Focal mechanism determination of Nima M_S6.6 and M_S4.8 earthquakes in Tibet in 2020

A M_S6.6 earthquake occurred at 4:07 am on July 23, 2020 in Nima County, Nagqu, Tibet Autonomous Region. The location of the main shock is 33.19°N, 86.81°E, and the focal depth is 10 km. At 18:50 on the same day, a strong aftershock of M_S4.8 occurred with a focal depth of 10 km. Based on the regional waveform data of Tibet, Qinghai and Xinjiang, we use the ISOLA full waveform method for local earthquakes to retrieve the focal mechanism of these two events. The results show that the best fault plane solution of Nima M_S6.6 main earthquake is as follows: strike 8°/dip 46°/rake −93° for nodal plane Ⅰ, strike 191°/dip 44°/rake −87° for nodal plane Ⅱ; the moment magnitude is M_W6.4, and the centroid depth is 7 km. The spatial orientation of the principal stress axis in the focal area is defined where the azimuth of the main pressure axis P is 220° and the plunge angle is 88°, and the azimuth of the main tension axis T is 99° and the plunge angle is 1°. The results show the best fault plane solution of the M_S4.8 aftershock with strike 12°/dip 47°/rake −106° for nodal plane Ⅰ and strike 214°/dip 45°/rake −74° for nodal plane Ⅱ; Moment magnitude is M_W5.0, and the best depth of moment center is 6 km. The spatial orientation of the main stress axis in the focal area is as follows: the azimuth angle of the main pressure axis P is 207° and the plunge angle is 78°, and the main tension axis T is 113° and the plunge angle is 1°. Focal mechanism inversion results show that these two earthquakes are mainly normal fault events, which are in good agreement with the previous focal mechanisms near the focal area. Combined with the surrounding geological structure and aftershock distribution, we believe the Nima M_S6.6 earthquake may be caused by the activity of a normal fault between the Riganpei Co fault and the fault on the western margin of the Yibug Caka Basin.