Abstract: In this paper, the phase velocity distribution images of Southwest Yunnan (21.0°N~29.5°N, 97.0°E~103.5°E) are obtained with continuous seismic records at 190 mobile stations in the first periods of ChinArray (2011-01—2014-06) and 35 broadband permanent stations of Yunnan regional network. The ChinArray was setup in the southeast of the Qinghai-Xizang Plateau. Inversion results reveal significant lateral heterogeneity in the velocity structure of the crust and upper mantle in the study area. Short period images (6~12 s) indicate a strong correlation between upper crust velocity and surface geological structures. The low velocity anomaly in Tengchong volcanic area and its southern side is related to the magmatic sac or partial melt in the upper crust, and the shape is different in depth. A high-velocity anomaly is observed between Chenghai fault and Puduhe fault, consistent with the body wave travel time inversion results. The inversion indicates a large high-velocity anomaly in Panxi area, potentially related to basalt uplift in this area. Phase velocity images with periods of 14~25 s reveal low-velocity anomalies in the Sichuan-Yunnan rhombic block and Tengchong volcanic area, suggesting the presence of lower crustal flow. Comparing 14~40 s images, due to variations in crustal thickness, the high-velocity anomaly gradually increases in the southern region. Furthermore, the low-velocity anomaly in the Tengchong volcanic area extends from the surface to the upper mantle, but it does not appear to be continuous with the low velocity anomaly in the Sichuan-Yunnan rhombic block. This low velocity anomaly may be a channel of mantle heat flow. The intersection of the Tengchong fault and Longling fault appears to be the boundary between high and low-velocity anomalies. Two M≥7 earthquakes in 1976 may occurred beneath this area.

Key words: Ambient noise tomography, Southwest Yunnan Area, Phase velocity, Crustal structure