基于余震重定位和震源机制解研究青海玛多M_(S)7.4地震序列的发震构造和断裂形态

Seismotectonics and fault geometries of the Qinghai Madoi M_(S)7.4 earthquake sequence:Insight from aftershock relocations and focal mechanism solutions

2021年5月22日02时04分(北京时间),青海果洛州玛多县发生M_(S)7.4地震,震后余震不断.地质调查和卫星观测对地表断裂痕迹有较好的约束.然而,对于理解区域应力场、地震的产生、传播和终止具有重要意义的地下断层几何结构的约束精度略显不足.利用国家地震台网的连续波形记录,本研究首先基于双差定位法对玛多地震震后25天的余震序列进行重定位,结果显示余震序列大致沿NWW向的江错断裂呈线性分布,位于主震震中两侧,延伸总长~170 km.主震东南侧存在一余震稀疏区,在断裂带东西两端余震分布转向且出现分叉现象,反映出发震断层的复杂几何形态,这与前人研究结果基本一致.进一步采用波形反演方法和P波初动极性反演方法,获得了玛多震源区132个中小余震的震源机制解与震源矩心深度,并基于此对该主余型地震的发震构造与断裂形态进行了初步分析.震源机制解结果表明,玛多M_(S)7.4主震的发震断裂主要为左旋走滑性质,余震与主震性质整体相同,在断裂带东段存在部分逆冲型余震.震源机制解约束的区域主应力方向约N60°E,与区内整体走滑断裂作用相一致.余震震源深度略微起伏,主要集中在10~12 km,且浅部余震较少,表明浅部应力可能主要通过主震释放,余震深度分布可能限定了主震同震破裂的下边界.玛多主震破裂起始于断裂带走向和倾向发生明显变化的位置,表明断裂带的复杂几何结构可能是此次玛多M_(S)7.4地震初始破裂空间分布的决定因素.主震破裂结束的两端都有"马尾状"构造(或次级断层),表明这种分叉断层复杂的几何形态可能控制着主震破裂的最终位置.

The 2021 M_(S)7.4 Madoi earthquake occurred in Madoi county,Guoluo Prefecture,Qinghai Province at 02:04(Beijing Time)on 22 May followed by notable aftershocks.Geological surveys and satellite observations provide important constraints on the surface traces of faulting.However,the subsurface fault geometries,which are important for understanding the regional stress field,earthquake initiation,propagation,and termination,are not well resolved.In this study,the double-difference earthquake location algorithm was employed to relocate the aftershocks within the first 25 days.The aftershocks mostly lie along a narrow compact straight strip,the Jiangcuo Fault,stretching NWW of the M_(S)7.4 hypocenter,indicating that the mainshock ruptured bilaterally with a total length of~170 km.To the east and west ends of rupture zone,the aftershocks bifurcates,indicating the complex geometry of the fault zone structure.The focal mechanism solutions of 132 aftershocks in the Madoi source area were obtained by the waveform inversion method and the P-wave first motion polarity inversion method.The focal mechanism solutions show that the mainshock occurred on a left-lateral strike-slip fault and the focal mechanism of the aftershocks is similar to that of the mainshock.Stress inversion shows that the maximum compressive stress is orientated about N60°E.The depth of aftershocks is mainly concentrated in 10~12 km.There are few shallow aftershocks,indicating that the stress may be largely released by the mainshock at shallow depth.Therefore,the depth distribution of aftershocks may limit the lower boundary of coseismic rupture of the mainshock.The rupture of the Madoi mainshock started at the location where the strike and tendency of the fault changed significantly,indicating the fault plane geometrical complexities may represent a favored site for the M_(S)7.4 event nucleation.There are multiple branches of faults at both ends of the mainshock rupture,indicating the fault geometric complex plays an important role in earthquake termination.