关键词: 摩擦强度, 速率依赖性, 辉长岩, 下地壳

Abstract: As most earthquake focal depths are concentrated in the upper crust (e.g. Chen et al.,1983),previous studies on rate dependence of frictional strength have focused on granite that is typical in the upper crust (Lockner et al.,1986; Blanpied et al.,1995). However,recent reassessment of focal depth of many earthquakes revealed that some of them have been generated in the continental lower crust (Maggi et al.,2000). Moreover,teleseismic imaging of the structure across the San Andreas Fault also suggested the existence of deep fault that cuts through the crust (Zhu,2000). With this background,we need to pay more attention to intermediate and mafic rocks in the lower crust. Frictional resistance and the related rate-dependence of such rocks are critical to understand the mechanical behavior of deep faults that cut the lower crust. Specifically,this is crucial for answering two important questions,namely: 1) Is it likely for earthquake to nucleate in the continental lower crust? 2) How much stress can the lower crust support? We conducted experiments on frictional sliding of oven dried gabbro gouge at elevated temperatures with a triaxial testing system using gas as the pressure medium. A gabbro gouge layer of 1 mm thick (particle size of the gouge<76μm) was placed along an inclined saw-cut (35°to the loading axis) in a 20-mm-diameter cylinder sample to simulate a fault with gouge. Two series of experiments have been conducted with normal stresses of 200MPa and 300MPa,respectively,with temperatures up to 616℃. For both normal stresses,the rate-dependence of friction is either positive or approaching to neutral rate-dependence. With normal stress of 300MPa,the values of steady state rate dependence are positive in most of the temperature range but show a minimum around 315℃ (approaching to zero). Variation of rate dependence under normal stress of 200MPa is less pronounced. Based on theoretical analysis on slip nucleation (e.g. Dieterich,1992) and these experimental results,it is unlikely that unstable slip nucleates in larger scales under similar conditions as set in the experiments.

Key words: frictional strength, rate dependence, gabbro, lower crust