Analysis of thermal infrared anomalies before Wenchuan，Yaan Lushan and Jiuzhaigou earthquakes by using amplitude fluctuation level of mean difference

Based on the thermal infrared phenomena before and after the 2008 Wenchuan M_S8.0 earthquake, the 2013 Yaan Lushan M_S7.0 earthquake and the 2017 Jiuzhaigou M_S7.0 earthquake, the amplitude fluctuation level of the mean difference of the original MODIS satellite 1 km high-resolution brightness temperature data was analyzed. ① Before these three earthquakes, there were significant difference amplitude changes in the area near the epicenter, which were low value anomalies, lower than 50% of the historical average, and then developed into high value anomalies, and the peak values of high value anomalies were all more than 3 times of the historical average. ② Wenchuan earthquake, Yaan Lushan earthquake and Jiuzhaigou earthquake occurred 55 days, 35 days and 22 days after the anomalous peak value, respectively. ③ The anomalous predominance is not located in the epicenter: The anomalous preseismic area of Wenchuan earthquake is mainly located in the Qaidam block in the northwest of the epicenter, and the anomalous preseismic area of Yaan Lushan earthquake and Jiuzhaigou earthquake is located in the Bayan Hala block in the west of the epicenter. Among them, the anomalous area before Wenchuan earthquake is much larger than the latter.