Earthquake Reaearch in China  2019, Vol. 33 Issue (1): 74-85     DOI: 10.19743/j.cnki.0891-4176.201901011
Intensity Distribution of the Mojiang MS5.9 Earthquake in Yunnan in 2018
LU Yongkun, ZHOU Yang, DAI Boyang, YANG Jianqiang, YAN Hang
Yunnan Earthquake Agency, Kunming 650203, China
Abstract: The distribution of the intensity of the Mojiang MS5.9 earthquake in Yunnan Province is expounded, and the damage characteristics of buildings and the damage ratio and seismic damage index of various building structures in each intensity area are compared with those of The Chinese Seismic Intensity Scale. The main basis and method of seismic intensity assessment are discussed in this paper. It is concluded that: ① The seismic intensity should be based on the earthquake damage of the housing structure, which takes up a high ratio in the seismic intensity assessment. It is recommended that seismic intensity is estimated by calculating the average seismic damage index. ② The highest intensity of the Mojiang MS5.9 earthquake is Ⅷ degrees, with the long axis trending in the north-west direction. The area above Ⅶ degrees is 5, 180km2. ③ The intensity distribution of the Mojiang MS5.9 earthquake meets the national standard and the distribution law of seismic intensity in Yunnan.
Key words: Mojiang MS5.9 earthquake     Seismic intensity     Damage ratio     Seismic damage index

INTRODUCTION

Earthquake intensity assessment is an important part of earthquake emergency response. The earthquake department is the main body to carry out seismic intensity survey and assessment. After a devastating earthquake occurs, the earthquake intensity assessment should be completed in time, which can be used to provide scientific basis for life rescue, emergency rescue, disaster resettlement, secondary disaster hazard avoidance and recovery and reconstruction. At 10:31 on September 8, 2018, an MS5.9 earthquake occurred in Mojiang County, Yunnan Province (23.28°N, 101.53°E). 28 people were injured in the earthquake, and the building and engineering facilities in the disaster area were damaged to varying degrees. After the earthquake, the Ministry of Emergency Management, the People's Repbublic of China and the China Earthquake Administration launched the Level Ⅳ response to earthquake relief, and the Yunnan Earthquake Agency initiated the Earthquake Emergency Level Ⅲ response. A total of 71 on-site staff members from the earthquake departments of the state, province, city, and county rushed to the disaster area to carry out earthquake emergency site disposal work 2.

2 Yunnan Earthquake Agency, 2018. The Disaster Direct Economic Loss Assessment Report on the Mojiang MS5.9 Earthquake, Yunnan.

In this paper, the distribution of the intensity of the Mojiang MS5.9 earthquake is expounded in detail. The seismic damage characteristics, average seismic damage indexes and damage ratios of each intensity area are compared and analyzed. The qualitative and quantitative methods of intensity evaluation are discussed.

1 EARTHQUAKE INTENSITY AND DISTRIBUTION

The Mojiang MS5.9 Earthquake Disaster Assessment Team surveyed the damage of 207 residential buildings in the disaster area and the damage of 66 infrastructures. The geologic structure background, aftershock sequence distribution and focal mechanism solutions were comprehensively considered. The seismic intensity distribution was determined. On September 12, 2018, the Yunnan Earthquake Agency released "The Mojiang MS5.9 Earthquake Intensity Distribution Map".

The intensity of the earthquake in the earthquake-stricken area reached Ⅷ degrees. The macro-seismic epicenter was located in the area of Bingbeng and Niuku Villages in Tongguan Town, Mojiang County. The isosceles line was elliptical and the long axis direction was northwestward (Fig. 1). The total area of the Ⅵ-degree area and above is about 5, 180km2, of which the total area of the Ⅷ-degree area is 32km2, involving Tongguan Town, Yayi Town and Lianzhu Town of Mojiang County. The total area of the Ⅶ-degree area is 438km2, involving Tongguan Town, Yutang Town, Yayi Town, Lianzhu Town and Longtan Town of Mojiang County. The total area of Ⅵ-degree area is 4, 710km2, involving 24 townships in Mojiang County, Ning'er County, Jiangcheng County and Yuanjiang County.

 Fig. 1 Intensity distribution map of the Mojiang MS5.9 earthquake, Yunnan
2 BUILDING DAMAGE CHARACTERISTICS 2.1 Earthquake Damage in Various Intensity Areas

After the disaster assessment team first learned about the basic situation of earthquake damage in the earthquake-stricken areas, the seismic intensity assessment standard was unified on the evening of September 8, 2018.

There are four types of housing construction in the disaster area: brick structures, brick-concrete structures, brick-wood structures and civil structures. The civil and brick-wood structures in rural areas account for more than 80%. Therefore, in this earthquake intensity assessment, the earthquake damage of brick-concrete structures forms the major basis for the macro-judgment in the seismic intensity assessment in rural areas. The earthquake damage of brick-concrete structures is used as the macro-judgment basis in Mojiang County. Most of the civil engineering structures in the disaster area are of the wooden weight-bearing frame and adobe wall, which is the common rural dwelling in Yunnan Province. The seismic performance is better than that of the wall-lifting structures in the northeastern part of the province. There are also a small number of hollow brick wall-lifting structures, which are classified as brick-wood structures, and such houses have poor seismic performance.

2.1.1 The Ⅷ-degree Zone

A few walls of brick-concrete structure houses have obvious cracks. Brick-wood structure houses generally drop tiles and open skylights, most of the walls of the houses are severely cracked (Fig. 2), and a few houses partially collapsed. Civil structure houses generally shuttle off the tiles, open skylights, and a few houses have wall collapses or partial collapses, wall cracks or flashes.

 Fig. 2 Cracks appear in the walls of most brick-wood structures in Jiuchong-Heping Groups in Tongguan Town

 Fig. 3 The collapse of a civil structure in Jiuchong-Heping Groups in Tongguan Town
2.1.2 The Ⅶ-degree Area

A few infill walls of the frame structure have obvious cracks, and some beams and columns are slightly cracked. A few load-bearing walls of the brick-concrete structure houses are cracked. Most of the brick-wood structure houses have corners, joints and other parts cracked, shuttled, and some old houses have partial collapses. Most of the walls of civil structure houses are cracked and shuttled, and a few walls partially collapsed.

 Fig. 4 Through cracks in the frame house walls in Heping Village of Yutang Town

 Fig. 5 Cracking walls of brick-concrete houses in Lao'ao Group of Mangga Village, Lianzhu Town

 Fig. 6 Partial collapse of the brick-wood house wall in Tuoluo Village, Yayi Town

 Fig. 7 Partial collapse of the wall of the beam house in Liangzizhai, Yutang Town
2.1.3 The Ⅵ-degree Area

Some walls and frame joints of the frame structure are cracked, and some fill-in walls are sheared and cracked. A small number walls of the brick-concrete structures have fine cracks, and some load-bearing walls are cracked. A few walls of brick-wood structure houses are cracked and with tile dropping. Some of the civil structure houses collapse partially, and some of the walls are cracked and with smashed tiles.

 Fig. 8 The destruction of the frame structure of the Welfare Center, Mojiang County

 Fig. 9 The cracking of the load-bearing walls of the brick and concrete clinic building of the Hospital of Traditional Chinese Medicine, Mojiang County
2.2 Average Seismic Damage Index and Damage Ratio

The average earthquake damage index and the building damage ratio are quantitative indicators for determining the intensity, and are also an important basis for the quantitative study of the earthquake damage of a building in a certain intensity zone or at a survey point (Lu Yongkun et al., 2014; Dai Boyang et al., 2017; Ming Xiaona et al., 2018).

2.2.1 Summary of Damaged Area of Houses at the Sampling Points and Earthquake Damage Index

The average earthquake damage index of a survey point can be calculated by obtaining the construction area of each damage level of various buildings at the survey site through on-site investigation. According to The Chinese Seismic Intensity Scale (2008), the average earthquake damage index of a certain type of structure is the sum of the product of the earthquake-damaged housing at each level and its corresponding seismic damage index:

 $D = \sum {{d_i} \times {\lambda _i}}$ (1)

where D is the average earthquake damage index of a certain type of building construction at a survey point (or area); di is the earthquake damage index of the type of housing damage level i(simple houses are basically intact, damaged or destroyed). The non-simple houses were basically intact, slightly damaged, moderately damaged, severely damaged or destroyed. λi is the proportion (%) of the floor area (or number of buildings) with the damage level i of this type of structure.

The damage indices of the damage levels of simple houses and non-simple houses are taken according to Table 1 and Table 2 respectively (China Seismological Bureau, 1998; Zhou Guangquan, 2011).

Table 1 Earthquake damage index corresponding to each damage level of simple houses (di)

Table 2 Earthquake damage index corresponding to each damage level of non-simple houses (di)

Table 3 to Table 6 give a summary of the damage areas and seismic damage indices of various types of houses (sampling points for participating in the assessment of calculated losses) obtained from the site survey of this earthquake.

Table 3 Summary of damage area (m2) and seismic damage index of civil-structured houses

Table 4 Summary of damage area (m2) and seismic damage index of brick-wood structure houses

Table 5 Summary of damage area and seismic damage index of brick-concrete structure houses

Table 6 Summary of damage area and seismic damage index of frame structure buildings
2.2.2 Damage Ratio of Each Intensity Zone

Based on the damage data of the houses in Table 3 to Table 6, the damage ratio of the buildings in each intensity zone and the earthquake damage index of different building structures in each intensity zone can be calculated, which are listed in Table 7 and Table 8.

Table 7 Summary table of simple house damage ratio and earthquake damage index

Table 8 Summary of non-simple house damage ratio and earthquake damage index

According to The Chinese Seismic Intensity Scale (2008), the corresponding relationship between seismic intensity and building damage index is given (Table 9). In Table 9, Class A houses are old-fashioned houses constructed of timber frames and earth, stone and brick walls. Class B is single- or multi-storey brick-walled houses without seismic fortification. Class C is single-layered for seismic fortification according to Ⅶ degrees or multi-storey brickwork houses. Combined with the site survey of this earthquake, the civil structure and brick-wood structure are Class A buildings, and some brick-concrete structures are Class B buildings. All frame structures and partial brick-concrete structures are Class C buildings.

Table 9 Correspondence between seismic intensity and earthquake damage index

The seismic damage index of civil and brick-wood structures in each intensity zone in Table 7 corresponds to the seismic damage index (Table 9) given in The Chinese Sesmic Intensity Scale(2008); the brick-concrete structure of the Ⅷ-degree and Ⅶ-degree zones in Table 8. The damage index is inferior to that in Table 9, and the Ⅵ-degree zone corresponds well, mainly because the frame structure and the brick-concrete structure are distributed in small numbers in rural areas, and the seismic performance is good and the damage is light. It also shows that the intensity assessment is carried out in rural areas and urban areas with different architectural structures. It is reasonable to use the seismic damage of the structure with the largest proportion of the area as the basis for assessing the intensity.

3 DISCUSSION

The Chinese Seismic Intensity Scale (2008) gives four categories of evidence for assessing seismic intensity: human perception, house damage, geological disasters and other seismic hazards and strong vibration records. When intensity reaches Ⅴ to Ⅸ degrees, housing damage is widely accepted as the main basis for assessing the intensity. However, at the same survey point, the seismic damage of different types of buildings is often inconsistent, that is, the seismic intensity assessed by different structural types at the same survey point is not necessarily the same. There is a view that the high-intensity area of Ⅷ-degree and above should be based on the damage of modern buildings such as brick-concrete structures and frame structures. It is even considered that there must be structural damage of brick-concrete and frame structures when the seismic intensity reaches Ⅷ-degree. We believe:

(1) When assessing the intensity, the earthquake damage of the type of housing with a relatively high percentage in the disaster area should be used as the main basis, even in high-intensity areas. For example, most rural areas in Yunnan are still dominated by civil structures and brick-wood structures. Most villages have only a few brick-concrete structures and frame structures. In many villages, there are no such structures. In this case, earthquake damage of civil and brick-wood structures should be used as the main basis for assessing the seismic intensity of this area. The damage of the civil structure and the brick-wood structure can better reflect the damage degree of the survey point. This can also better solve the problem that the average seismic damage index of several building structure types at the same survey point does not match the intensity value.

(2) The average earthquake damage degree (index) should be used as the main basis for assessing seismic intensity. The 1978 edition of The Chinese Seismic Intensity Scale began to introduce the concept of the seismic damage index. In addition to establishing a link between seismology and engineering, it also provided researchers with a method for quantitatively assessing seismic intensity and also solved the need to decide between the average damage degree and the degree of extreme earthquake damage in seismic intensity assessments. The average seismic damage index represents the average damage degree of the survey point, while the structural damage of the brickwork structure and frame structure in the aforementioned eight degrees represents the degree of extreme damage at the survey point. As shown in Tables 3 and 4, at the Ⅷ-degree survey points there are not many civil and brick-wood structures collapsed or destroyed, but most of the walls are cracked, partially collapsed, or tile dropping. The seismic damage index reflects that the survey point has reached Ⅷ-degree destruction. In Table 5, the survey of Group Ⅱ of the county town shows that there is a certain number of damaged and seriously damaged brick-concrete structures, but it can't be judged that the earthquake intensity of the County Town reaches the Ⅷ-degree.

(3) The degree of earthquake damage of a house is a method of qualitatively assessing the intensity. It relies too much on the experience of the operator. For most intensity assessors, it is recommended to quantitatively determine the seismic intensity by calculating the seismic damage index of the survey point.

4 CONCLUSION

By expounding the distribution of the intensity of the Mojiang MS5.9 earthquake in Yunnan, the seismic damage characteristics and damage ratio of houses in different intensity areas and the seismic damage index of the sample survey points are compared and analyzed. The following conclusions are drawn:

(1) Seismic intensity assessment should be based on the seismic damage of the housing structure types with relatively high percentage in the disaster area. It is recommended that seismic intensity is quantitatively assessed by calculating the average seismic damage index.

(2) The highest intensity and long axis distribution direction of the earthquake-stricken area is the primary factor that needs to be determined when the seismic intensity circle is scheduled. The intensity circle is mainly based on the earthquake damage data of the buildings obtained from the on-site investigation. At the same time, it refers to the seismic damage of the engineering structure, the seismic geological background, the strong vibration record, the aftershock distribution, and the focal mechanism solution. The highest intensity of this earthquake is mainly determined by the damage of houses. The long-axis northwest trend and the earthquake damage distribution, seismic geological background, aftershock distribution direction, strong earthquake focal mechanism solution and other scientific support results show good consistency. The corresponding relationship between the seismic damage index and the intensity of each intensity zone and survey site is good, indicating that the intensity of the Yunnan Mojiang MS5.9 earthquake is rigorous and well-founded, in line with the national standard of The Chinese Seiomic Intensity Scale (2008).

(3) According to the general law of seismic intensity distribution in Yunnan (Zhou Guangquan et al., 2006; State Seismological Bureau, National Bureau of Statistics, 1996; Department of Earthquake Monitoring and Prediction, China Seismological Bureau, 2001; Mao Yuping, Han Xinmin, 2003; Qiao Sen et al., 2000; Su Youjin et al., 2001; Zhou Guangquan et al., 2011), the intensity of the M5.8-6.2 earthquake in the extreme earthquake zone can reach the Ⅷ degree, and the area above the Ⅵ-degree zone is between 900 and 5, 500km2. The area of the earthquake-stricken area is slightly larger, possibly related to the terrain of the area which might amplify the disaster.

(4) Comparing the structural damage ratio of various buildings in the Ludian MS6.5 earthquake (Lu Yongkun et al., 2014), it can be seen that the damage of houses corresponding to the intensity areas of the Mojiang MS5.9 earthquake is significantly lighter than that of the Ludian earthquake area, and it proves additionally that the seismic performance of building constructions in southern and southwestern Yunnan is generally stronger than that in northeastern Yunnan. Judging from the earthquake damage of the buildings in the field investigation, frame structures of the earthquake-stricken area has the best seismic performance, followed by brick-concrete structures. Traditional brick-wood structures and civil structures have relatively poor seismic performance, while hollow brick-walled beam structures had the weakest seismic performance.

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