对南北地震带上的绝对重力数据和多期相对重力数据进行平差处理,获得了2018年9月8日云南墨江M5.9地震前的区域重力场变化图,分析了区域重力场变化及其与墨江M5.9地震的关系。结果 表明:①2015年9月—2017年9月,震区周边区域重力场变化呈现明显的四象限分布特征; ②2015年9月—2018年9月,在重力场变化的时空演化上,震区附近重力场变化显示出重力正变化→重力正变化扩展→重力零变化的平静期→重力变化呈四象限分布→重力场反向变化→临震前重力零变化的平静期的过程,反映出地震前震区附近的断层由单个独立活动向多个协同活动的演进过程,与处于亚失稳状态的断层活动具有一致性; ③地震前重力场四象限变化形态与震源机制显示的压缩区和拉张区一致,表明地震前震中附近构造应力作用下的物质迁移可能是区域重力场变化的成因。
The Mojiang M5.9 earthquake occurred in Yunnan province on September 8,2018,near the middle zone of the right-lateral strike-slip Honghe fault and the east branch (Mohei fault)of the left-lateral strike-slip Wuliangshan fault.We processed multi-period relative gravity data in North-South Seismic Belt based on the constraints of absolute gravity data,and got high precision regional gravity field changes before the Mojiang M5.9.The results indicate:①Between September 2015 and September 2017,regional gravity field changes near surrounding earthquake zone exhibits distinct four-quadrant distribution characteristics,controled by the Honghe fault,the Wuliangshan fault,and the Lancangjiang fault.The maximum gravity change is less than 50×10-8m/s-2 and the difference change is about 70×10-8m/s-2 in different quadrant.②During the two years,on the space-time evolution of half-year scale change about the gravity field,the gravity field change shows a special process before the M5.9 earthquake,which is positive gravity change at one place,then positive gravity change at another place,then zero gravity field change in the region,then four-quadrant distribution gravity field change,and then reverse gravity field change and zero gravity field change at last.The process inhibits fault activity evolution that is from independent activity a single fault to colaborative activity of mutiple faults and is consistent with faults activity in a sub-instability state.③The four quadrant characteristics of gravity field before the earthquake is consistant with compression zone and stretch zone displayed by the focal mechanism,we deduce the source of regional gravity field change may be mass migration under technic stress.
