At 22:24 p.m. on November 25, 2016, an earthquake with MS6.7 occurred in Akto County, the Kizilsu Kirghiz Autonomous Prefecture, Xinjiang Uygur Autonomous Region (the Akto earthquake below), with the epicenter at 39.27°N and 74.04°E and a focal depth of 10km. Preliminary analysis holds that the earthquake's seismogenic structure was the Muji Fault at the north end of the Gongur Extensional System. The Muji fault is a dextral strike slip fault due to Holocene activity, in a NWW strike and about 90km long. The epicenter is located in Muji Town of Akto County close to the border. The area belongs to the tectonic zone of Western Kunlun Mountains and often reports moderately strong earthquakes, including 2 earthquakes with M≥6.0 and 13 earthquakes with M≥5.0 within the area 100km around the epicenter since 2000. The latest earthquake nearby from the Akto earthquake was the MS6.7 earthquake that occurred in neighboring Kyrgyzstan (39.43°N, 73.40°E) on June 26, 2016, with a time interval of 152 days and a linear distance of about 58km. In addition, after the Akto earthquake, a series of aftershocks occurred with the highest magnitude of MS5.0, showing an active period of earthquake activity in nearly half a year. Due to different densities in the earth's interior, materials in crust suffer different gravity action, which will surely cause unbalance of energy accumulation in crust. At the same time, under stress action, material movements in crust, after being blocked, will form extrusion, push-over, thrust and strike-slip tectonics, creating conditions for earthquake generation and occurrence. The non-tide change information in the regional gravity field, material migration in crust, and crust structure and earthquake development process can all be reflected in mobile gravity re-measurement results, and changes of surface gravity fields can better reflect the information on tectonic activities of crust thickness difference, crust density change and deep material migration (Jia Minyu et al., 2000; Zhu Yiqing et al., 2001). Therefore, mobile gravity measurement is one of the effective approaches to understand earthquake generation and occurrence mechanisms. Systemic analysis and study of time-space change characteristics of gravity fields before and after the earthquake is of actual significance to recognize the law of earthquake generation and occurrence, catch earthquake precursors, and carry out applied studies of moderately strong earthquake forecasts (Zhu Yiqing et al., 2013). This paper plans to use the 2015-2016 mobile gravity observations to analyze changes of gravity field distributions in the measuring area and their relations with the MS6.7 earthquake in Akto on November 25, 2016.1 GRAVITY OBSERVATION AND DATA PROCESSING
The Earthquake Agency of Xinjiang Uygur Autonomous Region established a mobile gravity measuring network consisting of 40 mobile monitoring points in the Kashi-Jiashi area and its adjacent areas along the South Tianshan Mountains in 2005, and began to carry out repetitive observations at least twice a year (Wang Xiaoqiang et al., 2007). In 2007, in order to better understand the trend of mobile gravity field changes in the South Tianshan Mountains, the Earthquake Agency expanded the monitoring range to Wushi, Kalpin and Aksu on the basis of the existing Kashi-Jiashi Monitoring Network, and formed the mobile gravity monitoring network covering Southwestern Tianshan Mountains. Later, the Earthquake Agency of Xinjiang Uygur Autonomous Region further expanded the monitoring network to Taxkorgan, Hotan and Kuqa and enlarged the measuring network coverage to the Western Kunlun Mountains and Tarim Basin in 2013 (Ailixiati Yushan et al., 2014) (Fig. 1), and has carried out regular monitoring of the whole monitoring network in April and August every year since 2014.The observation instruments used includes two sets of CG5 high-precision gravimeters provided by the Institute of Seismology, China Earthquake Administration. After the Akto earthquake on November 25, 2016, Earthquake Agency of Xinjiang Uygur Autonomous Region organized implementation of regional emergency measurement and arranged for two field operation groups to carry out after-earthquake densified observations in the measuring areas with four sets of gravimeters.
This paper uses the 5-period mobile gravity observations made in 2015-2016 before and after the Akto earthquake, to study characteristics of the gravity changes before and after the Akto earthquake in the area over the past two years. The scope of study covers the Western Kunlun Mountains near the Akto earthquake area, the western section of the Southern Tianshan Mountains and the Western Tarim Basin.The observed values of all measuring sections meet the specification requirements, and past back measurement deviation is less than 25μGal, and the mutual deviation is less than 30μGal. Adjustment calculations were carried out by using the China Earthquake Administration's tackling software LGADJ. In the calculation, the absolute gravity values from three points of Kuqa, Wushi and Taxkorgan in the measuring area were used for classical adjustment calculations. The final adjustment calculation results show that the point value precision of adjustment results of all periods is between 7 and 13μGal, and the solution precision values satisfy the needs of the study of gravity field change characteristics. After getting the adjustment results, individual points damaged and transferred and the points with surrounding environment changed greatly are deleted. Finally, this paper calculates half-year-scale gravity change size in the study area respectively, and carries out interpolation calculation of gravity changes with the Kriging interpolation method, and getting the gravity change chorisogram of the whole study area. On the basis of the Akto earthquake and regional geologic structure, this paper analyzes the gravity field change characteristics and the relation between earthquake generation and gravity changes in the study area.2 CHANGE CHARACTERISTICS OF THE GRAVITY FIELD AND AKTO EARTHQUAKE
With the 5-period mobile gravity observation data from April 2015 to December 2016, this paper draws the maps of gravity field changes according to the South Xinjiang Measuring Network over the past two years and the maps of gravity changes in the measuring area before and after the Akto earthquake. In the maps, the red round dots represent the Akto MS6.7 earthquake on November 25, 2016, the red solid lines represent the zero-value lines of gravity changes, the black solid lines indicate changes of gravity positive value, the black dotted lines indicate changes of gravity negative value, the constant value line interval is 10×10-8m·s-2, and the light red solid lines show regional faults.2.1 Half-year-scale Gravity Field Change
In view of gravity changes during two periods from April to August 2015 (Fig. 2(a)), the zero line of gravity changes in Southern Tianshan Mountains forms a closed area along the Akqi-Wushi-Aksu gravity field, while the zero line of gravity changes in the Pamir area contracts westward and distributes along Kashi, Bulungkol and Taxkorgan. Changes in the gravity field in the Tarim Basin keeps increasing, namely the area that has the gravity field of positive value, while the Tarim Basin is in the trend of subsidence. Material movements and centralization make material thickness in the Tarim Basin keep increasing, showing that gravity field changes in the area present positive values, but the change is not high. Obviously, there is a gravity-field high-value concentration area occurring near Wuqia, with a change value of up to 80×10-8m·s-2. In view of gravity field changes in the Southern Tianshan Mountains — the Pamir area, the whole area shows the trend of increasing gravity changes.
The observations (Fig. 2(b)) obtained in April 2016 show that positive value changes are the main section of half-year-scale gravity field changes in South Xinjiang, with gravity field positive value changes occurring in the western section of the Southern Tianshan Mountains, Tarim Basin and the meeting area between Southwestern Tianshan Mountains and Western Kunlun Mountains, and gravity field negative value changes in the southern edge of the Tarim Basin and other areas in the Western Kunlun Mountains. The gravity field in South Xinjiang, centering on Bachu and Wuqia, becomes gradually poorer, and finally goes to two zero lines respectively along the Aksu-Wushi area and the Taxkorgan-Yecheng area. In which, there is a gravity change gradient zone that occurred in Wushi, Akqi, Wuqia, Yecheng and Hotan near the zero line. Gravity field anomalies have become poorer in the Wuqia-Taxkorgan area in Xinjiang to certain extent after two MS7.0 earthquakes in Tajikistan and Afghanistan, but the gravity gradient zone in the area still exists, with the highest change of 40×10-8m·s-2or so, which is obviously related to the MS4.1 earthquake in Wuqia County (39.80°N, 75.44°E) of China in 2016 and the MS6.7 earthquake in Kyrgyzstan (39.43°N, 73.4°E) on June 26, 2016, about 200km away from the anomalous area in Wuqia.
The observations in August 2016 (Fig. 2(c)) show that the gravity field distribution in South Xinjiang forms two zero lines along Aksu, Wushi and Akqi in the western section of the Southern Tianshan Mountains and Yecheng, Pishan and Hotan on the southern edge of the Tarim Basin. The gravity field along Aksu City, Wushi, Akqi and Karajül is in the positive value-negative value transition area, so it is accompanied with corresponding gravity anomalies. Near the gradient zone with positive-negative gravity anomalies, it is the area with significant material increase/decrease differences, where energy is liable to accumulate to cause earthquakes. The site with MS3.0-4.0 earthquakes frequently occurring near Akqi and Wushi last July-August, 2016 is in the area with positive/negative value conversions between Akqi and Wushi. Occurrence of those low-scale earthquakes is also accompanied with a change of crust medium density and migration of materials in deep crust in the area, making the zero line in the area shift northward. Occurrence of a larger-area negative value of gravity field changes in the Tarim Basin compared with the first half of the year shows that the Tarim Basin is in the stage of reverse adjustment, and outward spreading of materials makes the Basin's density change, leading to a negative value of gravity field change in the area. On the map of half-year-scale gravity change from April to August 2016, there is a gravity negative value change in the Wuqia-Bulunkou area near the epicenter of the Akto earthquake where gravity changes remained in a manner of positive value, and the gravity field distribution is generally in line with the fault structure in the area, following which the Akto MS6.7 earthquake occurred on November 25, 2016.
The distribution of the gravity field obtained after Akto earthquake is as shown in Fig. 2(d), in which the distribution of gravity field in the entire measurement area continues the trend of gravity changes before the earthquake (Fig. 2(c)), the change magnitude becomes stronger to certain extent, and the change in the Measuring Area during 2016-04-2016-08 is (-40~40)×10-8m·s-2, while the change in 2016-08-2016-12 goes higher to (-60~60)×10-8m·s-2. In addition to that, of the gravity field distribution in this period, there occurring a high gradient zone in the Bulungkol area and Kashi-Bachu area, which is basically in line with the tectonic characteristics in the area, namely the gravity constant value line and the fault run in Bulungkol-Wuqia area are south-north distributed consistently. In the Kashi-Bachu area, the direction of the gravity constant value line is basically in line with that of the Kalpin Tage Fault and others in the area. The gravity changes zero line, after extending to Wuqia along Wushi and Akqi in the western section of the Southern Tianshan Mountains, turns southward in the Wuqia area. As there is no mobile gravity measuring point in the epicenter area, it is unable to directly get the information on post-earthquake gravity field distribution in the area. According to findings of Jia Minyu et al. (2000), the time variance distance of the MS6.7 earthquake is about 190km (time variance distance means the volume reflecting the size of the range of gravity changes, the time variance distance in gravity 2-D plane change image or dynamic image of gravity means the distance from the center of the anomalous area to the epicenter), and the epicenter is about 100km from Bulungkol. For this reason, the gravity field distribution in Wuqia-Bulungkol area is influenced by the seismogenic process of the Akto earthquake.After the earthquake, the gravity field distribution in the Bulungkol area presents a more intensive trend.2.2 One-year-scale Gravity Field Change
Considering influences of seasonal change on gravity observation data, one-year-scale gravity change maps for the same time periods in spring and autumn before the Akto earthquake are selected.
From the one-year-scale gravity field change maps from April 2015 to April 2016 (Fig. 3(a)), it can be found that gravity changes in the time period are basically in line with background trend changes in the area. Namely, the mountains are in the negative value area of gravity changes, while the basin is in the positive value area of gravity changes. In view of gravity field distribution in the whole area, except for negative-value gravity changes that occurred in Hotan and Taxkorgan, all other areas present positive-value gravity changes. The positive-value change of the Tarim Basin gravity field is the background trend of crust deformation in the area. Tarim Basin, due to downthrust action of the Pamirs and the blockage of the Southern Tianshan Mountains from the north, is in a subsidence trend all the time, and the density keeps increasing thereof, so there comes a positive-value change in the gravity field. As for the high gradient zone of positive-value gravity changes in the Wuqia area, it is likely related to the MS5.0 and MS6.0 earthquakes that occurred in Kyrgyzstan near the area in June 2016.
The observations from August 2015 to August 2016 (Fig. 3(b)) show that Aksu, Baicheng and Kuqa to the east of Wushi in the western section of the Southern Tianshan Mountains and Tarim Basin mainly present negative-value changes, while in the Western Kunlun Mountains, the Kashi-Wuqia meeting area features positive-value change continuously, to finally form a zero line along Wushi, Bachu and Markit as well as Taxkorgan, and lead to a gravity change gradient zone in Wushi-Akqi and Taxkorgan near the zero line. In addition, there comes a zero line of gravity changes in the Bulungkol area. The gravity changes on the south and the north sides of the zero line are 40×10-8m·s-2 or so. From October 2015 to July 2016, several earthquakes higher than MS6.0 occurred in Tajikistan, Afghanistan and Kyrgyzstan nearby. Except for the MS6.7 earthquake on June 26, 2016 in Kyrgyzstan (39.43°N, 73.4°E), the other earthquakes had the epicenters far away from Bulunkou area, making little influences on gravity field in the area. Therefore, the gravity anomalies occurred in the Bulungkol area may possibly be the warning-sign anomaly of the Akto MS6.7 earthquake on November 25, 2016.3 ANALYSIS OF GRAVITY SECTION CHANGE
The gravity chorisogram drawn with the latest data in December 2016 shows a high gradient zone of gravity changes in the Bulungkol-Wuqia and Kashi-Bachu areas. Therefore, this paper analyzes gravity difference change in the manner of the profile of the gravity measuring lines in the area. The figure of changes of the gravity constant value line can better reflect larger-scope trend changes, while the figure of gravity section changes can better reflect actual changes of all measuring lines (sections) (Liu Daiqin et al., 2012). With the difference data through observations in three periods of April, August, and December 2016, this paper selects nine measuring lines from the South Xinjiang Measuring Network to draw the figure of difference changes from April to August 2016 and from August to December 2016, with the specific section distribution situations as Fig. 4, in which the period from April to August presents before-earthquake gravity changes (the first half of the year), and the period from August to December shows after-earthquake (including co-seismic) gravity changes (the second half of the year).
After-earthquake gravity changes of the Kashi-Tuopa- Baykurt measuring lines maintain the trend of change as before the earthquake, with a smaller change magnitude; the Wuqia-Biaoxiang-Wupar measuring line is a semicircle one and closer to the epicenter. The profile shows that the change trend of the measuring line before the earthquake is opposite to that after the earthquake, and the after-earthquake gravity change is smaller than that of the change before the earthquake; the Kashi-Wupar-Bulungkol-Taxkorgan measuring line is the second one closer to the epicenter of the Akto earthquake. Before the earthquake, gravity changes of the Kashi-Bulungkol measuring section is relatively gentle, and gravity changes from Bulungkol to Taxkorgan feature high positive-negative value fluctuations, with the range of change between (-20~20)×10-8m·s-2; gravity changes before and after the earthquake show obvious regularity, namely, taking Bulungkol as the boundary, gravity changes in the Kashi-Bulungkol line present an increasing trend, while gravity changes in Bulungkol Taxkorgan line show a slowly decreasing trend. In terms of gravity changes of the Kashi-Yengisar-Zepu measuring line, taking Yengisar as the boundary, positive and negative values alternate. The Kashi-Xiyitidun town measuring line is one extending southeastward to the Tarim Basin, and presents low gravity changes, the change trends in the first half and the second half of the year are basically identical. The Kashi-Sanchakou measuring line mainly presents positive-value change in the first half of the year but negative-value change in the second half of the year; the changes of the Artux-Karajül-Halabulak measuring line fluctuates between (-10~10)×10-8m·s-2 in the first half of the year, with the overall change trend gentle, but presents an opposite trend in the second half of the year, and the measuring line is in the trend of decreasing gravity changes; the Aksu-Wushi-Akqi measuring line is one across the north side of the Puchang fault. The profile of the measuring section shows that gravity changes on both sides of the Puchang Fault are opposite. The last section is the Aksu-Kalpin-Sanchakou measuring line across the south side of the Puchang Fault and parallel to the Kalpin Tage fault, which presents a basically identical change trend of gravity changes in two-period observations, with changes in the second half of the year smaller than that in the first half, basically within 10×10-8m·s-2. In view of gravity changes of the two sections across the south and the north sides of the Puchang Fault, the north section in the Puchang Fault has a greater influence on the gravity field distribution than the south section, and several large-sized faults in the Kalpin block make gravity field distribution characteristics in the area basically identical with the tectonic characteristics.4 ANALYSIS OF POINT VALUE TIME SEQUENCE
Gravity field distribution reflects gravity changes in a large area, and the gravity section reflects gravity changes of a measuring line, and time sequence change of gravity point value can reflect gravity changes of a point with time. This paper selects the three measuring points closest to the epicenter of the Akto earthquake (positions of the measuring points are as shown in Fig. 4 with red five-point stars), to carry out comparative analysis of gravity changes and the Akto earthquake. For point value time sequences, the gravity value observed in September 2013 is taken as the reference, to draw time sequence figures of gravity changes at the three points of KJ53, KJ54 (Piao'ertuokuoyi Township) and KJ56 (Bulungkol Township), respectively (Fig. 6), in which the rhombic points indicate gravity change values of corresponding time, and the error lines represent precision of the observed values. Of the three measuring points, KJ53 and KJ56 are base rock points.According to observations in recent years, the ambient environment of the three measuring points show no change and less interference factors, the observed data is of good quality.
In view of gravity changes of the three measuring points near the epicenter since September 2013, gravity values of the three points went up gradually after April 2015, reached the highest value in April 2016, and changed reversely in the following half year, but within a limited range, then the Akto MS6.7 Earthquake occurred on November 25, 2016, after which, gravity values of the measuring points went up again, with the values from KJ54 and KJ56 close to the epicenter increasing at a speed slightly higher than that from KJ53 slightly farther to the epicenter. The seismogenic structure of the earthquake is the Muji fault, southwest to which there are a series of north-west faults including the Gongur fault and the Western Kunlun Mountains north-edge fault. KJ53 and KJ54 are located in the north of the fault zones, while KJ56 is in the south. The above figures show that the range of gravity value changes of the KJ56 Measuring Point in the south of the fault zones is higher than those of KJ53 and KJ54 in the north of the fault zones, indicating that crust density change in the south of the faults is higher than that in the north. In view of the change trend, gravity value in the area near the epicenter began to go up gradually one year and half before the earthquake, but reversed to go down half a year before the earthquake, and the earthquake occurred in the process of reverse change in gravity, which conforms to the findings of Zhu Yiqing et al. (2016).5 CONCLUSIONS
Using the mobile gravity observations of 4 periods before the Akto MS 6.7 earthquake on November 25, 2016, and 1 period after the event, this paper readjusts and calculates gravity data of the measuring areas, and draws gravity field change maps of the area near the epicenter and other areas before and after the earthquake.Our analysis finds that, before the Akto MS6.7earthquake, there occurred the high gradient zone of gravity changes with the gravity field change that is consistent with tectonic zone characteristics of the Muji fault and Gongur Extension System. After the earthquake, there was a reverse change of gravity, namely the earthquake occurred in the process of reverse adjustment. In view of the gravity difference change of the measuring lines, the observation results of the two periods before and after the earthquake show that on both sides of the large-sized fault zone near the epicenter there existed obvious gravity change differences. Taking the Bulungkol area as the boundary, the north side presented gravity changes up, and the south side witnessed gravity changes down. No matter how is the gravity field distribution or measuring-line difference changes, the gravity-change zero line crossed the area near the epicenter, and high gradient zone of gravity changes occurred because the area with the zero line accompanying high gradient zone of gravity changes is a transitional zone with material thickness increasing or decreasing and severe differential movement of material increase or decrease, liable to generate shearing stress to first break and cause earthquakes. The gravity value time sequence of the measuring points near the epicenter visually shows the gravity evolution process in the epicenter area. A year and a half before the Akto earthquake, the area near the epicenter presented a gravity value increase, but met reverse change of the trend half a year before the earthquake, and obvious differences existed in gravity changes on both sides of the faults near the epicenter.ACKNOWLEDGEMENT
The authors thank the Department of Earthquake Monitoring and Prediction, CEA and the Institute of Seismology of China Earthquake Administration, and the Department of Monitoring, the Department of Emergency Response and the Surveying and Mapping Institute of Earthquake Agency of Xinjiang Uygur Autonomous Region for their support of our studies.
This paper has been published in Chinese in the journal of Inland Earthquake, Volume 31, Number 2, 2017.
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