Earthquake Research in China  2020, Vol. 34 Issue (4): 546-559     DOI: 10.19743/j.cnki.0891-4176.202004005
Relationship between the Landslides Triggered by the Tongwei M7 ${}^{1}\!\!\diagup\!\!{}_{2}\;$ Earthquake in 1718 AD and the Disappearance of Yongning Ancient Town
XU Yueren1),2), DU Peng1), LI Wenqiao1), TIAN Qinjian1), CHEN Lize1)     
1). Key Laboratory of Earthquake Prediction, Institute of Earthquake Forecasting, CEA, Beijing 100036, China;
2). Department of Earth Sciences, Durham University, South Road, Durham, DH1 3LE, UK
Abstract: Although China's historical earthquake documentation is relatively rich, it is not all based on scientific records. Therefore, the verification of the seismic information in historical records can effectively avoid exaggerating or underestimating the damage they produced. In this paper, we analyze the detailed information of the 1718 AD Tongwei M7 ${}^{1}\!\!\diagup\!\!{}_{2}\;$ earthquake through field surveys, document sorting, and manual visual interpretation of UAV images. Major conclusions are listed as follows:①The low-level terraces of Weihe River between Gangu and Wushan are fully developed with flatted surface, and the residents here are mostly killed by house collapses. In addition, the disappearance of Yongning Ancient Town is not directly related to the earthquake-induced landslides; ② In fact, "Yongning Town is entirely buried by the earthquake" in the historical records describes the phenomenon that loess dust has pervaded the entire Weihe Valley. These dust grains are produced by the sliding of earthquake-induced landslides. Thus, there is no possibility that large-scale landslides have slipped over Weihe bedrock and buried Yongning Town; ③After the earthquake, survivors abandoned the ruins and selected a new site to live. They built a new town named "Pan'an (means always peaceful in the future)". earthquake-induced landslides may be the reason of burying the residential areas on valley-side slopes, while those locations inside the valley are associated with the amplification effect of ground vibration. On the basis of compiling historical seismic data, scientific methods are used to explore the real meaning of these documents, for the purpose of providing basic data for the risk evaluation of strong historical earthquakes.
Key words: 1718 AD Tongwei M7 ${}^{1}\!\!\diagup\!\!{}_{2}\;$ earthquake     Yongning Ancient Town     Earthquake-triggered landslides     Loess Plateau    

INTRODUCTION

Compared with many other countries in the world, China has relative rich historical earthquake documentation with a longer time span of ~3000 years. Therefore, the time, relative location, number of casualties and even the destruction process of numerous strong earthquakes (M > 7) are accurately recorded, providing valuable references for modern earthquake scientific research (Gu Gongxu, 1983; Department of Earthquake Disaster Prevention, State Seismological Bureau, 1995). However, the earthquake literature does not accurately record earthquake phenomena with modern-time criteria (i.e. scientific terms). Some of the information may be exaggerated or reduced to a certain extent, and even there are various inferences, deductions, misrepresentations, and hearsay in the transcripts. If these unchecked seismic historical materials are used as evidence to determine the focal parameters of strong historical earthquakes, it will artificially exaggerate the regional seismic risk, or leads to a serious underestimation of the seismic risk. Thus, it is critical to use new technology to verify the information recorded in historical seismic documents (Gu Gongxu, 1983; Department of Earthquake Disaster Prevention, State Seismological Bureau, 1995; Lanzhou Institute of Seismology, State Seismological Bureau, 1989).

As an important means to investigate the characteristics of strong earthquakes, the remote sensing interpretation method plays an important role in understanding the source characteristics in modern time. Thus, it can be used to study the damage and focal parameters of historical cases and check the literature's correctness or accuracy (Xu Chong et al., 2016; Yuan Renmao et al., 2013; Dadson S.J. et al., 2004; Guzzetti F. et al., 2009) In the arid Loess Plateau of Northwest China, the earthquake-triggered disasters are relatively completely preserved and easily interpretable tofor seismic-related research. Because of the locality of human activities and the scarcity of rainfall, a large amount of earthquake damage information is retained. It is a feasible method to re-evaluate the focal parameters and key seismic damage data of strong earthquakes. Existing studies have shown that the spatial distribution characteristics of loess landslides triggered by strong historical earthquakes can revise the focal parameters and intensity lines of some previously given cases(Lei Zhongsheng et al., 2007; Xu Yueren et al., 2018a, b).

On June 19th, 1718 (the 57th year of Kangxi in Qing Dynasty), an M7 ${}^{1}\!\!\diagup\!\!{}_{2}\;$ earthquake occurred between Tongwei and Gangu counties in Gansu Province (Department of Earthquake Disaster Prevention, State Seismological Bureau, 1995) with MW7.3 (Cheng Jia et al., 2017), resulting in more than 70 000 deaths and triggering a large number of loess landslides (Liu Baichi et al., 1984; Lanzhou Institute of Seismology, State Seismological Bureau, 1989; Sun Ping et al., 2017; Xu Yueren et al., 2020a). Some scholars believe that the seismogenic structure is the West Qinling fault, but there is no corresponding evidence from paleoseismology(Wu Zhao et al., 2016; Shao Yanxiu et al., 2011). Several other scholars suggest that the newly discovered Tongwei fault is the possible seismogenic fault, but the evidence of the latest activity of the fault is currently insufficient (Wang Aiguo et al., 2018). Field investigations have discovered abundant earthquake-induced landslides (Liu Baichi et al., 1984). However, systematic analyses regarding the relationship between the earthquake-induced landslides and the destruction of residential areas are lacking (Xu Yueren et al., 2020a). For example, Sun Ping et al. (2017) propose that there is a huge earthquake-triggered landslide corresponding to the description of "Mountain in the north side moves from north to south (crossing the Weihe River), burying the whole Yongning Ancient Town (34.754597°N, 105.147506°E)". Such event has caused tens of thousands of deaths (Sun Ping et al., 2017). However, this view is not consistent with what we observed in the fieldworks. Therefore, it is necessary to study the relationship between the spatial distribution characteristics of the earthquake-triggered landslides and its possible damage to the local residential areas, and further develop the true role of the Tongwei earthquake-triggered landslide in the entire destruction.

With the support of historical earthquake analyses, high-resolution satellite image interpretations, UAV aerial photography, and field survey methods, this paper confirms the possible seismogenic structures through the spatial analysis of the relationship between the local active faults and densely distributed landslides. Based on the analysis of the spatial relationship between the interpreted landslides and the current residential areas, the role of the Tongwei earthquake landslide in the earthquake damage (personnel casualties, settlement changes) is discussed, for the purpose of giving more than 300 years of occurrence from the perspective of historical earthquake-induced landslides. Furthermore, we discuss the cautious attitude that scholars should have when referring to historical earthquake literature.

1 DATA AND METHODS

In this paper, we use the 2000-2018 Google Earth images to extract landslides triggered by the Tongwei earthquake and nearby other secondary disasters through machine-assisted visual interpretation method at 3-D view in Google Earth. The interpretation work still need to be verified combined with the field survey and literature information analysis. When an earthquake occurs at current time, scholars can use pre- and post- earthquake high-resolution images to extract detailed landslide distribution through comparative analysis. We cannot obtain the same images related to the historical earthquakes as modern time, and instead use the images formed decades, hundreds, even thousands of years after the earthquake. Thus, we should confirm that the co-seismic landslides triggered by these cases can be extracted by the modern images (Xu Yueren et al., 2020a, b). Firstly, The Tongwei earthquake had triggered plenty of landslides which have been clearly recorded in seismic literature (Lanzhou Institute of Seismology, State Seismological Bureau, 1989). Besides, there are still remained landslides that can be interpreted from the Google Earth for their bigger-size. Secondly, each historical earthquake has its macro epicenter which is the reason for the dense co-seismic landslides around it. On the contrary, it is difficult to trigger dense landslides far away from its macro epicenter (Xu Yueren et al., 2020a). In other words, the occurrence of dense landslides must have been triggered by local strong earthquakes and is unlikely to be triggered by other low-intensity seismic events with a longer distance. Therefore, we can confirm that the interpreted dense landslides are triggered by the Tongwei earthquake but not others. Thirdly, the moderate seismic events cannot trigger the regional dense landslides for their limited energy releasing (Xu Yueren et al., 2020b). Fourthly, with the passage of time, a large number of co-seismic landslides with an area ranges from 0 to 104 m2 cannot be identified due to surface process and local human activities (i.e. farming).However, those with am area ranges from 104-106 m2 are preserved and be further identified, representing the overall characteristics of co-seismic landslides (Du Peng et al., 2020). Finally, the study area is located in arid and semi-arid regions, in which rainfall-triggered landslides are formed by heavy rain events in summer. As a result, these events can be effectively distinguished from earthquake-triggered landslides in terms of their different sizes and durations (Xu Yueren et al., 2020b).

The study area is located between Gangu and Tongwei Counties (Fig. 1). Our research goal is to investigate the possible impact of other adjacent strong historical earthquakes in the study area except the Tongwei earthquake by the literature checking. The 734 AD Tianshui M7 ${}^{1}\!\!\diagup\!\!{}_{2}\;$ earthquake has not been recorded in the two counties, indicating that the damage is relatively slight in the study area. For the 1654 AD Lixian M8 earthquake in Gangu, documents say that the earthquake has caused landslides and cracks, subsequently leading to ~3 000 deaths. However, a death toll of less than ~300 is indicated by Lanzhou Institute of Seismology, State Seismological Bureau(1989), indicating that the local damage was not serious compared with its macro epicenter. As for the 1718 Tongwei earthquake, some documents record that houses collapsed with several people and livestock injured, indicating relatively slight damge during the earthquake (Lanzhou Institute of Seismology, State Seismological Bureau (1989); (Yang Xiaoping et al., 2015). During the 1920 Haiyuan M8 ${}^{1}\!\!\diagup\!\!{}_{2}\;$ earthquake, 1 365 people died in Gangu. Numerous cracks appear with only limited landslides. 10 206 people were killed in Tongwei, where the western county was destructed most, while the cities were slightly damaged. In addition, some of the triggered landslides blocked the rivers, forming temporal dammed lakes., Based on the above documents, it is indicated that the two earthquakes in 734 AD and 1654 AD have made relatively slight influence. Even though the 1920 Haiyuan earthquake had killed tens of thousands of people, the record of the corresponding landslides is rare. Thus, the objects in the study area are mainly induced by 1718 AD Tongwei earthquake. These landslides in the study area are mainly triggered by the Tongwei earthquake, which have resulted in mountain movement along a distance of more than 50 km and buried Fuqiang (Gangu) County (Just based on the local recorded description), indicating serious destruction in Gangu during the earthquake. Such disaster in the villages along the northwest of Gangu has caused more than 30 000 deaths. The southern part of Tongwei County is severely damaged with numerous loess landslides triggered by the Tongwei earthquake, leading to a total death of 40 000, which is more than four times of the death toll in Haiyuan earthquake. As a consequence, the local government had to abandon the original collapsed site and select another workplace. Compared with the landslide database of 2008 Wenchuan earthquake, Xu Yueren et al. (2020b) suggest that among the co-seismic landslides triggered by the Tongwei earthquake, those with an area range of < 104 m2 are destructed, while those having an area range of 104-106 m2 can be well preserved. In other words, the interpreted landslide database can reflect the scope of the macro epicenter.

This paper utilizes a comprehensive method which combines the historical literature reanalysis, the interpretation of densely distributed landslides and field surveys. Through the analysis of the distribution characteristics of earthquake-triggered landslides adjacent to Yongning Ancient Town, we determine the role that Tongwei earthquake and landslides played in the disappearance of Yongning Ancient Town.

2 RESULTS 2.1 Distribution of Interpreted Landslides Triggered by 1718 AD Tongwei Earthquake

Since the available Google Earth's image data in the study area is imaged from 2000 to 2018, with a spatial resolution of 1 m or better, most regions in this study have 3-4 sets of available data. These data can be used for cross-checking to extract the boundary of each large landslide, thereby obtaining a better angle of view than field survey. Fig. 1 shows that the landslides triggered by the Tongwei earthquake are concentrated in the area north of the Weihe Valley and south of Tongwei County. A large number of dense landslides which are interpreted in the study are also located in the lower right part of Fig. 1. These landslides are triggered by the Tianshui earthquake (Lei Zhongsheng et al., 2007).

Fig. 1 Distribution of interpreted earthquake-triggered landslides and the location of Yongning Ancient Town The blue polygons are the interpreted earthquake-triggered landslides, the thick red dotted line is the dense area of the Tongwei earthquake-triggered landslide, and the black dotted line is the intensity of > X (Department of Earthquake Disaster Prevention, State Seismological Bureau, 1995). Note that another intensive landslide region at the right bottom is triggered by the 734 AD Tianshui earthquake

The interpreted landslides triggered by Tongwei earthquake are densely distributed in the range of 50 km from east to west and 60 km from north to south. The total area of interpreted landslides is 635 km2 (Xu Yueren et al., 2020a). These landslides are closely related to the Tongwei fault within 15 km with 3 615 landslide bodies, accounting for more than 70% of the total number. It shows that the seismogenic mechanism of the Tongwei earthquake is the Tongwei fault rather than the West Qinling fault (Xu Yueren et al., 2020a). According to the statistics of the area range distribution of the landslides, about 60% of the landslides have an area range of < 105 m2. The length ranges of the landslides are concentrated at 100-400 m, accounting for more than 60% of the total number; the width is concentrated at 100-300 m, accounting for more than 80% of the total number; and the landslide elevation values are concentrated less than 200 m, accounting for more than 90% of the total. In addition, the size of the interpreted landslides is adapted to loess terrain in the study area (Liu Baichi et al., 1984; Xu Yueren et al., 2020b).

The distribution of the dense area of landslides in the study area is spatially coincident with the intensity > X but meanwhile shows obvious difference. The dense landslide area both extends to west and north by about 20 km, but there is no landslide distribution in the north-central part of the X intensity area, suggesting that the dense landslides interpreted by remote sensing technique can effectively modify the spatial distribution of the previous intensity line (Xu Yueren et al., 2020a).

2.2 Landslides Along the Weihe Valley and the Disappearance of Yongning Ancient Town

The understanding of the literature record of "Mountains (Beishan in Chinese) (in the left bank of Weihe River) moved southward, buried the whole Yongning Town" is an equivocal issue that is currently debated in news reports and scientific research. One main view, a giant landslide has slipped from Beishan, burying the Yongning Ancient Town (Sun Ping et al., 2017). Here we combine the interpretation of the relationship between the landslides and the villages on both sides of the Weihe River Valley for further analysis.

Fig. 2 is the interpretation of the relationship between the interpreted landslides and the existing residential areas on both sides of the Weihe River Valley. It can be seen that most of the residential areas in the county and townships are not affected by the coseismic landslides. The main affected areas by the earthquake triggered-landslides are the residential areas on the loess slopes. There are 6 places whose names are currently known as "Xieshan (means slope landsliding and movement in Chinese)", "Tashan (means loess slope collapsed in Chinese)", and "Yeshan (means haulage in Chinese)" at the north of Gangu county (Fig. 2), and plentiful earthquake-triggered landslides have occurred in the 6 villages named with landsliding. Although the exact number of village (residential areas) destruction on or around the landslides during the seismic event is unknown, we consider that the distribution of these landslides has importantly influenced the location of residential areas based on the fact that current villages are far from or at the corner of the landslide bodies.

Fig. 2 Distribution map of dense landslides triggered by Tongwei earthquake along Weihe Valley and its residential areas, see location at Fig. 1

The Yongning Ancient Town is located in the place of the current village Sishilipu (20 km west of Gangu County) on the G316 National highway, which is known by local people as the "Official Road" for a long history. After the collapse of the Yongning ancient town, local people moved to a new place, which is currently called Wushilipu (25 km west of Gangu County). This village is also located on the G316 National Highway, but has been renamed to "Pan'an (means pray for the safety in Chinese)" for avoiding the future earthquake damages (Lanzhou Institute of Seismology, State Seismological Bureau, 1989) (Fig. 2). Fig. 3 shows that the Beishan (slopes of Loess Plateau) on the left bank of Weihe River has developed a large number of landslides, but the size of these landslides are relatively small, and the average thickness of the landslides is in a range of of 20-30 m. Meanwhile, the height of the trailing scarps and side scarps of the landslide are also at a similar height. These landslides are developed on the slopes of the A-F branches of Weihe River that existed before the Tongwei earthquake. All basins along the slopes remain their basic shape, indicating that the landslides have limited moving distance slight impact (Fig. 3).

Fig. 3 Distribution of the interpreted landslides triggered by the Tongwei earthquake at left bank of Weihe River and related topographic profile, UAV images at 2018 The red lines show the trailing scarp boundaries of the landslides, the dashed green lines show the boundaries of the river basins along the slopes, the pink polygons show the flatted surfaces of the terraces of Weihe River, the residential areas are named with "XXX -ping", i.e. Peijiaping (means the topography surface is much flatted in Chinese). Note that the Yongning Ancient Town is located at the right bank of Weihe River, nearly 10 km far away from the drilling section X-X' by Sun Ping et al. (2017)

There are at least four evidences that do not support the phenomenon that large-scale landslides (Sun Ping et al., 2017) has slipped over the Weihe River and buried the Yongning Ancient Town. ① There are a large number of small-scale landslides which contain sliding materials accumulating on the slope surface with limited distance.② The ground of the terraces of the Weihe River, which is situated at the foot of Beishan, is well developed. In addition, there is no accumulation of landslide materials. The villages on these terraces are mostly named after "XXX-ping (means their village locating on the flatted ground)" (Fig. 2). We check the literature and find that except for the relocation of Yongning Ancient Town, most of the villages have been reconstructed at the same place. Thus, these villages indicate that they have not been struck by landslides on the nearby slopes. ③ Terrain profiles in Fig. 3 show that the first-order terraces on the left bank of the Weihe River Valley are flat, and there are no large-scale undulating sedimentary features that should be available after the occurrence of landslides. ④ Some documents record that the Weihe River is blocked by earthquake-triggered landslides (Lanzhou Institute of Seismology, State Seismological Bureau, 1989). Besides, some scholars infer that such phenomenon is related to the large-scale landslides in Beishan (Sun Ping et al., 2017). In fact, from the Weihe riverbed north of Yanjiazhuang to the foot of Beishan, at least two landslides are located at the foot of the Beishan slope, and one of the landslides has an estimated volume of approximately 2×106 m3 (Fig. 4(a)). Such landslide volume is sufficient to block the river. However, due to the loose loess landslides, it will soon be washed away by the silted river water (Lanzhou Institute of Seismology, State Seismological Bureau, 1989). Moreover, the side erosion of the river will erode the landslide accumulation of loose loess sediments. Significantly, because of the embankment and terrace reconstruction, it is hard to see the landslide accumulation on the current terrains. However, trailing scarps are clearly visible (Figs. 4(b), 5(a)), indicating that medium-size landslides, not large-scaled ones, can also lead to short-term damming of the Weihe River (Fig. 3).

Fig. 4 The profile of the landslide near the left bank of Weihe River and its volume calculation. See location in Fig. 3

Fig. 5 Photos of earthquake-triggered landslides on both sides of Weihe River (a) The trailing scarp of the landslide which has blocked Weihe River at its left bank; (b) The possible location of the Yongning Ancient Town (yellow dashed rectangle), which is the Sishilipu Village. On its south, there are several landslides along the slope. Note that the valley surface is flatted to farming and housing; (c) The dense rainfall-triggered landslides (dashed yellow ovals) along the trailing and side scarp of the earthquake-triggered landslides on the left bank of Weihe River. Note that there is a significant difference between the two triggered landslides types

In fact, the description of "Beishan moved southward" describes a large number of landslides with limited size occurring on the loess slopes. Most of these landslides have slipped locally on the slopes. Therefore, there is no possibility for the landslide to slip nearly 10 km to the Yongning Ancient Town (now at Sishilipu). Then, is there a possibility for the landslides occurred in the south of Sishilipu to bury the town? It can be seen in Fig. 5(b) that there is a landslide with a width of ~1 km, but its trailing scarp is only 10-20 m. In addition, the amount of sliding has not reached the volume of the northern wall of Yongningbao. Also, the terrain at Sishilipu is flat for farming and there is also no undulating landslide accumulation at the front of the slope. Therefore, it can be confirmed that no sliding material from the loess slope of Beishan has reached the township at right bank of Weihe River.

This paper gives two possible interpretation of the literature description of "the landslides in the Beishan area moved straight to the southward, burying the people in the surrounding towns" (Department of Earthquake Disaster Prevention, State Seismological Bureau, 1995; Lanzhou Institute of Seismology, State Seismological Bureau, 1989)? ① The Yongning Ancient Town has suffered severe damage during the Tongwei earthquake by the strong ground vibration and the site magnification effect of the loose sediment of the terraces of Weihe River (Wang Lanmin et al., 2017; Zhang Maosheng et al., 2011). In addition, the reason for the township relocation is that the survivors have abandoned the "ruins" of the Yongning Ancient Town and selected the open terrain of the current site to rebuild the new town named "Pan'an". ② When the 2008 Wenchuan earthquake occurred, the Wenchuan County in the valley of Minjiang River encountered co-seismic landslides, the actual experience of the entire town was filled with dust particles (Du Peng et al., 2020). The Tongwei earthquake generated loess dust particles along the Weihe River Valley, bringing about a large number of loess landslides on both sides of the river. The dust grains covered the whole river valley, causing the survivors to recall the loess dust grains as the "giant loess landslide" burying "Yongning Ancient Town". Fig. 6 is a schematic diagram of the loess dust particles generated by numerous co-seismic landslides in Beishan, which moves upward and southward along the slopes. As a result, a muddy air environment similar to a "sandstorm" was formed.

Fig. 6 A possible sketch model of earthquake triggered landslides along the slopes at both banks of Weihe River and related loess dust particles covering the valley space during the earthquake
3 DISCUSSION

Conducting a landslide borehole survey on the left bank of Weihe River (Fig. 3), Sun Ping et al. (2017) argue that there should be a large-scale landslide with a thickness of more than 90 m in Beishan. The landslide moved southward and slipped over Weihe River, diverting the river and overwhelming the Yongning Ancient Town. However, this view point is debatable. ① The Yongning Ancient Town is not located at the front edge of the "large-scale landslide", near ZK5 location (Fig. 3) (Sun Ping et al., 2017), but locates at Sishilipu (Fig. 2) which has a distance of 10 km to ZK5. That is, the "large-scale landslide" cannot bury the township spatially. ② Only one of the five boreholes has been found with a sliding surface at a depth of 90 m. Thus, scholars are supposed to be cautious about whether the thickness can be used as the average thickness of the entire landslide body. The model diagrams given by Sun Sun Ping et al. (2017) cannot explain the flat surface of the first and second terraces of Weihe River (Fig. 3), and the villages distributed on the terraces are named as "Xilijiaping" and "Wujiaping". Furthermore, no evidence has shown the undulating landslide accumulation along the river, and there is also no ancient river channel because the borehole did not reveal the sedimentary layer of the terraces. Obviously, Sun Ping et al. (2017) have only based on the deep sliding surface to estimate that the "large-scale landslide" has slipped over the river and buried the Yongning Ancient Town. However, it is difficult to achieve such estimation from the linear spatial distribution of earthquake damage (Li Gen et al., 2014; Meunier P. et al., 2008; Parker R.N. et al., 2011); ③ Most of the landslides triggered by the Tongwei earthquake are within the loess sediments, which are limited by loess sediments and the shape of slopes (Li Weile et al., 2015). The average thickness of earthquake loess landslides is generally from a few meters to tens of meters (Chen Yongming et al., 2006; Li Weile et al., 2015; Peng Jianbing et al., 2015). As can be seen from the profile in Fig. 3, the average thickness of the seismic landslide found in Tianshui City is about 40 m (Zhang Shuxuan et al., 2017).

The spatial distribution of the landslides triggered by the 1718 Tongwei earthquake is closely related to the Tongwei fault, which is a Holocene active fault on the fifth generation earthquake zoning map. It is mainly caused by the destruction of the original villages on the loess slopes, and has little impact on the village landslides in the river valley. The casualties in these residential areas are mainly caused by the collapse of houses or floods resulted from dammed rivers. The survivors abandoned the original ruins and selected a new location to rebuild their homes, instead of the original village buried by landslides.

Abundant strong historical earthquakes have occurred during the history in the Loess Plateau (Lanzhou Institute of Seismology, State Seismological Bureau, 1989), and most of them have caused co-seismic loess landslides. For example, the 1920 Haiyuan earthquake in Haiyuan and Xiji counties has led to numerous loess landslides and the subsequent dammed lakes, and the field traces are still prominent even 100 years after the event (Xu Yueren et al., 2020b). Therefore, the landslides we interpret in this study should be triggered by the Tongwei earthquake. It is worth noting that some scholars have mistakenly classified the Tongwei earthquake landslide as the Haiyuan earthquake when passing through Tongwei County (Xu Yueren et al., 2020b).

Wang Jiading et al. (1999) propose that the earthquake-triggered landslides have deformation and failure mechanisms such as slope disintegration, oblique tossing, and dusting effects. This model provides a mechanism for the rational explanation of the loess dust phenomenon that occurs over the Yongning Ancient Town (Wang Jiading et al., 1999). Similar to the "Loose solid liquefaction" feature, the liquefied loess dust moves rapidly along the slope to form a traditional massive landslide body, but the material of the landslide is significantly different from the traditional bedrock landslides. Meanwhile, a large amount of dust generated by sliding is diffused in the air, pervading the entire valley; the landslide cannot slip over Weihe River to bury the town due to the fact that the sliding distance of Beishan's landslide is limited and has nothing to do with the destruction of the Yongning Ancient Town.

4 CONCLUSIONS

This paper combines historical literature re-examination, remote sensing interpretation, and on-site verification, and analyzes the relationship between the 1718 Tongwei earthquake-triggered landslide and the disappearance of "Yongning Ancient Town". Several conclusions are obtained as follows:

(1) Based on Google Earth interpretation, the landslides triggered by the Tongwei earthquake are densely distributed along the both sides of the Tongwei fault, which is located on the north side of the West Qinling fault. The main residential areas affected by the landslides are those on the loess slopes. The co-seismic landslides have made little effects on the residential areas in the river valley, in which the destruction is mainly from the strong ground vibration and site amplification.

(2) According to the spatial distribution of the Tongwei earthquake-triggered landslides and the residential areas and the relevant historical documents, the Tongwei earthquake has indeed triggered dense loess landslides. On the other hand, we need to use scientific methods to extract the scientific meaning of those historical materials, verify the corresponding authenticity, and provide basic data for the reasonable seismic risk assessment of strong historical earthquakes;

(3) The terraces at all levels of Weihe River are fully developed. The disappearance of the Yongning Ancient Town is related to the magnification effect of the river terrace site and is not directly related to the co-seismic large-scale landslide. There is no possibility of the super-large thick landslide to Weihe River block. The disappearance of the Yongning Ancient Town was caused by earthquake-triggered landslides. Furthermore, these landslides can lead to the overall disappearance of the villages and towns at loess slopes. The residential areas at the river valley are highly damaged by ground vibrations.

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