| Mechanical behaviors of frictional sliding of rocks can be described well by rate- and state- dependence friction laws. The parameter a - b in the friction laws is important in controlling stability of frictional sliding. Analysis of small perturbation around steady state sliding for a spring-slider system (linear analysis) shows that the condition for self-sustained instability to occur is a negative rate dependence of friction (a - b < 0).Our laboratory has performed experimental studies on frictional sliding of gabbro, a typical mafic rock in the lower crust. Based on the results, analysis has been made on the possible cause of earthquakes in the lower crust. Focusing on plagioclase and pyroxene, the major minerals of gabbro, in this study the author conducted 3 sets of experiments of frictional sliding on plagioclase and pyroxene under different loading conditions. It has been found from these experiments that frictional sliding of pyroxene is more difficult to approach a steady state than that of plagioclase, and the constant normal stress control makes it easier for pyroxene gouge to approach the steady state. Research on the frictional stability of plagioclase and pyroxene draws conclusions as follows: (a). two major constituents of gabbro, plagioclase and pyroxene codetermine the parameter a-b critical for sliding stability of gabbro, but quantitative contributions by the two constituents on sliding stability of gabbro is not available. (b). Compared to the positive a-b values of plagioclase, negative values of a-b occurred for gabbro and pyroxene in the higher temperature range. Velocity weakening or stick-slip behaviors of gabbro under higher temperature are consequently determined by the property of frictional sliding of pyroxene. (c). If data for frictional sliding of plagioclase and pyroxene are employed to analyze sliding stability of other rocks, such as diorite, contributions of different mineral constituents of rocks should be considered.
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