In the Capital Circle area, located in the north of the North China Plain, the NWW-striking Zhangjiakou-Bohai fault zone is the main active tectonic zone (Zhang Peizhen et al., 2003), trending northwest and extending for about 700km in length. The tectonic zone, with a steep dip angle, showing various trending directions and left-lateral strike-slip movements, is a Cenozoic active fault (Xu Jie et al., 1998). The Zhangjiakou-Bohai fault zone, from northwest to southeast, consists of the Zhangjiakou fault, Xinbao'an-Shacheng fault, Shizhuang fault, Sunhe-Nankou fault, Yongdinghe fault, Langfang-Wuqing fault, Baodi fault, Jiyunhe fault and Haixi faults (Fig. 1). The Zhangjiakou-Bohai fault zone corresponds to the crustal thickness variation belt, with a thicker crust in the north of the fault zone, thinner in the south. Multiple seismic reflection profiles and magnetotelluric profiles reveal that this fault zone is a deep-seated fault cutting the lithosphere (Lai Xiaoling et al., 2004). The NNW-striking Zhangjiakou-Bohai fault intersects with NNE-striking faults in many areas, which is one of the typical tectonic characteristics in the Capital Circle area, and moderate-strong earthquakes and violent earthquakes often occur in these areas (Xu Xiwei et al., 1998). Prior earthquakes in this area of high seismic risk include the 1679 Sanhe-Pinggu MS 8.0 earthquake, the 1976 Tangshan MS 7.8 earthquake, the 1998 Zhangbei MS 6.2 earthquake and the 2006 Wen'an MS 5.1 earthquake etc. (Fig. 1).
Quantitative study of regional present-day crustal deformation is the basis for the understanding of its dynamic process. In recent years, geodetic technique is widely used in the study of present-day crustal deformation, including GPS technology, which is best used to acquire more accurately the crustal horizontal motion deformation over a large area. Since the 1990s, for the research of earthquake prediction and earthquake disaster mitigation, the Crust Movement Observation Network of China deployed GPS monitoring network in North China where the Capital Circle area is located, and a great deal of GPS observations have been carried out and abundant observation data has been accumulated (Li Yanxing et al., 1998), which provided conditions for the study of regional crustal horizontal motions, deformation characteristics, activity characteristics of principal fault and geodynamic features. A lot of research (Jiang Zaisen et al., 2000; Yang Guohua et al., 2003; Liu Xiaoxia et al., 2010; Shen Z.K. et al., 2002; Jin Honglin et al., 2012; Wu Weiwei et al., 2015; Fang Ying et al., 2008, 2009) has been carried out using GPS data to study horizontal motions and strain characteristics in the Capital Circle area and even North China area where the Zhangjiakou-Bohai fault zone lies. Most of these studies describe regional large-scale deformation characteristics from the macroscopic angle and qualitatively/quantitatively analyze the overall movement deformation of the Zhangjiakou-Bohai fault zone (Liu Xiaoxia et al., 2010; Shen Z.K. et al., 2002; Jin Honglin et al., 2012), or study only some secondary faults of Zhangjiakou-Bohai fault zone (Fang Ying et al., 2009), neglecting the segmented movement properties of the Zhangjiakou-Bohai fault zone. In this article, using mobile observation GPS data during the period of 1999-2007 from basic stations and regional stations of the Crust Movement Observation Network of China, we emphatically analyze segmentation characteristics of motion and deformation of the Zhangjiakou-Bohai fault zone based on the analysis of present-day horizontal motion and deformation features in the Capital Circle area, and in combination with seismologic and geological results, we discuss movement properties, tectonic significance and dynamic sources.1 DATA AND ANALYSIS
GPS data used in our research mainly comes from 163 continuously monitoring GPS sites and mobile sites of the Crust Movement Observation Network of China deployed in the Capital Circle area where the Zhangjiakou-Bohai fault zone lies (Fig. 2). Four observation periods are carried out at mobile sites (1999, 2001, 2004 and 2007) and continuous observation of at least 72 hours was performed in each observation.
GAMIT/GLOBK/QOCA software is used for data processing, and for processing procedures, see references (Wu Yanqiang et al., 2009; Liu Zhiguang et al., 2013; Zhan Wei et al., 2013). For convenience, rates in the ITRF framework are converted to motion rates relative to the Eurasian plate. The Eurasia reference frame is defined by difference disposal applied between rates in the ITRF frame and rotating motion of the Eurasia plate. Usually by inversion of a set of station coordinates, a stable Eurasian plate reference can be obtained, but this group of points needs to be distributed on the Eurasian plate and located in the stable interior of the continent. Accordingly, in this article when conducting transformation of the reference frame, by conversion of GPS, VLBI and SRL data provided by Altamimi et al. (2007), we get Euler vectors of the Eurasian plate, 56.330±0.549° and -95.979±0.969°, and its rotation rate 0.261±0.003°/Ma. By removing the integral rotation of the Eurasian plate from coordinates in ITRF2005 frame, we get the velocity field relative to the Eurasian plate frame, shown in Fig. 2. In this article, GPS horizontal velocity field data (1999-2007) is used to explore crustal motion and deformation characteristics along the Zhangjiakou-Bohai fault zone and its adjacent area. During observation, an MS 8.0 earthquake occurred at the west pass of the Kunlun Mountains in 2001. After the earthquake, co-seismic dislocation and fault creep deformation obtained from relatively dense observations of crustal deformation by use of GPS data show that the influence area of crustal deformation is roughly 88°-97°E (Ren Jinwei et al., 2005), which is far away from the study area in this article. Thus it can be seen that co-seismic dislocation and fault creep deformation after the west Kunlun mountain pass earthquake have little effect on crustal deformation in the study area.2 ANALYSIS OF HORIZONTAL MOTION VELOCITY FIELD
Global positioning system observations provide high-precision, large-scale and quasi-real time crustal movement data, and crustal velocity field formed by these data is an important basis for the revelation of the form, pattern and amplitude of present-day tectonic deformation. Fig. 2 provides GPS velocity field in the Zhangjiakou-Bohai fault zone and its adjacent area relative to Eurasia frame, by which the southeastward movement of the whole study area is clearly shown. It can be seen from Fig. 2 that motion rates from GPS observation stations in the whole Zhangjiakou-Bohai fault zone and its adjacent area are relatively small. Fig. 3 shows that for 163 GPS observation stations in the study area, velocity component values at GPS stations in E direction which are between 0-3mm/a account for 90%; velocity component values in the N direction are mainly concentrated at 1-3mm/a, occupying 93%. Crustal horizontal movements in the study area reflected by displacement rate vector distribution changes have the following characteristics:
(1) From the perspective of direction of motion, except for individual points around Beijing and Tianjin which move eastward or northeastward, other sites inside the study area all move to the southeast, reflecting that horizontal movements in the study area may still be controlled mainly by the collision between the India plate and Eurasia block.
(2) From the view of GPS station velocity, velocities at GPS stations on the southwest side of the Zhangjiakou-Bohai fault zone are greater than that on the northeast side, and the dominant motion directions of sites at the two sides are also different. Dominant motion direction of GPS sites at the interior of the North China Plain block is SE35° and dominant motion direction of GPS sites at the interior of the Yanshan block is SE21.8°.3 ANALYSIS OF CRUSTAL STRAIN RATE FIELD
The inconsistency of the spatial distribution of the horizontal velocity field is the direct reflection of crustal deformation, while strain field is the main parameter for crustal deformation and an important indicator to describe regional deformation, which is not affected by the reference frame and can reflect regional deformation characteristics from resolutions (Meng Guojie et al., 2009). By strain analysis with GPS data, the change process of the strain field in the study area with time and space can be monitored (Yang Guohua et al., 2002; Ren Jinwei, 2002). In this article, by use of GPS data during 1999-2007, the principal strain rate field along the Zhangjiakou-Bohai fault zone and its adjacent area is obtained by the least square collocation method (Fig. 4). The results of principal strain rate (Fig. 4) show that crustal deformation characteristics along the Zhangjiakou-Bohai fault zone mainly resulted from NEE principal compressive strain or NNW principal extensive strain, of which, GPS strain field shows that principal strain rate field along the Zhangjiakou-Bohai fault zone has a feature of obvious segmentation. Although principal compressive strain along all secondary faults in the Zhangjiabao-Bohai fault zone is basically in the NEE same direction, principal compressive strain value changes are more obvious. Principal compressive strain along secondary faults in the northwest segment of the Zhangjiakou-Bohai fault zone is much smaller than that in southeast segment, and along the northwest segment of Zhangjiakou-Bohai fault zone, there is mainly NEE principal compressive strain, while at the southeast segment, obvious NEE squeezing and NNW extension are both displayed. The northern Shanxi fault zone mainly suffers from NW principal extensive strain, while the old Tangshan seismic region suffers from NE principal compressive strain and NW principal extensive strain at the same time.
The results of areal dilation rates (Fig. 5) also show obvious segmentation features along the Zhangjiakou-Bohai fault zone, including the Zhangjiakou, Xinbao'an-Shacheng, Sunhe-Nankou, Langfang-Wuqing and Baodi faults in the compression zone, among which the segment between Zhangjiakou-Bohai fault zone and Langfang-Wuqing fault has the highest degree of compression, and Shizhuang fault, Yongdinghe fault, Jiyunhe fault and Haixi fault are all in the dilation zone. Among all fault segments, the region in which Haixi fault lies is with the highest dilation values. The results of the maximum shear strain rate (Fig. 6) show that during 1999-2007, zones of high shear deformation values in the study area mainly included Xianghe County of Hebei Province at the east of Beijing, Wen'an County in Hebei Province at the southwest of Tianjin and Tangshan in Hebei Province, while along the Zhangjiakou-Bohai fault zone, shear strain is highest at the northwestern section of Langfang-Wuqing fault.
Earthquakes are an important manifestation of fault movement in the earth's crust, and characteristics of fault movement are closely related to attitudes of faults and regional tectonic stress state (Working Group of M7, 2012). The study of the state of regional tectonic stress and its effect laws has become an important part in revealing the geodynamical process, exploring interactions between faults and analyzing earthquake preparation and the occurrence process. Seismic waves contain important information about mode of fault motions and state of regional stress field. By analysis of earthquake source mechanism using seismic waves, we can obtain the mode of motions of seismogenic faults and state of regional tectonic stress fields. The regional stress field along the Zhangjiakou-Bohai fault zone and its adjacent area in North China obtained from data of focal mechanism solution during different periods (Li Ruisha et al., 2008; Wu Minjie et al., 2011) shows that the direction of maximum regional principal stress is NEE, the minimum principal stress is in the NNW, which is basically consistent with the directions of principal compressive strain of regional strain field obtained by the least square collocation method based on GPS data in this article. Ma Xingyuan (1989), using the method of geology and seismography and crustal stress reliese method, provided directions of horizontal principal compressive stress axis in some areas in the Chinese mainland, and his results for direction of horizontal principal compressive stress axis in North China are consistent with the results in this article.
For several great earthquakes happening in North China in the last 50 years, such as the 1966 Xingtai MS 6.2 earthquake, the 1967 Hejian MS 6.3 earthquake, the 1975 Haicheng MS 7.3 earthquake, the 1976 Tangshan MS 7.8 earthquake, Luanxian MS 7.1 earthquake and the 1998 Zhangbei MS 6.2 earthquake, spreading directions of ground fissures and mode of motions in their epicenter areas indicate that these earthquakes happened under the action of NE-NEE principal compressive stress. Surface ruptures caused by the 1976 Tangshan earthquake have a maximum horizontal offset of 2.3m, showing NNE right-lateral strike-slip features; the 1966 Xingtai earthquake also shows NNE right-lateral strike-slip features (Working Group of M7, 2012). Characteristics of strong earthquake source dislocation show that NE-NNE seismogenic faults display right-lateral dislocation, and NW-NNW seismogenic faults show left-lateral dislocation. Statistical results of mode of motions and focal mechanism solutions for seismogenic faults in North China indicate that most earthquakes in the region are strike-slip earthquakes, accounting for 72%, and normal faults account for 19%, and thrust type 9%. And there exist two groups of predominant directions in the nodal planes, one group is in the NNE direction and the other in the NWW direction (Working Group of M7, 2012), of which, the NWW direction reflects that the present-day crustal tectonic activities along the Zhangjiakou-Bohai fault zone and its adjacent area in North China are represented mainly by horizontal movement.4.2 Motion and Deformation Characteristics of All Segments along the Zhangjiakou-Bohai Fault Zone
Profile projection is a simple and effective method for the study of the tectonic deformation field. Velocity component profiles at GPS stations obtained by the projection method can directly reveal the changes of rates at stations with distances between stations and faults caused by strike-slip (compression and extension) movement of faults. To analyze motion and deformation characteristics of each segment of the Zhangjiakou-Bohai fault zone, and to more clearly acquire spatial features shown by velocities at GPS stations on both sides of the Zhangjiakou-Bohai fault zone, GPS profile analysis is conducted for different fault segments. Fig. 2 provides the GPS profile range and distribution of GPS stations. Projections along the direction of profile and perpendicular to the direction of profile are respectively done for velocities at GPS stations on the two sides of the fault zone, and the difference between average velocity values for GPS stations on the two sides of faults is used to estimate slip or extension/compression rate of faults. Using the law of error propagation, the error of rate is calculated. According to strike and distribution characteristics, from northwest to southeast, we divide the Zhangjiakou-Bohai fault zone into three segments, namely the northwest, middle and southeast segment. It can be seen from GPS velocities (Fig. 7) that are parallel to the fault that all three segments of the Zhangjiakou-Bohai fault zone show features of left-lateral strike-slip movement, with slip rates 0.9±0.7mm/a, 0.9±0.7mm/a and 1.8±0.7mm/a, respectively. GPS velocities (Fig. 7) perpendicular to the fault indicate that all segments of Zhangjiakou-Bohai fault zone display features of compression movement, and extension and compression movement rates for all secondary faults are presented in Table 1. Shen et al. (2000), using GPS observation data in North China, revealed that the left-lateral slip rate along the Zhangjiakou-Bohai fault zone is 1.8±1.0mm/a. Jin Honglin et al. (2012), using GPS data based on the Savage one-dimensional dislocation model, held that the Zhangjiakou-Bohai fault zone has a left-lateral strike-slip rate of around 2.0mm/a. The above analysis on integrated movement characteristics of the Zhangjiakou-Bohai fault zone and our research results on movement features of its segments both show that motion rates along the Zhangjiakou-Bohai fault zone are rather low, usually no more than 2mm/a. Also, results of GPS principal strain, areal dilation rate and the maximum shear strain all indicate obvious segmentation of movement along the Zhangjiakou-Bohai fault zone (Fig. 5, Fig. 6).
Late Cenozoic and present-day tectonic deformation in the Chinese mainland is characterized by block motions, and movements of active blocks and their interactions directly control the generation and occurrence of strong earthquakes in the Chinese mainland (Zhang Peizhen et al., 2003). According to active block division in the Chinese mainland (Zhang Peizhen et al., 2003), the Zhangjiakou-Bohai fault zone and its adjacent area mainly suffer from the effects of the Yanshan active block in the north and the North China block in the south, and from west to east, respectively, are the Ordos block, North China Plain block and Ludong-Huanghai block, among which, the Yanshan active block, located at the southwest corner of the northeast China active block, is a nearly EW wedged block. The Ordos active block lies in central China, except that its southwest corner suffers from strong compression from the northeastern margin of the Qinghai-Tibetan Plateau. All other sides are surrounded by fault basins, and its interior region shows weak deformation features. The North China Plain active block suffered from strong extensional and rift forces in early Cenozoic, forming a series of NNE-striking normal faults, horsts and grabens. Since the Pliocene, the rifting of the North China active block has ceased, and the North China Plain started an integral subsidence deformation, forming right-lateral strike-slip faults on the foundation of NNE normal faults; the Ludong-Huanghai active block contains Ludong, Huanghai and the southern Korean Peninsula, with Tancheng-Lujiang fault zone as its western boundary. Assuming that all blocks are as rigid as possible, with no or very little deformation inside blocks, and that deformation is mainly concentrated along the boundary faults, by use of GPS velocity field relative to the Eurasia frame, the characteristics of rigid movements are calculated for all blocks (Fig. 8, Table 2).
The Zhangjiakou-Bohai fault zone lies between the Yanshan block and the North China Plain block. Translational motions of both blocks are in SE direction, and translational motion rate of the North China Plain is significantly greater than that of Yanshan block, thus the difference between translational motion rates of two blocks becomes the direct dynamic source for the left-lateral strike-slip movement of the Zhangjiakou-Bohai fault zone. From the perspective of broad regional tectonic background, the Chinese mainland is located at the intersection of the India plate, the Eurasian plate and the Pacific plate, and its internal tectonic movement and tectonic deformation are the result of interactions between these plates. By the southwest of the Chinese mainland, the India plate advances towards the Eurasia plate at a rate of about 50mm/a (Xu Xiwei et al., 1994), which makes the Qinghai-Tibetan Plateau and its adjacent area suffer from obvious NE squeezing force. In eastern China, the subduction of the Eurasia plate to the Pacific plate and back-arc spreading of the Sea of Japan form westward or SW squeezing force. Therefore, combined actions of the India plate, the Eurasia plate and the Pacific plate form an overall stress pattern in the Chinese mainland, i.e., the principal compression stress axis shifts regularly from near-SN in the west to NEE in the east. The massive NE squeezing force of the India plate causes the northeast edge of the Qinghai-Tibetan Plateau, the eastern part of the Gansu-Qinghai block and the west of the North China block to move to the north and northeastward, and motion amplitude and velocity gradually decrease eastwardly, which has led to passive large-scale clockwise rotation of the North China block since the Paleogene period (Xu Xiwei et al., 1994). All active blocks in the study area move in the SE direction, which may be the results of the broad regional tectonic movement. Considering the differences of interactions and motion directions between the India plate, the Eurasia plate and the Pacific plate, and by analysis, we believe that the collision between the India plate and the Eurasia plate and the later NE movement is the dynamic source for movements of the Ordos, North China Plain, Ludong-Huanghai block and Yanshan blocks, and is also the ultimate dynamic source for the significant left-lateral strike-slip movement along the Zhangjiakou-Bohai fault zone.5 CONCLUSION
By use of repeated measurement data during 1999-2007 from basic stations and regional stations of the Crust Movement Observation Network of China, we analyze the spatial distribution characteristics of the velocity field and strain field for present-day crustal horizontal movement along the Zhangjiakou-Bohai fault zone and its adjacent area, study the motion and deformation characteristics of all segments of the Zhangjiakou-Bohai fault zone, and combined with previous research, discuss the dynamic sources for its motion and deformation. The main conclusions are as follows:
(1) From the perspective of motion directions of GPS stations, in the reference Eurasia frame, other sites inside the study area mostly move to the southeast; from the view of GPS station velocity, velocities at GPS stations on the southwest side of the Zhangjiakou-Bohai fault zone are greater than that on the northeast side, and dominant motion directions of sites at two sides are also different. Dominant motion direction of GPS sites at the interior of the North China Plain block is SE35° and dominant motion direction of GPS sites at the interior of the Yanshan block is SE21.8°.
(2) GPS strain field shows that principal compressive strain along all secondary faults in the Zhangjiakou-Bohai fault zone is basically at the same direction, all in NEE direction, the principal compressive strain values vary obviously. Principal compressive strain along secondary faults at northwest segment of the Zhangjiakou-Bohai fault zone is much smaller than that in the southeast segment; the results of area dilation rates also show obvious segmentation features of the Zhangjiakou-Bohai fault zone; the results of the maximum shear strain rate show that during 1999-2007, high shear deformation value zones in the study area mainly include Xianghe County of Hebei Province east of Beijing, Wen'an County of Hebei Province southwest of Tianjin, and Tangshan in Hebei Province, and among all secondary faults along the Zhangjiakou-Bohai fault zone, the highest shear strain appears at the northwestern section of Langfang-Wuqing fault.
(3) By analysis of GPS profile across the fault zone, it is discovered that all segments of the Zhangjiakou-Bohai fault zone display left-lateral strike-slip features with squeezing movement, and the left-lateral slip rates are respectively 0.9±0.7mm/a, 0.9±0.7mm/a and 1.8±0.7mm/a.
(4) Using rigid block model, rigid motion characteristics are analyzed for the Yanshan block, the Ordos block, the North China Plain block and the Ludong-Huanghai block in the North China region where the Zhangjiakou-Bohai fault zone lies. All blocks move horizontally to southeast at the same time with rotational movement. The dynamics of the motion styles of Zhangjiakou-Bohai fault may directly come from the relative movement between the Yanshan block and the North China plain block, and the ultimate dynamics may be the results of the collision between the Indian and Eurasian plates, and the persistent northeastward extrusion of the Indian plate.
This paper has been published in Chinese in the journal of Earthquake, Volume 36, Number 1, 2016.
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