青藏高原东北缘—鄂尔多斯地块地壳上地幔S波速度结构

Crust and upper mantle S-wave velocity structure across Northeastern Tibetan Plateau and Ordos block

1999～2 0 0 0年从青海玛沁到陕西榆林,横跨青藏高原东北缘和鄂尔多斯布设了一条由4 7台宽频带数字地震仪组成的长约10 0 0km的流动地震台阵观测剖面.利用记录到的远震体波波形资料和接收函数方法获得了剖面下0～10 0km深度的地壳和上地幔S波速度结构.结果表明,沿观测剖面地壳结构显示了明显的分块特征;地壳厚度自东向西由4 0km增加到6 4km左右;在海原地震带下方和西秦岭断裂以西到日月山断裂之间的区域Moho间断面结构复杂;在192 0年海原震区及其西侧,上地壳存在明显的低速层,在该地区的绝大部分地震分布在该低速层东边界偏向高速区一侧;祁连山东缘Moho面有约4km的深度间断,壳内向西逐渐减薄的低速层内有大量微震发生,沿祁连山的逆冲加走滑的构造运动在深度上已经穿透了Moho面;在玛沁断裂和日月山断裂之间,上地壳存在厚度很大的低速层,同时该区域下地壳也明显加厚.研究结果表明,青藏高原东北缘与鄂尔多斯地块之间的过渡带地壳变形强烈,地壳结构较为破碎,这与该地区地震频发相一致.

A 1000km long passive seismic observation profile consisting of 47 broadband portable seismometers was deployed from Maqen in northeastern Tibetan Plateau to Yulin in central Ordos. Using the teleseismic waveforms recorded by the passive seismic experiment, receiver function method was used to investigate the S wave velocity structure. The results showed that the crustal structure is clearly divided into blocks along the profile. From central Ordos to northeastern Tibetan Plateau, the crustal thickness varies from 40km to 64km. Under the Haiyuan earthquake region and west of Northern Qinling Fault, the Moho discontinuity seems quite complicated. In the Haiyuan earthquake region, recent earthquakes tend to occur around the upper crust low velocity zone. Moho under the east edge of the Qilian mountains has a depth jump and seismicity is closely related with the Qilian fault, indicating that the fault has reached under the Moho. Crust thickening is obvious in the region from Maqen to the Riyueshan fault. Our results show that the crust structure of the transition zone between Tibetan Plateau and Ordos is strongly deformed and fragmented, which is consistent with the occurrence of earthquakes in this region.