PN Wave Velocity Structure and Anisotropy in the Central Tien Shan Region

Abstract

Pn wave velocity structure and anisotropy in the central Tien Shan are inversed by using seismic arrival data from the broadband temporary network (GHENGIS) and the Kyrgyzstan seismic network (KNET). Results reveal lower average Pn velocities and a strong heterogeneity at the uppermost mantle beneath the central Tien Shan, which are often found in tectonically active zones. Particularly, very low Pn velocities are found in the south-central Tien Shan, which is believed to be underlain by an unusual mantle with higher heat flows. There is a clear correlation between seismic activity and Pn velocity variation. Most earthquakes are concentrated in the north-central Tien Shan with hight Pn velocities, while almost no earthquakes located in the south-central Tien Shan with very low Pn velocities. Our result suggests that the very low Pn velocity beneath the southcentral Tien Shan is caused by high temperatures produced by heating from mantle upwelling, and that this heat has been conducted well up into the crust, inducing a ductile rheology that prevents significant seismic activity. Moreover, strong anisotropy is observed in the north-and south-central Tien Shan. Fast Pn velocity anisotropy in the south-central Tien Shan is nearly in N-S directions, which is consistent with the SKS wave anisotropy concerning mantle flow. However, fast Pn velocity directions in the north-central Tien Shan show a southward rotation variation. We estimate that it is probably related to the inserting of the crust of the Chu basin from the southern Kazakstan platform to the central Tien Shan, which caused a change of stress fields and lithosphere deformation.

Crustal Structure of the Chuan‐Dian Block Revealed by Deep Seismic Sounding and its Implications for the Outward Expansion of the East Tibetan Plateau

“…The Pn wave velocity is faster in stable tectonic units, such as old basins and cratons, than in active or mobile tectonic units, such as active orogenic belts, extensional basins and volcanic areas (Christensen and Mooney, 1995;Li et al, 2006). Li et al (2011) have used Pn travel time picks to invert Pn velocity variation and anisotropy beneath the CDB, the results being consistent with the surface geological observation. Our results show that: the average Pn wave velocity beneath the CDB is 7.91 km/s, lower than the global average velocity of 8.09 km/s (Christensen and Mooney, 1995), which implies that the tectonic activity in the CDB is relatively strong, particularly in the Cenozoic.…”

“…A NS-trending deep Pn wave low velocity zone is developed along Liangshan-Panzhihua-Chuxiong-Simao (Fig. 6), which was also found by the inversion of the Pn travel time picks from the earthquake (Li et al, 2011). This zone may represent the present boundary between the Yangtze block and the Sanjiang tectonic belt, or the Panxi rift (Ma, 1989).…”

“…The Pn wave velocity is 7.81 km/s beneath the BSB, 7.72 km/s beneath the TCB, 7.82 km/s beneath the ZDB, 7.95 km/s beneath the LSB and 7.94 km/s beneath the WYB. The Pn wave velocity is relatively lower in areas with intense tectonic activity, such as the TCB, than areas with rigid crust, such as the Songming Basin (Huang et al, 2003;Li et al, 2011). A NS-trending deep Pn wave low velocity zone is developed along Liangshan-Panzhihua-Chuxiong-Simao (Fig.…”

“…The variation of the Pn wave velocity, reflecting differences in lithospheric strength (Wang et al, 2002), is of great significance to understanding the deep tectonic background of earthquakes and identifying the stability of the different tectonic units (Christensen and Mooney, 1995;Wang et al, 2002;Deng et al, 2011;Li et al, 2011). The Pn wave velocity is related to the strength and fluidity of the material in the crust as well and may even be a characteristic of the temperature at the top of the upper mantle (Xu et al, 2010;Deng et al, 2011).…”