Earthquake Reaearch in China  2017, Vol. 31 Issue (3): 360-367
Discussion on the Seismic Intensity of the Hutubi MS6.2 Earthquake on December 8, 2016
Luo Ju, Chen Jianbo, Chang Xiangde, Li Shuai, Sun Jing, Yao Yuan
Earthquake Administration of Xinjiang Uygur Autonomous Region, Urumqi 830011, China
Abstract: Based on the field investigation of 182 seismic hazard survey sites, combined with analysis of the aftershock sequence, focal mechanism, and seismo-tectonic background, we produced the seismic intensity map of the Hutubi MS6.2 earthquake. The seismic intensity of the magistoseismic area is degree Ⅷ, with the orientation of long axis of isoseismic contour lines east-west. The Qingshuihezi fault is considered as the seismogenic fault of Hutubi MS6.2 earthquake
Key words: Hutubi MS6.2 earthquake     Damage ratio     Earthquake hazard index     Seismic intensity

INTRODUCTION

A MS6.2 earthquake happened in Hutubi County, Changji Hui Autonomous Prefecture of Xinjiang Uygur Autonomous Region on December 8, 2016 at 13:15 p.m. Beijing time2, with epicenter located at 43.83°N, 86.35°E and hypocentral depth of 6km. The epicentral altitude is 1500m, which is about 60km from Hutubi County, 54km from Manas County and 57km from Shihezi City. It was intensely felt in places such as Changji Hui Autonomous Prefecture, Shihezi City, Urumqi City and Karamay City. After the earthquake, 40 specialists from 18 units of the earthquake system and 51 staff from the Earthquake Administration of Xingjiang Uygur Autonomous Region came to provide relief in the distress area, carrying out tasks such as observation of the earthquake, evaluation of the aftershock trend, research of loss of the earthquake and assessment of intensity. Under the command of headquarters, staff were deployed to carry out research on disaster areas including 7 counties (cities) and 35 townships (towns, villages and streets), travelled more than 20, 000km and working for 6 days. They set 182 points in total to record and provide basic data to draw the intensity map.

1 SEISMICITY 1.1 Seismic Sequence

According to records from the Xinjiang Observation Station, there were 1, 329 aftershocks for the MS6.2 earthquake in Hutubi, until midnight on December 12, 2016, among which were 3 earthquakes of magnitude from MS4.0 to MS4.9, 10 earthquakes of magnitude from MS3.0 to MS3.9 with the strongest aftershock being MS4.0(Fig. 1). The seismic sequence is distributed almost along the EW direction and is also consistent with the fault trend of Qingshuihezi, where the main shock occurred nearby.

 Fig. 1 The aftershock sequence of the Hutubi MS6.2 earthquake on December 8, 2016

Table 1 The focal mechanism solution of the Hutubi MS6.2 earthquake

 Fig. 2 The focal mechanism solution of the Hutubi MS6.2 earthquake
1.2 Focal Mechanism Solution

According to the focal mechanism solution (Table 1) of the earthquake offered by USGS, P-axis of principal compressive stress is directed NW (357°), with dip angle 26° and T-axis dip angle 64°. The type of the hypocentral fault is reverse fault, with trends similar to the trend of fault in Qingshuihezi. Referring to the structure of the fault belt in Qingshuihezi, the earthquake fault is the nodal plane Ⅱ (Table 1).

2 SEISMOGENIC STRUCTURE

The stricken area is located in a folded belt of northern Tianshan fault, the northern branch of the orogenic belt of Tianshan ranging from Xinjiang to middle Asia. The Tianshan orogenic branch is mainly the product of Hercynian plate tectonic movement (The Paleozoic Era). Since the Paleocene period, the Indian plate has been colliding with the Eurasian plate and Tianshan orogenic belt has been successively moving in its entirety, manifesting with Tianshan itself uplifting and the piedmont depositional hugely thick molasse forming. Activity along Tianshan orogenic belt is quite intense particularly in the neogene-quaternary.

The region mainly affected by the earthquake is the boundary of the Northern Tianshan high terraces and middle and low terraces, which structurally belong to the southern depressed margin of the Urumqi piedmont, where three line folded fault belts have developed, similar to the EW trend. From south to north, they are the Qigu late Pleistocene folded fault belt, the Manas-Tugulu Holocene folded fault belt and the Dushanzi-Anjihai Holocene folded fault belt. The folded fault belts in the upper crust are comprised of the fault ramp, fault flat, faulted and extended fold and other complicated nappe structures, finally including the Qingshuihezi fault belt in the south. The Qingshuihezi fault belt is the demarcated fault of the Paleozoic erathem and the Cenozoic erathem of the Northern Tianshan fault belt, totaling 280km with a trend of 280° and fracture surface of 45°-75°, belonging to the Holocene thrust active fault. Although the belt is a line of fault in the figure (Fig. 3), in fact it is distributed in lines of width from 4km-6km and a belt made of imbricated thrust fault slanting towards south, where microscopic epicenters and macroscopic epicenters are all in the middle. In the west, was the MS7.7 Manas earthquake in 1906, while this quake occurred in the east, more than 40 away from preparation zones of intensive geological disasters including mudslides and dammed lakes caused by the quake in 1906, indicating that there was again an intense quake after 110 years of quietude. Around the epicenter of this quake, although there is no obvious sign of the earth surface rupture belt, through site observation and research, there is an extended fault in the north of the epicenter, for example around the fault of Tulugu (Zhang Peizhen et al., 1994).

 Fig. 3 The seismic-tectonic map of the Hutubi MS6.2 earthquake
3 THE DISTRIBUTION OF EARTHQUAKE INTENSITY

According to the degree of destruction, the distribution of victims and types of housing structures of the affected areas (Gu Guoliang et al., 2016), the disaster area was initially divided into three intensity zones (Fig. 4). The intensity of the strong earthquake is degree Ⅷ, for which the long axis is northwest and westward. The Ⅷ area is 140km2, with the long axis 17km and short axis 10km, including Que'ergou town in Hutubi County and Taxihe Kazakh town in Manas County; the Ⅶ area is 1, 570km2, with a long axis of 58km and short axis of 38km, including Que'ergou town and Shitizi Kazakhstan village in Hutubi County, Taxihe Kazakhstan village and Qingshuihe Kazakhstan village in Manas County; the Ⅵ area is 10, 740km2, with the long axis 134km and short axis 116km, which is added up to the whole area of 12, 450km2. The disaster area included up to 900, 000 people and 260, 000 households3.

 Fig. 4 The seismic intensity map of the Hutubi MS6.2 earthquake on December 8, 2016

After the earthquake, the method of quickly, scientifically and reasonably ascertaining the intensity of the earthquake, the long and short axes of the intensity area and their trends, and the area of the intensity area are of great significance to post-earthquake disaster loss assessment and post-disaster recovery and reconstruction (Dong Man et al., 2015). In order to further improve the accuracy of earthquake intensity in the field investigation, the field team first proposed the concept of a hazard index and applied it for the first time in the "China Earthquake Intensity Table" after the MS7.7 earthquake in Tonghai County, Yunnan Province (Hu Yuxian, 1988).

Table 2 The earthquake hazard index of A-type houses

The main types of housing structures in the earthquake-stricken areas are mainly of simply constructed houses of earth, wood and stone or frame structure houses made of brick, wood and concrete. According to "Post-earthquake Field Works (Part 4): Assessment of Direct Loss" (General Administration of Quality Supervision, Inspection and Quarantione of PR.China, 2012), the disaster area housing damage levels are divided into destruction, serious damage, moderate damage, minor damage and basically intact, with reference to the "China Earthquake Intensity Table" (GB/T 17742-2008) and the opinions of the expert group, which are that simple structure houses and brick-wood structure houses for assessing the earthquake intensity are classified as Class A, considering the houses' quake-proof performance of the affected area and site conditions. In this discussion, the damage levels and hazard indices of these houses are shown in Table 2

The "China Earthquake Intensity Table" (GB/T 17742-2008) defines the average hazard index as follows:

The average value of the hazard indexes an architectural group or architectures in some regions, i.e., the ratio of architectures in every hazard index plus the corresponding hazard indices' multiplication, calculated by the formula:

 $\overline {{d_i}} = \frac{{\sum {{d_{ij}}{n_{ij}}} }}{{\sum {{n_{ij}}} }}$ (1)

In which, dij is the hazard index of the i housing damage degree j; nij is the number (or the area) of the i housing damage degree j.

It could also be accounted for as the weighted average hazard index (Shi Weihua et al., 2007), because the weight coefficient's statistical significance is obvious. The higher the housing structure A's various damage degrees are in the ratio, the more reliable the average result of statistical data.

According to the statistical data of the seismic hazard sampling point and formula (1), the average hazard index of each survey point is calculated (each point usually contains multiple sampling points). This calculation uses representative sampling point data. In the selection of calculation data, the same point also contains different types of destructed houses, excluding points containing only destroyed or intact houses. According to this, brick-and-concrete structures, frame structures and welfare houses are basically complete, so they are not included in the calculation. Table 3 shows the selected representative data and the results of the survey.

Table 3 The average earthquake hazard index and seismic intensity of all survey sites

The ratio (damage ratio) and earthquake hazard index of houses of every degree of the earthquake hazard in 166 sampling points of the calculated intensity area are shown in Table 4 (Shihezi, Manas, Hutubi's houses are almost complete involving no hazard indices, with 80% of welfare houses in the disaster area).

Table 4 The damage ratio and earthquake hazard index of different seismic intensity zones

According to the provisions of the "China Earthquake Intensity Table"(GB/T 17742-2008), the average earthquake hazard index of the same kind of houses of different earthquake hazard indices at some survey site is: degree Ⅷ of 0.29-0.51, degree Ⅶ of 0.09-0.31, degree Ⅵ of 0.00-0.11. It can be seen from Table 3 that the average hazard index in area Ⅷ is 0.37-0.44, while in Table 4 in area Ⅷ the earthquake hazard index is 0.38, which is in the range of the "China Earthquake Intensity Table" degree Ⅷ; in Table 3, the average hazard index in degree Ⅶ is 0.28-0.29, but the hazard index in the degree Ⅶ area in Table 4 is 0.20, which is slightly lower than the range of degree Ⅶ of the "China Earthquake Intensity Table". The average hazard index of the Ⅵ survey point in Table 3 is 0.03-0.11, and the result of the damage index in the Ⅵ area in Table 4 is 0.07, which is included in the intensity table within the specified range. Considering the qualities, site conditions and construction age of all sampling points' houses, the average earthquake hazard index of the survey site in the low and strong seismic zones is in agreement with the seismic intensity of the specified range. After discussion by the expert group, the results of Fig. 4 were obtained.

According to the provisions of the Article 4.3 of the "China Earthquake Intensity Table"(GB/T 17742-2008), when seismic intensity is assessed by the hazard degree and the average earthquake hazard index of buildings with extremely poor constructive quality, the assessment value can be appropriately reduced. On the contrary, using better-quality housing hazard degrees and the average earthquake hazard indices to assess the intensity, the assessment value could be improved properly. The site survey indicates that in the densely populated area, building quality is relatively good; some construction site conditions are better and some are relatively poor.The epicenter is relatively remote, where sampling points are limited, with poor-quality houses. According to the results in Table 3 and Table 4, the south of Que'ergou and the south of the Kazakhstan village in Taxihe are categorized into the Ⅷ area, and the north of the middle of them is categorized as area Ⅶ (Fig. 4); in the range of 29km-67km of the long half axis and in the range of 19km-58km of the short half axis. This is the Ⅵ area.

4 PRELIMINARY KNOWLEDGE

(1) Before the proposal of the hazard index, through site investigation, the accuracy of intensity was not high (Shi Weihua et al., 2007). The seismic hazard index was proposed and quantified so that the intensity of the assessment could be stable and the precision higher. The results are more scientific.

(2) From Fig. 1 and Fig. 4, the distributional trend of the aftershock sequence coincides with the trend of Qingshuihezi fault and the long axis of the intensity circle.

(3) It can be seen from focal mechanism 2 and Fig. 4, the thrust fault plane is basically consistent with the overall trend of Qianshuihezi fault. Surface Ⅱ is the fault plane. The trend of the Qingshuihezi fault and the long axis of the intensity circle can be confirmed.

(4) The seismogenic structure of this earthquake is the Qianshuihezi fault zone, which consists of several parallel distributions of the thrust fault. The epicenter is located along the middle segment of the fault zone. In the western section of the fault zone, the Manas MS7.7 earthquake occurred in 1906 while the Hutubi MS6.0 earthquake occurred in the eastern section of the fault zone, which is more than 40km away from the concentrated zone of earthquakes and geological disasters such as landslides and dammed lakes caused by the Manas earthquake in 1906. Considering the distribution of the aftershock sequences and the fault plane of the focal mechanism solution with the tectonic model of the main earthquake, the trends of long and short axes of the intensity contour are primarily confirmed.

The Hutubi MS6.2 earthquake's long axis follows a trend of NWW. The main axis of Ⅷ and Ⅶ areas is located along the Qingshuihezi fault zone, which is in good agreement with the distribution of aftershocks in the earthquake area, the mechanism solution of the source and the seismogenic structure, providing strong support for the drawing of the intensity contour.

This paper has been published in Chinese in the journal of Technology of Earthquake Disaster Prevention, Volume 12, Number 1, 2017.

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