岷县漳县强震前的跨断层短临异常及亚失稳状态特征

CROSS-FAULT SHORT-TERM AND IMPENDING ANOMALIES BEFORE THE MINXIAN-ZHANGXIAN STRONG EARTHQUAKE AND THE CHARACTERISTICS OF META-INSTABLE STATE

2013年岷县漳县M S6.6地震发生前2~4d,距震中37~70km的四店、毛羽沟、麻界滩跨断层流动短水准场地观测到以正断层型突跳为主的短临异常,尤其是四店场地,变幅高达5.5mm,异常相当显著。文中对这3处场地所在的西秦岭构造区的跨断层短水准观测资料进行分析,并结合GPS连续观测数据与小地震活动信息对其异常机理进行探讨。结果表明:岷县漳县震前数天,西秦岭构造区跨断层短水准观测显示的断层活动微动态变化量增大、特性明显改变及靠近震区的“局部化增强”等现象,符合构造力学实验显示的亚失稳状态特征。

2~4 days before the Minxian-Zhangxian M S6.6 earthquake on July 22,2013,short-term and impending anomalies dominated by normal fault type abrupt change were observed at mobile cross-fault short-leveling sites,Sidian,Maoyugou and Majietan,within 37~70km from the epicenter,especially at the Sidian site,where the amplitude of the anomaly reached up to 5.5mm.In this paper,based on the analysis of cross-fault short-leveling observations in the western Qinling Mountains tectonic area,where the above three sites are located,and the experimental results of structural mechanics for the existence of meta-instable stress state before fault instability,the anomaly mechanism is discussed,combined with the information of GPS continuous observation and small earthquakes activities.The results show that:1)Several days before the earthquake,obvious“changes”in the micro-dynamic variation magnitude and characteristics of fault activities were observed,which revealed that the regional stress had changed.New synchronous anomalies occurred at Sidian,Maoyugou and Majietan sites,especially at Sidian site where the anomaly was large in amplitude,and the number and magnitude of the anomalies increased obviously,Moreover,the dynamic distribution of anomalies of the observational curves showed an obvious change process of the characteristics of tectonic fault activities.In the western Qinling tectonic area,the change was mainly dominated by reverse faulting,but this reverse fault type change was not dominant several months before the earthquake,and till a few days before the earthquake,the normal fault type variation was absolutely dominant,which is contrary to the long-term background of the reverse fault favorable for stress concentration in this tectonic area;In addition,temporal variation of trend accumulation rate,which reflects the feature and magnitude of stress-strain accumulation,showed the accumulation firstly slowing down in several months and turning obviously in several days before the earthquake.2)Obvious anomaly variation was observed at the sites near the earthquake region several days before the earthquake,the crustal stress evolved from“regional change”to“localized enhancement”.During the four observational periods before the earthquake,it was observed that the cross-fault anomalies were gradually concentrated to the local tectonic region where the earthquake occurred,also the grey relativity index which reflects the general trend of fault activities displayed a larger turning magnitude in this local tectonic region,Next,the local fault segment mostly near the epicenter not only maintained the reverse fault type change consistent with the earthquake dislocation,significant accelerating anomalies were also observed at Majietan site,which may be the relatively strong stress concentration position.The auxiliary data of continuous GPS observation and small earthquake activities were also of some supporting significance to above phenomena.The comprehensive analysis shows that the temporal and spatial evolution process of fault activities and anomalies obtained from the cross-fault short-leveling observation in the western Qinling tectonic area before the strong earthquake accorded with the characteristics of meta-stable(deviating from linear stage)-meta-instable state revealed by the structural mechanics experiment.