Earthquake Reaearch in China  2017, Vol. 31 Issue (1): 116-124
Analysis on Characteristics of the Hazards of the 2015 Pishan MS 6.5 Earthquake in Xinjiang
Liu Jun1, Liu Aiwen2, Sun Jianing1, Song Lijun1, Tan Ming1, Chang Xiangde1     
1 Earthquake Administration of Xinjiang Uygur Autonomous Region, Urumqi 830011, China;
2 Institute of Geophysics, China Earthquake Administration, Beijing 100081, China
Abstract: The disaster area of the Pishan MS6.5 earthquake in 2015 is located in the southern margin of the Tarim Basin, where the natural condition are harsh, and the economy is extremely backward. Moreover, because of a large number of residential housing with poor seismic performance in the disaster area, the damage and economic losses are serious. Since the most disaster area is located in the piedmont overflow, with poor site conditions such as shallow groundwater level and soil foundation, the magnifying effect of ground motion has a significant impact on the damage. In conclusion, we believe that investment in anti-earthquake housing projects should be increased in post disaster reconstruction. Furthermore, for the north of the disaster area, with the dense population, poor conditions like soft soil foundation and poor engineering geological conditions, we recommend that in the future construction of anti-earthquake housing projects, more attention should be paid to strengthen the foundation treatment and precaution measures.
Key words: Pishan MS6.5 earthquake     Earthquake disaster     Emergency expedition     Anti-earthquake housing project    

INTRODUCTION

At 09:07, July 3, 2015 (Beijing time), a MS6.5 earthquake occurred in Pishan County, Hotan Prefecture, Xinjiang Uygur Autonomous Region (37.6°N, 78.2°E), with a focal depth of 10km. The epicenter was only 7km from Pishan County and 75km from Yecheng, Kashi. The earthquake was strongly felt in the Hotan, Kashi and Aksu regions.

The main contents of disaster investigation and evaluation for this earthquake include housing damage, human casualties and damage to lifeline projects (Sun Jingjiang et al., 2008, 2011). By analyzing and processing data from field investigations, an earthquake intensity distribution map was plotted after several revisions. At 12:00 a.m., July 10, a review meeting on seismic disaster loss from the Pishan MS6.5 earthquake was held by the Earthquake Administration of Xinjiang Uygur Autonomous Region. According to the assessment, the affected region mainly covers Pishan County, Moyu County, the 14th Regiment Division and 3rd Regiment Division in the Hotan region and Yecheng County of the Kashi Prefecture, with a total area of about 14, 580km2, a population of 653, 608 and 168, 523 households. Altogether 155, 664 people from 38, 916 households lost their homes due to destruction or serious damage of their houses. The earthquake caused a direct economic loss of 5.43 billion yuan (RMB), and funds needed for reconstruction of earthquake-stricken area amounted to 6.71 billion yuan (RMB).

1 SEISMIC TECTONIC BACKGROUND AND BASIC SITUATION 1.1 Seismogenic Structure

The earthquake zone is located in the transition zone between West Kunlun Mountains uplifts and the Tarim Basin. With neotectonic movement, the West Kunlun Mountains keep rising, while the Tarim Basin subsides continuously, and multiple active faults have developed in the piedmont of West Kunlun Mountains (Earthquake Administration of Xinjiang Uygur Autonomous Region, 2015). The Poskam buried fault and southeast Pishan buried fault have developed in the border zone of Tarim Basin, and the microscopic epicenter for this earthquake lies between the two buried faults (Fig. 1).

Fig. 1 Seismo-tectonic background of the Pishan earthquake

The Poskam fault, approximately 210km in length, stretching from Shache to the west, passing through Poskam to Duwa, is a relatively large-scale buried fault in the southwest of Tarim Basin, which is in the NW-NWW direction, SW-dipping, with a steep fault surface. During the disaster investigation, at about 10km in the southwest of the microscopic epicenter, a series of NWW-NW-striking linear images were developed, about 20km in length and 2km-4km in width. The Pixina fault was developed consisting of more than 80 faults, with the single fault being 0.5km-3km in length and spacing 0.2km-1km. Research shows that Pixina fault has dislocated terrace Ⅱ (altitude 15m-20m) and terrace Ⅲ (altitude 60m-80m) of both banks of the Pixina River, and a branch in the north of the Pixina fault belt has formed a 0.5m-high fault scarp on terrace Ⅱ, which is high in the south and low in the north, striking 300°. The fault scarp height shows a tendency of gradually lowering from south to north.

Combined with focal mechanism solutions for this earthquake, seismogenic structure is dominated by NWW-striking thrust movement, with a small amount of right-lateral strike-slip component, which is basically consistent with the strike of Pixina fault. Combining the analysis of active fault age and performance of fault surface activity, we believe that this earthquake has a close relationship with Pixina fault belt.

1.2 The Site Conditions in the Earthquake Area

The earthquake zone is located in the plain area in northern piedmont of the Kunlun Mountains, where geography is high in the south and low in the north, with flat terrain, dipping to the north with a slope of 1°-3°. In the plain area, rivers spread like a fan, no gully has developed, and sedimentary particles gradually grow finer from south to north. Near the piedmont are mainly gravel plains, and the underground water level also rises gradually, forming oases. In the north are wind-drift plains, most of which belong to movable sand dunes (Liu Jun et al., 2014). By collecting surrounding materials, we believe site classification in this geomorphic unit can be categorized to class Ⅱ-Ⅲ, as shown in Fig. 2. People affected by the disaster mainly belong to this area. The earthquake zone is located mostly in the piedmont overflow area, and site conditions such as shallow underground water level and weak soil foundation amplify ground movements (Field et al., 2000), thus the sphere of seismic effect is relatively large.

Fig. 2 The site condition map in the earthquake area
2 ANALYSIS OF CHARACTERISTICS OF EARTHQUAKE DISASTERS 2.1 Analysis of Characteristics of Earthquake Disasters

Building structures involved in this earthquake include civil structures, brick-wood structures, brick-concrete structures and frame structures, and structural areas of various types of housing structures in the earthquake area are shown in Table 1 (Earthquake Administration of Xinjiang Uygur Autonomous Region, 2015). Those that collapsed were mainly civil structure houses and several old brick-wood-structure houses. Civil structure houses were all destroyed, 60% of which collapsed in the Ⅷ-degree area. Collapsed brick-wood-structure houses were all old houses, 15% of which collapsed in the Ⅷ-degree area. Old brick-concrete-structure multistoried buildings in counties were destroyed in a large area, 15% of which have reached the point of median or greater damage. Several structure units of frame-structure houses were destroyed, most of which are slightly damaged and large areas of shearing cracks appeared on infilled walls. In the disaster area, civil-structure houses were widely distributed, and load-bearing structures include adobe masonry walls and wooden framed structures (Tang Lihua et al., 2003). In the Ⅷ-and Ⅶ-degree areas, almost all civil-structure houses are destroyed and collapsed over a large area. For most wooden framed structure houses, one side or two sides of walls and roofs collapsed, and for adobe wall houses, gable walls in front and back all collapsed, with no remaining complete walls.

Table 1 List of various types of housing structures of the earthquake impacted region

Brick-wood structure houses accounted for about 40% of the total area of houses in villages and towns, most of which were anti-earthquake houses (Zhang Yong, 2005), and some wetre old brick-wood structure houses. Collapsed houses in the disaster area were all old brick-wood-structure houses, and anti-earthquake houses are slightly damaged. Except for the collapsed old brick-wood-structure houses, another 10% of brick-wood-structure houses were severely damaged. Medium damage to houses mainly included oblique or vertical cracking of walls, from top to bottom. In the Ⅷ-degree area, about 30% brick-wood-structure houses weare basically well preserved, all of which were anti-earthquake houses.

Brick-concrete-structure buildings were mainly concentrated in counties and township government public buildings, accounting for 50% of the total area of houses in counties. Frame-structure buildings accounted for only a small proportion, mainly schools, hospitals and public houses newly built after 2010, taking up about 20% of public houses, and no large structural damage areas appeared. However, infilled walls of 30% frame-structure houses in counties show shearing cracks, and in the meantime, a large amount of indoor facilities and equipment fell and collapsed, causing great disaster losses.

Altogether 222 investigation sites were set up after the earthquake, and by the method of wide-range uniform sampling, 131 sample points are selected to investigate damage extent of houses. Sampling sites were basically evenly distributed in disaster areas. Ratios of housing damage in the earthquake area are shown in Table 2.

Table 2 Summary of damaged houses with various structure types
2.2 Earthquake Induced Geological Disaster

Two ground fissures can be seen in orchards in Pixina and Kumuqiake village, about 8km from the Pixina fault belt, which were shearing type, showing right-lateral features, of which the ground fissure surface of the northwest branch is nearly vertical (Fig. 3(a)), striking 300°, about 20m in length and 30cm in width, and a 7cm right lateral oblique displacement is observed (Fig. 3(b)); The ground fissure of the southeast branch strikes 310° on the whole, relatively straight (Fig. 3(c)), about 18m in length and 2cm-10cm in width, with a right lateral 2cm ground fissure observed (Fig. 3(d)).

Fig. 3 Geological hazards caused by the earthquake

In Team No.5 of Pixina village, multiple ground cracks appeared on footpaths between fields, with varying widths. Most cracks were about 1cm-2cm in width, the widest reaching about 10cm. Extended length can be up to 50m-100m, on and off. Ground cracks show strong directional features, striking within a range of 90°-110°. In nearby farmland, there were several earthquake water spraying and sand emitting remains, one of which was about 1.5m in diameter, with a central sandblasting mouth of about 30cm in diameter. About 2.5km to north of Yapuquan in Guma Town, several earthquake fissures and water spraying and sand emitting mouths appeared. Ground fissures were 60m-120m long, 10cm-20cm wide, striking 285°.

The phenomenon of water spraying and sand emitting is found in the north of Yapuquan in Guma town, forming numerous sand emitting mouths, and collapse of the loess layer occurs beside the pond. In Oytorak village of Bashenlangan in Pishan County, earthquake-induced collapse occured along vertical terrace on shore, and a large number of gravelly clay fell to the foot of slope.

2.3 The Damage of Infrastructures and Lifeline Engineering 2.3.1 The Damage to the Transportation System

The earthquake caused a certain degree of damage to part of the transport facilities in Pishan County and Pishan farm, including medium damage to the railway station in Pishan County, and dsubgrade settlement of parts of railway sections and fissures on bridges and culverts. 99 bridges and culverts are damaged, and earthquake-induced landslides caused road damage 279km long. The earthquake caused 5km of road damage in the 14th Regiment Division, 23 landslides along the mountain road and damage to 15 bridges.

2.3.2 The Damage of Municipal Facilities

The earthquake causes a certain degree of damage in municipal facilities in Pishan County and urban area of the 14th Regiment Division, including different degrees of damage to the urban pipe network (water supply, drainage, heating, natural gas and pipeline), which affects normal life for residents in the disaster area, while 14071m long public walls are severely damaged.

The earthquake caused infrastructure damage to the 14th Regiment Division. The 13km water supply network and 7.2km drainage network in the new town and 9.7km drainage network in the old town were damaged.

2.4 The Damage of Water Resource System

The earthquake caused a certain degree of damage to some of the water conservancy facilities in Pishan County and Pishan farm, including the reservoir dam foundation, water diversion culverts, water towers, water pipes and 111 pump wells in the 14th Regiment Division, 100m of dam seepage and fissures on the waste gate of Yuejin reservoir and cracks along 27.3km of the water delivery canal.

2.5 The Damage in Agriculture, Animal Husbandry and Other Fields

The earthquake caused a certain degree of damage to agriculture, animal husbandry in Pishan County and Pishan farm of the 14thRegiment Division, mainly including cracks on greenhouses, damage to animal pens and livestock deaths. 9573 heads of livestock were killed, and 6612 livestock pens and 640 greenhouses in Pishan County were destroyed in the earthquake.

721 greenhouses and 3200 livestock pens in the 14th Regiment Division were damaged and 705 heads of livestock were killed in the earthquake. 200 greenhouses in Yecheng 2nd ranch of the 3rd Regiment Division outside the assessment area were destroyed. In addition, some houses in Yecheng 2nd ranch of the 3rd Regiment Division were damaged in the earthquake, 161 livestock pens in Yecheng were destroyed and 72 heads of livestock were killed.

In the meantime, the earthquake caused a certain degree of damage to the food system, including a different degree of deformation and cracks to warehouses, and roof collapses and wall cracks in the grain supply center offices. A total of 15 warehouses collapsed, covering an area of 8019m2, of which 7 were severely damaged, with an area of 2264m2.

3 EFFICIENCY ANALYSIS OF DISASTER REDUCTION OF ANTI-EARTHQUAKE HOUSING PROJECTS

In historical destructive earthquakes, anti-earthquake housing projects and anti-earthquake houses have played an important role in protecting life and property safety in the disaster area (Tan Ming et al., 2014). Collapsed houses were all abode houses in disaster area and anti-earthquake houses built in 2005 were mostly wood-framed structures, with a higher degree of coverage. After several earthquakes with MS≥5.0, 35% wood-framed houses in the Ⅶ-degree area suffered from only grass-mud loss, and structures were not damaged and still able to be used after basic repairs. Anti-earthquake houses built according to high specifications in the Xinjiang rural areas after 2011 were not damaged, showing good seismic performance.

After the earthquake, by sample investigation in disaster area, the proportion of anti-earthquake houses and civil-structure, brick-wood structure and brick-concrete structure houses reconstructed according to anti-earthquake housing project are calculated. According to data on civil-structure, brick-wood and brick-concrete-structure houses before anti-earthquake housing reconstruction in the disaster area, combined with the earthquake damage matrix for anti-earthquake houses in Xinjiang, the effectiveness of disaster reduction for the Pishan MS6.5 earthquake is calculated and analyzed. In the assessment of disaster loss in this earthquake, the area of anti-earthquake houses is replaced with the area of simple houses before reconstruction to calculate the effectiveness of disaster reduction from anti-earthquake houses. Specific data is shown in Table 3.

Table 3 Comparison of damage and economic loss with/without anti-seismic countermeasure
4 CONCLUSIONS

(1) The epicenter of this earthquake was located 7km west of the Pishan County, with focal depth of only 10km. The whole county is located within the range of Ⅷ-degree of earthquake intensity; therefore this earthquake is close to the type of earthquake occurring directly beneath cities. All types of buildings in Pishan County, with a high concentration of buildings and houses, were extensively damaged, and houses without anti-earthquake measures were all destroyed. Although most of the old brick-concrete-structure houses in town did not collapse, the interiors were severely damaged and difficult to repair. This earthquake caused the most serious disaster in the Xinjiang region in the last decade.

(2) The epicenter intensity is Ⅷ degrees (0.2g), already breaking the local basic seismic fortification level (Ⅶ degrees, 0.10g), which is one of the factors causing casualties in the area. However, the coverage of anti-earthquake houses reached 40%, which is an important reason for the relatively fewer casualties and injuries in the disaster area.

(3) The disaster area is located in the southern margin of the Tarim basin, with poor natural conditions and being economically backward. Except for houses built by the anti-earthquake housing project, there were a large number of houses with poor seismic performance in villages in the disaster area, in particular some old civil-structure and brick-wood-structure houses built in the 1990s, which is an important reason for the severe damage caused by this earthquake.

(4) Site formation lithology in most parts of the disaster area was silty soil, and site soil was a type of medium soft soil, with relatively shallow underground water level, which amplifies ground motions. This is also one of the causes for the aggravated earthquake damage.

This paper has been published in Chinese in the journal of Technology for Earthquake Disaster Prevention, Volume 11, Number 3, 2016.

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2015年7月3日新疆皮山MS 6.5级地震震害特征分析
刘军1, 刘爱文2, 孙甲宁1, 宋立军1, 谭明1, 常想德1     
1 新疆维吾尔自治区地震局, 乌鲁木齐 830011;
2 中国地震局地球物理研究所, 北京 100081
摘要:2015年7月3日新疆皮山MS6.5级地震灾区地处塔里木盆地南缘,该地自然条件恶劣,经济极为落后,灾区农村的民房除了抗震安居工程建设的房屋外,还有大量抗震性能较差的民居,在地震中受损严重,经济损失较大。灾区大部分位于山前溢出带,地下水位浅、地基土层软弱等场地条件对地震动有放大作用,震害影响范围较大。灾后重建中应继续加大安居富民工程的经费投入力度。灾区北部人口密集,地基土层软弱,工程地质条件差,建议在今后的安居富民建设中加强地基处理。
关键词皮山MS6.5级地震    震害    应急科考    安居富民工程