S-wave Velocity And Azimuthal Anisotropy Of The Crust And Uppermost Mantle In North China And Yunnan Region

In this study, we proposed the method of cross-correlation and multi-earthquake stacking to measure interstation surface wave dispersion, which could improve the accuracy of dispersion measurement. We used teleseismic data recorded by the portable seismic network deployed in North China and the permanent seismic network in Yunnan region, and measured the interstation Rayleigh wave phase velocity (10- 60 s). We constructed both the isotropic and the anisotropic S-wave velocity structure of the crust and uppermost mantle in North China and Yunnan region. The traditional two-step inversion scheme was adopted for isotropic S-wave velocity inversion. Firstly, we imaged the isotropic Rayleigh wave phase velocity distributions using the continuous Tarantola inversion method. Secondly, we inverted the 1-D S-wave velocity structure with a grid spacing of 0.°×0.3° and constructed the 3-D isotropic S-wave velocity structure of the North China. For anisotropic S-wave inversion, a similar two-step inversion scheme is adopted. We discussed the key geodynamic problems in North China and the Yunnan region and obtained some meaningful conclusions.In North China, the S- wave velocity structures in the northwestern and southwestern sides of the North-South Gravity Lineament (NSGL) are obviously different. The upwelling asthenosphere above the stagnated Pacific plate may cause the destruction of the Eastern Block and form the NSGL. A prominent low-V anomaly exists around Datong from 50 to 110 km, which may due to the upwelling asthenosphere originating from the mantle transition zone beneath the Western Block. The upwelling asthenosphere beneath the Datong may also contribute to the destruction of the Eastern Block. In the depth range of 30-40 km, a prominent low velocity belt is shown on the southwest of Zhangjiakou-Penglai fault zone (ZPFZ) and the fast direction is sub-parallel to the strike of the low velocity belt. The ZPFZ may cut through the lithosphere and act as a channel of the upwelling asthenosphere. A noticeable low-V zone also exists in the lower crust and upper mantle lid (30-50 km) beneath the Beijing-Tianjin-Tangshan (BTT) region, which may be caused by the upwelling asthenosphere through the ZPFZ. The anisotropic results reveal that the azimuthal anisotropy varies both horizontally and vertically. Obvious stratified azimuthal anisotropy is shown in the Central Orogenic Belt, where the fast direction is NE-SW to NNE-SSW in the depth range of 20- 40 km and changes to NW-SE in the depth range of 60- 110 km. Distinct lateral variations of azimuthal anisotropy are clearly shown at 110 km. Historical strong earthquakes (M> 6.0) mainly occurred in the transition zone between low and high velocity anomalies in the upper and middle crust. The upwelling asthenosphere may prompt the generation of large earthquake.In Yunnan region, the crustal low-V flow and crust-mantle coupling mechanism are of great interest. Our result shows that the crustal low-V zones do exist in Yunnan region, and the low-V zones are connective and are not separated by the main faults. In the low-V zone, the fast axis of S-wave is parallel to the strike of the low-V zone, which coincides with the crustal channel flow model. The crust-mantle coupling mechanism in Yunnan region is quite complex and different regions may have different patterns. The N- S trending Luzhijiang fault can separate the Dianzhong btock to two parts. In the eastern part of the Dianzhong block and the Yangtze Craton, there is uniform fast axis of S-wave in the depth range of 20-110km, which suggests that the crust and mantle are coupled in this region. However, in the western part of the Dianzhong block, the fast axis of S-wave changes with depth, which suggests that the crust and mantle are decoupled. The Baoshan block has different S-wave and azimuthal anisotropy patterns in different depths, which suggests that the crust and mantle are decoupled there. The S-wave velocity structure shows that the Dianzhong block and the Yangtze Craton have similar structure and both show high-V feature at 80-110km. We suggest that the Dianzhong block belongs to the Yangtze Craton. The Tengchong block, Baoshan block and Simao block show low-V feature at the depth of 80-110km, which may due to the upwelling of asthenosphere from deep mantle. At 20-60km, there is a very obvious high-V anomaly in the southern Baoshan block, and the shape of the high-V is consistent with that of the Lancang River fault. We suggest that the Baoshan block may extrude Simao block in the northeast direction in the geologic time and form the shape of the Lancang River fault. In Yunnan region, most of the historical large earthquakes occurred along the main faults. Most earthquakes were located in the Sichuan-Yunnan diamond block and in the western edge of the study region. Most earthquakes occurred in the low-V zone or velocity transition zone in the upper-middle crust, and there is almost no large earthquake in high velocity zone. There are few large earthquakes near the Red River fault and the Ailaoshan fault, which may due to the similar S-wave structure of Simao block and Dianzhong block.