2014年秭归MS4.5和MS4.9地震震源与发震构造特征

A DISCUSSION ON THE SOURCE AND SEISMOGENIC STRUCTURE OF M_S4.5 AND M_S4.7 ZIGUI EARTHQUAKES IN 2014

2014年3月27日和30日湖北秭归县发生了三峡水库蓄水以来该地区最大的2次地震,文中通过多种方法分析了2次地震的震源与发震构造特征。采用Kiwi方法反演2次地震的矩张量解,该方法中使用了湖北和重庆测震台网14个宽频带波形记录和6层地壳速度结构模型,反演结果显示,2次地震观测谱和波形与理论谱和波形拟合得比较好,非拟合误差数〈0.57,表明反演结果是可靠的。2次地震均为走滑兼少量逆冲错动,但前一地震为左旋走滑,后一地震为右旋走滑,矩张量解中DC成分偏少而ISO成分多可能是库水对地下介质物性影响的反映。同时也使用三峡台网15个子台记录的波形资料,采用双差定位法重新精定位了从3月27日至4月27日时段内超过500次地震序列事件,结果显示余震序列分布方向为NNW向和NE向,但主要集中在NE向,并分别沿NNW和NE向作了深度剖面,剖面显示震源深度为4.5-10.0km,余震在深部呈现2个较明显的断面,与震源机制解NE向节面产状一致。野外现场宏观烈度调查指出,Ⅴ度极震区等震线为一椭圆,长轴NWW向,短轴NE向,结合野外现场考察结果和震源区地质构造背景,综合推断仙女山断裂北端的NE向破裂面为2次地震的发震断面,余震序列的NE和NNW向分布、剖面上发震层的形态和深度特征表明,这次地震活动受到了仙女山断裂和九畹溪断裂活动的控制与影响。

On 27 and 30 Mar. 2013,an MS4.5 and MS4.7 earthquake occurred at Zigui County of Hubei Province,which were two larger earthquakes in this region since the impoundment of the There Gorges Reservoir in 2003. Characteristics of the focal and seismogenic tectonic of the two earthquakes are analysed and discussed in this paper. Moment tensor solutions of two earthquakes are inverted by the Kiwi method which uses the wideband waveform of 13 stations from Hubei and Chongqing seismic network and a six-layer crustal velocity structure model. The inverted results show that there is a good fit between the observed waveform spectrum and theoretical waveform spectrum,and the non-fitting errors of two earthquakes are less than 0.57. These indicate that the inversion results are reliable.Rupture modes of the earthquakes are both strike-slip with a small amount of thrust component,but the former earthquake is of sinistral strike-slip and the latter is the dextral one,and the fact that there is less DC component and more ISO component in their moment tensor solutions is likely the manifestation of the effect of the reservoir water on physical properties of crustal strata. More than 500 earthquakes from Mar. 27 to Apr. 27 in this sequence are relocated by Double-Difference Method using the waveform recording of 15 sub-stations from the Three Gorges Seismic Network,and the results show that aftershocks are distributed along NNW and NE,but mainly along NE direction. Two depth profiles along NNW and NE direction show that focal depths of the sequence are from 4.5 to10 km. Two significant planes in the deep strata are formed along NE direction by these aftershocks,which are in accord with the occurrence of the NE-striking nodal plane of the focal mechanisms. The field intensity survey points out that the isoseismal line of the intensity Ⅴ meizoseismal area is an ellipse,the major axis is along NNW direction and the minor axis is along NE direction. Combining the results of field intensity survey with the tectonic setting of the epicenter area,we deduce that the rupture plane along NE direction at the north of Xiannüshan Fault is the seismogenic fault plane of the two earthquakes,the distribution of aftershocks along NE and NNW,the shape of the seismogenic fault plane and the characteristic of the depth profiles all indicate that these earthquakes are controlled and influenced by Xiannüshan Fault and Jiuwanxi Fault.