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

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 Haiyuan-Liupanshan 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.