岷县漳县6.6级地震前区域地壳运动变形背景与断层形变异常特征

Regional Crustal Movement Background and Cross-fault Deformation Anomaly Characteristics Prior to the Minxian-Zhangxian M_S6.6 Earthquake

利用甘肃岷县漳县6.6级地震区所在的青藏块体东北缘地区的区域水准、GPS、流动重力和跨断层短测线等地形变监测资料,结合地质构造、动力环境和已往的研究结果,分析了不同类型资料反映的震前区域性地壳变形背景、断层形变异常特征和可能的机理。结果认为:(1)岷县漳县6.6级地震前西秦岭、六盘山等构造区不同程度地存在着GPS水平挤压闭锁高应变积累、垂直隆升异常高梯度带和重力升、降差异剧烈变化等中长期背景;(2)震前到震时发震断裂附近及其外围相关构造区域断层形变异常在空间和时间上的起伏波动变化显著,尤其是汶川地震以来的波动变化在一定程度上反映了与本区构造比邻的龙门山断裂带剧烈右旋错动对本区的影响,与本次岷县6.6级地震过程有关;(3)本区有地形变监测资料积累以来缺乏6级以上震例,虽然存在不同程度的中长期形变背景异常,但何时进入短期-短临阶段确实很难把握,需要不断积累总结和探索提高。

In this study, the regional crustal movement background is analyzed using various types of regional crustal deformation data including vertical deformation, GPS, mobile gravity, and cross-fault short line observation in the Minxian--Zhangxian Ms6.6 earthquake zone located at the northeastern margin of the Qinghai--Tibet block. On the basis of regional geological struc- tures, dynamic environment, and existing research findings, the anomaly characteristics of cross- fault deformation and its possible mechanism also are discussed. The results show varying de- grees of medium- and long-term seismogenic backgrounds in the west Qinling and Liupanshan tec- tonic province such as a GPS horizontal compresso-locked zone with high strain accumulation and an anomalous GPS vertical uplift belt with a high gradient, in addition to an abrupt change in the pull of gravity prior to the Minxian--Zhangxian earthquake. Moreover, a fault deformation a- nomaly near the Lintan--Minxian fault caused the earthquake, and its peripheral-related structurerealm showed significant temporal and spatial fluctuation up to the time of the earthquake. Par- ticularly since the Wenchuan Ms8.0 earthquake, the abnormal fluctuations of cross-fault deform- ation have reflect that the influences from the active dextral rupture of Longmen Mountain seis- mogenic structure adjacent to the regional tectonic belt have promoted the formation of this tec- tonic province and the seismogenic process of the Minxian--Zhangxian earthquake. Further, var- ying degrees of medium- and long-term seismogenic backgrounds exist in the area; therefore, it is difficult to determine the impending earthquake stage because few earthquakes above 6 have oc- curred since the data of crustal deformation in this region has been accumulated. Nonetheless, earthquake data must be continually accumulated, evaluated, and improved.