A study on 3-D velocity structure of crust and upper mantle in Sichuan-Yunan region, China

Abstract

Based on the first arrival P and S data of 4 625 regional earthquakes recorded at 174 stations dispersed in the Yunnan and Sichuan Provinces, the 3-D velocity structure of crust and upper mantle in the region is determined, incorporating with previous deep geophysical data. In the upper crust, a positive anomaly velocity zone exists in the Sichuan basin, whereas a negative anomaly velocity zone exists in the western Sichuan plateau. The boundary between the positive and negative anomaly zones is the Longmenshan fault zone. The images of lower crust and upper mantle in the Longmenshan fault, Xianshuihe fault, Honghe fault and others show the characteristic of tectonic boundary, indicating that the faults likely penetrate the Moho discontinuity. The negative velocity anomalies at the depth of 50 km in the Tengchong volcanic area and.the Panxi tectonic zone appear to be associated with the temperature and composition variations in the upper mantle. The overall features of the crustal and the upper mantle structures in the Sichuan-Yunnan region are the lower average velocity in both crust and uppermost mantle, the large crustal thickness variations, and the existence of high conductivity layer in the crust or/and upper mantle, and higher geothermal value. All these features are closely related to the collision between the India and the Asia plates. The crustal velocity in the Sichuan-Yunnan rhombic block generally shows normal value or positive anomaly, while the negative anomaly exists in the area along the large strike-slip faults as the block boundary. It is conducive to the crustal block side-pressing out along the faults. In the major seismic zones, the seismicity is relative to the negative anomaly velocity. Most strong earthquakes occurred in the upper-mid crust with positive anomaly or normal velocity, where the negative anomaly zone generally exists below.

Heterogeneous strain regime in the eastern margin of Tibetan Plateau and its tectonic implications

“…Seismic velocity models indicate that the crustal thicknesses of the western Sichuan plateau and the Sichuan basin are 62 and 43 km, with lower crustal thicknesses at 27 and 15 km, respectively (Wang et al 2002). Therefore, the top surface of upper mantle displays great fluctuation in this region (Wang et al 2002). Density models constructed from the seismic velocity models are consistent with observed Bouguer gravity anomalies ).…”

“…The focal mechanisms of contemporary earthquakes in this region reveal a NNE horizontal maximum compressive stress (Kan et al 1977;Xu et al 1989). Seismic velocity models indicate that the crustal thicknesses of the western Sichuan plateau and the Sichuan basin are 62 and 43 km, with lower crustal thicknesses at 27 and 15 km, respectively (Wang et al 2002). Therefore, the top surface of upper mantle displays great fluctuation in this region (Wang et al 2002).…”

Double-difference relocation of earthquakes in central-western China, 1992–1999

“…The results obtained in this study indicate that the seismogenic layer in central-western China reaches, on average, a depth of about 20 km. With an average crustal thickness of about 45 km to 50 km and an average upper crustal thickness of about 20 km in this area of China (Kan and Lin, 1986;Liu et al, 1989;Sun and Liu, 1991;Fan and Chen, 1992;Wang et al, 2002Wang et al, , 2003, most of the stress appears to be released by brittle failure in the upper crust. It is worthy pointing out that the estimates of seismogenic depths depend on the initial focal depth estimates and the completness of the catalog.…”

Finite Difference Tomography of Crustal Velocity Structure Beneath Tengchong, Yunnan Province

“…Kan and Zhao have proved that a magma chamber underlies the Tengchong volcano on the basis of recent volcanic movement, different physical properties and focal mechanisms associated with magma impact [1] . The tomographic results by geophysicists show the existence of magma chambers or the paths of magma intrusions in the deep crust and mantle of this area [2,[9][10][21][22]24] . There are abundant surface manifestations in the Tengchong area, including the high-K calc-alkaline basalt, to indicate that the low-V band may be closely associated with the path through which the mantle magma upwells [33] .…”