Imaging the crustal structure beneath the Yinchuan Basin in the western north China Craton using normalized full gradient and 3D gravity inversion

Abstract

• The crustal structure beneath the Yinchuan Basin is apparently heterogeneous. • The Yellow River fault is divided into two segments, with the boundary at Yongning. • The primary causative structure of the 1739 Pingluo earthquake is the Yinchuan fault. The Yinchuan Basin is located along the transition belt between the Alxa Massif and Ordos Basin, and is characterised by a complex geological structure. However, several important scientific problems remain unresolved, such as, the causative fault of the 1739 Pingluo Ms 8.0 earthquake. In order to provide new insight into the geodynamic processes, the normalized full gradient (NFG) method and 3-D gravity inversion are utilised to image the crustal structure beneath the Yinchuan Basin in the study. The NFG results indicate that the upper crustal structure is complex, particularly at depths shallower than 20 km. The inversion results infer that the locations of large faults are consistent with the high–low gradient zones in horizontal direction. The integrating NFG and density contrasts imaging results reveal that the south-western boundary of the Ordos Basin is the Yellow River Fault which is divided into two segments with the boundary at Yongning; the East Helanshan and Qingtongxia–Guyuan faults are the south-eastern boundary of the Alxa Massif; the Yinchuan fault is the causative fault of the 1739 event and the focal depth of the event is ∼15 km. These findings provide a scientific basis for studying the crustal dynamics and seismogenic mechanisms of strong earthquakes.