Velocity structures of the crust and uppermost mantle beneath the northeastern margin of Tibetan plateau revealed by double-difference tomography

We assemble the local earthquake data recorded by 71 permanent stations and 418 portable stations around the northeastern margin of the Tibetan plateau, and then utilize the double-difference tomography method to image high-resolution 3D P- and S-wave velocity structures of the crust and uppermost mantle in the study area along with hypocentral relocations. The resultant P- and S-wave velocity models fit both body-wave traveltime and Rayleigh-wave phase velocity dispersion data better than previous global models. Significant low-velocity anomalies of P- and S-waves appear in the depth interval of 20 similar to 40 km beneath the Songpan-Garze block and Qilian orogenic belts. The low-velocity layer below the Songpan-Garze block may be related to the crustal partial melting, while that of the Qilian orogenic belt possibly indicates the crustal thickening. The hypocenter depths of both the Minzhang M6. 7 earthquake and Jiuzhaigou M7. 0 earthquake after relocation lie around the high-and-low velocity transition zone within the brittle upper crust, showing both events occurred at the junction of distinct blocks. The low-velocity layer beneath these two earthquake areas may be partially melted or easier to creep, and the brittle upper crust is more likely to accumulate elastic strain, resulting in the occurrence of earthquakes.