摘要：

Evolution of fault-zone strength and its weakening mechanisms during an earthquake are critical for understanding of earthquake rupture process. We report dramatic weakening of dry simulated faults in Carrara marble at seismic slip-rates, with frictional coefficient as low as 0.04 (probably the lowest record as rock friction). Calcite decomposition was confirmed by in-situ CO2 detection and other methods and the weakening may require new weakening mechanisms other than currently suggested ones such as frictional melting, thermal pressurization and silica gel formation. We conducted rotary-shear friction experiments on Carrara marble at slip-rates (V) of 0.09-1.24 m/s and normal stresses (σn) of 2.5-13.4 MPa. For preventing a thermal fracturing and applying a high normal load, we used solid cylindrical specimens jacketed with aluminum tubes. Narrow gap was left between the two aluminum tubes to avoid metal-to-metal contact. Our main results can be summarized as follows: (1) Slip weakening occurs in all experiments except for the runs at the lowest V (0.09 m/s); (2) Steady-state friction coefficient (μss) decreases as slip-rate and normal load increase; (3) At the highest V (1.13-1.24 m/s) and σn = 7.3 MPa, the average friction coefficient of initial peak friction (μp) is 0.61 (± 0.02), but the average μss is 0.04! (± 0.01) which is much lower than μp; (4) Decrease in average temperature of sliding surfaces corresponds to increase in friction, and strength recovery occurs very rapidly and completely upon cooling of specimens; (5) XRD and EPMA data show that the gouge for the specimens at V > 0.09 m/s is composed of calcite, lime (CaO) and/or hydrated lime (Ca(OH)2); (6) CO2 gas was detected with sensors during the weakening; (7) Decomposed calcite forms a fault zone consisting of ultrafine-grained gouge, but no melt or amorphous material was identified by optical microscopy or XRD analysis. Calcite decomposition clearly indicates that temperature in the fault zone reached above about 900° C due to frictional heating. The weakening is unlikely due to the formation of weak phase along slipping zone because the initial peak friction is fully recovered in tens of seconds upon cooling of the specimens. Those results strongly suggests that temperature rise in fault zone was a primary cause of dramatic strength reduction at high slip rates. There is no time for lime grains to crystallize to large grains, and we suspect that ultrafine-grained lime gouge is very weak at high temperature although its exact deformation mechanism is unclear at present. TEM observation of gouge is thus planned in the future. In view of our experimental results, decomposition due to frictional heating may be a common phenomenon in natural fault zones.

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