Parameters of the AD 849 earthquake east of Baotou City in Inner Mongolia and the adjustion of the potential seismic sources zone spatial distribution functions

1.   Introduction

Volume 18 (Xuanzong Ji) of the Old Book of Tang records the following: “Xinsi Age in October of the third year of Dazhong Emporer in Tang Dynasty (October 20, AD 849), the Jingshi (Xi'an City) felt earthquake shock and Hexi, Tiande, Ling, Xia felt especially serious. Thousands of people were killed”. This indicates the historical occurrence of a great earthquake in the north of China, but the scant historical records and the nearly 1200 years that have elapsed since the earthquake mean that little trace of it remains. Nie and Li (1991), and Nie et al. (1996, 2008, 2010) studied the AD 849 earthquake in detail and suggested the Daqing Mountain piedmont fault as the causative fault. His work showed that the earthquake fault starts at Haolaigou in the west and extends eastward intermittently to the Baiyunchahan area of the Tumed Left Banner. The length of the surface rupture zone is about 100 km. The accumulated vertical displacement measured by Nie et al. (2010) was more than 2.3 m, from which he judged that the epicenter intensity could have been as high as intensity of Ⅹ degree.

However, Nie et al. (2010) estimated the death toll being only in the dozens, which seems somewhat inconsistent with the scale of economic and population damage caused by a major earthquake at that time. In the present study, data about this earthquake were collected systematically and then sorted and examined. Here in, we discuss the epicenter location, the seismogenic structure and isoseisms, the number of death toll, and the magnitude of the earthquake. Having revised the seismic parameters, we adjust the boundary and parameters of some seismic potential sources zone in the Baotou area.

2.   Epicenter of AD 849 Baotou east earthquake

To determine the epicenter of the AD 849 east Baotou earthquake, we begin by establishing the present locations of some of the places named in historical books. Specifically, Jingshi or Shangdu refers to the capital City of the Tang Dynasty, which is now Xi’an City in Shaanxi Province. Meanwhile, Zhenwu and Tiande were important military organizations on the northern frontier of the middle to late Tang Dynasty (AD 755–907) and were also known as the Zhenwu Army and Tiande Army, respectively.

The Zhenwu Army, established in the second year of the Jinglong Age of the Tang Dynasty (AD 708), located in Tuchengzi township, Helinger county, Inner Mongolia, according to the Eighteenth Geographical records of the Old Book of Tang (Liu, 1975 a, b).

Tiande Army, belonging to Guan-nei Dao, Tang Dynasty, was stationed on the south bank of Wuliangsu Sea, the Urad Front Banner, Inner Mongolia.

Lingwu, namely, Lingzhou City of Tang Dynasty was located on the East Bank of the Yellow River in the northwest of Wuzhong City, Ningxia Hui Autonomous Region (Lu et al., 1993).

Yanzhou City is located in Dingbian County in Shaanxi Province.

Xiazhou is now Baichengzi Village of Hongdunjie Town in Jingbian County in Shaanxi Province.

Yunjia Town, namely, Yunjiaguan pass, this showed that Yunjiaguan pass belonged to Jinhe County. It should be in the area of Helinger County of Inner Mongolia (Table 1).

Table  1.  Ancient places name and its modern location and destruetion associated with AD 849 earthquake

No.	Ancient place name	Modern location	Destruction	Intensity
1	East Shouxiang City	Old Town of Tuoketuo County, Inner Mongolia	Thousands of soldiers were dead	X
2	Chanyu Grand Frontier Command (location of Zhenwu Army)	Tuchengzi Township, Helinger County, Inner Mongolia	Army towns and farm houses destroyed	≥VIII
3	Shengzhou City	South bank of Yellow River in Zhungeer Banner, Inner Mongolia
4	Mid Shouxiang City	North bank of Yellow River in south of Baotou City, Inner Mongolia
5	Tiande Army	South coast of Wuliangsu Sea in Wulat Qian Banner, Inner Mongolia
6	Yunjia Town	Helinger County, Inner Mongolia	Two captains and tens soldiers were killed
7	West Shouxiang City	Wulat Qian Banner of Inner Mongolia	Earthquake had sound, farm houses destroyed	VII
8	Linzhou	Shenmu County, Shaanxi Province
9	Ma Yi	Shuozhou City, Shanxi Province
10	Lingzhou	East bank of Yellow River in northwest of Wuzhong City, Ningxia Hui Autonomous Region	Earthquake was strong and may have caused some damages	VI
11	Xiazhou	Baichengzi Village, Hongdunjie Town, Jingbian County, Shaanxi Province
12	Yanzhou	Dingbian County, Shaanxi Province
13	Jingshi	Xi′an City, Shaanxi Province	Felt a little shock	<VI
14	Tongzhou	Dali County, Shaanxi Province	Felt a little shock
15	Yulin	Yulin City, Shaanxi Province	Felt a little shock
16	Huaiyuan	Hengshan County, Shaanxi Province	Felt a little shock
17	Xining	Xining City, Qinghai Province	Felt a little shock
18	Zhili	Hebei Province	Felt a little shock
19	Shanxi	Shanxi Province	Felt a little shock

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In summary, the locations of heavy death toll due to the earthquake were all in and around the Hetao Basin. Considering the seismic geological conditions of the Hetao Basin and its surrounding areas, we conclude that the earthquake almost certainly occurred in the Hetao Basin, a conclusion that is supported by previous research (Nie and Li, 1988, 1991; Nie et al., 1996, 2008, 2010).

3.   Fracture range and location of epicenter region

Next, we discuss the seismogenic structure and more-accurate epicenter location of this earthquake based on the results of recent seismogeological studies. Nie et al. (1996) uncovered 6 ancient earthquake events since the Holocene by digging a trench in an aluminum plant. In the middle and western segments of the fault, traces of seismic deformation about 1000 years ago have been found in many places. The main relics are as follows.

1) A fault scarp of 2.6–2.7 m in height is developed in the rear edge of the level-I platform in the west of Xuehai Gully, whose strike approaches nearly E-W and extends 500 m. The latest activity of faults controlling steep ridges disrupted the grayish-brown sandy soil layer with a ¹⁴C age of 1640±112 a, with a fault displacement of 1.2 m (Nie et al., 2010).

2) In the north trench trough of the Baotou Aluminum Plant (Nie et al., 1996), the latest fault activity is a stratum with a staggered moluminescence age of 1280±70 years ago.

3) There are two distinct preserved seismic scarps on the level-I platform (newest terrace) of Ashangoumen Village, extending in the ENE-EW directions, with a visible length of over 500 m and heights of 0.28 m and 0.42 m, respectively. The distance between them is around 30 m. A trench was excavated across the front of a steep ridge, and it could be seen that the steep ridge is controlled by a south-dipping normal fault dipping southward. The gray-black paleosoil layer on the top of the staggered platform has a ¹⁴C age of 2507±122 a, and the vertical displacement of the fault is 0.85 m (Nie et al., 2010).

4) North of the expressway bridge of Yongfu Village, a Holocene alluvial fan (with a ¹⁴C age of 4610±90 a at the top) is arched and deformed, and a group of more than 10 faults with different scales has developed (Nie et al., 2010).

5) On the front of the Hazigai diluvial fan in Goumen Town of Tumed Right Banner, the fault dislocation strata have a ¹⁴C age of 2310±100 a (Jiang et al., 2001).

6) In the Shangdalai trench in the Tumed Left Banner, the fault dislocation was around 1 m from the surface. The layer contains Han Dynasty pottery tablets, and its ¹⁴C age is 2115±105 a (Wu et al., 1995).

7) At the front of the Baiyunchahan estuary alluvial fan in Qianzhan Village of Tumed Left Banner, the age of ¹⁴C to fault dislocation was 1160±100 a (Jiang et al., 2001).

The aforementioned seismic deformation traces along the Daqing Mountain piedmont fault are distributed between the Donghe District of Baotou City and the Tumed Left Banner of Hohhot City, with a length of around 80 km from east to west and concentrated in the area from the Baotou Aluminum Plant to Yongfu Village. Therefore, we consider these to be traces of the AD 849 earthquake and that the Daqing Mountain piedmont fault is the seismogenic fault of the AD 849 earthquake, whose epicenter was located in the area from the Baotou Aluminum Plant to Yongfu Village. We suggest that the epicenter should be revised to 40.6°N, 110.2°E, which is consistent with the above description (Figure 1). Nie et al. (2010) gave the geographic coordinates as 40.4°N, 110.2°E, but that location is around 20 km far from place of the Daqing Mountain piedmont fault and is inconsistent with the reference geographical names mentioned above (Baotou Aluminum Plant to the Yongfu Village area).

Figure  1.  Distribution map of meizoseismal areas

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Damaging range: Baotou to Yanzhou (Dingbian), Xiazhou (Jingbian), Lingzhou (Wuzhong), had an average radius of around 400 km. Xining City was the farthest place in which the earthquake was felt shock, with a radius of around 830 km (Figure 2).

Figure  2.  Intensity of AD 849 Baotou east earthquake

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4.   Analysis of death toll in AD 849 earthquake

The earthquake shock was felt in Xi′an, the capital of Tang Dynasty, with no obvious damage. The damage occurred mainly in Hexi, Tiande, Lingzhou, and Xiazhou, where thousands of soldiers were crushed to death. However, the historical data do not include the number of inhabitants who were killed; if that number was included, then the death toll could be several times greater.

The New Book of Tang (Ouyang, 1975a) records the following: “The earthquake shocks were felt in Shangdu. In Zhenwu, Hexi, Tiande, Lingwu, Yanxia and other city the earthquake damaged houses, killed dozens of people.” The earthquake impact range recorded in the New Book of Tang (Ouyang, 1975b) is basically the same as that in the Old Book of Tang, but the death toll is quite different. The Old Book of Tang was published in the second year of the Kaiyun Age of the HouJin Dynasty (AD 945) and edited on the basis of the past national history (Chen, 2006, Yan et al., 1992). Soon after the Tang Dynasty, the sources of information were abundant and detail. The New Book of Tang (Ouyang, 1975c) was completed in the fifth year of the Jiayou Age of Emperor Renzong in the Song Dynasty (AD 1060), more than 100 years after the Old Book of Tang. Therefore, the death toll recorded in the New Book of Tang may be incorrect. Moreover, an earthquake with such a large impact would have been one with a large magnitude and that could not have killed only dozens of people. Moreover, “ten” may have been a miswriting of “thousand”. “Ten” and “thousand” are closer in Chinese writing. The writing of the Chinese characters “Ten” and “Thousand” has not been changed.

Regarding the earthquake, the Institutional History of the Tang Dynasty records the following in particular: “(The earthquake) Crushing military bases and houses, the captain of Yunjia Town and the captain of soldiers and horses from Hubei Province for prevention of robbery in autumn and tens soldiers were killed.” Dozens of people died in Yunjia Town, so the number of people killed in this earthquake was not as little as the dozens recorded in the New Book of Tang. We reason thousands of soldiers as having been killed, which is closer to the truth than tens of people. How many troops were stationed in the Hetao area at that time? If thousands of soldiers died, what was the death rate of the entire soldier?

Shuofang warlord had 7,000 soldiers and 1,700 horses. Zhenwu Army had 9,000 soldiers and 1,600 horses. The Tiande Army had more than 10,000 soldiers. The Anbei Protectorate had 6,000 soldiers and 2,000 horses. In summary, the total troops of the Hetao area was around 32,000 soldiers, with around 5,300 horses. If thousands of people died, the death rate corresponds to around one tenth of the troops.

At that time, in addition to the troops stationed in Hetao, were there also inhabitants living there? The history of Hetao shows that during the Warring States Period, Zhao State troops swept across northern Shaanxi and southern Inner Mongolia and drove the Huns to the north of Ura Mountain. They established Yunzhong County of today’s Inner Mongolia, and built the Great Wall along Ura Mountain, which was a section of the famous Great Wall of Zhao. That is to say, a large city was built in the southern part of Hadmengou Pass, namely Jiuyuan City, in which many troops were stationed. This made it a military town in the northwest of Zhao State, effectively guaranteeing the safety of the northern part of Yunzhong Prefecture. The Qin Dynasty established Jiuyuan Prefecture, one of 36 Prefectures (Sima, 2005).

In 127 BC, Jiuyuan Prefecture was renamed Wuyuan Prefecture, which had 16 counties under its jurisdiction. The immigration of inland immigrants, the development of agriculture, and the use of advanced production methods meant that this area became highly developed. Wuyuan Prefecture became an important agricultural reclamation area in the Han Dynasty, and Jiuyuan City also developed accordingly. By the end of the Western-Han Dynasty, because of the rise of the Huns and the decline of the Western-Han Dynasty, as well as the incorrect foreign policy of Mang Wang when he was in power, the Huns invaded continuously southward, forcing the residents of Jiuyuan City to flee southward, and the city went into decline. Because of the decline of the Han Dynasty, by the end of the Eastern-Han Dynasty, the northern nomadic people were strong. Facing constant attacks from those nomadic people, the residents of Jiuyuan City abandoned it and moved southward.

Up to AD 856, this shows that the early Tang Dynasty ruled the Hetao area effectively. In the mid-Tang Dynasty, the Tiande Army and Zhenwu Army were added to defend the area, and Mid Shouxiang City was built on the original site of Jiuyuan City, becoming a transportation hub and important military town connecting east, west, north, and south. This shows that the Hetao area was relatively prosperous and economically developed at that time, and there would have been tens of thousands of people living there. Therefore, on the basis of a death toll of 10%, we estimate that at least several thousand common people would have died.

Analyzing the above historical records confirms that a large earthquake occurred in northern China in AD 849. The earthquake killed thousands of soldiers in the Tiande Army and Zhenwu Army stationed in the Hetao Basin and thousands of inhibitants. Overall, the earthquake may have killed nearly ten thousands people.

5.   Magnitude of AD 849 earthquake East of Baotou

Previous views differ regarding the magnitude of this earthquake. Based on the epicenter intensity, the length of the surface rupture zone, the range of damage, and the range of feeling shock, Sun (1985), Wu (1989), Huang (1989), Department of Earthquake Disaster Prevention, China Seismological Bureau (1995), and Nie et al. (2010) gave earthquake magnitudes in the range of 7.3–8.2.

Clearly, the magnitude of the AD 849 earthquake has been recognized differently. From areas in which the earthquake intensity could be determined, a series of empirical relations has been established using the statistical analysis of magnitude. These empirical relations can then be used to calculate the magnitude where the earthquake intensity cannot be determined. However, if the earthquake fault has been investigated fully, the magnitude can also be obtained by using the established statistical empirical relationship based on the dislocation and length of fault surface rupture zone. Deng et al. (1993) studied earthquake faults in China and noted that only earthquakes with M ≥ 7 can have earthquake fractures. Therefore, the magnitude of the earthquake in AD 849 was undoubtedly greater than M7. Based on the characteristics of the earthquake intensity and surface rupture zone, we determine the magnitude of the AD 849 earthquake east of Baotou as follows.

1) Determine the magnitude according to the seismic intensity.

a) Calculate the magnitude (M) according to the epicenter intensity (I₀);

M = 0.67I₀ + 0.66, M = 7.36 (Liu, 1989)

M = 0.61I₀ + 1.42, M = 7.52 (Wu, 1989)

M = 0.58I₀ + 1.5, M = 7.3 [(Institute of Geophysics, China Seismological Bureau and Institute of Chinese Historical Geography, Fudan University, 1990)]

b) Calculate the magnitude (M) according to the epicentral distance (R) of areas of different intensity.

M = 5.02 + 1.21log(R_Ⅷ), R_Ⅷ = 140, M = 7.62

M = 1.38 +2.32log(R_V), R_Ⅴ = 450, M = 7.53

M = 0.25 + 2.54log(R_Ⅳ), R_Ⅳ = 830, M = 7.66( Liu, 1989)

M = 1.93+2.06log(R_Ⅴ), R_V = 450, M = 7.39 (Wu, 1989)

c) Calculate the magnitude (M) according to range of the feeling shock (D)

log(D) = 0.22M+1.11, D = 830, M = 8.22

7.5 < M < 8 (Institute of Geophysics, China Seismological Bureau and Institute of Chinese Historical Geography, Fudan University, 1990)

2) We determined the magnitude according to the surface rupture zone. For that relationship, the type of rupture zone of the earthquake has been discussed previously. Much work has been done on the relationship between magnitude and type, length of fracture, and displacement, and various empirical relationships have been established, including that by Deng et al. (1993). The empirical relationship is M=7.20+0.31log(L) + 0.57log(D), which gives M=8.0 with L = 80 km and D = 2.3 m. Based on that result, we determined the magnitude of the AD 849 earthquake east of Baotou finally as 7.62±0.5. From the magnitude rule of historical earthquakes in China, we suggests that the magnitude of this earthquake is 7½±0.5.

6.   Potential seismic sources zone border and parameters adjustment in Baotou Area and its significance

The neotectonic movement in the Baotou area was characterized mainly by the strong subsidence of the Hetao fault basin and the uplift of the surrounding blocks. It can be divided into three neotectonic units, namely the Yinshan Mountain uplift, the Hetao basin, and the Ordos uplift (Figure 3).

Figure  3.  Regional geological structure in Baotou. F₁: Daqing Mountain piedmont fault; F₂: Wula Mountain piedmont fault; F₃: Serteng Mountain piedmont fault

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The Yinshan Mountain uplift area is in the north of the region, which comprises a series of near-EW mountains, including Serteng Mountain, Wula Mountain, and Daqing Mountain. Controlled by the active faults in front of it, Yinshan Mountain has been uplifted to form a tilted fault-block mountain with south-north tilts. Since the Cenozoic, the piedmont Hetao Basin has been subsiding continuously and forming the broad Hetao Plain, and the geomorphic boundary between the mountain and the plain is obvious.

Hetao Basin is a compound basin that can be divided into three depressions in the left and right steps. These three depressions are controlled by the piedmont fault zones of ① Lang Mountain and Serteng Mountain, ② Wula Mountain, and ③ Daqing Mountain and are basically dustpan-like fault depressions that are deep in the north and shallow in the south. The Hetao fault depression zone is an active seismic zone with large magnitude and high frequency.

The Ordos uplift is a relatively complete uplift area with weak internal deformation. The block is surrounded by faults and fault basins. As a stable area, the Ordos block has been in a state of uplift since the neotectonic period. There has been no active structure or strong earthquake in the uplift area. It is a continental block with strong integrity and no structural activity differentiation.

We identified four potential seismic sources zone along the Daqing Mountain piedmont fault (namely Hohhot M7, Tumed Left Banner M7.5, Tumed Right Banner M8, and Baotou M7.5) two along the Wula Mountain piedmont fault (namely Baotou M7 and Urad Front Banner M7), and three along the Serteng Mountain piedmont fault (namely Hassatu M7, Dashetai M7, and Wujia River M7.5). The maximum probable earthquakes of the seismic potential sources zone in the other areas are M6.0 or M6.5.

We used the above research results to adjust the boundary between the potential seismic sources zone of the Tumed Right Banner and Baotou City (Figure 4). The potential seismic source zone of the Tumed Right Banner expanded to the west, including the epicenter of the AD 849 earthquake.

Figure  4.  Distribution map of potential source areas in Baotou Area

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Based on Holocene paleoearthquake events in Daqing Mountain (Gao et al, 2014), there may have been three paleoearthquakes in the Baotou section since the Holocene, with age values of 7.6 Ka BP, 5.5 Ka BP, and 1.2 Ka BP from old to new. The spatial distribution functions of the potential seismic sources zone of the Tumed Right Banner were increased accordingly, as given in Table 2. It can seen that the recurrence period of the original M8 earthquake before adjustment was 2450 a, which exceeded the shortest recurrence period of paleoearthquakes by 2100 a but became 1500 a after this adjustment. The recurrence period of the original M7.5 earthquake was adjusted from 1070 a to 830 a.

Table  2.  Annual incidence rate and spatial distribution function adjustment.

Seismic belt/Potential source		M7.0–7.4				M ≥ 7.5
		Spatial distribution function	Annual incidence rate	Recurrence cycle(year)		Spatial distribution function	Annual incidence rate	Recurrence cycle
Yinchuan-Hetao seismic belt		1	0.00891	112		-	0.00316	316
Tumed Right Banner	Before adjustment	0.10489	0.0009346	1070		0.1286	0.0004065	2450
	After adjustment	0.1349	0.001202	830		0.2086	0.000659	1500
Baotou		0.1714	0.00160	630		-

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7.   Conclusions

1) The range of continuous distribution of seismic surface rupture traces is about 80 km. It is the same as the length of seismic surface rupture zone in the Daqing Mountain piedmont fault zone. These identified Daqing Mountain piedmont fault is the seismogenic structure of the AD 849 earthquake.

2) According to the research and historical data of the Daqing Mountain piedmont fault zone, the epicenter of the AD 849 earthquake was in the area from the Baotou Aluminum Plant to Yongfu Village, with geographic coordinates of 40.6°N, 110.2°E and a magnitude of 7½. The suggestion is that this earthquake be named the AD 849 earthquake East of Baotou 7½ magnitude in Inner Mongolia.

3) According to historical data, the AD 849 earthquake East of Baotou damaged the houses of the military towns, killed nearly 10,000 people, with an epicenter intensity of X degree.

4) Using the place of epicenter of the AD 849 earthquake, the boundary of the Tumed Right Banner potential source zone was adjusted. Because the magnitude of AD 849 earthquake was increased from M7 to M7½, the recurrence period was decreased. Recurrence period of M8 were adjusted to 1500 a and M7.5 was adjusted to 830 a.

Acknowledgments

This work was supported by a subproject of the strategic leading science and technology project (A) of the Chinese Academy of Sciences (No.XDA20070302)”and we thank the reviewers and editors for their suggestions and opinions.