Crustal seismic imaging of Northeast Tibet using first and later phases of earthquakes and explosions

A new crustal 3-D P-wave velocity model beneath NE Tibet is determined by jointly inverting 62 339 high-quality first P wave and later PmP-wave arrival-time data from local earthquakes and seismic explosions. Resolution tests show that the use of the PmP data can effectively improve the resolution of crustal tomography, especially that of the middle-lower crust. Widespread but intermittent low-velocity anomalies are revealed in the lower crust beneath NE Tibet, and the Kunlun fault acts as a transfer structure. High-velocity zones are visible in most parts of the crust below the transition zones bordering the southwestern Ordos basin between 105 degrees and 106 degrees E longitude. We think that they form an important transpressive boundary to absorb sinistral strike-slip and thrust faulting deformation, like a western frontline of the Ordos basin, which is compatible with the latest GPS observations in the region. Considering the tectonic features and deformation of the Liupanshan fault, we need to pay much attention to the seismic risk of the fault zone from now. Our results reveal different structural features of the major blocks and their boundary faults, indicating the complexity of the Cenozoic crustal deformation in NE Tibet that partitioned between steep strike-slip shear zones and thrust faults.