安宁河断层地震成核条件研究——来自天然花岗岩断层泥摩擦实验的启示  

Earthquake nucleation conditions of Anninghe Fault:constrains from frictional experiments on natural granite gouge

作　　者：雷蕙如 周永胜[1] 姚文明[1] 马玺 何昌荣[1] 党嘉祥[1] 苗社强 戴文浩 LEI HuiRu;ZHOU YongSheng;YAO WenMing;MA Xi;HE ChangRong;DANG JiaXiang;MIAO SheQiang;DAI WenHao(State Key Laboratory of Earthquake Dynamics,Institute of Geology,China Earthquake Administration,Beijing 100029,China)

机构地区：[1]中国地震局地质研究所地震动力学国家重点实验室,北京100029

出　　处：《地球物理学报》2022年第3期978-991,共14页Chinese Journal of Geophysics

基　　金：国家重点研发项目(2018YFC1503400)资助。

摘　　要：安宁河断层历史上发生过多次强震,现存的地震空区反映了其断层中部近30年来的断层闭锁和应变积累情况,是潜在的大地震危险区.本研究采用断层带内的新鲜花岗岩断层泥样品,在水热条件下开展了摩擦滑动实验,以研究安宁河断层摩擦特性和地震成核条件.实验有效正应力200 MPa,孔隙水压30 MPa,温度25~600℃,剪切滑移速率在1μm·s^(-1),0.2μm·s^(-1),0.04μm·s^(-1)之间切换.实验结果显示,在100~400℃,摩擦系数随着温度的增加而增大(约0.663~0.704),温度高于400℃时略有降低.从200℃开始,花岗岩断层泥的摩擦滑动表现出由速度强化向速度弱化转变,并随温度升高速度弱化程度增强,在600℃时仍然具有显著的速度弱化,具备地震成核条件.本研究获得的花岗岩断层发生不稳定滑动的温度范围为200~600℃,远大于已有研究中给出的350~400℃这一不稳定滑动的温度上限.本实验首次发现,在相同温度条件下花岗岩断层滑动稳定性与加载速率转变方向有关,向慢速切换或低速下(0.04μm·s^(-1))可以促进花岗岩断层由稳定滑动向不稳定滑动转变.基于本实验结果,结合川滇地区的地温梯度,推测得出安宁河断层中北段的地震成核深度为10~30 km,该断层的地震空区是断层闭锁的具体表现.The“Anninghe seismic gap”reflects fault-locking and associated strain-accumulation along with the central part of the Anninghe Fault over the last 30 years,and is considered a high possibility location for future earthquakes.In this study,we performed frictional experiments of natural granite gouges from the Anninghe Fault core under hydrothermal conditions.Velocity-stepping experiments were performed at effective normal stress of 200 MPa,pore fluid pressure of 30 MPa,temperature from 25 to 600℃,and shear velocities between 0.04μm·s^(-1) and 1μm·s^(-1).Results show that the friction coefficient(about 0.663~0.704)increases at 100~400℃followed by a decrease at 400~600℃.The slip stability response is characterized by velocity strengthening at temperatures of 25~100℃and velocity weakening at temperature of 200~600℃.The upper temperature limit for unstable sliding of granite gouges is 600℃,which is much higher than the 350~400℃upper limit of unstable sliding temperature given by previous data.The results also show that the stability of granite fault is related to the loading rate at the same temperature,which the velocity-weakening region is more likely to occur at the lower rate or slow velocity(0.04μm·s^(-1)).Based on the experimental results and combined with the geothermal gradient in Sichuan-Yunnan Block,it is inferred that the depth of seismic nucleation is 10~30 km in the middle and northern section of the Anninghe Fault,which implies that the earthquake gap should be a kind of interseismic locking along the Anninghe Fault.

关 键 词：花岗岩断层泥 安宁河断层 摩擦强度 速度依赖性 地震成核 

分 类 号：P313[天文地球—固体地球物理学]