安宁河断裂假玄武玻璃摩擦滑动实验研究

The frictional-slip experiments on pseudotachylytes gouge of Anninghe Fault

我们在安宁河断层带西侧的花岗片麻岩和碎裂岩中发现假玄武玻璃.并开展了假玄武玻璃的摩擦滑动实验,为国际首次.在化学成分和微观结构分析的基础上,本研究使用粉碎后的假玄武玻璃粉末作为模拟断层泥,在有效正应力250 MPa、温度100~500℃、孔隙水压30 MPa和干燥条件下开展了摩擦滑动实验.为获得假玄武玻璃摩擦系数的速度依赖性参数(a-b),实验中的轴向加载速率在1μm·s^(-1),0.2μm·s^(-1),0.04μm·s^(-1)之间进行切换.实验结果显示,在100~500℃假玄武玻璃的摩擦系数随着温度增加而增大,含孔隙水压条件下摩擦系数比干燥条件大.在加载速度为1~0.2μm·s^(-1)条件下,100~300℃温度范围内,假玄武玻璃表现为速度强化型稳定摩擦滑动;在300~400℃的温度范围内,假玄武玻璃的摩擦滑动行为表现出由速度强化向速度弱化转变的趋势,在400~500℃时其表现出显著的速度弱化行为,从而在该温度范围内具备自发的地震成核条件.本实验还发现假玄武玻璃的滑动稳定性与加载速率有关,在相同温度条件下以及100~300℃的温度范围内,假玄武玻璃在实验室标速条件下(1~0.2μm·s^(-1))表现出速度强化的摩擦滑动行为;而在低速(0.2~0.04μm·s^(-1))条件下,表现出速度弱化.因此在100~500℃的温度条件下,降低剪切滑动速度可以促进假玄武玻璃的不稳定滑动.对比花岗岩与假玄武玻璃的摩擦强度与滑动稳定性表明,在100~500℃(5~30 km深度),假玄武玻璃和花岗岩都具备地震成核的条件.假玄武玻璃的摩擦强度比花岗岩高,由此推测,被假玄武玻璃愈合的同震破裂,原地再次发生同震滑动的可能性小,但如果断裂带内存在假玄武玻璃,不仅能增加断层不稳定滑动和地震的风险,而且会促进同震动态破裂传播.

Pseudotachylyte was found in graintic gneiss-cataclastic rocks at the western side of Anninghe fault.In this study,we performed frictional experiments of pseudotachylyte gouges under hydrothermal and dry conditions.Velocity-stepping experiments were performed at effective normal stress of 250 MPa,pore fluid pressure of 30 MPa,temperature range of 100~500℃,and shear velocities between 0.04,0.2 and 1μm·s^(-1).Experimental results show that at 100~500℃,the friction coefficient of pseudotachylyte gouges increase with the increasing of temperature,and friction coefficient for dry samples is lower than that of hydrothermal conditions.The slip stability is characterized by velocity strengthening at temperatures of 100~300℃and velocity weakening at temperatures of 400~500℃under normal loading rate(1~0.2μm·s^(-1)).The results find that loading rate can also affect the stability of pseudotachylyte gouges.At the temperature range of 100~300℃,pseduotachylyte exhibits velocity strengthening behavior at experimental standard velocity of 1~0.2μm·s^(-1) and converts to velocity weakening behavior at slow loading rates(0.2~0.04μm·s^(-1)).Based on the experimental results,it is inferred that the depth of earthquake nucleation is 10~30 km at the Aninghe fault.Comparing with the frictional experiments of granite gouge,we found that the frictional strength of pseudotachylyte is higher than that of granite,implying that it is impossible for the large earthquake to re-occur at the same location if the last co-seismic rupture was healed by pseudotachylyte.However,it could increase the possibility of unstable slip and nucleation of earthquake,also promote propagation of co-seismic dynamic rupture if there exists pseudotachylyte in fault zone.