PHASE VELOCITY TOMOGRAPHY OF RAYLEIGH WAVE IN QINLING-DABIE AND ITS ADJACENT AREAS USING AMBIENT SEISMIC NOISE

The Qinling-Dabie orogenic belt is located between the northeastern margin of the Tibetan Plateau and the Tan-Lu fault; it was formed by the collision of Yangtze and North China blocks. We obtain the phase velocity of Rayleigh wave at the periods of 8∼35 s in the Qinling-Dabie orogenic belt using ambient seismic noise recorded at 160 broad-band stations from China digital seismic network. 24 month data have been cross-correlated to yield the empirical Rayleigh wave Green's functions. Phase velocity dispersion curves are measured for each interstation path by frequency-time analysis. The Rayleigh wave phase speed maps agree well with each other and show clear correlations with major tectonic structures. The Dabie is characterized by high velocity anomaly at 8 s but slow velocity at 14 s, indicating the influence of high pressure (HP)/ultrahigh pressure (UHP) metamorphic regions in the upper crust. At 25 s, the velocity varies from slow in the west to high in the east across the gravity gradient zone in the Taihang-Wuling Mountains. This pattern mainly reflects the effect of crust thickness which is thicker in the west and thin in the east. The southern segment of Tan-Lu fault shows different features across it at 14∼35 s, suggesting that the fault zone may extend down to the crust-mantle boundary. The slow velocity close to the fault is probably caused by the hot material upwelling. More constraints are needed by further study. Obvious slow velocities at periods of 14∼25 s in South Qinglin and northeast Sichuan basin are observed. We could not determine whether this low velocity zone is due to the Tibetan lower crustal flow and/or the delamination of the South Qinglin now. Study of the 3-D shear wave velocity structure in the crust and upper mantle is necessary to constrain the geodynamics of this region in the future.