According to the China Earthquake Networks Center, the MS6.7 earthquake (herein after referred to as the Akto earthquake) occurred in Akto County (39.27°N, 74.04°E), Xinjiang Uygur Autonomous Region, at 22 :24 :30 on November 25, 2016, with a focal depth of about 10km. This earthquake is a strong earthquake that occurred in the intersection area between the western part of Southern Tianshan and the Pamir since the Wuqia M5.1 earthquake in 2012.
This earthquake occurred in the plateau mountainous area. Due to the inconvenience of traffic and the complexity of fault structures near the epicenter, effective deformation data cannot be obtained in time by traditional surveying methods. Since its application in seismic deformation in 1993, synthetic aperture radar interferometry (InSAR) has been widely used (Qiao Xuejun et al., 2014; Ji Lingyun et al., 2017; Zhao Qiang et al., 2017). Especially in the low vegetation coverage areas of Qinghai-Tibetan Plateau, the co-seismic deformation field can maintain good coherence for a long time. In addition, the Pamir arc tectonic syntaxis, which was subjected to the northward compression and nappe of the Indian plate since the Cenozoic, is one of the most dynamic and seismically active regions in the Chinese mainland (Burtman V.S. et al., 1993; Burtman V.S., 2000; Yin Jinhui et al., 2001; Pan Jiawei et al., 2009). Therefore, InSAR technology is an important means to carry out the 2016 Akto MS6.7 earthquake research, obtain the coseismic deformation field of this earthquake, invert the source parameters and the distribution of the fault slip, help to explore the relation between the earthquake and regional tectonic movement, and provides important basis for determining the earthquake situation of the earthquake hit area as well as the region.1 OVERVIEW OF THE RESEARCH AREA
The 2016 Akto MS6.7 earthquake occurred in the juncture area between the western section of the southern Tianshan Mountains and the Pamir, near the arc-top area of the Pamir syntaxis and the western end of the Muji fault basin. It is the west antenna area of Indian plate's northeastward collision to the Eurasian plate, where a series of large geological structures are developed (Fig. 1). On the north of the epicenter is the Pamir main thrust fault, the Karakul graben is in the west, the Karakoram strike-slip fault is in the south, and the Kongur extensional tectonic system is in the east (Fan Guangwei et al., 1994; Schoenbohm L. M. et al., 2011; Sippl C. et al., 2013; Schurr B. et al., 2014).
Strong earthquakes are frequently occurred in the history in the region. Seventeen earthquakes with magnitude 6.0 or above were recorded in the range of 100km around the epicenter, including 4 earthquakes with magnitude 7.0 or above, i.e. the south of Wuqia M7.0 earthquake on September 28, 1944, the M7.0 double earthquakes in the west of Wuqia on April 15, 1955 and the southwest Wuqia M7.3 earthquake on August 11, 1974. As on 10 :00 a.m. Agust 26, 309 aftershocks were recorded, including 3 earthquakes with magnitude 4.0-4.9, 31 earthquakes with magnitude 3.0-3.9 and the largest aftershocks with magnitude 4.0. After the earthquake, the focal mechanism solutions given by different institutions show that the earthquake is a unilateral-rupture event dominated by strike-slip, but there are some differences in the seismic fault parameters given by different institutions (Table 1, Fig. 2). What's more, the main shock rupture process of the Akto earthquake is complicated, and the ground motion caused by it is inevitably complicated.
Because the epicenter locates at high altitudes, the area is sparsely populated and the natural conditions are harsh, the conventional crustal deformation observation technology based on ground observation stations is difficult to implement. Therefore, SAR image becomes an important data source for obtaining the coseismic deformation field of this earthquake. ESA's Sentinel-1 has been providing free, widely available, short-cycle, short-baseline C-band SAR data since its launch in 2014. After the Akto earthquake, we downloaded four scenes of SAR image data (SLC format) of ascending/descending orbit covering the entire seismic zone, which formed two interference pairs with short time-space baselines. The basic parameters are shown in Table 2.
The InSAR processing of Sentinel-1 image is based on GAMMA commercial software platform (Werner C. et al., 2000), and co-seismic deformation interferogram is generated by two-track method. The orbital parameters of SAR data are refined by Sentinel-1 POD regression orbital data released by ESA, and the terrain phase is simulated and removed by SRTM DEM data with 30m resolution taking into account the difference of geoid. The multiple Goldstein filtering is used to improve the signal-to-noise ratio of the interferogram, the minimum cost flow (MCF) algorithm is used for phase unwrapping, and the polynomial model and terrain correlation method are used to remove the orbit errors and weaken the atmospheric delays for the remaining orbit errors and atmospheric phase delays in the interferogram. Finally, the geocoded high-resolution co-seismic surface deformation field (in LOS direction) of the Akto earthquake is obtained, as shown in Fig. 3.
It can be seen from the obtained coseismic LOS deformation field (Fig. 3) that the Sentinel-1 satellite data completely covers the entire coseismic deformation region of the earthquake, and the entire interferometric deformation phase is continuous and the features are clear and obvious. The T107 descending orbit interferogram (Fig. 3(a)) shows a semi-butterfly LOS sinking zone in the southwest part (about 32km long and a maximum width of about 10km), with a maximum deformation of about 20cm, and a uplift zone in the northeast part (about 36km long axis and 16km short axis), with a maximum rise of about 9cm. The T23 ascending orbit interferogram (Fig. 3(a)) shows a deformation trend that is opposite to the descending orbit interferogram. The southwest sinking zone shown by the descending orbit interferogram shows an uplift in the ascending orbit interferogram, with a maximum rise of about 12cm. The NE-directed uplift zone shown in descending orbit interferogram shows sinking in the ascending orbit interferogram, with a maximum subsidence of about 6cm.
Such an opposite deformation state shown in the ascending and descending orbit interferogram indicates that the surface deformation caused by the earthquake was dominated by horizontal deformation (Ji Lingyun et al., 2017), which coincides with the main features of the strike-slip earthquake deformation and is consistent with the seismological results. Based on the analysis of the InSAR co-seismic deformation pattern, the aftershock distribution and the relative spatial distribution of the existing faults, the Muji fault is initially determined as the seismogenic fault of the earthquake.3 INVERSION OF FAULT PARAMETERS AND SLIP DISTRIBUTION 3.1 Fault Geometric Parameters Inversion
The Okada dislocation model constructs the functional relation between the subsurface fault parameters and ground deformation data, mainly simulating the observed interferometric deformation field and estimating fault parameters (Wang Rongjiang et al., 2006). Generally, if there is no other prior information, the fault geometry parameters of the uniform slip distribution model can be inverted by the nonlinear inversion method and the Okada model (Wen Yangmao et al., 2012).
In order to effectively obtain the coseismic slip distribution characteristics of the Akto MS6.7 earthquake, this paper firstly uses the uniform grid sampling method to down sample the co-seismic deformation field of the ascending/descending orbit, and obtains a total of 5, 839 points of the ascending orbit and 4, 841 points of the descending orbit for the following inversion, and calculates the actual satellite incident angle and its orbital azimuth according to the obtained sampling point position. Using Okada uniform elastic half space dislocation model, combined with GCMT focal mechanism solution and the accurate location results of the Akto earthquake obtained by Fang Lihua et al. (2016), a nonlinear inversion was performed for the fault geometry (longitude, latitude, strike, dip, rake, depth, length and width of the fault). In order to fit the possible residual satellite orbit error, 6 additional orbital parameters are used for the linear estimation of orbit error, in which, the Levemberg-Marquardt least squares optimization algorithm is used for the iteration, and 8 geometric parameters and 6 orbital parameters are solved.
It can be seen from Table 3 that the strike of the seismogenic fault is approximately NWW, with an angle of about 100°, the epicenter location of (74.13E, 39.24N) and the dip angle of 80° by single fault inversion. The fault parameters are similar to that of plane Ⅰ in the focal mechanism solution given by GCMT. However, the focal mechanism of strong aftershocks shows that the aftershocks on the east-central side of the earthquake have an obvious normal component, indicating that the dynamic process of the source of the earthquake is relatively complex (Zhang Xu et al., 2017). There are at least two source rupture processes of the main shock (Chen Jie et al., 2016; Fang Lihua et al., 2016). Therefore, the seismogenic fault is divided into two parts according to the interferograms and aftershock distribution map, and the geometric parameters of the seismogenic fault are obtained by nonlinear inversion as shown in Table 4. According to the formula for magnitude calculation, the moment magnitude range of the earthquake is MW6.62-6.75.
The geometric model of the seismogenic fault shows that there is a linear relationship between the slip on the fault plane and the surface deformation (Ji Lingyun et al., 2017). Therefore, on the basis of the Okada dislocation model inversion to obtain the geometric parameters of the seismogenic faults, the SDM program package (Wang Rongjiang et al., 2011, 2013) is further used to obtain the fine slip distribution on the fault plane.
Based on the deformation field obtained from InSAR data and the fault parameters inverted from Table 4, the two-fault model is used to determine the seismogenic fault. In general, the upper limit of the rupture area of the main shock can be determined by the distribution range of aftershocks. Therefore, it is considered to extend the fault along the strike to 50km and 26km respectively, and along the dip to 40km and 38km respectively, and to divide the fault plane into 747 sub-faults according to 2km×2km. The fine slip distribution of seismogenic faults is inverted by ascending/descending orbit interferograms (Fig. 3) with joint constraints. The two interferograms are given the same weight. In the inversion process, the regional crustal stratification structure is determined according to CRUST 1.0 model. For the slip of adjacent fault slices, the stress-drop smoothing constraint is applied.
The blank points are filled by the Kriging interpolation method, and the results are shown in Fig. 4. On the whole, the deformation field fitted by the distributed slip model can simulate the observed deformation field well, and the two main deformation features can be best fitted. However, it is also found that over-fitting exists in the deformation area, especially in the north part (Fig. 4(c), (d)), which may be due to the residual errors caused by noise such as atmospheric delay or snow cover.
Fig. 5 shows the results of the coseismic slip distribution of the Akto earthquake. It can be seen from the figure that the co-seismic rupture length of the Akto earthquake is about 70km, and the coseismic slip distribution is mainly concentrated in the depth of 0-20km along the dip direction, which is a typical shallow source tectonic earthquake. The fault rupture is mainly dominated by right-lateral strike-slip. The maximum slip in western section is 0.84m, which is located at 7.1km deep and has no obvious normal component. The maximum slip amount in the eastern section is 0.68m, which is located at 6.6km deep, and has a normal component with the maximum slip along the dip direction 0.38m. The moment magnitude derived from the slip distribution is MW6.61-6.67, which is basically consistent with the magnitude shown in Table 1, which is slightly smaller than the result of uniform slip inversion.
Based on the above analysis results, the following discussion is carried out:
(1) Analysis of epicenter location. After the Akto earthquake in 2016, although CENC, GCMT and USGS have calculated the location and focal mechanism solutions of the earthquake by using far-field waves data, the location of the epicenter is uncertain due to the scarcity of seismic stations in Asia and the inhomogeneity of the crust in the Qinghai-Tibetan Plateau (Table 1). According to the location calculated by single fault inversion (Table 3), the epicenter of the coseismic rupture of the earthquake is roughly determined to be (74.13°E, 39.24°N), which is about 11.7km different from that determined by USGS, 3.4km from that determined by GCMT and 6.4km from that determined by CENC. This difference may be due to the fact that the epicenter obtained from seismology inversion is the location of initial repture, which InSAR inversion results in the location with the largest sliding magnitude.
(2) Analysis of focal mechanism. From InSAR co-seismic deformation images (Fig. 3) and dislocation inversion results (Fig. 4 and Fig. 5), it can be seen that the surface deformation of this earthquake is mainly horizontal deformation, and the length of co-seismic rupture is about 70km. The strike of the seismogenic fault is approximately NWW, with an angle of about 100° and a dip of about 80°. The fault parameters are similar to that of plane 1 of the focal mechanism given by GCMT, and coincide with the Muji fault. The moment magnitude of the earthquake is MW6.62-6.75 from the inversion of the two-fault homogeneous slip model, and MW6.61-6.67 from the inversion of the distributed slip model, which are basically consistent with the magnitudes given by various institutions. The fine slip distribution on the fault plane proves that the source rupture of this earthquake is mainly of dextral strike-slip, and only in the eastern segment, there is some normal faulting components.
(3) The relationship with regional tectonic movement. The Pamir tectonic syntaxis is one of the areas in the Chinese mainland where plate dynamic action is the strongest and seismicity is the most frequent. The GPS data (Yang Shaomin et al., 2008; Mohadjer S. et al., 2010; Ge Weipeng et al., 2015; Zhou Yun et al., 2016) indicate that the overall northward push rate of this tectonic syntaxis is as high as (23±2)mm/a, and the present-day tectonic deformation is characterized by shortening and strike-slip of the crust in the front and extension in the interior of the tectonic syntaxis. The internal extension is presented mainly by the EW-trending Kongur extensional system in the east and the Karakul graben in the west (Brunel M. et al., 1994; Robinson A. C. et al., 2004, 2007). The seismogenic fault is located in the northernmost segment of the Konggur extensional system and the northern margin of the Muji Basin. The EW-trending extension amount of the Kongur extensional system shows a deceasing trend from north to south. The Muji basin in the northernmost section is about 30km (Robinson A. C. et al., 2007). By comparing the focal mechanism solutions of earthquakes with magnitude 7.0 or above in the range of 100km around the epicenter (Qiao Xuejun et al., 2014; Metzger S. et al., 2016), it is found that the deformation of the upper crust in the Pamir Plateau is still dominated by near EW-trending extension, and the NE-trending pushing of the Indian plate is enhanced.
(4) The Okada model is widely used in co-seismic inversion because of its fast calculation speed and high accuracy, but the fault depth obtained by semi-infinite space model inversion is larger than the true fault depth, and the inversion slip is also larger. The Okada semi-infinite space model used in this paper is only for inversion of geometric parameters of faults. Meanwhile, the SDM program based on stratified medium model developed by Wang Rongjiang et al. (2006) is used to invert the underground dislocation of this earthquake, in order to obtain more accurate information about the distribution of underground dislocation and stress. However, this earthquake is in a high-altitude area, which makes field investigation difficult. Therefore, it is difficult to accurately determine the distribution of dislocations in the underground and the surface by a single measurement method (Li Jin et al., 2016; Ji Lingyun et al., 2015). A joint inversion of surface rupture and underground dislocations in combination with strong earthquake and other data will more accurately solve the relationship between the modeling and the actual earthquake rupture, and effectively improve the accuracy of the focal mechanism solution.5 CONCLUSION
On November 25, 2016, the Akto earthquake occurred in the alpine region on the southwestern edge of Xinjiang. Due to natural conditions, it is difficult to carry out field geological surveys and geophysical data collection. In this study, Sentinel-1 data was used for InSAR processing to obtain high-quality coseismic surface deformation fields of the earthquake, and the geometric parameters and fine slip distribution characteristics of the seismogenic faults were jointly inverted.
(1) The earthquake occurred in the arc top area of the Pamir syntaxis, and the Sentinel-1 data can completely cover the entire coseismic deformation field of the earthquake. It can be seen from the co-seismic LOS deformation field of the ascending/descending orbit that the earthquake is a right-lateral strike-slip earthquake, and the surface deformation is dominated mainly by horizontal deformation, which shows a longitudinally symmetric distribution, and is mainly in the area near the seismogenic fault. The maximum LOS deformation is 20cm. Combined with the analysis of aftershock distribution, the Muji fault is determined as the seismogenic fault of the earthquake.
(2) The main shock rupture process of the Akto earthquake is complicated, and a large number of geological disasters such as slope instability, rockfall and rolling stone occarred on the ground near the epicenter. Therefore, the uniform grid sampling method is adopted to effectively suppress the influence of large individual errors on the deformation field characteristics.
(3) The dynamic processes of this earthquake source are more complicated. There were at least two main shock rupture processes, so the two-fault model is used to invert the fine slip distribution on the fault plane. The coseismic slip distribution is mainly concentratedin the depth of 0-20km along the dip, which is a typical shallow-focus tectonic earthquake. The normal component is not obvious in the western section of the rupture, and the maximum slip is 0.84m, which is located at the depth of 7.1km. The maximum slip amount in the eastern section is 0.68m, located at the depth of 6.6km, and has normal component with the maximum slip along the dip 0.38m. It conforms to the structural characteristics of the Muji fault which is nearly upright and has a normal component.ACKNOWLEDGEMENTS
Thanks are extended to ESA for providing the Sentinel-1 A/B satellite data for this study, Research Professor Fang Lihua of Institute of Geophysics, China Earthquake Administration for providing the relocation results of the aftershocks, and to Professor Wang Rongjiang of GFZ for offering the SDM code.
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