HR: 1340h
AN: S53B-0198    [Abstracts]
TI: Crustal Structure of China and Surrounding Regions from P and S Wave Travel-time Tomography
AU: * Sun, Y
EM: youshun@mit.edu
AF: massachusetts Institute of Technology, 77 Mass Ave, Cambridge, MA 02139 United States
AU: Toksoz, N
EM: toksoz@mit.edu
AF: massachusetts Institute of Technology, 77 Mass Ave, Cambridge, MA 02139 United States
AU: Lei, J
EM: leijs@sci.ehime-u.ac.jp
AF: Ehime University, Bunkyo-cho 3, Matsuyama, 790-8577 Japan
AU: Zhao, D
EM: zhao@sci.ehime-u.ac.jp
AF: Ehime University, Bunkyo-cho 3, Matsuyama, 790-8577 Japan
AB: We have constructed a 3-D P and S velocity structure of the crust and uppermost mantle of China and surrounding areas using high quality first arrivals extracted from the Annual Bulletin of Chinese Earthquakes (ABCE). We used 345,000 P wave, and 230,000 shear wave arrivals. A preliminary 3-D model has been generated by combining 2400 1-D models (Sun et al., 2004). By applying the tomography method (Zhao et al., 1992), we obtain a detailed 3-D P and S wave velocity model of the crust and uppermost mantle in China with a spatial resolution of 1$\deg$ $\times$ 1$\deg$ in horizontal direction and 10 km in depth. This method is adaptable to a velocity structure which includes several complex-shaped velocity discontinuities and allows 3-D velocity variations everywhere in the model. Velocity discontinuities represent known geological boundaries such as the Moho and/or a subducting slab boundary, etc. A 3D grid net is set up in the model to express the 3D structure. Velocity perturbations at the grid nodes are taken as unknown parameters. The velocity perturbation at any point in the model is calculated by linearly interpolating the velocity perturbations at the eight grid nodes surrounding that point. To calculate travel times and ray paths accurately and rapidly, an efficient 3D ray-tracing technique is employed to iteratively use the pseudo-bending technique (Um and Thurder, 1987) and Snell's law. Station elevations and the sediment layers are taken into account in the ray tracing. The LSQR algorithm (Paige and Saunders, 1982) with a damping regularization is used to solve the large and sparse system of observation equations. Our tomographic model provides new insights into the geological structure and tectonics of the region, such as the lithological variations and large fault zones across the major geological terrenes. The velocity images of the upper crust represent the surface geological and topographic features accurately. Basins and the Tibetan Plateau are clearly depicted in our model. The crust beneath the Precambrian regions (Tarim Basin, Ordos Basin, Sichuan Basin and western half of Songliao Basin) shows high velocity anomalies. The highest velocity anomaly is beneath the Sichuan Basin. The Bohai Gulf shows both high and low velocity anomalies due to a major Cenozoic rift system through it. P velocities are lower in the lower crust beneath the northern part of the South China Block than that beneath the southern part of the South China Block. The Indochina Block demonstrates low velocities both in the crust and uppermost mantle due to volcanism. The Pn velocities in the Tibet area are higher than those in other areas largely due to thicker crust thickness. Both 3-D travel time and broadband seismogram fits show that our model is well determined and can be applied to determine the source parameters of earthquakes and to generate synthetic travel times.
DE: 1734 Seismology
DE: 0935 Seismic methods (3025)
SC: Seismology [S]
MN: 2004 AGU Fall Meeting