2021年固原震群重定位及发震构造分析

Relocation and seismogenic structure of the 2021 Guyuan earthquake swarm

通过初至P震相定位法测定了2021年固原震群中ML≥3.0地震的震源深度,同时根据多阶段定位和微震匹配定位方法对该震群序列进行了重定位,基于Snoke方法和HASH方法计算了震群序列中ML≥2.8地震的震源机制解,并根据gCAP方法反演了震群中最大的地震即2021年6月22日固原ML3.6地震的震源机制解及矩心深度,确定了ML≥2.8地震基于各种方法所得震源机制的中心解,然后根据重定位结果拟合了断层面,最后初步分析了固原震群的发震构造。结果表明:微震检测到901次ML0.0以上地震事件,约为区域地震台网原始目录的2.6倍,震群的震中区呈WNW向优势分布,长约8 km,宽约3 km,震源深度处于8—17 km之间,平均深度为13 km;ML≥2.8地震主要发生在震群序列12—14 km震源深度的相对密集区,其震源机制解均为走滑型,大多数地震略带一点逆冲分量,P轴优势方位近EW向,与六盘山地区的局部构造应力场主压应力轴方向基本一致。结合重定位、震源机制和断层面拟合认为,固原震群的主要发震构造可能是由两个WNW走向而倾向相反的隐伏断层控制,即NE倾向和SW倾向的断层构成了向上分叉的花状结构,其中ML≥2.8地震均位于不同倾向断层的交会部位,均具有左旋走滑的运动性质,整个震群序列的小震活动大体呈现以NE倾向断层活动为主、以SW倾向断层活动次之的特征。结合已有研究,发现震群序列处于六盘山地区上地壳底部厚约11 km的低速异常透镜体内,初步分析认为固原震群的孕震环境可能与深部流体作用有关。

According to the Ningxia seismic network,Guyuan earthquake swarm activity occurred from June 11 to July 15,2021.The 2021 Guyuan earthquake swarm had the most earth-quake frequency ever recorded in historical swarm events of southern Ningxia,and hence attracted more attentions of relevant departments.Therefore,studying on the earthquake swarm provides fundamental data for the further comprehension of the seismic generation,evolution process of the earthquake swarm in this region and the investigation of the earthquake mechanism,thereby offering relevant supports for the earthquake tracking mission.In this research,ML≥3.0 earthquakes of 2021 Guyuan earthquake swarm were determined using the methods of first arrival P phase location,and the earthquake swarm was also relocated by using multi-stage location and microearthquake matching location.Meanwhile,the focal mechanism solutions of the ML≥2.8 earthquakes from the swarm were calculated based on Snoke method and HASH method.Focal mechanism and centroid depth of the ML3.6 Guyuan earthquake on June 22,2021,the largest in the swarm,was also determined by gCAP inversion method,and the central solutions of various focal mechanisms for the ML≥2.8 earthquakes were determined.Then the fault plane was fitted according to the relocation results and the seismogenic structure of Guyuan earthquake swarm was preliminarily analyzed.The results show that 901 microseismic events above ML0.0 were detected,which is about 2.6 times as much as those in the original catalog of the regional seismic network.The epicenters area is about 8 km long and 3 km wide,spreading in WNW direction predominately,with focal depths ranging from 8 km to 17 km,and the average depth of 13 km.ML≥2.8 earthquakes are mainly concentrated in the earthquake dense area with focal depth of 12−14 km.The focal mechanism of ML≥2.8 earthquakes show that all of them are strike-slip type,and most of them have a little thrust component,and the dominant azimuth of the P-axes is in nearly EW direction,which is basically consistent with the direction of the principal compressive stress of the local tectonic stress field in Liupanshan area.Combined with relocation,focal mechanism and fault plane fitting,it is suggested that the main seismogenic structure of Guyuan earthquake swarm may be controlled by two WNW-striking hidden faults with opposite dipping.The NE-dipping and SWdipping faults form upward-branching flower structure,and the ML≥2.8 earthquakes are located at the intersection of faults with different dip,and all may have left-lateral strike-slip movement.As for the whole earthquake sequence,the NE-dipping fault activity is dominant,and the SW-dipping fault activity is secondary.According to previous studies,it is found that the Guyuan earthquake sequence is located in the low-velocity anomalous lens with about 11 km thickness at the bottom of the upper crust in Liupanshan area.The crust-mantle interaction deep fluid action and high Poisson’s ratio zone due to the upwelling of the upper mantle in the Liupanshan area are the possible seismogenic reasons for the formation of the low-velocity lens where Guyuan earthquake swarm is located.The preliminary analysis suggests that the seismogenic environment of Guyuan earthquake swarm may be principally related to the action of deep fluid.This research is of significance for the deep insight into the relationships between earthquake relocation,focal mechanism and seismogenic structure.