鄂尔多斯块体及其东南缘剪切波速度结构与波速比研究

Study on Shear Wave Velocity Structure and Velocity Ratio Under the Ordos Plate and Its Southeastern Marginal Areas

利用深源远震的纯S波波形拟合方法研究了鄂尔多斯块体及其东、南缘的剪切波速度结构。结果显示 ,鄂尔多斯块体内部榆林台站下方速度结构相当稳定 ,没有很明显的高低速层变化 ,随着深度的增加 ,剪切波速度稳步升高 ;在该台下方 2 5km深处有一个厚度达 12km的层位 (VS=3 90km s) ,该层的波速度相对其东部同等深度处的波速度偏低 (VS ≥ 4 0 0km s)。鄂尔多斯地块东部边缘的地壳厚度都比榆林厚 ,而且呈现高、低速层相间结构 ,低速层明显增多 ,与块体内部比较一致的特征为下地壳是一个厚达 10km的速度较低的下地壳 (Vs=3 8km s) ,但埋深在 35km左右。另外反演得到了各个台站的P波速度结构 ,并对相应的波速比 (VP VS)进行了研究 ,发现位于鄂尔多斯块体内部的榆林台波速比平均值仅为 1 6 8,上地壳波速比处在非常低的状态 (1 6 0左右 ) ,中下地壳一直到上地幔的波速比稳定在 1 73左右 ,反映介质均匀稳定 ,处于一种刚性状态 ;而鄂尔多斯块体东、南边缘的台站 ,都有几个波速比较高 (1 80左右 )的层位 ,其中东部边缘苛岚和离石地区的波速比平均值系统超过一般的弹性体波速比平均值 1 732 ,反映了边缘地区地壳活动相对比较活跃 ,介质层中可能有其他物质的影响。

Using pure S wave fitting method, the authors studied shear wave velocity structure under the Ordos plate and its southeastern marginal aeras. The results show that the velocity structure in inner the Ordos plate is relatively stable, where it hasn't apparent change between high and low velocity layers and the shear wave velocity steadily increase with the depth. There is a 12km thick layer at a depth of 25km beneath the Yulin station, with a lower velocity (V S=3.90km/s) than that at the same depth under its southeastern areas(V S≥4.00km/s). The crust under eastern area of the Ordos plate is thicker than that under the Yulin station, and the structure is alternated by high and low velocity layers, more low velocity layers occurring, but with the same characteristics that is the 10km thick lower crust(V S=3.8km/s). Moreover, they studied the velocity ratio(V P/V S) beneath each station based on the result of P velocity inversion. The results show the average velocity ratio of the Yulin station at the inner Ordos plate is only 1.68, with its upper crust being very low(about 1.60), and mid and lower crust being steady (about 1.73), which indicates the media under this station is even and stable, in a state of rigidity. But under southeastern part of the Ordos plate, the stations all have some high velocity ratio layers(about 1.80), in which under the Kelan and Lishi station at eastern margin, their velocity ratios are systemically higher than average velocity ratio of elastic body(1.732), which reflects that the crust under marginal areas is relatively active, and maybe there is other material existing in these layers. Finally, the relationship among earthquakes, velocity structure of stations and faults have been discussed.