As a case study of spatial and temporal variations in earthquake stress drops between the 2008 Ms 8.0 Wenchuan and 2013 Ms 7.0 Lushan earthquakes,we computed 1828 stress-drop values for earthquakes with magnitudes 1.7 ≤ ML ≤ 5.0 during an eight-year time span before and after major earthquakes.We divide the study area into three subregions (the southern segment of the Longmen Shan fault zone;the southwestern junction of the Longmen Shan and Sichuan Basin;and the southwestern margin of the Sichuan Basin) and calculate individual event stress drops in each.The results show that regions of alternating high and low stress drop are found on either side of the southwestern segment of the Longmen Shan fault zone.During the two-year period after the 2008 Ms 8.0 Wenchuan earthquake,the stress state of the southern Longmen Shan fault shows no significant change.A marked increase in stress level appears about 18 months before the 2013 Ms 7.0 Lushan earthquake near the Lushan hypocenter zone.Two months after the Ms 7.0 event,the stress drops suddenly attenuate,with significantly less seismic energy release per event.We find that changes in the patterns of high and low stress drop values are consistent with the process of stress accumulation or transfer from the pre-mainshock to postmainshock periods.The results indicate that major earthquakes are the dominant cause of temporal and spatial evolution in stress levels.Stress drop variations show obvious temporal and spatial patterns that may suggests subtle changes in the character of stress fields on faults and spatial variations related to local intense compression and tectonic effects.