漾濞Ms6.4地震三维精细速度结构与地震活动研究

Three-dimensional high-resolution velocity structure imaging and seismicity study of Yangbi Ms6.4 earthquake

本研究利用漾濞及其周边地区32个固定台站和94个临时台站记录到的5506个地震的体波到时数据,使用具有一致性约束的双差层析成像方法联合反演了漾濞Ms6.4地震序列的震源参数及三维精细的Vp、Vs、Vp/Vs结构模型。结果显示,重定位后的震源深度大多为3-10km范围,呈明显的近水平状分布,集中在高速体的薄弱区域或高低速体过渡带偏向高速体的一侧,与速度结构之间表现出了很好的对应关系。漾濞地区的速度结构存在明显的不均特性,地震活动发育区在近地表处以容易发生脆性破裂的高速体为主,随深度增加开始有低速异常体出现,在地震活动密集条带的上下两侧波速比值存在明显差异。根据漾濞Ms6.4地震的震源机制和精细的三维速度结构,本文认为漾濞Ms6.4地震是西北方向坚硬的高速体将强大区域构造应力传递到相对薄弱区,导致应力集中位置发生破裂所产生的。Ms5.6前震打破了区域应力的固有平衡,促进了漾濞Ms6.4主震的发生,随后又通过大量余震调整区域应力到新的平衡状态,形成了前震-主震-余震型的地震活动模式。基于漾濞Ms6.4地震序列活动规律和三维速度结构分布特征,本文推测此次漾濞地震的发震构造是位于维西-乔后断裂西南侧3-10km深度可能存在的北西向走滑型隐伏断层。

In this study,based on the body wave arrival data of 5506 earthquakes recorded by 32 fi xed stations and 94 temporary stations in Yangbi and surrounding areas,the source parameters of Yangbi Ms6.4 earthquake sequence and three-dimensional(3-D)fi ne Vp,Vs,and Vp/Vs were inverted by using the consistency-constrained double-diff erence tomography method.The results showed that the focal depth after relocation was mostly in the range of 3–10 km,evidently nearly horizontally distributed,and concentrated in the weak area of the high-velocity body or at the side of the high-low-velocity body transition zone toward the high-velocity body,showing a good corresponding relationship with the velocity structure.The velocity structure in the Yangbi area has remarkably uneven characteristics.The seismic activity area is dominated by high-velocity bodies prone to brittle fracture near the surface.As the depth increases,low-velocity anomalies appear.A signifi cant diff erence was observed in the wave velocity ratio between the upper and lower sides of the seismically dense strip.Based on the focal mechanism of the Yangbi Ms6.4 earthquake and the fine 3-D velocity structure,this article concludes that the Yangbi Ms6.4 earthquake was caused by a strong regional tectonic stress concentrated in the relatively weak area by hard high-velocity bodies on the northwest sides.The Ms5.6 foreshock broke the inherent balance of regional stress and promoted the occurrence of the Yangbi Ms6.4 mainshock.Afterward,the stress was adjusted to a new equilibrium state through a large number of aftershocks,forming a foreshock–mainshock–aftershock type of seismic activity model.Based on the activity law of the Yangbi Ms6.4 earthquake sequence and characteristics of the 3D velocity structure distribution,this paper speculates that the seismogenic structure of the Yangbi earthquake was possibly a northwest strike-slip buried fault with a depth of 3–10 km on the southwest side of the Weixi–Qiaohou fault.