鄂尔多斯地块北部及邻区Pn波速度结构与各向异性

Pn-wave velocity structure and anisotropy beneath northern Ordos block and its adjacent areas

利用鄂尔多斯地块北部及其邻区2008-2018年期间固定台网的地震波形记录,手动拾取出高质量的Pn波到时资料,反演获得了研究区上地幔顶部的Pn波速度结构及各向异性。结果表明:鄂尔多斯地块北部及其邻区上地幔顶部的Pn波速度存在明显的横向不均匀性,与区域地质构造和地震活动相关;研究区内平均Pn波速为8.18 km/s,鄂尔多斯地块内部表现出大范围的高速异常,阿拉善地块的高速异常体中存在低速异常现象,河套断陷带、阴山-燕山造山带、银川-吉兰泰断陷带和海原-六盘山弧形断裂带区域均表现为显著的低速异常,河套断陷带下方存在向鄂尔多斯地块内部延伸的明显低速异常条带,大同火山群下方存在强低速异常;多数历史强震均发生在低速异常区或高低速异常过渡带上;鄂尔多斯地块内部Pn波各向异性快波方向西部为近NE-SW向,而东部为近NW-SE向,河套断陷带和鄂尔多斯地块西缘、青藏高原东北缘与阿拉善地块的交界带以及阴山-燕山造山带的各向异性快波方向总体均呈现为NW-SE向,而阴山-燕山造山带东部则呈NE-SW向。

This paper picked up the high-quality Pn-wave arrival times from the waveform data recorded at the digital seismic networks in the northern Ordos block and its adjacent areas from 2008 to 2018,and obtained Pn velocity structure and anisotropy structure in the uppermost mantle of the region by performing tomographic inversion. The results show that there exist obvious lateral heterogeneities in the study area. Such lateral heterogeneities not only exist inside the block but also on its margins,which shows a close correlation to the regional tectonics and earthquakes. The average Pn velocity in the region can amount up to 8.18 km/s. The Ordos block shows a broad high velocity anomaly,whereas the Alxa block is imaged as a mixture of both high and low velocity anomalies. Obvious low velocity anomalies are observed beneath the Hetao graben,Yinshan-Yanshan orogenic belt,Yinchuan-Jilantai fault zone,and HaiyuanLiupanshan arc-shaped fault zone. Beneath the Hetao graben the low velocity anomaly extends southward to the Ordos block. A strong low velocity anomaly exists beneath the Datong volcano. Most strong historical earthquakes in the study region occurred in the low velocity anomalous areas or in the high-to-low velocity anomalous transition zones. The fast direction of Pn anisotropy shows nearly a NE-SW direction in the western Ordos block and a NW-SE direction in its eastern parts. The fast wave of anisotropy is in NW-SE in the Hetao graben,junction zone of the Alxa block and the western margin of the Ordos block and the northeastern margin of the Tibetan block,and it is mainly in NW-SE in the Yinshan-Yanshan orogenic belt, except for a NE-SW direction in the eastern Yinshan-Yanshan orogenic belt.