震前突变——水准资料反映的2013年岷县-漳县M_S6.6地震孕震特征

Sudden Change before Earthquake-Seismogenic Characteristics of 2013 Minxian-Zhangxian M_S6.6 Earthquake as Reflected by Leveling Data

利用兰(州)天(水)武(都)水准监测网1973~2014年水准资料,获取2013年岷县-漳县MS6.6地震前后震中及其所在区域的垂直形变场。结果表明:1)从1999~2006年区域垂直形变场来看,本次地震位于NNW-NWW向隆起带至近EW向相对下沉带的转换位置,也是反映岷山与秦岭之间构造活动的差异带附近。2)从震中以西附近沿NNE向的陇西-岷县水准剖面演化过程来看,垂直形变具有明显的阶段性变化特征,最终表现为震前突变。由分时段结果来看,1973~1993年,在漳县至岷县之间出现最大上升幅度近40mm的压性隆起区;1993~2008年,隆起区消失且转平,期间剖面整体呈波动状态,幅度在±10mm之间,西秦岭断裂、临潭-宕昌断裂两侧无明显的垂直差异运动;2008~2011年,相对陇西盆地,漳县至岷县之间跨临潭-宕昌断裂两侧出现近40km长的下沉梯度带,其中距震中16km以内的武定80、武定78、武定77等3个水准点同步出现向下突变,接近或大于1973年以来正常变化幅度的2倍;2011~2014年,漳县、岷县间相对陇西盆地恢复以逆冲隆升为主,但震前临潭-宕昌断裂两侧出现的形变梯度带已不存在,而上述3个水准点也恢复至震前波动区间。3)在总结岷县-漳县MS6.6地震前形变前兆的基础上,针对目前关于震前是否出现形变加速或突变特征的争议认为,只要形变资料的时空尺度足够,至少对某些构造区域的中强地震还是可以捕捉到震前的突变现象。

The vertical deformation field of the epicentral region before and after the 2013 Minxian-Zhangxian M_S6.6 earthquake is obtained by using the leveling data of Lanzhou-Tianshui-Wudu regional leveling from 1973 to 2014. The results show that:(1) From 1999~2006 regional vertical deformation field, the earthquake is located at the transition zone from the uplift zone of NNW-NWW to the subsidence zone near EW, and is also a reflection of the tectonic activity difference zone between the Minshan and Qinling mountains.(2) From the evolutionary process of the Longxi-Minxian leveling section along the NNE direction near the west of the epicenter, the vertical deformation has obvious characteristics of stage deformation, and it finally shows the mutation before the earthquake. Among them: in 1973~1993, there is a pressure uplift area with the maximum rise of nearly 40 mm between Zhangxian and Minxian. The uplift area disappears and turns flat. the overall profile of the section fluctuates, and the range is about 10 mm from 1993~2008. There is no obvious vertical difference movement on both sides of the west Qinling fault and Lintan-Tanchang fault. A subsidence gradient zone near 40 km appears on both sides of the Lintan-Tanchang fault between Zhangxian and Minxian relative to the Longxi basin in 2008~2011. Among them, three leveling points, Wuding 80, Wuding 78 and Wuding 77, which are within 16 km of the epicenter, have a downward mutation synchronously, close to or more than twice the normal range of changes since 1973. Relative to Longxi basin, the recovery between Zhangxian and Minxian is mainly thrusting and uplifting. In 2011~2014, the deformation zone on both sides of the Lintan-Tanchang fault before the earthquake did not exist, and the above 3 leveling points are also restored to the interval before the earthquake.(3) On the basis of summing up the deformation precursors before Minxian-Zhangxian M_S6.6 earthquake, in view of the current controversy about the characteristics of deformation acceleration or mutation before the earthquake, we infer that if the spatio-temporal scale of the deformation data is sufficient, at least the moderate strong earthquakes in some tectonic regions can be caught before the earthquake.