Stress-Induced Strong Microwave Radiation of Rock: The Recent Experimental Results

Microwave Brightness Temperature (T-B) anomalies preceding strong earthquakes have been frequently observed by satellite microwave remote sensing. To reveal the physical mechanism of the pre-earthquake T-B anomalies, rock mechanics experiments have been conducted through microwave radiation observation at low stress levels, and only weak microwave radiation has been detected, which is insufficient to detect by satellite microwave sensors. This is inconsistent with the strong pre-earthquake T-B anomalies monitored by satellite microwave remote sensing. In this study, a microwave observation experiment of rock under tiered cyclic loading with a gradually increasing cyclic stress was conducted to investigate the T-B variation throughout the elastic deformation and failure process. The experimental results illustrate that the T-B and stress continuously maintain a positive correlation, with a linear correlation coefficient greater than 0.90. A stronger T-B variation of 3.07 K than the previous experimental results is observed at a high stress change of 96 MPa, which is quantitatively in the same order as the T-B anomalies detected by satellite microwave remote sensing. With the stress increases, the T-B variation decreases shortly preceding rock failure due to the formation of fractures, which is in agreement with the short-term weakening of pre-earthquake T-B anomalies. The outcome of this study promotes the understanding of pre-earthquake T-B anomalies detected by satellite microwave remote sensing.