Ambient noise tomography in the Ordos block and its surrounding areas

Based on the 2-year seismic observation data of 461 seismic stations deployed in the Ordos and surrounding areas by the ChinArray, we use the ambient noise tomography method to obtain the Rayleigh surface wave phase velocity （5−46 s） images whose resolution is up to 0.3×0.3 degree. Compared with the seismic surface wave imaging results based on the Eikonal equation, we find that ambient noise tomography has a clear advantage in shorter periods （5−16 s） and higher resolution imaging. The short-period （5−10 s） phase velocity distribution reveals that the Hetao basin, Taiyuan basin and Yuncheng basin all exhibit significant low-velocity anomalies, indicating that these basins have thicker Cenozoic deposits. And in the Linfen basin, there is a weak low-velocity anomaly and a relatively thin sediment layer. Also, the Ordos block has a low-velocity anomaly, consistent with the Mesozoic thick deposit in this area; the Datong volcanic area shows a high-velocity anomaly, which may be caused by the distribution of Cenozoic basalt in this area. The mid-long period （20−44 s） phase velocity imaging results show that the Ordos block exhibits an obvious high-velocity anomaly. The anomaly can extend eastward to the Taihang mountain through the Lingshi uplift and Linfen basin. We suspect that the area is with less damage in the evolution process of the North China Craton and retains part high-velocity lithosphere roots. The Datong volcanic area and Hetao basin area have low-velocity anomalies. As the period increases, the low-velocity anomalies are gradually concentrated near the Datong volcanic area. We speculate that the deep thermal effect in the Hetao basin may originate from the deep mantle near the Datong volcanic area in the east.