In order to determine the specific location, geometric structure, tendency, width and burial depth of the QF, we acquired an approximately N-S shallow seismic line ∼6 km east of Sanyuan County (Figure 1D). The details of the shallow seismic survey are listed in Table 1.

The shallow seismic exploration data were processed using GeoEast software. The main processing module flow of the reflection data included: (1) elevation static correction; (2) amplitude compensation; (3) deconvolution; (4) velocity analysis and residual static corrections; (5) post-stack noise reduction and an offset test.

The survey line direction is nearly perpendicular to the QF. The survey line was laid along a road, where surface stimulation and geophone receiving conditions are good. The seismic detection zone is flat and located in an alluvial plain. Good Holocene strata are exposed on both banks of the Qinghe river. As shown in Figure 2A, the horizontal bedding has typical alluvial-colluvial characteristics, providing a good marker strata (Figure 2B). The sediments in the seismic detection zone are relatively thick, with well-developed sedimentary facies, which correspond to abundant acoustic impedance contrasts.

Figure 3 illustrates our seismic profile after processing. The time profile shows that the signal-to-noise ratio in the profile is high, and reflections are well developed, which is helpful for tracking the stratigraphic horizons. A total of five reflections (T1, T2, T3, T4, T5 in Figure 3) are examined, all of which are within the Quaternary sediments (Figure 3). From the profile, the strata as a whole are slightly inclined southward. The closer to the fault, the dip angle gradually increases, accompanied by drags and folds (Figure 3).

The exploration results show (Figure 3) that a large-scale main fault dipping to the south has developed along the Qinghe river. On the south side of the main fault, five secondary faults dip to the north (f1, f2, f3, f4, f5 in Figure 3), together constituting a small composite graben, with a width of the fracture zone in the plane of the section of about 4 km. The dip of the faults is about 70°–80° in the upper section of the profile, and gradually decreases.

In order to verify the geophysical interpretations, and document activity on the QF, five boreholes were drilled on the fault zone, roughly perpendicular to the fault and parallel to the seismic profile. The location of the boreholes is shown in Figure 1D. The details of the boreholes are found in Table 2.

The first paleosol layer (S₁) in the boreholes is well-developed, continuous horizontally, and has characteristics different from the upper and lower strata. In addition to the bright reddish-brown color, the S₁ has a well-developed soil architecture (Figure 4), which also provides a nice seismic marker.

Extensive research has been done on the ages of the regional S₁ paleosol layers (Table 3; Sun and Xu, 2007; Lei and Yue, 1997; Lei et al., 1992; Chen et al., 1999; Ding and Liu, 1989; Liu and Ding, 1990; Ma et al., 2019; Yang et al., 2021). Five levels of fluvial terraces are developed in the Weihe graben, and these terraces are composed of the riverbed facies at the bottom and the Quaternary aeolian sediments above. The loess layers at the bottom of the five terraces are L₃₃, L₁₅, L₉, L₆ and L₂. On the basis of magnetostratigraphy and climatic stratigraphy, the above-mentioned loess layers correspond to the deep-sea oxygen isotope stages 104, 36, 22, 16 and 6, respectively; the ages of the terraces are 2.60 Ma, 1.20 Ma, 0.90 Ma, 0.65 Ma, and 0.15 Ma, respectively (Sun and Xu, 2007). The geological environment of the study area belongs to the alluvial plain of the Weihe river, and the landform belongs to the T1 terrace. Thus, the S₁ paleosol of the T1 terrace, located in the upper part of the L₂ loess, close to ∼0.15 Ma. Lei and Yue. (1997) concluded that the S₁ palaeosol formed in the late Pleistocene of the Guanzhong Basin in Shaanxi during the period of about 130–75 ka B.P. (the last interglacial period) was composed of four layers of paleosol and three layers of loess. The L₁ loess accumulated during the 75–10 ka B.P. period (last glacial period) can be divided into three layers of loess and two layers of weakly developed soil.

In order to visualize the location and activity of the fault in the study area, a profile of the borehole descriptions was made (Figure 5). As shown in Figure 5, the boreholes ZK4 and ZK5 in the southern area are 800 m apart, and the bottom elevations of the S₁ paleosol layers are 363.91 m and 362.67 m, respectively, leaving a difference of 1.3 m. While the ZK3 and ZK4 are only 300 m apart, the bottom elevations of the S₁ paleosol layers are 366.50 m and 363.91 m, respectively. An offset of 2.6 m thus exists between ZK3 and ZK4, consistent with a late Pleistocene fault displacing the S₁ (Figure 5).

Similarly, ZK2 and ZK3 in the middle part of the study area are 1,600 m apart, and the bottom elevation of the S₁ is 362.93 m and 366.50 m, respectively. The bottom elevation of the S₁ paleosol layer of the ZK2 on the northern side is 3.5 m lower. This implies a fault between ZK2 and ZK3, active since the late Pleistocene (Figure 5).

A geological field investigation was carried out along the main fault and secondary faults interpreted from the shallow seismic profile. A large number of structural tensional fissures were found developed in Holocene strata (Figure 6), striking east-west. Most of these fissures are generally perpendicular to the ground surface or strata, and are filled with dark brown consolidated veins (Figures 6, 7), causing sharp contrast with the strata on both sides. The veins in the tectonic fissures can often be the silty and fine sand produced by the sand liquefaction accompanying the earthquake process. The scale of these fissures ranges from a few centimeters up to 10 cm to tens of centimeters. Where the vertical exposure is better, they can be seen to cut through Holocene strata (Figure 6A). The fissures formed by tectonic stress are called tectonic fissures. Since tectonic stress has a certain direction in a region, the distribution of various tectonic fissures formed by tectonic stress in nature is regular, and the arrangement direction is certain. In addition, tectonic fissures have good extension in both horizontal and vertical directions, mainly concentrated in and near the fault zone, and generally parallel to the fault strike. The direction of cracks generated by non-tectonic stress (oil processes/internal deformation of unconsolidated materials) can be random or in multiple groups, and the distribution is relatively local. The tectonic fissures in the field are mainly located inside the Qinghe fault zone, outside the fault zone, these tectonic fissures are not observed. Therefore, we can be sure that these must be due to tectonic faulting.

In addition, we also observed signs of two phases of activity (Figure 7), indicating that the fault is quite active. The fissures formed by an early earthquake event (T1) were filled with veins, and the veins were subsequently consolidated; the fissures formed by a later earthquake event (T2) cut through and staggered the veins formed earlier (Figure 7). In some locations, the Holocene gray-black soil layer is vertically offset (Figure 8), demonstrating Holocene activity on the fault.

The Quaternary stratigraphy of this area was established by the early standard borehole detection data. The results show that the bottom of the Holocene is roughly 6–8 m, that of the late Pleistocene is roughly 78–104 m, and that of the middle Pleistocene is roughly 470–620 m (Table 4; Figure 9; The third census exploration team of the State General Administration of Geology, 1970s). In addition, according to the 1:200,000 geological map of this area, the thickness of the alluvial deposits in the lower and upper Holocene are 1–5 m and 5–29 m respectively. The lower Holocene constitutes low terrace sediments, mainly composed of loess-like sandy clay, loam, and sand-gravel layer. The upper Holocene constitutes a modern riverbed, which is mainly composed of alluvial sand, gravel and a small amount of loam. The structural fissures and outcrops of displaced strata we observed in the field are all located on the surface, it is thus certain that the latest activity of the QF is the Holocene.

Our seismic line is consistent with previous petroleum geophysical data (Figure 10; Xijie Feng, personal communication), with respect to the largest main fault situated in the Qinghe river area, parallel to the Qinghe river, which strikes east-west. The difference between the two results is that the multiple secondary faults discovered this time on the south side of the main fault were not identified because the oil exploration target layer was beyond this group of secondary faults.

The rupture characteristics of the Qinghe fault could be manifested in two ways, and its geometry structure is instructive for study and comparison to the internal faults in other basins (Zhang, 1991; Shi et al., 2007; Feng et al., 2008; Lei et al., 2008; Li et al., 2008; Shi et al., 2009; Rao et al., 2014; Rao et al., 2015; Cheng et al., 2018; Ma et al., 2019; He et al., 2020; Yang et al., 2021). One is that the Qinghe fault zone is larger in scale. Many secondary reverse faults are the same size as the main fault, that together with the main fault, forms a “Y”-shaped composite graben (Figure 3). On the other hand, the width of the fault zone of the Qinghe fault reaches about 4 km in the plane of the section (Figure 10), which is rare on other faults in the basin. However, the deformation on both sides of the Qinghe fault is most controlled by the main fault.

Based on the above characteristics, we interpret the deformation in the area has concentrated on the main fault of the Qinghe fault zone, and the strain release is mainly borne by the main fault. The secondary faults absorb and accommodate the release of stress and strain in the interior of the fracture zone, dominated by post-seismic and inter-seismic deformation. The scale of the secondary faults limits the capacity to generate a large earthquake alone compared with the main fault. Such a wide fracture zone and complex structure might be attributed to the repeated dislocations caused by multiple earthquakes.

Although the Qinghe fault was discovered as early as the 1960s, its detailed shallow structure has once been ascertained in recent years. Due to the limited work on the activity of the fault, its seismic risk has not received much attention. Our detailed field surveys show that the Qinghe fault developed many structural fissures on the surface that cut Holocene strata (Figures 6, 8). These observations suggest the Qinghe fault is an active Holocene fault. Combined with the multi-stage fissures we have observed (Figure 7), the Qinghe fault appears to have been active since the late Pleistocene to Holocene and may have the potential to generate moderate to strong earthquakes.

As earthquake magnitude is well-known to be positively correlated with fault rupture features (Wells and Coppersmith, 1994), the evaluation of rupture characteristics of potentially seismogenic faults is important for seismic risk assessment. Some recent research shows that normal fault earthquakes have smaller upper limits to their magnitudes than thrusts, because their relatively steep dips produce intersections at the base of the seismogenic layer at smaller downdip widths than gently dipping thrust faults: for a given seismogenic thickness, a normal fault dip-slip rupture reaches the full limit of the seismogenic layer at a smaller distance than a thrust, thereby limiting the maximum size of the earthquake (Middleton et al., 2016; Xu et al., 2018). However, in order to limit the upper bound of the highest earthquake magnitude in the study area, we adopt a maximum deformation parameter as a criterion for the seismic fortification of engineering buildings. The width of the Qinghe fault zone is about 4 km, so we refer to this value as the subsurface rupture width. Utilizing the empirical relationship between the magnitude and subsurface rupture width of Wells and Coppersmith (1994), an earthquake magnitude of 5.2–5.5 could be a conservative estimate.

In addition to the strong ground motion caused by the earthquake, the liquefaction of sand is one of the common secondary disasters of large earthquakes, especially the plains or basins distributed in the Quaternary and close to the earthquake area, and these areas are often densely populated and built. Therefore, an in-depth understanding of the earthquake sand liquefaction phenomenon is an important part of disaster reduction and prevention (Liu-Zeng et al., 2017). Liquefaction of sand refers to the phenomenon in which water-saturated loose silty fine sand are suddenly destroyed under the vibration and become liquid. Due to the increase of pore water pressure and the decrease of effective stress, the sand changes from solid to liquid state. Sand liquefaction can occur on a large scale during earthquakes and cause serious damage. In China’s 1966 Xingtai earthquake, 1975 Haicheng earthquake and 1976 Tangshan earthquake and other major earthquakes, the damage to some buildings was caused by the liquefaction of sand. The liquefaction of sand is also common in the active fault zone of the Weihe basin (Wang et al., 2018; Ma et al., 2019; Yang et al., 2021). Considering the developed loose late Quaternary sediments in the study area and a large number of fissures found in the field, a detailed investigation and study of the Qinghe fault and its fissures are important for preventing earthquake’s secondary disasters. In addition, seismic-induced sand liquefaction is also an important means to study historical earthquakes and paleo-earthquakes, and the results are of great significance for evaluating the earthquake hazards in the region.

Next, we briefly discuss the tectonic significance of the Qinghe fault from the perspective of continental rifts. The Weihe rift and other rifts (Shanxi rift system, Yinchuan rift, Jilantai rift, Hetao rift) around the Ordos block differ significantly in tectonic activity (Deng et al., 1984; Deng and Liao, 1996; Lei et al., 2008; Middleton et al., 2017; Liang et al., 2021; Su et al., 2021). The main differences can be summarized as follows. First, the active faults inside the Weihe rift are well developed, and most of them have experienced historical earthquakes. They are mostly active since the late Pleistocene with intense tectonic activity, while few active faults developed in other rifts’ interior. Second, the Weihe rift developed multiple groups of faults with different tectonic trends, and moderate to strong earthquakes were not only distributed on boundary faults but also on those located inside the rift. The tectonic strikes of other rifts are simple and unitary, and large earthquakes only occur on basin-controlling boundary faults. Third, the topography and geomorphology of the Weihe rift is controlled by its internal active structures and can be divided into multiple faulted-blocks. The topographic differences of other rifts are mainly located at the border between the basin and mountain, controlled by the frontal main fault.

The above evidence shows that the Weihe rift has a more complex structure, and that its tectonic evolution is affected by multiple tectonic events in time and by different tectonic stress fields in space. The discovery of the Qinghe fault not only confirms the tectonic activity inside the Weihe rift, but also reveals an active intracontinental rift. Active faults in the Weihe rift play an important role in the process of absorbing and accommodating regional tectonic deformation, which is a major factor that cannot be ignored. This is of great significance for re-determining the regional seismic hazard and understanding the evolution model of continental rifts.