青藏高原中东部地壳和上地幔顶部P波层析成像

为获取青藏高原中东部地壳和上地幔顶部的精细结构，本文基于1万4 484条天然地震的P波（Pg和Pn）到时数据，对青藏高原中东部地壳和上地幔顶部进行P波三维速度结构层析成像，获取了该区域内地壳P波、上地幔顶部Pn波的速度结构和地壳厚度信息。层析成像结果显示，青藏高原中东部地壳P波速度范围为5.2—7.2 km/s，上地幔顶部Pn波速度范围为7.7—8.4 km/s，地壳厚度范围为48.0—68.6 km，地壳和上地幔顶部存在强烈的横向不均匀性，与地质块体分布有较好的对应关系。地壳P波速度结构显示，研究区中、下地壳分布有较大范围的低速区，上地壳与中下地壳P波分布存在明显的差异:羌塘地块和巴颜喀拉地块在上地壳主要表现为高速异常，随着深度增加逐渐表现为低速异常；而柴达木地块在上地壳主要表现为低速异常，下地壳则表现为高速异常；柴达木地块和拉萨地块在上地幔顶部表现为较高的Pn波速度，最高约为8.4 km/s，而在巴颜喀拉地块和羌塘地块东部，Pn波总体上表现为低速，最低约为7.7 km/s。研究区内地壳厚度的总体特征表现为南厚北薄，其中羌塘地块东部和拉萨地块的地壳较厚，而柴达木地块和巴颜喀拉地块东部的地壳相对较薄，羌塘地块西部存在局部的地壳变薄现象，反映了印度板块对欧亚板块北向俯冲作用下的岩石圈变形特征。 

Crustal and uppermost mantle velocity structure beneath the central-eastern Tibetan Plateau from P-wave tomography

Based on joint inversion with 14 484 P-wave （Pg and Pn） first arrival times, we obtained regional 3-D P velocity structure beneath the central-eastern Tibetan Plateau, including crustal P velocity, uppermost mantle P （Pn） velocity, and Moho depth. The results show that the crustal P velocity ranges from 5.2 to 7.2 km/s, the Pn velocity ranges from 7.7 to 8.4 km/s, and the crustal thickness ranges from 48.0 to 68.6 km. The crustal and uppermost mantle structure is strongly heterogeneous, which generally correlates with geologic blocks. The crustal P velocity structure shows that prominent low velocity zones exist in the middle and lower crust. There are great difference in the distribution of high and low velocity anomalies between upper crust and mid-lower crust. High velocity anomalies primarily exist in the upper crust of the Qiangtang block and the Baryan Har block, while become lower as depth increases. The Qaidam basin shows low velocity anomaly in the upper crust while high in the lower crust. High Pn velocity exists in the Qaidam basin and the Lhasa block with a maximum velocity of about 8.4 km/s, while the Baryan Har block and the Qiangtang block mainly show low Pn velocity anomalies, which reach as low as 7.7 km/s. Crustal thickness is mainly characterized by thick in the south and thin in the north. The crust is thicker in the eastern Qiangtang block and the Lhasa block, while thinner in the Qaidam basin, the eastern Baryan Har block and the western part of the Qiangtang block, which shows the characteristics of lithospheric deformation due to the northward subduction from the Indian Plate to the Eurasian Plate.