基于背景噪声研究青藏高原东缘三维横波速度结构

本文收集了四川、重庆、甘肃、青海、陕西地震台网的105个宽频带地震台站在2017年8月—2018年8月期间记录的连续波形数据,通过对台站记录的长时间背景噪声进行互相关计算,从而提取出瑞利面波经验格林函数,共获得5416条周期在5~40 s内的基阶瑞利面波频散曲线,用于反演青藏高原东缘地壳和上地幔顶部三维横波速度结构.研究结果表明青藏高原东缘地下速度结构呈现非常明显的横向不均匀性,地下浅层速度结构特征与研究区域的主要地质特征和地块单元非常吻合.青藏高原东缘下方的低速体在向东延伸过程中受到四川盆地刚性地壳的阻挡作用,一部分向南进入川滇地块,并在下方30 km处形成厚度约为15 km的大范围低速异常;另一部分向北延伸,并在塔藏断裂下方上涌和堆积,造成局部莫霍面下沉.此外,青藏高原东缘下方的低速体在东北方向跨越东昆仑断裂带进入柴达木地块,并到达西秦岭断裂带附近.其中,巴颜喀拉地块下方30 km处存在厚度约为15 km大范围的低速体,与青藏高原东缘隆起地形和地壳增厚有一定联系.且该低速体在柴达木地块下方变窄,与祁连地块地下30 km处厚度约为10 km的小范围低速体存在一定的连通性.但祁连地块低速体速度值略高于巴颜喀拉地块低速体速度值,这可能意味着祁连地块低速体相当于巴颜喀拉地块低速体的早期发育阶段.

Three-dimensional shear wave velocity structure in the eastern margin of the Tibetan Plateau based on the ambient noise

In this paper, we collected continuous waveform data recorded by 105 broadband seismic stations in Sichuan, Chongqing, Gansu, Qinghai and Shanxi seismic network during August 2017 to August 2018, and extracted empirical Green's functions of Rayleigh surface-wave based on the cross-correlation calculation on the long-time ambient noise data recorded by station pairs to obtain 5416 fundamental-order Rayleigh surface-wave dispersion curves with periods in the range of 5 s to 40 s. The three-dimensional shear wave velocity structure of the crust and uppermost mantle beneath the eastern margin of the Tibetan Plateau was inverted. Our results show that the velocity structure beneath the eastern margin of the Tibetan Plateau exhibits very pronouncedly lateral heterogeneity, and the characteristics of the velocity structure of underground shallow layer fit well with the main geological characteristics and block units in the study area. The low-velocity body beneath the eastern margin of the Tibetan Plateau is blocked by the rigid crust of the Sichuan basin during its eastward extension. Part of the low-velocity body penetrates the Sichuan-Yunnan block southward, forming extensively low-velocity anomalies of about 15 km in thickness at 30 km below, and the other part extends towards the north and then upwells and accumulates beneath the Tazang fault, resulting in the local Moho's subsidence. In addition, the low-velocity body under the eastern margin of the Tibetan Plateau crosses the East Kunlun fault zone into the Qaidam block in the northeast direction, and reaches the vicinity of the West Qinling fault zone. Among them, there is a large-scale low-velocity body of about 15 km in thickness at 30 km beneath the Bayan Har block, which is associated with the uplifted topography and crustal thickening on the eastern margin of the Tibetan Plateau. Furthermore, the low-velocity body narrows below the Qaidam block, and may be connected with a small low-velocity body of about 10 km in thickness at 30 km beneath Qilian block. However, the value of the low-velocity body of the Qilian block is slightly higher than that of the Bayan Har block, which may indicate that the low-velocity body of the Qilian block is in the early developmental stage of the Bayan Har block.