Abstract
On May 21, 2021, an MS6.4 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 ML ≥ 1.5 from 2009–2019 and the Yangbi sequence in May of 2021 were used to invert the three-dimensional (3D) structures for both vP and vP/vS. 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 vP velocity was verified using airgun signals excited by the Binchuan Airgun Transmitting Seismic Station (BATSS). The resulting vP and vP/vS 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-vP and high-vP/vS values. From 3–15 km, vP and vP/vS 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 (MS5.6), mainshock (MS6.4), and largest aftershock (MS5.2) occurred near the boundaries where both vP and vP/vS have clear contrasts. (3) Small earthquakes are also concentrated in the transition zone between high- and low-vP and vP/vS anomalies, and are biased toward low-vP/vS zones. (4) Boundaries in vP and vP/vS are observed at 20 km west of the Weixi-Qiaohou-Weishan fault, indicating that there may exist one hidden fault.
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