to overcome this mechanical barrier, fault slips will stop. The elapsed time Te to the partial melting regime is expressed as, $\hspace*{10mm}$Te = 40 k$\rho$c Tm$^{2}$ (V$\sigma$)$^{-2}$ --------- (2). Applying eq. (2) to our stick-slip experiments (Koizumi and Otsuki, this session), Te is calculated at 27 $\mu$s, close to the observation of 17 $\mu$s. Assuming V=1 m/s and $\sigma$=275 MPa for seismic slips, Te=7.5 ms (7.5 mm slip), but it will be much larger when the slip is diffused in the fault zone with a finite width. 3) Full melting region: small $\mu$ (ca. 0.4-0.1) and W at large V. When the barrier of the partial melting is overcome, fault slip will run away.
DE: 8010 Fractures and faults
DE: 8030 Microstructures
DE: 7209 Earthquake dynamics and mechanics
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting