CURRENT CRUSTAL MOVEMENT IN CHINA
Institute of Seismology, China Seismological Bureau, Wuhan 430071, China
The current Asian tectonics is characterized by the large-scale and intensive crustal deformation in response to the continental collision of Indo-Asian at Himalaya and India's subsequent penetrating beneath southern Tibet that resulted in the uplift of Tibetan Plateau at an averaged elevation of 4500-5000, extruded Indochina southeastward and reshaped Tianshan and Baikal in the farther north. Distinguished in its complexity and vast region involved, active tectonics in Asia, especially east and central parts are of critical importance to deepen the knowledge of global plate motion and continent dynamics. China covers, in territory, most of southeastern part of Eurasian continent, including most of the Tibetan Plateau, Tarim Basin and eastern Tianshan. Thus, research on active deformation in China plays a significant role in Asian tectonics and dynamics. As yet, widespread use of spaced-based techniques such as GPS, SAR and so on, Chinese scientists have made remarkable advances on measuring present-day crustal deformation during 1999-2002. In particular, Crustal Movement Observation Network of China (CMONOC)( Ma Z. et al., 2001; Niu Z. et al., 2002) ,one of the National Key Infrastructure Projects has helped improve our understanding to the ongoing deformation in eastern Asia, thanks to its densification and coverage. As yet, a unified horizontal velocity field of continental China and vicinity on the Eurasia-fixed reference frame was obtained through measurements at approaching 1000 GPS sites (Wang M et al., 2003). The velocity field delineates patterns of subplate motion and intraplate deformation in Asia The long-term endeavor on space-based geodesy in mainland China offers, in unprecedentedly detail, the feature of the tectonic deformation process induced by indentation of the Indian plate. The quantitative description of surface displacement on the upper crust provide kinematic constraints for modeling dynamic process of continental lithospheric deformation beneath.
This review begins from a brief introduction of the ongoing project of CMONOC, proceeds through an overall description of current deformation in China inferred from GPS measurements , investigations on current displacement, strain-stress field in Tianshan, Yunnan, Sichuan, Gansu, Qinghai and north China , which were done by various groups in the past 4 years, to the study of kinematic model of intraplate blocks on a tectonic frame of east Asia, and finishes with some work using SAR images on co-seismic displacement fields in Tibet and north China.
CMONOC was inaugurated in 1998 as the national scientific infrastructure designed for monitoring current crustal deformation with space geodesy to facilitate mitigating potential seismic risks in China. A consortium from China Seismological Bureau(CSB), Chines Academy of Science(CAS), and National Bureau of Surveying and Mapping(NBSM) is responsible for construction of the scientific facility in four-year and maintaining its operation after. The key geodetic technique involved is Unite States developed Global Positioning System(GPS), complemented by Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR). The surface absolute and relative gravity measurement are added to enhance function and capability of the facility. The CMONOC consists of fiducial, basic and regional networks and one data center, constituting an multi-technique geodetic observation system with a nationwide coverage.
The fiducial network is consisted of 25 continuous tracking GPS sites with 2-meter steel-enforced concrete pillar embedded into bedrock. The existing IGS sites at Wuhan, Shanghai, Beijing, Lhasa and Urmuqi are affiliated to the CMONOC. Collocation SLR (permanent and mobile) and VLBI observations are taken at some sites (Shanghai, Beijing, Wuhan, Kunming, Changchun, Urumqi and Lhasa). The continuos sites are distributed rough-evenly throughout China with an average spacing of 700 kilometer, aiming at studying the tectonic block motions (There are six blocks suggested from geological investigation and earthquake records)。Excerpt South China Sea, a minimum of 3 continuos sites are located in each block (Tibet, Tianshan/Tarim, South, North, Northeast China). Such site configuration ensures a possibility of determining precisely relative velocities between neighboring blocks.
Most of the sites are equipped with ASHTECH Z12 receivers, Choke-Ring antenna and data communication devices. The tracking data each site is downloaded daily via telephone line link or communication satellite。The absolute gravity measurement are performed at every fiducial site using FG5 gravimeter. So far, a 4-year period of tracking operation of 25 continuos sites show that changes of baseline connecting a pair of adjacent sites are determined with a precision of 1.3 mm/yr or better. Twice occupations of absolute gravity measurement were performed at each site over a typical session of 1-2 days during 1999-2001 . The uncertainty of gravity acceleration ranges 1-5μgal.
The fiducial network is augmented by a basic network that 56 GPS/gravity sites are distributed uniformly over the mainland China. An average spacing of the basic sites plus the continuos sites is 350 kilometer. The configuration of the basic sites is designed to constrain the internal deformation on the framework of a large block as well as to define secular gravity changes at a national scale. Since 1998, 5-epoch field surveys at an annual interval are completed and yield a baseline repeatability of 3 mm for horizontal component and 10 mm for vertical, at average, corresponding to 1×10－8 relative positioning precision. A total of 3 campaigns of relative gravity measurement connecting continous and basic sites are conducted with LaCoste-Romberg Model-G gravimeters. The relative gravity measurement between sites of two networks has an uncertainty of 20-30μgal。A large –scale gravity change is obtained at the first time in the mainland
The fiducial and basis networks are densified greatly by the so-called regional network that is composed of 1000 roving sites. 700 of such sites spread over the eastern and northeastern margins of the Tibetan Plateau and north China plain ) where major and moderate earthquakes happened frequently in the history and any potential event with M> 7 will pose serious thread against the local population and properties. The remaining 300 sites are distributed in Tibet, Tianshan, south China and northeast China. The regional network aims primarily at providing the detailed strain partitioning along the faults and block boundaries as to assess the seismic risks or potential site of earthquakes. GPS observation in campaign mode were arranged irregularly for all roving sites in a time or a part of these some times according to forecast of potential seismic risk. In 1999, the first epoch survey was conducted and all roving were occupied, whereas 90% of these sites were re-visited in 2001. In 2000 and 2002, only small part of sites (50-60 each year) were selected for re-measurement to detect plausible anomaly of displacement associated with potential earthquake in Yunnan and obtain the co-seismic deformation produced by the Kunlun Shan earthquake (Ms 8.1) November 14, 2001.
II. OVERVIEW OF PRESENT-DAY CRUSTAL MOVEMENT
Late Quaternary active faults with diverse styles and natures are widespread over the continental China. The upper crust has been sliced into a number of tectonic blocks by the major active faults. There is no or minor internal deformation within crustal blocks. Most of deformations are commonly localized on the boundaries of the tectonic blocks where devastating earthquakes mostly occur in the history. Each tectonic block has its own style, amount, and evolution history of deforming. The latest velocity field integrating CMOMOC and other data collected 1991-2001 reveals the various features of tectonic process[3-7].
Inferred from the GPS site velocities, Nepal moves roughly northward with a little eastward component relative to stable Eurasia. The velocities of these sites range usually 35-40 mm/a, in a good agreement with the independent estimates by other groups in the world. The GPS-derived velocity field for the Nepal Himalaya demonstrates exclusively a subsequent penetration of India subcontinent beneath the Tibetan Plateau since the Indo-Asia collision about 50 Ma ago.
The southern Tibet is subjected to east-west extension. A series of north-south trending normal fault and graben systems characterizes the ongoing extension. The GPS measurements indicate that the southern Tibet moves to N30°-47°E at a rate ranging 27-30 mm/a. The rate of extension of 14.5 mm/a is inferred simply from difference of velocities between a site in the western Tibet and one in the east. This GPS-derived rate is slightly larger than rate of 10±5 mm/a from the Quaternary faulting , but is less than the rate of 18±9 mm/a inferred from a summation of a century-long seismic moment tensors. Given some extension occurring in farther east and farther west, the rate above may be a lower bound on the extension in the southern Tibet, for which an estimation of 15-20 mm/a seems plausible .
In the high and north parts of the Tibetan Plateau, 4 active blocks (from south to north, Qiangtang, Kunlun, Qaidam and the Qilian Shan) are bounded by several NWW trending left-lateral strike-slip faults. GPS sites in the Qiangtang show consistently a N60°E movement at a rate of 28±5 mm/a relative to stable Siberia. To the north, GPS sites in the Qaidam basin indicate a decreased rates of 12-14 mm/a with an orientation similar to that in the Qiangtang . However, in the Qilian Shan, most sites exhibit a motion of NEE direction with a lowering rates of 7-14 mm/a. The velocity field implies that these blocks within the Tibetan Plateau might require for distinct deformation behavior and mechanism
Active faulting and folding are widespread throughout the Tianshan despite far from the collision zone at the Himalaya to the south, obviously as a consequence of the collision between India the rest of Eurasia. High seismicity within the mountain belt attest to continued crustal shortening. Velocity field across the Tianshan between 81° and 85° E shows a gradual decrease from 19 mm/a in the south Tianshan to ~13 mm/a in the northern. Total shortening rate across the eastern Tianshan is ~8 mm/a, generally consistent with geological estimates of ~6 mm/a averaged over a few million years. Farther east, the rate of convergence is reduced further to 2 mm/a. In the western, early GPS observations outside China postulated a contraction rate of 20 mm/a by extrapolating the velocity field to the south to cover the entire western Tianshan. The direct measurement across the segment of the Tianshan corroborate the shortening rate of 20 mm/a. The crustal shortening across the Tianshan shows a tendency of eastward decrease, which is consistent with that inferred from theoretical modeling and geological investigations.
The North China region includes the Erdos and the northern China Plain. Despite variations in rate and orientation of velocity vectors, the GPS measurements indicate a relatively uniform strain field. Most of sites have motion rates of 8-13 mm/yr toward N90°-110°E in a Eurasia-fixed reference frame. Deformation happens apparently in two regions, one on a Quaternary fault trending EW, north of Beijing with 3~4 mm/a left-lateral displacement and another on the Shanxi graben with 3-4 mm/a NW-SE extension. In light of the existing GPS velocity, extension across the entire basin or individual faults is absent, contrary to the geological and seismological study. Such inconsistence may require either low strain accumulation on these faults in the past decade or that the ongoing deformation isn't characterized exclusively by crustal extension.
The South China is stable block in the views of geologic structure and seismicity and behaves as a coherent tectonic unit. Very Long Baseline Interferometry (VLBI) measurements at Shanghai inferred that the South China block might have a 8±1 mm/a of N116°E-oriented motion with respective to stable Siberia. The early GPS studies from velocities of few GPS sites also suggested that the South China moves south-southeastward as a rigid block at a rate of 6-10 mm/a. The new estimation of velocity with respect to stable Eurasia confirms the coherent motion at a rate of 11-14 mm/a in the orientations of N90°-120°E. Thus, the South China indeed behaves as a rigid block with few internal deformation.
The Chundian block consists of western Sichuan and central Yunnan. It is one of the most seismically active regions in the mainland. The block is bounded to the east by the left-lateral Xianshuihe and the Xiaojiang faults, which offset ridges and valleys with an average slip rates of 10-15 mm/yr. The southwest boundary of the block is the right-lateral Red River fault, with and its average slip rate is 7-8 mm/a at a geological scale. Active tectonic studies suggested that the Chundian block may have a southeastward motion but the deformations not be necessary to be uniform. GPS sites in the northern part of the block, especially along the Xianshuihe fault, move towards ~N120°E. whereas sites in the southern part of the block have velocity vectors oriented to ~N160°W. Furthermore, the Chundian block itself exhibits a clockwise rotation around the Eastern Himalaya Syntaxs superimposed on the integrated southeast movement.
1. The Deformation in Tibet and Adjacent Regions
A joint group at NBSM and Wuhan University reported that crustal movement in the Everest , Himalayas was in the direction of NEE at an extra fast velocity rate of 60-70 mm/a based on two-epoch GPS observations in 1992 and 1998 (Chen J. et al., 2002). Moreover, the group at Wuhan University demonstrated independently a velocity profile across from the Himalaya to the Qaidam Basin from 3-epoch GPS measurements performed between 1993 and 1997[9,10]. The velocity profile illustrated a north-south crustal shortening, east-west extension as well as significant uplift within the Tibetan Plateau. For instance, the convergent rate of the plate boundary at Himalayas is about 19±2 mm /a. An extension of 6±6 mm /a was applied to a small EW extent of the southern Tibet. The rock uplift is estimates to average rate of 8±5 mm /a from 5 sites in the central Tibet, which seem to be consistent with independent evaluation of 10 mm/a from absolute gravity observation 1993-1999 at Lhasa, reported by Institute of Geodesy and Geophysics CAS (Zhang W. 2001)。The southern and central Tibet move at a rate 9±5 mm /a relative to the Qaidam, coupled with an averaged eastward component of 9±6 mm /a. The velocities of sites in the interior of Tibet indicate the eastward extrusion of crust materials out of northward path of Tibet relative to Siberia. Compression strain prevails in the Himalaya. The extension strain dominates in the central Tibet, manifested by a maximum 16±6 mm /a of lateral extrusion relative to the Qaidam basin to the north。
The group at Institute of Seismology CSB gave a geodetic constrain on the overall convergent deformation across the entire Tianshan based GPS data collected from 1992 to 1999 (Wang Q. et al., 2000). The shortening rate of 20 mm/a is applied to the western segment with a width exceeding 400 km and 4 mm/a for the eastern Tianshan, a much narrow zone. The convergent rate along the Tianshan decreases progressively from west to east as similar, in view of topography, as an eastward tapering mountain. The Tarim basin have no internal deformation in general and rotates clockwisely as a rigid block relative to Siberia, pushing on Tianshan in a direction perpendicular to the range trending by reverse faulting and folding. The differential velocity between the Zhunggar basin and Hazak Platform is significant. The Zhunggar Basin might be regarded as a tectonic block with different kinematics behavior from the Kazak Platform and others nearby.
The many groups have investigated the current crustal deformation in north China using GPS measurements from 1992 to 1999 (Jiang Z. 2000; Yang G., Xie J., and Han Y., 2001; Xu C. et al., 2002; Xu C. et al., 2000; Wu J. et al., 2001). Wuhan University report that the North China moves at a rate of 3-12 mm/a with respect to stable Siberia. The First Crustal Deformation Monitoring Center at CSB revealed a change of strain field during 1992-1999. The Second Crustal Deformation Center suggested an apparent contrast in the stress field between eastern and western parts of North China using discontinuous deformation analysis technique. They conformed to that the current stress field characterizes mainly a compressive pattern in the north China. The Shanxi graben as a block boundary to the west demonstrates an east-west extension.
The Second Crustal Deformation Center at CSB reported a GPS-based study of velocity field of horizontal crustal movement from 1993 to 1999 in the northeast margin of Tibetan Plateau (Jiang Z. et al., 2001). Their results consisting of 31 GPS site velocities showed a integral SSE motion at the rate of 9 mm/a in a Eurasia-fixed frame. The Gansu-Qinghai block to the south moves much faster, whereas the Alashan block to the north move slowly, with a rate of 6 mm/a less than the Gansu-Qinghai. The sinistral slip is outstanding on the Haiyuan fault where a M8.5 event occurred last century. The NNE in the west and NEE compression in the east characterizes in the stress field stress. The NWW extension exhibits within Gansu-Qinghai and Alshan block.
The GPS studies by a joint team of Changed institute of Mineral resource and Massachusetts Institute of Technology reflected the complexity of active deformation in the southeastern China (Chen Z et al., 1999; Chen Z. et al., 2000). The multiple surveys conducted in a duration of 1991-1997 resulted in a precise constrain on the crustal deformation pattern, clockwise rotation of tectonic blocks and non-uniform slip on the major faults. The GPS sites in the Chuandian Block and regions far west move at 5-10mm/a relative to Chengdu. Chuangqing block and Yangtze Block, a regions east of Xianshuihe and Xiaojiang faults have a minor relative motion of 1-7mm/a Furthermore, the behavior of a clockwise vortex movement seems apparent for the these regions. However, the their current GPS velocity don't illustrate the eastward extrusion or escape of crustal materials in the southeastern China in contrary to the other independent research there. Institute of Seismology revealed that the Red River and Xianshuihe Fault have absorbed a lot of deformation by high-rate slips based on a geodetic inversion of GPS data . If the deformation were accumulated by elastic energy were released primarily by potential earthquake, it would produce a moderate event with Ms~6 along the faulting zones (Shen C. et al., 2002). Moreover, the group at Institute of Seismology has studied the crustal movement in southeast China using GPS (Zhou S. et al., 2000).
2. Tectonic Blocks and Kinematic Model
The group at Shanghai Astronomical Observatory (CAS) developed a kinematic model of 10 tectonic block on the International Terrestrial Reference Frame ITRF96, using 28 GPS sites in China and its contiguous areas to derive their Euler vectors (Zhang Q., and Zhu W., 2000). The model was quite consistent with those from geologic data. The model was modified by dividing some large blocks into sub-blocks (15 in total) and incorporating new (79 in total) GPS site velocities on ITRF97 (Fu Y. et al., 2002) . The group CSB has proposed a new model based on a velocity field of 900 CMONOC sites (Zhu W. et al, 2000). The model delineates 9 tectonic blocks and 2 wide deformation zones among which Euler poles and relative rotation rates are derived. The good fit to the GPS velocity field suggests that there are 3 categories of deformation patterns in eastern and central Asia. The first one, associated with the interior of the Tibetan plateau and the Tianshan orogenic belt, exhibits a widely diffusive deformation . The last , associated with the Tarim basin and the region to east of the North-South Seismic Belt (95°E-102°E) , represents block-like motion similar to plates. The deformation is accommodated primarily within several narrow zones of block boundary. The second category, mainly associated with the margins of the Tibetan plateau such as the Qaidam, Qilian, Xining (eastern Qinghai) and rhombus-shaped (western Sichuan and Yunnan) blocks, has manifested the deformation pattern between the first and the third.
3. Seismic Displacement by SAR Measurement
On 10 January, 1998 an earthquake of Ms 6.2 occurred in Zhangbei-Shangyi, north China. Institute of Remote Sensing Applications, CAS used ERS-1/2 SAR images through the 3-pass differential interferometric technique to obtain surface co-seismic deformation (Wang C. et al., 2001; Zhang H. et al., 2002). The line-of-sight displacement mapping by SAR indicated that the maximum displacement induced by the event was located at E114°20', N40°57', with surface uplift up to 25 cm. The area extent of coseismic displacement is around 300 km2. Based on a half space elastic dislocation model,. the optimum focal parameter set is derived from inversion of the line of sight displacements in the SAR interferogram, The inversion suggested that the earthquake required a thrust slipping on a rupture plane dipping 30° southwestward with a large right-lateral striking N95°E The mainshock took place on a rupture surface of 12 km long by 14 km wide at N40°58', E114°21' down to 7.5 km in depth. The co-seismic slip is estimated to 0.73 m with a seismic moment of 2.7×1018 N·m.
A group at Institute of Geology CSB has recently investigated by SAR the co-seismic deformation caused by Mani M7.6 earthquake on November 8, 1997. The three-pass differential interferometric method were applied to ERS1/2 SAR data to yield a interferogram covering whole surface trace of rupture plane (150 km, Shan et al., 2003) displacement caused by the earthquake reached to 8 m. The mainshock rupture may be divided into four segments, of which two segments in the middle, 27 km and 37 km in lengths and both 35 km in depths are bigger with the average slips of 6.5 m and 6 m respectively. In comparison with the middles, the west and east end segment of the rupture are relative minor in length 23 km and 26 km, and in depths 20 km and 18 km, respectively. The coseismic slip is slightly small, 4m for the west and 5.8 m east.
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