廊固凹陷深部剪切破裂构造的地震学证据

基于区域地震台网观测数据，采用近震波形反演方法，确定2018年2月12日河北永清M4.3地震的最佳双力偶源震源机制解为：节面Ⅰ走向297°，倾角58°，滑动角-32°；节面Ⅱ走向45°，倾角63°，滑动角-144°；是一个略带正断分量的右旋走滑地震.结合近震转换波测定主震的震源深度在19 km附近.地震序列的双差定位结果显示：永清地震序列震中呈北东向窄带展布，表明此次地震主要向北东向破裂；深度集中分布在17~19 km，整体形态近于铅直，显示发震断裂具有走向北东、倾向南东、倾角陡立的特征，与节面Ⅱ的性质比较吻合，推测节面Ⅱ为发震断层面.将发震断层面参数与震源区附近断裂性质进行对比分析，形成了关于廊固凹陷附近区域地震构造的一些认识：（1）推测永清地震的发震构造不是地壳浅部发育的先存正断裂，而是震源区下方一条地壳尺度的深断裂，该深断裂为新生断裂，具有右旋走滑正断性质，倾角陡峭、近于直立、宽度较大，向上与夏垫断裂相通.（2）综合震源区附近多条深地震反射剖面探测结果，推测永清地震的发震断裂与新夏垫断裂同属一条断裂，称为：新夏垫深断裂.该断裂从夏垫向西南方向延伸至文安，并可能与霸县-束鹿-邯郸断裂带相联系，总长度超过150 km.（3）基于2006年文安M5.1地震与2018年永清M4.3地震在震源机制上的相似性及震源位置上的关联性，结合区域构造条件，认为两次地震的发震构造均为新夏垫深断裂.（4）根据研究区几次显著地震的震源深度分布特征，参考区域断层构造、电性结构和流变学模型，推测活化克拉通块体新生断裂的脆韧性转换界面深度在15 km附近.

Seismological evidence for a deep-seated shear zone in the Langgu Depression

The focal mechanism of 2018 Yongqing M4.3 earthquake was resolved by applying CAP method to local waveforms, showing that the best double-couple solution is as follows:the strike, dip, and rake angles of the two nodal planes are 297°, 58°, -32°, and 45°, 63°, -144°, respectively. The centroid depth of mainshock was resolved as about 19 km with locally converted waves, and the locations of aftershocks were also solved by the HypoDD method. The result from relocation shows that the epicenters of events are distributed along northeast, and the focal depths ranges from 17 to 19 km, distributed within a straight zone, indicating that the causative fault should be a strike-slip fault which strikes to the northeast and dips to the southeast. From the mechanism and spatial distribution, we infer that, (1) the seismogenic tectonic character of the earthquakes should not be caused by normal faults in the shallow crust, but by the deep-seated Cenozoic dextral strike-slip fault, which cuts through the whole crust vertically from the shallow to the deep portion of the crust; and that (2) the causative fault should be one of the branches of the Xiadian fault zone, which strikes from Xiadian to Wen'an with a total length of more than 150 km, and maybe related to the Baxian-Shulu-Handan fault zone; and that (3) the 2006 Wen'an M5.1 earthquake and 2018 Yongqing M4.3 earthquake should be both caused by the Xiadian fault zone. Furthermore, based on the comparison of the distribution of focal depth of events, regional fault zone, electrical structure and rheological model in the area under investigation, we posit that, the brittle-ductile transition depth of the newly-generated fault in the activated craton should be around 15 km.