## 1 Introduction  
Crustal stress is one of the most important properties of the crust, and it is closely related to various geological tectonic phenomena (including shallow earthquakes) as well as the accompanying physical phenomena in the surface and internal crust. Detecting the crustal stress condition and distribution is the basis for solving scientific problems relevant to geodynamics. Currently, most crustal stress monitoring methods adopted are contact-type and scattered, which are not usable for monitoring the crustal stress field in a large area, and the identification of potential catastrophes is limited. Since the 1980s, the rapid development of satellite technology has led to significant developments in seismic monitoring and geological disaster research. The satellite technology, including Global Navigation Satellite System (GNSS) [1], Synthetic Aperture Radar Interferometry (InSAR) [2; 3], Gravimetry [4], and Remote Sensing (RS), has been used to detect crustal activity and to identify the relationship between anomalies and fault activities. In 1988, [PERSON] et al. observed thermal infrared radiation (TIR) anomaly phenomena before a strong earthquake in Central Asia and set the stage for using satellite thermal infrared remote sensing to detect pre-earthquake thermal anomalies [5]. Since then, this phenomenon has been repeatedly verified all over the world [6; 7; 8; 9; 10; 11; 12; 13; 14]. The positive temperature anomaly has become well known because of the frictional heating that occurs during earthquake faulting [15; 16]. Using not only the frictional heat, [PERSON] et al. [17; 18; 19; 20] proposed a new method of using TIR data from satellites to investigate fault activities and performed experimental studies on the evolution characteristics of TIR in response to stress changes on fault-bearing samples. The use of thermal infrared remote sensing for monitoring pre-earthquake anomalies has become a popular topic in seismic research. The temperature response properties associated with stress changes in rocks are key to improving understanding of temperature anomalies. To reveal the internal mechanisms of this phenomenon, the TIR observation experiments for loading rocks were conducted. [PERSON] et al. [21] performed TIR observations of 34 rock samples with 26 types of lithology under uniaxial compression. They found that the infrared radiant temperature changes with stress during the rock-loading process and proposed a new method of detecting the stress state and rock stability with the concept of remote sensing rock mechanics (RSRM). [PERSON] et al. [22; 23; 24] conducted wide research on the spatiotemporal evolution characteristics and anomalous precursors of TIR for loaded rocks under different stress conditions; two main thermal effects were proposed for explaining the experimental phenomenon. One is the thermoelastic effect, which is present throughout the entire loading process [20; 25]; the other one is the frictional-thermal effect, which only occurs during microfracturing and fracturing stage [26; 27; 28; 29]. Therefore, the stress condition and fracture process of loaded rock can be monitored in the experimental condition. The infrared thermography has been widely employed for the analysis of the instability process for rock engineering [30; 31; 32]. However, studies have shown that the TIR changes induced by the thermoelastic effect during loading are relatively small with the increasing temperature rate of approximately 3 mK/MPa under the normal temperature and pressure conditions [22; 23; 24; 25]. Additionally, the wavelength features in the spectral radiance change for loaded rocks were observed and can be used for stress monitoring [33; 34; 35; 36; 37]. The TIR change in the loading process is caused by both the physical temperature and emissivity change. The crustal stress condition could be evaluated by the emissivity change based on the accurate acquisition of stress-related TIR information [38].