An MS6.2 earthquake occurred in Hutubi County, Xinjiang Uygur Autonomous Region at 13:15:03 p.m. on December 8, 2016, with geographical coordinate of 43.83°N, 86.35°E. The epicenter2 was located in Que'ergou Town, Hutubi County, at a depth of 6km, where seismic intensity in the meizoseismal area is Ⅷ degree. Based on the data of the surrounding geological structure, the Qingshuihezi (south margin of Junggar Banner) fault is determined to be the seismogenic fault for this earthquake. Up to 10:00 a.m.on December 12, 2016, altogether 1, 329 aftershocks were recorded after the Hutubi MS6.2 earthquake, including 3 MS4.0-4.9 aftershocks and 10 MS3.0-3.9 aftershocks, the largest with a magnitude of 4.0, distributed mainly on the north side of the southern margin of the Junggar Basin in the nearly east-westward direction, with focal depths in the range of 4km-12km. By means of the double-difference earthquake location method, we obtain the length of the earthquake source region to be about 14km and width 8km, and the characteristics of aftershocks basically are consistent with the fracture mode of the Qingshuihezi fault3. The microscopic epicenter of this earthquake was located in the southern mountainous area of Que'ergou, 54km from Manas County, 60km from Hutubi County and 103km from Urumqi, and the earthquake was strongly felt in the North Tianshan Mountains. The earthquake caused injury to three people, and resulted in varying degrees of damage to a large number of civil structure houses in the earthquake zone, Altogether 38, 580 people were rendered homeless. An earthquake field work team from the China Earthquake Administration, Earthquake Administration of Xinjiang Uygur Autonomous Region and Earthquake Administration of Changji Prefecture, through investigation and analysis of earthquake damage at 182 survey points of 25 townships (towns and sites), drew out the seismic intensity map with the long axis of isoseismal lines distributed in the NWW direction, and determined the disaster area to be 12, 450m2. In this article, by referring to previous statistical data for earthquake damage (Chang Shuai et al., 2013; 2014; Yao Yuan et al., 2016), under the guidance of the standard "Post-Earthquake Site Works-Parts 4: Assessment of Direct Loss", we summarize the seismo-tectonic background and geomorphic features of the earthquake zone. The investigation of earthquake damage provides an important basis for earthquake relief work and scientific reconstruction.1 SEISMO-TECTONIC BACKGROUND AND GEOMORPHIC FEATURES IN THE EARTHQUAKE ZONE 1.1 Seismo-tectonic Background
The earthquake zone is located at the intersection of the North Tianshan Mountains and the Junggar Basin, with complex geological structures and intense tectonic activities. Earthquakes are mainly concentrated in the mountainous and piedmont area of the North Tianshan Mountains, with high intensity and frequency. Nearly EW, NW-NWW and NEE-striking active faults developed in the earthquake zone and its surrounding areas, most of which strike in the nearly EW and NW-NWW directions. They generally have a longer development history and larger scale, and fracture surfaces are mostly inclined to the south. MS≥5.0 earthquakes have occurred many times along these faults, such as the southwest Hutubi MS5.0 earthquake on April 25, 1953, the southwest Hutubi MS5.0 earthquake on November 29, 1953 and the Manas MS5.7 earthquake on November 6, 1980.
Through years of exploration and research on seismic tectonics of the North Tianshan Mountains (Yang Xiaoping et al., 1998; Wang Chunyong et al., 2001), seismotectonic models in the earthquake zone are divided into two types: (1) The strike-slip fault-seismogenic model inside the Tianshan Mountains, mainly including the Boluokenu-Aqqikkuduk fault, the Lilian Habilis fault and the Bao'ertu fault; and (2) the piedmont reverse fault-fold model.
The piedmont reverse fault-fold seismic model, mainly distributed in the transitional zone of the basin and mountainous area, manifested as a reverse fault-fold tectonic zone in the Urumqi piedmont depression, is a complex nappe structure made up of fault ramp, fault flat and fault-propagation fold. Strong earthquakes which occurred at the root of the nappe structure, and the Qingshuihezi fault developed at the root of the reverse fault-fold belt in the North Tianshan Mountain foot (southern Junggar fault), which serves as the source region of strong earthquakes, occurred in the reverse fault-fold tectonic zone in the piedmont depression, for example, the Manas MS7.7 earthquake in 1906 occurred along this tectonic zone. The Hutubi MS6.2 earthquake also occurred along this tectonic zone, and according to its focal mechanism solutions and aftershock distribution characteristics, the seismogenic tectonic mode for its main shock also conforms to the seismotectonic model of the North Tianshan nappe tectonic belt.1.2 Geomorphic Features in the Earthquake Zone
The epicentral area is located in the middle and high mountainous area of the Illian Habilis Mountain, and to the north of the epicenter are hilly areas in the northern slope of North Tianshan Mountains and alluvial-proluvial plains of the Hutubi and Manas Rivers. In the middle and high mountainous areas, the overall mountain range spreads in NWW-SEE direction, exhibiting ridged and beam-like tops, plenty of ravines and gullies, steep side slopes on both sides of gullies and topographic slope greater than 30°, and some parts of the area, affected by river and wind erosion, are nearly vertical. The hilly area on the northern slope of the North Tianshan Mountains is a nearly EW-spreading long-striped uplift developing along its piedmont structure, at an altitude of 800m-1200m, and the uplift is cut by SN-stretching rivers, mainly the Manas, Ningjia, Taxi and Hutubi rivers, forming broad river valleys, wide and flat, where multi-level river terraces have developed. This is also a densely populated area. In the alluvial-proluvial plains of the Hutubi and Manas Rivers, old and new proluvial fans distribute dispersedly. The terrain gradually flattens out, and the overall landform is flat and wide. Lobate gullies are distributed in the upper part of the proluvial fans, and ground water is gradually exposed in the lower part. The formation lithology is gradually changed from coarse grained gravel and boulders to round gravel and fine particles and sandy soil. This region is densely populated, forming relatively large cities (Fig. 1).
This earthquake mainly caused housing damage, and destruction of public service facilities such as the education system and infrastructure such as the electric system. In the earthquake zone, some earthquake-induced geological disasters occurred, such as earthquake rock fall and collapse. In addition, seismic fortification intensity for houses in the earthquake zone is in the Ⅶ-Ⅷ degree, and is improved by 1 degree for some major projects. A brief analysis of housing destruction caused by the earthquake is presented below.2.1 Earthquake Damage Investigation of Houses
Through investigation and summary analysis, it is believed that the population distribution is affected by geomorphology in the earthquake zone. The population is small and scattered in the middle and high mountainous area in the south, and restricted by the cost of housing and materials, houses are mainly self-built mud-wood structures. In hilly areas and alluvial-proluvial plains to the north of the epicenter, houses of various structures increase gradually as the population increases, gradually transitioning from simple mud-wood structures to brick-concrete structuress, frame structure and urban and rural seismic safe houses with better seismic behavior. This earthquake caused the destruction of houses of various structures, including mud-wood, brick-wood, brick-concrete and frame structures, as well as urban and rural seismic safe houses. Photographs of house damage are shown in Fig. 2.
(1) Mud-wood Structure Houses
Mud-wood structure houses are mainly distributed in rural and township areas far from cities, occasionally in counties, which were most seriously damaged by the earthquake. According to construction materials and methods, mud-wood structure houses are composed of adobe houses and brick-mud structure houses (the interior of brick-mud houses is composed of adobe walls, while the outside is brick walls. The two have no effective connection). This earthquake caused the collapse of walls and roofs of some adobe houses in areas with Ⅶ-Ⅷ degree seismic fortification intensity, and most houses had vertical fractures of varying degrees in the junction of vertical and horizontal walls and inclined cracking at the corners and openings of doors and windows, while in the Ⅵ degree area, this earthquake caused only slight damage such as vertical fractures of varying degrees in the junction of vertical and horizontal walls. Although brick-mud structure houses did not collapse, there was no effective connection between the inside adobe walls and outside brick walls, which lead to the peeling of external brick walls and the serious cracking of the interior adobe walls of some houses, and some load-bearing walls tilted outward. In areas of Ⅶ-Ⅷ degree seismic fortification intensity, 4.2% of mud-wood structure houses were destroyed by the earthquake, 20.4% were badly damaged, 37.1% and 26.3% were moderately damaged and slightly damaged, while in areas of Ⅵ degree seismic fortification intensity, 2.6% of mud-wood structure houses were damaged, and 6.3% seriously damaged.
(2) Brick-wood Structure Houses
Brick-wood structure houses, more widely distributed in the earthquake zone, are mostly peasants' self-constructed residences, and due to various building ages and construction quality, houses were damaged by this earthquake to varying degrees, exhibiting mainly inclined cracks at the corner and opening of doors and windows, vertical cracks of walls and fractures in the junction of vertical and horizontal walls. Surveys show that in areas of Ⅶ-Ⅷ degree seismic fortification intensity, 3.5% of brick-wood structure houses were destroyed, 17.4% seriously damaged, and 32.6% and 31.4% moderately and slightly damaged respectively. In the Ⅵ degree area, 1.7% houses were destroyed and 5.1% seriously damaged.
(3) Brick-concrete Structure Houses
Brick-concrete structure means that the vertical load-bearing structure of a building is constructed with bricks or building blocks, while constructional columns and horizontal load-bearing beams, floor slabs and roof boarding adopt a reinforced concrete structure, with good seismic performance. Brick-concrete structure civil houses damaged in the earthquake zone are mainly distributed in the Ⅵ degree areas, and brick-concrete structure buildings of industrial and mining enterprises are mainly concentrated in the Ⅶ-Ⅷ degree areas. Earthquake damage survey shows that this earthquake caused less damage to brick-concrete structure houses, no serious and moderate damage, and there were no collapses. Only 4.9% of brick-concrete structure houses were slightly damaged, mainly distributed in the Ⅵ degree areas, showing widened inclined cracks at the corner and opening of doors and windows, cracks in the external insulating layer, peeling walls inside the house and corner cracks of the plastic/steel window frames. The destruction of brick-concrete structure buildings of industrial and mining enterprises is manifested as a large scale of slant and lateral-shearing penetrating cracks on single-layer walls of brick-concrete buildings (this type of damage is more concentrated in Ⅶ-Ⅷ degree areas), slant cracks at the corner and opening of doors and windows and horizontal (vertical) cracks on walls.
(4) Frame Structure Buildings
Frame structure buildings are concentrated in counties, cities and industrial and mining enterprises. The destruction of frame structure buildings in counties is manifested as cracks in the junction of in filled walls and beam columns, slant cracks on in filled walls, with slight damage; The destruction of frame structure buildings of industrial and mining enterprises inthe Ⅶ degree area shows X-shaped shearing cracks on in filled walls, shearing or slant cracks at the corners and openings of doors and windows, cracks in the junction of in-filled walls and beam columns, land subsidence, decoration works and equipment damage, which are all slight without main structural failure.
(5) Urban and Rural Seismic Safe Buildings
The earthquake-proof and Comfortable Housing Project was started in the Xinjiang area in 2004, which was later replaced by the Urban and Rural Seismic Safe Buildings Project in 2010. Up to now, the Urban and Rural Seismic Safe Buildings Project in 2010 has been implemented for more than 10 years, during which the project has achieved certain effectiveness, especially with performance in moderate-strong earthquakes. For example, in the Xinyuan-Hejing MS6.6 earthquake on June 30, 2012, the Yutian MS7.3 earthquake on February 12, 2014 and the Pishan MS6.5 earthquake on July 3, 2015, buildings all showed good seismic performance, which provides a security guarantee for life and reduces the economic loss of the nation. In this earthquake, the seismic safe buildings were mainly distributed in rural areas, which show good earthquake resistant capabilities, and no damage was caused.2.2 Destruction of Other Industrial Facilities
Lifeline systems are divided into transportation, water supply, gas supply, electricity and communication. According to the investigation of lifeline systems in the earthquake zone, it can be seen that the electric, transportation and hydrological systems were relatively seriously damaged. Also, the earthquake zone was located in the southern mountainous area, where grassland and coal mine enterprises are more densely distributed, so the livestock system and industrial and mining enterprises also suffered damage to varying degrees, which is also described here.
(1) Electric System
This earthquake caused severe damage to office buildings and equipment houses of the State Grid 35kV Xigou Substation in Que'ergou, Hutubi County. Office buildings in the Xigou Substation are brick-concrete structure type, only about 1km from the microscopic epicenter. Penetrating X-shaped fissures appeared along gable walls and the corners of windows, while door frames were badly deformed due to the earthquake (Fig. 3(a)).
(2) Transportation System
Research found that varying degrees of ground cracks and other damage was caused along the Huque highway, Huqi highway and S101 fork (K64) 106 coal mine-Qilihu(Z537) accommodation highway, 14 bridges and culverts in Manas County were damaged, and a 4km-long pastoral way in towns and villages in 3 pasturing areas were ruined (Fig. 3(b)-(d)).
(3) Hydrological System
Hydrological systems such as the Shimenzi reservoir in Taxihe in the earthquake zone are investigated, and no mass destruction was found in any reservoirs, and only individual reservoirs were abandoned due to subsidence and fracture of original water release culverts, which are estimated to be more severely fractured after the earthquake and in need of reinforcement. The earthquake also caused varying degrees of damage to some water conservancy facilities such as the diversion irrigation, and the consolidation grouting and curtain at the joint surface of both sides of the reservoir dam body of individual reservoirs and rock are damaged. In addition, the most typical damage caused by this earthquake is the chamber located at the Taxihe reservoir, the lower part of which is a concrete frame structure and the upper part brick-concrete structure type. The lower concrete frame remains intact, while on the upper brick-concrete walls, a 24cm thick, penetrating slant and horizontal fractures can be observed along gable wall on the northwest side and penetrating X-shaped fissures on the gable wall on the southeast side, and 2m-3m horizontal fissures can be seen on these two longitudinal walls. The damage is relatively serious (Fig. 3(e)).
(4) Livestock System
The earthquake zone is located in the middle and high mountainous area in the North Tianshan Mountains, and a small part extends into hilly areas. Grassland is widely distributed in this area, and livestock sheds are mostly constructed from old mud-wood structure houses with simple structures. This earthquake caused the collapse and destruction of some livestock sheds in areas of Manas, Shihezi and Hutubi (Fig. 3(f)).
(5) Industrial and Mining Enterprises
Coal mine enterprises are mostly concentrated in mountain valleys in middle and high mountainous areas, and office buildings, living quarters (including dormitories and dining halls) and production workshops are built in mining areas. This earthquake caused slight-medium damage to frame structure buildings and reinforced brick-concrete structure buildings of some industrial and mining enterprises. Frame structure buildings mainly show shearing and lateral cracks on infilled walls, and some small vertical cracks along load-bearing columns; and reinforced brick-concrete structure buildings show some small vertical cracks along load-bearing walls, and slant fissures along the corners of doors and windows. In addition to the destruction of buildings, some production facilities were also damaged. For example, the draught fan at Hongsangou coal mine went off orbit due to earthquake rockfalls, and an earthquake rock-fall knocked out a hole with a size of 0.5m×0.8m in the air shaft tunnel at Xiaoxigou coal mine. However, because of the small earthquake magnitude and small sizes of earthquake rockfalls, industrial and mining enterprises maintained normal production.2.3 Investigation of Earthquake-induced Geological Disasters
The earthquake epicenter is located in the middle and high mountainous areas, where plenty of ravines and gullies have developed, with steep side slopes on both sides of the gullies. According to field investigation, earthquake-induced geological disasters such as collapse and rolling stones can be found in many places in the earthquake zone, but the scales is small. Rolling stones are mainly concentrated in gullies in the middle and high mountainous areas, the largest of size 2m×2m×2m, which is mostly rubble left over in rills or ditches on the half way of slopes, or rock mass that is about to disintegrate under physical weathering, shaking, loosening and rolling, causing damage to buildings or industrial facilities. For example, after the earthquake, rolling stones were found at Baiyanggou coal mine, with a size about 1m×1m×1m. The rolling stones crashed through the buildings in living quarters, however, because the production was shut down at that time, no casualties were caused. In addition, rolling stones were also found after the earthquake at Hongsangou coal mine, which hit the draught fan and pushed it off the track by 0.5m (Fig. 4, Fig. 5).
Earthquake-induced collapses are mainly distributed along the S101 provincial road. From Liuhuanggou town in the east to Qingshuihe village in the west, collapses occurred within a total range of 100km, all in small scales, with the largest covering only up to a hundred square miles, which occurred near the east Miaopu S101 provincial road in Xigou of Sanpianqu, Que'ergou town (Fig. 6(a)). The collapse body is mainly composed of sandstone, and the rock mass at its location is rather steep, with natural slope of 40°-50°, partially upright. Strong physical weathering in this region caused the formation of wind-eroded caves on the side slope rock mass. After field investigation, it is believed that the collapse is the rock mass suspended on the half way of the slope, loosening, disintegrating and ultimately falling under seismic action. The maximum size of the collapsed rocks is 2m×1.8m×1.8m, which caused no vehicle damage or injuries. In the meantime, there were small collapses on the north and south sides of the collapse, pilling up at the foot of the slope. Additionally, a rock collapse body developed on the side of the highway near Tianfu coal mine, and the landform at the location of the collapse body was artificially modified due to road construction. Side slopes on both sides of the highway have a grade of 30°-40°, without retaining structure and supporting procedures, therefore, rocks that disintegrated due to road construction, which remain on side slops, fell from the parent body under seismic action, and collapsed rubble covered the road and piled up at the foot of the slope (Fig. 6(b)).
In addition to rock collapses in the earthquake zone, there were also some small-scale collapses of soil mass near the epicenter, which are mainly distributed along the road, such as the side slope collapse along the west Shimenzi S101 provincial road in Kangjia and collapse on the side of S101 provincial road near Qingshuihezi village (Fig. 6(c), (d)). Analysis suggests that these soil mass collapses share some common characteristics, that is, in the process of road construction, after cutting original side slopes, under physical erosion such as rainfall, the internal stress of soil mass presented local variations, and the soil body became relatively loose, and after the earthquake, the soil mass collapsed with the help of seismic action. However, because the relative height difference of soil slopes is not large, and seismic action is not strong, there were no large-scale collapses after the earthquake.3 CONCLUSIONS
By investigating and sorting out earthquake damages caused by this earthquake, we have obtained some knowledge, which are summarized as follows:
(1) The total earthquake effected area is 12, 450km2. And houses involved include mud-wood, brick-wood, brick-concrete, frame structures and urban and rural seismic safe buildings, of which, mud-wood structure and brick-wood structure houses are mainly distributed in rural areas, brick-concrete and frame structures mostly in towns, counties and cities. This earthquake caused damage mainly to mud-wood and brick-wood structure houses. The walls of mud-wood structure houses were mostly made of adobe, and the walls of a few houses were built with compacted cohesive soil. Some adobe walls were lined with clay brick walls on the outside, and some contained wood. Brick-wood structure houses were mostly peasant-constructed residences, and the walls are built of clay bricks, without ring beams and constructional columns, therefore, it is believed that the cause of more severe damage to the mud-wood and brick-wood structure houses is that houses of these two structural types took no earthquake-resistant constructional measures and do not possess good performance in earthquake resistance.
(2) Through the investigation of lifeline engineering such as the electrical system and hydrological system, it is concluded that after the earthquake, the damage to some important structures of reservoirs in the earthquake zone, such as the dam, diversion tunnel and power house, are not serious, and only some reservoir facilities were slightly damaged, which did not affect the normal use of the reservoir. Most of the equipment and buildings of the electrical power system remain intact, and only a few buildings have been damaged, which does not affect the normal use of equipment. The above situation shows that seismic design for lifeline engineering such as the electrical and hydrological systems is reasonable, and earthquake resistant measures are appropriate. However, because this earthquake occurred in winter, it is not possible to carry out an investigation on the concealed works of reservoirs and coal mines due to cold weather, therefore, further investigation on the damage of these facilities should be carried out.
(3) The earthquake occurred in the northern slope of the middle section of the North Tianshan Mountains, in the economic zone of the North Tianshan Mountains in Xinjiang. Unlike other earthquake zones in Xinjiang, more factories and mining enterprises are distributed in this region, where there are working and living quarters composed of reinforced brick-concrete and frame structure construction. After this earthquake, varying degrees of damage such as vertical cracks on vertical and horizontal walls, slant fissures at corners and opening of doors and windows can be found on both the brick-concrete structure and frame structure buildings, and no casualties were caused, indicating that the local government has successfully implemented policies of seismic resistance and disaster prevention of our state and autonomous region, and all seismic resistance and disaster prevention measures have been comprehensively implemented.
(4) This earthquake caused not only the destruction of houses, but also some earthquake-induced geological disasters, such as collapse and rock falls. According to the survey, it is found that these geological disasters occurred mainly near highways, factories and mines, which were small scale but easily blocked roads, smashed industrial facilities and destroyed houses. Therefore, in road construction, the landform in the mountain areas should be considered, and construction measures such as slope support should be taken, with regular inspections to avoid the traffic stoppage caused by earthquake-induced geological disasters.
(5) The earthquake occurred in the middle and high mountainous areas of the middle section of the North Tianshan Mountains, in the economic zone of the North Tianshan Mountains in Xinjiang, where the population distribution is relatively concentrated, with a high coverage of urban and rural seismic safe buildings. This earthquake caused only three injuries, indicating that anti-seismic buildings and urban and rural seismic safe buildings possessed good aseismic performance and played an important role in protecting the safety of lives and properties of the people in the disaster area.
Many thanks to the hard working of the on-site team participating in the earthquake damage investigation, who provided the materials used in this paper.
This paper has been published in Chinese in the Journal of Technology for Earthquake Disaster Prevention, Volume 12, Number 1, 2017.
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