On the precursor times of thermal infrared radiation anomalies before earthquakes in Taiwan

Twenty-seven Taiwan’s earthquakes with local magnitudes ML=5.6−7.3 and focal depths d=8.0−215.8 km were proceeded by the thermal infrared radiation (TIR) anomalies. The data set is good for us to explore the precursor times and mechanisms of TIR. Let TD and TN be the largest number of days when the anomalies observed in the day-time and the night-time, respectively. The precursor time, T, for an event is the larger value of TD and TN. The data points of T versus ML and those of TD versus ML are scattered. Regardless of two data points with abnormally large TN values, a positive correlation seems able to be recognized from the data points of TN versus ML. Among the 27 earthquakes, 8 events were proceeded by Rn concentration changes with precursor times TRn=7.4−171 days. For most of the 8 events, TRn is much longer than either TD or TN. Hence, the model of outflow of gases from the depths to the ground surface seems unable to interpret the TIR anomalies. Based on rock mechanics experiments, the temperature rise, ΔT, is generated by frictional heating due to preseismic slip. That the TIR anomalies discontinuously appeared before earthquakes and the occurrence times vary for different events might be due to the generation of frictional heating caused by preseismic slip on a few shallow sub-faults in the source areas before the earthquakes.