Abstract: We determined a 3D P-wave velocity model of the middle and upper crust under the northwest Yunnan (25°~28.2°N, 99.5°~101.5°E) by using double-difference seismic tomography and jointly inverting 60471 absolute arrival times and 196465 differential arrival times from 7349 local events recorded by permanent seismic network and ChinArray. Our results show that a low P wave velocity zone is existed above 15 km in the northern section of the Jinshajiang-Red River fault zone, and majority of hypocenters are located in low velocity anomalies and focal depths are similar on both sides of the fault zone. We speculate that the role of the Jinshajiang-Red River fault zone as the southwest boundary of the Sichuan-Yunnan diamond block is weakening, and low velocity anomalies below the fault zone may helpful to dynamic transmission across the fault zone. There are large range of low velocity anomalies on both sides of the southwest section of the Lijiang-Xiaojinhe fault zone, and we deem that these low velocity anomalies may be the Tibetan plateau material which have been extruded. On the east of Chenghai fault zone, there are obviously P wave high velocity anomalies from near surface to 25 km depth, and these high velocity anomalies may block the southeastern escape of the plateau material.

Key words: Double-difference seismic tomography, 3D P-wave velocity structure, Jinshajiang-Red River fault zone, Lijiang-Xiaojinhe fault zone, Chenghai fault zone