Statistical Analysis of the Largest Aftershocks in Chinese Mainland

CHEN Daqing; SHAO Ye; XU Binin; YANG Xue; WANG Xiaona; ZHONG Qiu

doi:10.13512/j.hndz.2025.03.02

Abstract：Based on the data of 134 earthquakes with M≥6.0 in Chinese mainland since 1976,the paper presents a statistical analysis of the rupture types,sequence types,magnitude difference between the main shock and the largest aftershock,time interval,as well as the relationship between sequence types and small and medium-sized seismicity in the pre-mainshock epicentral region.It is found that 90%of the largest aftershocks occurred within 90 days after the main shock.According to the classification of rupture types,the proportion of main and after types in each type is more than 60%,and the proportion is higher in strike-slip type and normal fault type,while the proportion of isolated type is lower in strike-slip type,there is no isolated earthquake in the normal fault type.Isolated earthquakes usually occur in regions where earthquakes are relatively inactive,and with the increase of the magnitude of the main earthquake,the proportion of isolated sequences decreases significantly.Based on the relationship between the largest aftershock within 24 hours after the main shock and the largest aftershock,it is found that the larger the largest aftershock within 24 hours,the smaller the subsequent largest aftershock.If the magnitude difference between the largest aftershock within 24 hours and the main shock is less than one magnitude or more than 2.5 magnitude,the probability of a subsequent larger aftershock is 18.2%for the former and 89.5%for the latter.These statistical rules can be used to estimate the largest aftershocks in different time periods before and after an earthquake.