2021年5月21日云南漾濞M_(S)6.4地震序列重新定位、震源机制及应力场反演

Relocations,focal mechanisms and stress inversion of the May 21th 2021 Yangbi M_(S)6.4 earthquake sequence in Yunnan,China

2021年5月21日云南省大理州漾濞县发生了M_(S)6.4地震,引起了社会的极大关注.本研究利用双差定位法对云南漾濞M_(S)6.4地震序列(2021年5月18—24日)进行了重新定位,获得331个地震重新定位结果,主震震源位置为(99.869°E,25.689°N,8.8 km).利用远场Rayleigh面波振幅的频谱陷波相,通过理论地震图的波形拟合,测定了主震震源深度为7.5 km.由于使用了远场数据,基本避免了区域速度模型对深度定位产生的影响,确定的震源深度准确度优于传统的走时定位.本文使用云南省区域地震台网波形数据,对主震及29个M_(L)3.5以上余震进行震源机制反演.反演结果表明,主震为一次走滑型地震,震源机制为节面I走向143°/倾角80°/滑动角-165°,节面II走向50°/倾角75°/滑动角-10°,矩震级为M_(W)6.0.根据余震序列分布情况,推断节面I为发震断层面.利用地震序列震源机制解的力轴数据,反演得到震源区的主压应力方向为近NS向(7°).本文研究表明,震源机制反演得到的主压应力方向与上地壳各向异性揭示的主压应力方向有高度的一致性.

The M_(S)6.4 earthquake occurred in Yangbi County,Dali Prefecture,Yunnan Province on May 21 th,2021.It has drawn great attention.Using double difference algorithm,the M_(S)6.4 Yangbi earthquake sequence is relocated,and 331 earthquakes relocation results are obtained.The main shock location is(99.869°E,25.689°N,8.8 km).In order to obtain a more accurate focal depth,the notch phase of the far-field Rayleigh surface wave amplitude spectrum was used to locate the main shock.This method used the position of the notch phase to compare with the theoretical seismogram to accurately determine the focal depth in frequency domain.Because of the far field Rayleigh wave used,the influence of the errors of regional velocity model on depth location is avoided,and the accuracy of depth determination is higher than the traditional travel time locating.After depth location,the focal depth of the main shock was 7.5 km.Using the waveform data from Yunnan regional seismic network,the focal mechanisms of main shock and 29 aftershocks(M_(L)≥3.5)were inversed.The inversion results show that the main shock is strike-slip earthquake and the moment magnitude M_(W)=6.0.The nodal plane 1 of the double couple is strike=143°/dip=80°/rake=-165°;The nodal plane 2 of the double couple is strike=50°/dip=75°/rake=-10°.According to the distribution of the aftershock sequence,it is inferred that the nodal plane 1 is the seismogenic fault plane.Appling the focal mechanisms of the earthquake sequence,the principal compressive stress direction is nearly north-south(7°)in the source regions.In this study,it is shown that the principal compressive stress direction obtained by inversion of focal mechanism is highly consistent with that revealed by anisotropy of upper crust.