The spatio-temporal characteristics of earth tide modulation ratio before moderate earthquakes in northwest China

Based on the shallow-focus earthquakes lunar catalog in Chinese mainland from 1900 to 2015, modulated lunar date and threshold of modulation ratio are defined respectively according to seismic active characteristics. The modulation ratio method is employed to carry out spatio-temporal scanning for three moderate earthquakes in northwest China, and the spatio-temporal characteristics of modulation ratio of regional small earthquakes are analyzed. Results indicate that there are modulation ratio anomalies around epicentral area before Minxian-Zhangxian M_S6.6 earthquake and Alxa M_S5.8 earthquake, evolution time of modulation ratio anomaly is relatively long for Menyuan M_S6.4 earthquake. All three moderate earthquakes happened around the area with higher earth tide modulation ratio according to the spatio-temporal scanning results, and the length of time window and the abnormal area size of the earth tide modu-lation ratio may have correlations with the magnitude of moderate earthquakes.