Seismological Evidences of the Crustal Structures and Tectonics in the Yinchuan Downfaulted Basin

Abstract

A high resolution deep seismic reflection profile of 68.9 km long across the Yinchuan faulted basin has been accomplished which for the first time yields the fine crustal structures, characteristics of deep fault system (Yellow River fault, Yinchuan fault and Eastern piedmont fault of Helanshan) of faulted basin in graben style, and the relationship between shallow and deep structures in Yinchuan basin. The results show that the upper crust is the region above a reflector with 8 s two‐way traveling time (about 20 km deep), there are many strata in the upper part of the upper crust where the continuity of different segment of stratum is good, and there is no obvious layered feature in the lower part of upper crust, where the geological structure is simple. The reflection energy is weak in the lower crust (8~13 s), where reflection events are not obvious. The crust‐mantle transitional zone (around 13 s) below the lower crust consists of a group of reflection sequences that have stronger energy and longer duration (1.5 s), and the thickness is about 4.5 km. Luhuatai fault and Yinchuan fault merge into the Eastern piedmont fault of Helanshan in the depth of 12~12.5 km and 18~19 km, respectively, the Eastern piedmont fault of Helanshan merges into Yellow River fault in the depth of 28~29 km, and Yellow River fault is a deep fault cutting the Moho. Yinchuan graben is a negative flower structure that is assembled mainly by Yellow River fault and secondarily by other faults. Based on the relationship between the Eastern piedmont fault of Helanshan and Yinchuan fault, it is thought that the Eastern piedmont fault of Helanshan played a controlling role in 1739 M=8 Pingluo‐Yinchuan earthquake.

Active normal faulting and the seismogenic fault of the 1739 M ∼8.0 Pingluo earthquake in the intracontinental Yinchuan Graben, China

“…Previous studies have identified two other inferred faults in the graben: the Luhuatai and Yinchuan-Pingluo faults, identified using seismic reflection data (Fig. 1b and c;Cai et al, 2006;Wang et al, 2007;Fang et al, 2009;Feng et al, 2011;Yang et al, 2009). However, the topographical analysis and field investigations of the present study show no conspicuous topographical or geologic evidence for tectonic deformation features on the alluvial surfaces, including the terrace risers and alluvial fans, which formed during the Holocene along the Luhuatai and Yinchuan-Pingluo faults (Fig.…”

“…Seismic reflection data show that the HHFZ, which runs along the eastern margin of the Helan Mountains, is the main fault forming the western boundary of the graben (Fig. 1b and c;Fang et al, 2009;Feng et al, 2011). The Huanghe Fault (HHF), along which the Yellow River flows to the NNE, is inferred to be a blind fault and its recent activity is unclear (Deng and Liao, 1996).…”

“…Therefore, investigating the activity and kinematic characteristics of active normal faults in the Yinchuan Graben should provide a better understanding of the seismotectonic features and tectonic evolution of active extensional grabens around the Ordos Block. The Yinchuan Graben has long attracted research attention on account of its high levels of historical paleoseismicity and its normal faulting regime sustained since the early Eocene Li and Wan, 1984;Zhang et al, 1986;Deng and Liao, 1996;Zhao et al, 2007;Fang et al, 2009;Feng et al, 2011).…”

Gravity anomaly and crustal density structure in Jilantai rift zone and its adjacent region

“…Earthquake activity is strong in this region; the Pingluo M 8 earthquake occurred in 1739, but its seismogenic structure has been controversial. Mao and Yao (1982), Li and Wan (1984) and Wu et al (2015), and many other scholars think that the earthquake seismogenic structure is the Yinchuan blind fault, but others believe the active Helandonglu fault is the seismogenic structure (Fang et al 2009;Lei et al 2015;Wang et al 2015). The current understanding of the blocks is obscure or ambiguous, which makes it difficult to analyze the mutual block movement and deformation in-depth.…”

“…Chai et al (2006) gave the blind fault activity characteristics in Yinchuan and the surrounding area comprehensively. Fang et al (2009) studied the relationship between the Huanghe, Yinchuan, and Helanshandonglu faults and their shallow and deep structure using a deep seismic reflection method. Other researchers (Zheng 2011;Yin et al 2013) then obtained the electrical structure of the Ordos block and JiangyuanNingxia region.…”