A 3-D P-wave upper-mantle tomography model under eastern Tibet is inferred using a large number of high-quality arrival-time data collected from seismic stations in southwestern China. The results show that high-velocity (high-V) anomalies are revealed down to 200 km depth under the Sichuan basin and Ordos and Alashan blocks, whereas low-velocity (low-V) anomalies are imaged in the upper mantle under the Kunlun-Qilian and Qinling fold zones, Songpan-Ganzi, Qiangtang, Lhasa, and Chuan-Dian diamond blocks, suggesting that eastward moving low-V materials are obstructed by the Sichuan basin, Ordos and Alashan blocks, and they could be extruded through the Qinling fold zone and Chuan-Dian block to eastern China. In the mantle transition zone (MTZ), broad high-V anomalies are visible, and they are connected upward with the Wadati-Benioff seismic zone beneath Burma arc. Some large earthquakes occurred on the boundaries of high- and low-V anomalies in the upper mantle, suggesting that the crustal large earthquakes of the region are related to the upper-mantle structure. These results could be helpful to better understand the rock stress around the source areas.
The Tibetan plateau, being called the third pole of the world, is characterized by a highly deformed zone with a very broad area, due to the Indo-Eurasian collision since 50–60 Ma (e.g., Tapponnier et al., 1981; England & Houseman, 1986; Yin & Harrison, 2000). The present study region, the eastern Tibetan plateau, contains the stable Sichuan and Qaidam basins and Ordos and Alashan blocks as well as several large active faults, such as the Longmenshan fault zone and the Red-River fault (Figure 1). Along these fault zones, many moderate-to-strong earthquakes have occurred so far, the recent ones are the 12 May 2008 Wenchuan (Ms 8.0), the 14 April 2010 Yushu (Ms 7.1), the 20 April2013 Lushan (Ms 7.0), the 22 July 2013 Minxian-Zhangxian (Ms 6.6), and the 3 August 2014 Ludian (Ms 6.5) earthquakes. Although the Ludian earthquake is of a moderate size, it is the largest and damaged one in Yunnan since 2000, which killed over 589 people and caused at least 2,400 injuries by 6 August 2014 (www.cea.gov.cn). Although in recent years many researchers have conducted several seismic tomographic models, it is still unclear if these strong earthquakes are related to the upper mantle structure.