青藏高原东北缘地壳上地幔速度结构的地震层析成像研究

Seismic Tomography of the Velocity Structure of the Crust and Upper Mantle in Northeastern Margin of the Qinghai-Tibet Plateau

本文收集了甘肃、青海、宁夏、陕西和四川台网的130个台站1980-2002年间记录的3 229个区域地震(MS≥1.5)和168个远震资料,从55 024个区域地震震相中挑选出了51 210个最大走时残差为3.0s的震相,选取了2 651个远震震相。层析成像结果显示:(1)青藏高原东北缘地区下地壳存在大范围的P波速度低速异常,上地幔顶部多数地区平均P波速度为8.05km/s左右,接近于大陆下方全球的Pn波平均速度8.1km/s,使得莫霍间断面比较清晰,莫霍面反射波能量较强;(2)研究区内发生大震的震中大多位于深度图中10km的低速区、30km的高速区附近和50km的低速区附近,表明这些区带为孕震区;(3)青藏高原东北缘地区的主要断裂带均是逆冲兼走滑断裂,多数位于速度正负异常的过渡区上,且很可能是超壳断裂;(4)从张掖经海原、平凉再向南拐的弧形地带可能就是青藏高原的边缘地带;且预示着阿拉善地块有地台活化的迹象;(5)从层析成像结果中切出的二维速度扰动剖面与人工地震测深剖面吻合较好。

The seismic data including 3 229 local earthquakes （Ms≥l. 5） and 168 teleseismic events occurred during 1980 to 2002 recorded by 130 seismic stations in the seismic networks of Gansu, Qinghai, Ningxia and Sichuan province are collected, and from 55 024 local phases we selecte 51 210 with the maximum travel time residual of 3.0 s, and 2 651 teleseismic phases. The tomographic inversion results show that ： （ 1 ） Lower P-wave velocity anomaly exists widely in the lower crust beneath the northeastern margin of the Qinghai-Tibet plateau. Average P-wave velocity in most of the top of upper mantle is 8.05 km/s that is close to the global continental average Pn-wave velocity with 8.1 km/s. It indicates the existence of a clear Moho interface and strong reflections from Moho interface. （2） The epicenters of most strong earthquakes occurred in the research region are located in the lower velocity areas in the 10 km depth range, around thehigher velocity areas in the 30 km depth range and lower velocity areas in the 50 km depth range, which suggests that these areas could be the possible earthquake pregnancy regions. （3） Major fault belts in the region are reverse thrust with strike slip, locate on the transition zones between lower and higher velocity anomalies, and are possibly ultra-crustal faults. （4） The arc belt from Zhangye to Haiyuan and Pingliang and turns to south is possibly the marginal belt of Qinghai-Tibetan plateau, and indicates the platform activation in Alashan platform. （5） The 2D velocity perturbation profile taken from the seismic tomographic results shows fairly agreement with deep seismic sounding profile.