Xu Y C,Guo X Y,Feng L L. 2022. Relocation and focal mechanism solutions of the MS6.9 Menyuan earthquake sequence on January 8,2022 in Qinghai Province . Acta Seismologica Sinica,44(2):195−210. DOI: 10.11939/jass.20220008
Citation: Xu Y C,Guo X Y,Feng L L. 2022. Relocation and focal mechanism solutions of the MS6.9 Menyuan earthquake sequence on January 8,2022 in Qinghai Province . Acta Seismologica Sinica,44(2):195−210. DOI: 10.11939/jass.20220008

Relocation and focal mechanism solutions of the MS6.9 Menyuan earthquake sequence on January 8,2022 in Qinghai Province

  • On January 8, 2022, an earthquake with MS6.9 occurred in Menyuan County, Haibei Prefecture of Qinghai Province. The earthquake ruptured a nearly 22 km long surface fracture zone and strong tremors were felt through the regions of Qinghai, Gansu and Ningxia regions. In this paper, the early events of the MS6.9 Menyuan earthquake sequence from January 8 to 12, 2022 were relocated by multi-step locating method. Meanwhile, the focal mechanisms and focal depths of the main shock and MS≥3.4 aftershocks were calculated by gCAP method. Based on focal mechanism result of the main shock, the relative shear stress and normal stress on the two nodal planes of the focal mechanism solutions were also calculated under the existing stress field system. The result indicates that the initial rupture depth of MS6.9 Menyuan main shock was 7.8 km, and the centroid depth was 4 km. The dominant initial rupture depths of early earthquake sequence were mainly between 7 km and 8 km, while the centroid depths of MS≥3.4 aftershocks varied between 3 km and 7 km. The focal depth profile shows that the sequence length within 24 hours after the main shock was about 25 km, which was roughly consistent with the length of the surface rupture zone, and the overall sequence length was about 30 km. The MS6.9 main shock and MS5.1 aftershocks on January 8 were located in the western part of the aftershock region, and the MS5.2 earthquake on January 12 was located in the eastern part of the aftershock region. The focal mechanism solution of the MS6.9 main shock was strike 290°, dip 81°, rake 16° for the nodal plane Ⅰ , and strike 197°, dip 74°, rake 171° for the nodal plane Ⅱ . Based on the spatial distribution of aftershocks, it is estimated that the strike of the fault plane is 290º, indicating that the earthquake is a left-lateral strike slip event on a nearly vertical fault plane. The results of focal mechanism solutions of MS≥3.4 aftershocks show that these earthquakes were mainly strike-slip, and from the west to the east of the aftershock region the P-axis azimuths varied from NE to EW. Under the current stress field system, the relative shear stress generated on the nodal plane Ⅰ of focal mechanism of the MS6.9 Menyuan earthquake is 0.638, while on the nodal plane Ⅱ is 0.522. The two nodal planes of focal mechanism are not the maximum released nodal plane of the tectonic stress field, which is obviously different from the thrust focal mechanism of the MS6.4 Menyuan earthquake in 2016, which is the optimal released nodal plane of tectonic stress field. Combined with the geological structure, focal mechanisms and aftershock distribution, the seismogenic structure of MS6.9 Menyuan earthquake on January 8, 2022 may be the western segment of Lenglongling fault, and its seismic dislocation mode is left-lateral strike-slip. According to the results of relocation, the magnitude-rupture relationship and the shear stress, it is concluded that there is a certain stress accumulation and stress has not been fully released in Menyuan area, and the risk of strong earthquakes still exists in this area.
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