The Karakoram Fault is located in the west of the Qinghai-Tibet Plateau and crosses Kashmir, Xinjiang and Tibet in China. It is a large normal dextral strike-slip fault in the middle of the Asian continent. As a boundary fault dividing the Qinghai-Tibet Plateau and the Pamir Plateau-Karakoram Mountains, the Karakoram Fault plays a role in accommodating the collision deformation between the Indian plate and the Eurasian plate and in the tectonic evolution of the western Qinghai-Tibet Plateau. The fault trace in Ngari area is clear and the faulted landforms are obvious, which show strong activity characteristics in late Quaternary. As a large active fault, only one earthquake of magnitude 7 has been recorded on the Karakoram Fault since the recorded history, namely, the Tashkurgan earthquake of 1895 at its north end. There are no records of strong earthquakes of magnitude≥7 along the rest of the fault, and no paleo-seismic research has been carried out. Ages of recent strong earthquake activity and earthquake recurrence intervals are not clear, which greatly limit the accuracy of seismic risk assessment. In this study, we investigated the fault geometry and faulted landforms in Ngari area, collected OSL samples of the faulted landforms and sag ponds in Zhaxigang, Menshi and Baga towns and preliminarily discussed the ages of recent strong earthquake activity.

Study shows that the fault can be divided into three sections by Zhaxigang town and Suoduo village, and the structure and properties of each section are significantly different. In west Zhaxigang town section, the fault is dominated by dextral strike-slip with certain vertical movement, it is almost straight on the surface, with river terraces, alluvial-proluvial fans and water system faulted ranging from tens to hundreds of meters. In Zhaxigang town to Suoduo village section, the normal faulting is remarkable, the main fault constitutes the boundary fault between Ayilari Mountain and Gar Basin; fault facets and fault scarps are common along the fault line, there are also secondary faults with the same or opposite dip as the main fault developed near the piedmont basin. In east Suoduo village section, the main part of the fault is located at the south foot of Gangdise Mountain, and in addition to the piedmont fault, several approximately parallel faults are also developed on the southern alluvial-proluvial fans and moraine fans which are mainly dextrally faulted with certain vertical component.

According to the analysis of the faulted landforms and dating of the OSL samples collected from the sag ponds and faulted landforms in the west of Zhaxigang town, the east of Menshi town and the east of Baga town, the ages of recent strong earthquake activity on the fault are analyzed as follows. In the west of Zhaxigang town, the age of recent strong earthquake activity of the fault is constrained to be close to 2.34kaBP according to the average OSL dating results of KKF-3 and KKF-4. In the east of Menshi town, the recent earthquake activity age of fault f₂ is 4.67~3.01kaBP, but closer to 3.01kaBP according to the OSL dating results of KKF-11 of the youngest faulted geomorphic surface and average OSL dating results of KKF-6 and KKF-13 collected from sag ponds. In the area near Angwang village, Baga town, it is inferred that the recent strong earthquake activity age of the fault is close to 2.54kaBP according to the OSL dating results of KKF-2 collected from sag pond. If the faults of above three places are active at the same time, the age of recent strong earthquake activity of the fault is close to 2.63kaBP. The Karakorum Fault in Ngari area has obvious segment boundaries, and the activity of each segment and in its internal branch faults is most likely to be independent.

The earthquake recurrence interval on the fault is estimated to be 2.8ka according to the slip rate and the amount of displacement. From the above analysis, it can be seen the time since the last strong earthquake activity of Karakorum Fault may have been very close to the interval of earthquake recurrence. If the fault is characterized by a quasi-periodic in-situ recurrence, the energy accumulation in the fault may have reached a very high degree and the risk of recurrence of strong earthquake events of the fault may be very high, so more attention should be paid and more detailed research on the paleo-earthquake events and recurrence intervals should be carried out as quickly as possible.

The Hetao depression zone, located to the north of Ordos block, is a complex depression basin that consists of two sub-uplifts and three sub-depressions. The depression zone is subject to the regional extensional stress field driven by the Indo-Asian continental collision and the westward subduction of the Pacific Plate. The Baotou uplift that separates the Baiyanhua sub-depression and Huhe sub-depression is mainly composed of Archean gneiss and is overlaid by Quaternary sedimentary strata. The two sub-depressions are bordered by the Wula Mountains and Daqing Mountains to the north, respectively. The bedrock exhumed in Wula Mountains and Daqing Mountains consists mostly of Precambrian granitic gneiss, and the piedmont depressions are infilled by thick Cenozoic strata. The Wulashan piedmont fault and Daqingshan piedmont fault extend along the range front of Wula Mountains and Daqing Mountains, respectively. The subsidence is controlled by the two boundary faults. Previous studies have preliminarily documented the characteristics of the northwest boundary fault of Baotou uplift. Combining shallow seismic exploration, active fault mapping, and geological drilling, this paper presents a detailed study on the tectonic characteristics of the Baotou uplift.
The shallow seismic exploration reveals that the Baotou uplift is an asymmetrical wedge with a steep southeast wing and a gentle dipping northwest wing. The Baotou uplift is wider in the northeastern part and narrows down towards the southwest. In seismic profiles, the Baiyanhua sub-depression and the Huhe sub-depression manifest as asymmetric dustpan-like depressions with south-dipping controlling faults. Baotou uplift is bounded by the Xishawan-Xingsheng Fault to the northwest and Daqingshan piedmont fault to the southeast. The two faults exhibit significant difference in many aspects, such as fault geometry, fault displacement, the latest active time, and so on. The southeast boundary fault of Baotou uplift is the Baotou section of the Daqingshan piedmont fault which is a Holocene active fault and the major boundary fault of Huhe sub-depression. East of Wanshuiquan, the fault strikes EW-NEE; west of Wanshuiquan, the strike changes to NW. The Daqingshan piedmont fault appears as a south-dipping listric fault in seismic profiles whose dip decreases with depth and cuts through all the sedimentary strata in Huhe sub-depression; the fault extends along the late Pleistocene lacustrine platform at surface with prominent geomorphological evidences. The Xishawan-Xingsheng Fault is a buried high-angle normal fault that mainly dips to the northwest and strikes NE. The fault strike changes to NNE at the eastern tip. Based on the results of seismic exploration and geological drilling, the Xishawan-Xingsheng buried fault is an early to middle Pleistocene Fault capped by late Pleistocene lacustrine strata. We reckon that the Xishawan-Xingsheng Fault is one of the synthetic faults that dip towards the main boundary fault of Baiyanhua sub-depression.
Similarities in lithology, geometry, and structural characteristics of south boundary faults all indicate that Baotou uplift is the western extension of Daqing Mountains. Multiple factors may contribute to the formation of Baotou uplift, such as tectonic subsidence and the development of large-scale river system and mega-lake. We suggest that the upwelling of asthenosphere may play a primary role in the evolution of Wulanshan piedmont fault and Daqingshan piedmont fault. Separated by the Baotou uplift, the Wulashan piedmont fault and Daqingshan piedmont fault can be regarded as independent seismogenic faults. The Hetao depression zone is featured by complex inner structures, and many scientific issues are subject to further researches. Thus, more attention should be paid to the secondary structures within the depression zone for a better understanding on the formation and evolution of Hetao depression zone.

Based on DEM data and ArcGIS software, we extract the geomorphic parameters of drainage basins and rivers that flow through the Huashan piedmont, which include stream length-gradient index (SL), stream-power incision model normalized channel steepness index (k_sn), hypsometric integral (HI), valley floor width to valley height ratio (Vf)and mountain front sinuosity (Smf). Study shows that all parameter indexes have obviously different distributions roughly bounded by Huaxian and Huayin. In the Huaxian to Huayin section, the stream length-gradient index has extremely high abnormal values near the fault, the values of river mean SL, mean k_sn, HI, Vf and Smf are concentrated in 500~700, 120~140, 0.5~0.6, 0~0.1 and 1.0~1.1, respectively. Between Lantian and Huaxian and between Huayin and Lingbao, the parameter indexes distributional characteristics are largely the same, with the values in 300~500, 100~120, 0.4~0.5, 0.2~0.6 and 1.2~1.5, respectively. Comprehensive analysis suggests that tectonic activity is the primary factor responsible for these differences. We divide each geomorphic parameter into three classes (strong, medium, and low)and calculate the relative active tectonics (Iat)of the Huashan piedmont. The results show that the Iat values in Huaxian to Huayin section are in 1.0~1.5, those at other places are in 1.5~3.0, indicating that the tectonic activity from Huaxian to Huayin is most intense, while that of other places are relatively weak. Field geological investigations show that the Huashan piedmont fault can be divided into Lantian to Huaxian section, Huaxian to Huayin section and Huayin to Lingbao section. In Huaxian to Huayin section the fault has been active several times since Holocene indicative of strongest activity, while in Lantian to Huaxian section and Huayin to Lingbao section the fault was active only in the late Pleistocene and its activity was weaker as a whole. Tectonic activity of the Huashan piedmont derived from river geomorphic parameters is consistent with field geological investigations, indicating that geomorphic parameters of rivers can be used to characterize activity of faults on a regional scale.

Based on the 1︰50000 active fault geological mapping, combining with high-precision remote imaging, field geological investigation and dating technique, the paper investigates the stratum, topography and faulted landforms of the Huashan Piedmont Fault. Research shows that the Huashan Piedmont Fault can be divided into Lantian to Huaxian section (the west section), Huaxian to Huayin section (the middle section) and Huayin to Lingbao section (the east section) according to the respective different fault activity.
The fault in Lantian to Huaxian section is mainly contacted by loess and bedrock. Bedrock fault plane has already become unsmooth and mirror surfaces or striations can not be seen due to the erosion of running water and wind. 10~20m high fault scarps can be seen ahead of mountain in the north section near Mayu gully and Qiaoyu gully, and we can see Malan loess faulted profiles in some gully walls. In this section terraces are mainly composed of T₁ and T₂ which formed in the early stage of Holocene and late Pleistocene respectively. Field investigation shows that T₁ is continuous and T₂ is dislocated across the fault. These indicate that in this section the fault has been active in the late Pleistocene and its activity becomes weaker or no longer active after that.
In the section between Huaxian and Huayin, neotectonics is very obvious, fault triangular facets are clearly visible and fault scarps are in linear distribution. Terrace T₁, T₂ and T₃ develop well on both sides of most gullies. Dating data shows that T₁ forms in 2~3ka BP, T₂ forms in 6~7ka BP, and T₃ forms in 60~70ka BP. All terraces are faulted in this section, combing with average ages and scarp heights of terraces, we calculate the average vertical slip rates during the period of T₃ to T₂, T₂ to T₁ and since the formation of T₁, which are 0.4mm/a, 1.1mm/a and 1.6mm/a, and among them, 1.1mm/a can roughly represent as the average vertical slip rate since the middle stage of Holocene. Fault has been active several times since the late period of late Pleistocene according to fault profiles, in addition, Tanyu west trench also reveals the dislocation of the culture layer of(0.31~0.27)a BP. 1~2m high scarps of floodplains which formed in(400~600)a BP can be seen at Shidiyu gully and Gouyu gully. In contrast with historical earthquake data, we consider that the faulted culture layer exposed by Tanyu west trench and the scarps of floodplains are the remains of Huanxian M_S8½ earthquake.
The fault in Huayin to Lingbao section is also mainly contacted by loess and mountain bedrock. Malan loess faulted profiles can be seen at many river outlets of mountains. Terrace geomorphic feature is similar with that in the west section, T₁ is covered by thin incompact Holocene sand loam, and T₂ is covered by Malan loess. OSL dating shows that T₂ formed in the early to middle stage of late Pleistocene. Field investigation shows that T₁ is continuous and T₂ is dislocated across the fault. These also indicate that in this section fault was active in the late Pleistocene and its activity becomes weaker or no longer active since Holocene.
According to this study combined with former researches, we incline to the view that the seismogenic structure of Huanxian M_S8½ earthquake is the Huashan Piedmont Fault and the Northern Margin Fault of Weinan Loess, as for whether there are other faults or not awaits further study.