Experimental Study Of Evolution Of The Thermal Field In The Meta-instability Stage Of Faults And Its Application To Earthquake Precursors Analysis

Meta-instability stage before a slip instability is the interval between peak stress moment O and instability moment B as captured from the differential stress vs. time curve during a fault slip experiment. Quasi-static stress release is predominant in the early phase of meta-instability stage, which transits to a quasi-dynamic phase in the final time interval up to instability, with point A as the turning moment. Identification of the meta-instable stress state from the whole process is of great significance for building up fundamentals of earthquake prediction.Previous studies suggested two primary mechanisms of temperature changes caused by tectonic deformation:one is temperature increase or decrease generated by accumulation or release of stress, the other is temperature increase due to sliding friction. Meta-instability is a stage containing both two mechanisms. The evolution characteristics of these two mechanisms could be one of the criteria to indentify the meta-instability stage.This thesis focuses on evolution of the thermal field in meta-instability stage of faults. Deformation of rock samples with different fault structures were conducted on a bilateral servo control loading system using an infrared imaging system as the major observation tool. The main results are summarized below.(1) Temperature decrease of rock is the main thermal indicator of entering the meta-instability stage. Temperature of the samples increases as a whole in the stage before peak stress. Location of the fault cannot be recognized from the thermal images. Large-scale temperature decrease occurs after entering the stage of meta-instability with slight temperature increase on some places of the fault.(2) Strengthening and expansion of temperature increase on the fault are the principal thermal signs of entering the quasi-dynamic stage. Sporadic slightly warmed parts on the fault in the quasi-static stage gradually increase, expand, and finally connect rapidly after entering the quasi-dynamic stage. The warming velocity increases in orders of magnitude until the instability occurs. Temperature on the fault is essentially an extension and connection of pre-slip zones, with friction as the main mechanism. The quasi-dynamic stage is not only the process of fault activity becoming collaborative, but also the stage of heating mechanism converting from stress dominant into friction dominant.(3) The partial fault region without temperature variation and continual high temperature in the rock near the fault could possibly be the instability place. The position where temperature increases highest when instability occurs is called the instability position. This study found that the temperature of the instability position keeps relatively high since the beginning of the stage of strongly off-linear stage. The heating rate of instability position rises abruptly in the meta-instability stage. Analysis shows that temperature increase before instability exists in the rock rather than the fault, indicating that warming is caused by high stress concentration in the surrounding rock, and the temperature in the fault paralleled to the warm area in the rock keep locked as the temperature does not increase. Thus the position satisfied with the locked fault and high stress concentration in the rock near the locked fault is possibly the instability position in the future.(4) Evolution of the thermal field of faults with different structures has a mix of commonness and differences. Characteristics of the thermal field mentioned above are most obvious in a planar fault. For the compressive en echelon fault, temperature decrease appears earlier in the strongly off-linear stage and temperature increase concentrates on the jog area. For the bending fault, both the temperature decrease on rock and increase on the fault are relatively weak due to the obstacle of the inflection area. Evolution of the thermal field of faults with different structures has a mix of commonness and differences, which means that the impact of fault structures should be considered when applying the experimental results to real situations in the field.A thermal field model of fault deformation and instability is also established in this thesis based on the analysis of the thermal field evolution and the mechanism analysis. According to the experimental results, temperature decrease of rock caused by the release of stress is a possible signal of entering the meta-instability stage. MODIS(Moderate-resolution Imaging Spectroradiometer) LST (Land Surface Temperature) products and weather data were used to analyze the surface temperature impending the 2008 Wenchuan Mw7.9 earthquake and temperature response of the Yilan-Yitong fault to the great Tohoku-oki Mw9.0 earthquake in 2011. Results of this study would be helpful to the observation and analysis of earthquake precursors.