平直断层黏滑过程热场演化及失稳部位识别的实验研究

EXPERIMENTAL STUDY OF TEMPERATURE EVOLUTION AND IDENTIFICATION OF INSTABILITY POSITION OF PLANAR STRIKE-SLIP FAULT DURING PROCESS OF STICK-SLIP

实验室断层黏滑伴随有温度变化,温度不但与摩擦滑动有关,还与样本的应力状态相关。文中利用红外热像仪进行全场观测,研究平直断层黏滑不同阶段的热场演化。实验不仅观测到了峰值前后从应力积累转变为释放导致的温度由升转降的现象,还观测到了失稳后断层升温和块体降温的相反变化。更重要的是通过分析沿断层各个部位的温度随时间的变化,发现了识别失稳部位的温度前兆:失稳部位的温度从强偏离线性阶段开始,一直是相对高值,在亚失稳阶段升温速率突然增大,远高于其他部位。分离摩擦和应力的作用后,发现亚失稳阶段失稳部位的升温发生于围岩上而非断层带上,说明断层处于闭锁状态而相邻的围岩区域处于应力集中状态,推测满足这2个条件的部位可能是未来的失稳部位。此现象或许有助于野外发震位置的判断。

Stick-slip of fault in laboratory accompanies change of temperature. Temperature change is not only concerned with sliding friction,but also with the stress state of the sample. In this article,we use infra-red thermal imaging system as wide-range observation means to study the temperature variation of different stages during the deformation of sample. The rock sample for the experiment is made of granodiorite from Fangshan County with a size of 300 mm × 300 mm × 50 mm. It is cut obliquely at an angle of 45°,forming a planar fault. Two-direction servo-control system was used to apply load on the sample. The load in both directions was forced to 5MPa and maintained constant（ 5MPa） in the X direction,then the load in the Y direction was applied by a displacement rate of 0.5μm / s,0.1μm / s and 0.05μm / s successively. The left and below lateral of the sample were fixed,and the right and top lateral of the sample were slidable when loaded. The experiment results show not only the temperature change from increase to decrease caused by conversion of stress accumulation to relaxation before and after the peak stress,but also opposite variation of temperature increase on fault and temperature decrease in rock during instability stage. Most important of all,we have found the temperature precursor identifying the position of instability through the temperature variation with time along the fault. It shows that rate of temperature increase of instability position keeps relative high value since the stage of strongly off-linear stage,and accelerates in stage of meta-instability. After separating the effect of friction and stress,we found that temperature increase occurs in the rock near the fault instead of on the fault,which means the mechanism of temperature increase is stress accumulation.Temperature of fault at the instability position does not increase,which means the position is locked.We speculate that the position of locked area on fault with high stress accumulation near the fault may be the future instability position. It is of significance of studying temperature variation during stick-slip to the monitoring of earthquake precursors. Heat caused by friction of earthquake needs long time to transfer to the surface and could not be detected as a precursor. While the stress of surface rock near the fault would change as the stress of interior rock changes, which could cause detectable temperature variations. The research purpose of this article is to find special change positions before instability. As the temperature variations are caused by stress and slip of fault,the results are also meaningful to analysis of stress and displacement data related to earthquake precursors.