I. INTRODUCTION

China continent crustal motion is still one of the hot spots of studying the intraplate crustal motion and continent dynamics. It undergoes the strong collision of Indian plate and the subduction of Philippine Sea plate and the Pacific plate. The intraplate deformation is very complicated and diversiform. So, it is important to study the crustal deformation around China for lithosphere dynamics and earthquake focal. To monitor and study the crustal motion in China, some nationwide GPS networks and several regional GPS monitoring networks have been set up in the 1990s. Using recent 10-year GPS measurement data of about 1000 sites from these networks and three-campaign GPS measurements provided by the Asia-Pacific Regional Geodetic Project (APRGP), a lot of monitoring results of crustal motion of China and its surrounding regions are obtained during this report time. The monitoring and investigating work of Chinese crustal motion have made great progress. In this report, the research work related to crustal motion measurement of China from 1999 to 2002 is highlighted.

II. DATA COLLECTION AND REDUCTION

In order to monitor and study the crustal motion in China, some nationwide GPS networks and several regional GPS monitoring networks have been set up since 1991. The National (Climbing) key project on basic research “ Investigation on Present-day Crustal Motion and Geodynamics” establishes the first nationwide GPS network named the Crustal Motion Monitoring Network of China (CMMNC). The CMMNC included 24 GPS sites that were situated in each geologic tectonic block in China with an average baseline length 1000 km. Four GPS campaigns in CMMNC were carried in 1992, 1994, 1996 and 1999 respectively. The project “Crustal Motion Observation Network of China (CMONC)” was started in 1997. It mainly relies on GPS technique and consists of a continuously operated state fiducial network of 25 sites, a repeated surveyed (once per year) state basic network of 56 sites  The first campaign of CMONC was carried out in August 1998. The fiducial network has been operated continuously since January 1, 1999. Beginning from the early 1990s, several regional GPS monitoring networks about 1000 stations for active tectonic studies were established in China, including Tibet-Himalaya, Tianshan-Tarim, Altun Mountain, Qilian Mountain, Sichua-Yunnan, North China and Fujian coast network, and carried out repeated measurement for several periods between 1991 and 2001 (Zhu et al., 2000a; Wang Q. et al.,2001a; Ma et al., 2001; Cheng et al., 2001). All these GPS measurement data have been processed by GAMIT/GLOBK or GIPSY software and used to monitor present-day crustal motion in China. The estimated accuracy of the horizontal velocity of the sites is shown in (unit: mm/a):

 

Table 1.  Estimated Accuracies of Horizontal Velocity of Sites

Applying recent 10-year GPS measurement data from these nationwide and regional networks in China and GPS data of three campaigns from the Asia-Pacific Regional Geodetic Project ( APRGP), a lot of monitoring results of Chinese crustal motion have been obtained in the period of 1999-2002. (Zhou et al., 2000; Wang Q. et al., 2000; Wang Q. et al., 2001a; Wang Q. et al., 2001b; Huang et al.,1999; Huang et al.,2002; Liu et al., 2001; Lai et al.,2001; Fu et al.,2002a; Fu et al.,2002b; Wang X. et al., 2002; Zhu et al.,1999; Zhu et al., 2000a; Zhu et al., 2000b; Zhu et al., 2002) The velocity fields of these monitoring results are defined in the different terrestrial reference frames such as ITRF94, 96, 97, ITRF2000 and some regional reference frames. In order to form a united velocity field of Chinese crustal motion, through the comparisons and the rotation transformations between different velocity solutions, a combined, consistent and unified velocity fields of China and its surrounding regions in ITRF 97 and ITRF 2000 are produced (Wang Q. et al., 2001; Zhu et al., 2002; Wang X. et al., 2002;).

III. ESTABLISHMENT OF PRESENT TIME GLOBAL PLATE MOTION MODEL

In order to study the characteristics of contemporary crustal deformation, some present-time plate motion models named ITRF96VEL, ITRF97VEL, ITRF2000VEL are established by incorporating ITRF96, ITRF97 and ITRF2000 velocity fields respectively. These velocity fields are derived from the combined solutions of the space geodetic data of VLBI, SLR, GPS and DORIS , and totally independent of any tectonic plate motion models (Zhang et al., 1999; Zhu et al., 2000c; Zhu et al., 2002; Xiong et al., 2000; Fu et al., 2002; Wang X. et al., 2002;). For the methods and criteria of establishing present-time global plate motion models can see Zhu et al., 2000c. Because the velocity fields of ITRF 2000 etc. represent the crustal motion of current 20 years span, these present-time plate motion models can better describe present-time features of global plate motion than the geological model NNR-NUVEL1A. In general these models are consistent with the geological model NNR-NUVEL1A, but there are differences of about 10 percent in rotation magnitude and 15 degrees between the Euler poles for some important plates. These differences are very significant for the measurement of present-time crustal deformation in 1mm/a accuracy. So, the plate models ITRF2000VEL etc. are more suitable as the background of deformation investigation for current short time scale than the NNR-NUVEL1A.

According to the following formula, the total angular momentum of all tectonic plates with regards to the ITRF96VEL, ITRF97VEL and ITRF2000VEL are calculated.

where L is the total angular momentum of all tectonic plates, and are the rotation tensor and Euler vector of the pth plate. Table 2 shows the magnitude of the total angular momentum for the different plate motion models.

Table 2. The Magnitude of the Total Angular Momentum

In Table 2, the magnitude L of the total angular momentum for the NNR-NUVEL1A should be zero in accordance with the definition of the NNR reference frame. The nonzero value is due to round-off error.  But the nonzero values of the total angular momentum with regards to ITRF96VEL etc. models show that the ITRF96, ITRF97 and ITRF2000 are not an NNR reference frame and are not sufficiently concordant with the definition of CTRF. Therefore, the consistency of the ITRF series can not be maintained. And the net rotation of ITRF etc. relative to the NNR reference frame will influence some studies about long-term variation of the Earth rotation parameters. Though the inconsistency and influence are smaller, they are still worthy of consideration with precision improvement of space geodetic techniques (Zhang et al., 1999; Zhu et al., 2000c; Zhu et al., 2002).

IV. FEATURE OF PRESENT-TIME CRUSTAL DEFORMATION IN CHINA

Taking the ITRF97VEL or ITRF2000VEL motion of the Eurasia plate as the background motion, the deformation velocities about 500 sites in China and its surrounding regions are determined (Wang Q. et al., 2001; Zhu et al., 2002; Fu et al., 2002a; Wang X. et al., 2002; Huang et al., 2002; Wei et al., 2002; Li et al., 2001). Their results are shown in Table 3 and Fig.1.

 

 

Fig.1.  Contemporary crustal deformation in China and surrounding regions relative to Eurasian plate from ITRF2000VEL

Table 3. Horizon Deformation Velocities of Tectonic Blocks and Fault Belts in China

From Table 3 and Fig.1, we can see that the Chinese crustal deformation is very inhomogeneous. The North-South seismic belt is an important boundary of the deformation. The crustal deformations in the west of China are stronger and more completed than those in the east of China, the motion direction also changes from north direction to south direction across this zone. The deformation velocities gradually reduce from south to north in the west of China by energy release in several W-E direction arc suture zones.

The Himalaya-Tibet is the most active region in China or even in global continent deformation. The Qinghai-Tibetan Plateau is shortening in north-south direction and extending in west-east direction due to the strong shove of Indian plate. The convergence rate of about 15 mm/a and 9-13 mm/a are accommodated across Himalayan block and the west Tianshan respectively. Within southern Tibet, between the longitudes of 80°E to 91°E, there is E-W extension of 20.2±1.2 mm/a. The slip rates of KJFZ in south Tibet and Altun Mountain fault are 2-3 mm/a and 4-6 mm/a respectively. Our GPS results indicate more than 50 percent of convergence between India  and Eurasia is absorbed by the crustal thickening in Himalayan and Tianshan and there is a lesser than 7 mm/a shortening across the Longmen Mountain fault and its adjacent foreland. These results support the supposition of crustal thickening (Wang Q. et al., 2001; Wang X. et al., 2002; Shen et al., 2001; Zhu et al., 2002; Ren et al., 2002).

The Heilongjiang block is the most stable blocks in China. The South-China block also is a stable block. The North-China block consists of three sub-blocks. The Ordos sub-block is its main body, although its boundary is very active, its inner is ‘cool’ and steady. The deformation velocities of North-China block are slower than that of South-China block, the difference of motion between the South China block and the North China block can not identify whether crust thickening dominates the lateral transfer; but other results (such as the slip rate on the Altun Mountain fault and KJFZ, and contraction rate across Longmen Mountain fault) support the supposition of crust thickening.  The action of Australian plate on Eurasian plate is absorbed rapidly by Java island arc and almost do not affect the crustal deformations of China. The west subduction of Pacific plate is absorbed rapidly by Japan island and Japan straits and do not affect the crustal deformations of China; but, the Philippine plate has great effect on crustal motion of Taiwan. Therefore, combining the results from geology and seismology, we can say, the present-time crustal motion in China is affected not only by Indian plate but also by Philippine sea plate. But, it is not related to the Pacific plate and Australian plate. In addition, the results of GPS measurements show the new division rule of blocks is more reasonable than that of the old. Especially, the introduction of Lhasa sub-block and Qiangtang sub-block in Tibet block can interpret the GPS results, which are not interpreted by the old one Tibet block.

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