Three-dimensional velocity structure around the focal area of the 2021 MS6.4 Yangbi earthquake

On May 21, 2021, an M_S6.4 earthquake occurred in Yangbi, Yunnan province, China, which exhibited typical foreshock-mainshock-aftershock characteristics. To better understand the velocity structure of the focal area and adjacent fault zones, Pg/Sg travel times at 12 seismic stations for the local earthquakes with M_L ≥ 1.5 from 2009–2019 and the Yangbi sequence in May of 2021 were used to invert the three-dimensional (3D) structures for both v_P and v_P/v_S. The obtained structure extends deeply to 15 km for area (25°N–26.5°N, 99.5°E–101°E) at a horizontal resolution of 10× 10 km, and the accuracy of the v_P velocity was verified using airgun signals excited by the Binchuan Airgun Transmitting Seismic Station (BATSS). The resulting v_P and v_P/v_S images correlate with existing fault zones and the Yangbi sequence, including: (1) The shallow velocity structure at 0 km agrees with local topography, where the Binchuan basin exhibits low-v_P and high-v_P/v_S values. From 3–15 km, v_P and v_P/v_S show variations, and the boundaries are consistent with the main faults (e.g., the Weixi-Qiaohou-Weishan, Honghe, and Chenghai faults). (2) The largest foreshock (M_S5.6), mainshock (M_S6.4), and largest aftershock (M_S5.2) occurred near the boundaries where both v_P and v_P/v_S have clear contrasts. (3) Small earthquakes are also concentrated in the transition zone between high- and low-v_P and v_P/v_S anomalies, and are biased toward low-v_P/v_S zones. (4) Boundaries in v_P and v_P/v_S are observed at 20 km west of the Weixi-Qiaohou-Weishan fault, indicating that there may exist one hidden fault.