GPS Crustal Deformation Observation And Its Application In The Great Triangular Seismotectonic Region Of Central Asia

Although most major earthquakes occur on plate boundaries, including ocean mid-ridges, trenches, and transform faults, some of continental interiors are also seismically active, such as central Asia. A map of epicenter distribution in this region delineates a huge triangular area with relatively densely populated quakes (M≥6.0) which account for 4.4% of all events since 1898. It is called the "Great Triangular Seismic Region". It covers 60°-120E and 0°-60°N. Viewed as a plane, it as three boundaries. The southwestern boundary of the triangle lies parallel to the Himalayas. The eastern boundary lies roughly along 105°E, and tectonically along the eastern edges of South China and the Ordos block, and extending northward to Lake Baikal. The third side is the northwestern boundary, which roughly begins at the Pamirs and continues northeastward through the Tian Shan Mountains, the Altai, and onwards to Baikal.Since 1900,121 quakes greater than M7.0 have occurred in this region, including twenties of greater than M8.0. Almost all these earthquakes happened in the Tibetan Plateau, Tien Shan orogen and Baikal rift zone. In recent years, the devastating events in this region include the Mani Ms7.9 in Qinghai on November 8,1997, Kunlun Ms8.1 in Qinghai on November 14,2001, Yutian Ms7.3 in Xingjiang on March 21,2008, Wenchuan Ms8.0 in Sichuan on May 12,2008 and Yushu Ms7.1 in Qinghai on April 14,2010.This great triangular seismotectonic region is a particular natural domain formed by densely distributed major and large earthquakes, laced with plateaus, mountains, basins, and grabens, and complicated active faults, indicative of intensive crustal movement and tectonic deformation. In tectonics it mainly comprises the Tibetan plateau block and Xiyu (West domain) block. A series of large-scale and active arcuate fault systems characterize the Tibetan plateau, such as the West Kunlun fault, Karakorum-Jiali fault, Xianshuihe fault, East Kunlun fault, Altyn Tagh fault and the Red River fault. In the Xiyu block exist the Tienshan fault, Gobi-Altay fault, Altay fault and Bolnay fault and so forth.This thesis focuses on data processing and analysis of high-precision and large-scale GPS measurements. Combined with data of active faults and major earthquakes over 100 years, this work attempts to reveal present-day crustal deformation of the great triangular seismic region in central Asia and its relationship with seismicity. Detailed approaches and research results are described as follows.1. Establishment of a set of high-efficiency and high-precision GPS data automatic processing system. Based on the GP programs developed by USGS, this work has absorbed and adopted the latest models and methods of GNSS data processing. By utilizing and modifying the software TEQC, GIPSY, Ambizap and QOCA, a complete set of perfect pre-processing and post-processing GPS data analysis system was formed. It realizes a series of automatic procedures, including GPS data preparation, GPS data processing, joint adjustment and visual displaying of results. All these have facilitated GPS data processing with high efficiency.2. Integrating GPS data from many channels abroad and determination of the GPS velocity field for the great triangular seismic region in central Asia. This GPS field was determined with two methods:①field campaign GPS observations,②The GPS data of different times and under various frameworks from varied institutions and regions are reduced to a same reference frame using the common points among the data, and rotation transform on the Euler sphere. By consistency tests and eliminating the data of sites that were too dense in distribution, of bad quality or obviously inconsistent with surrounding sites. Finally, the rates at 1647 GPS sites are obtained. It is the first time to show a GPS velocity field in the great triangular seismic region in central Asia and its surroundings.3. Based on the resultant high-accuracy and high-density GPS velocity field, combined with the outlined active tectonics in the great triangular seismic region in central Asia, this thesis summarizes the following main characters of the GPS velocity field and differential motions in the study area:①With respect to the stable Eurasian plate, the northeastward horizontal crustal velocities become smaller successively from-40mm/a at the south Himalaya to-3mm/a at north of Baikal, which reflect the uplift and shorten in the great triangular seismic region in central Asia plunged northeastward by the Indian plate.②In the whole great triangular seismic region in central Asia, the most remarkable differential horizontal crustal motions appear in the Tibetan plateau. Due to hampering of the rigid Tarim and Alashan blocks and the quasi-rigid Ordos block as well as the natural impedance of the northern plateau to the southern regions, notable eastward extrusion-like escape of the Tibetan plateau occurs as it experienced northeast directed compression and shortening.③In the southeastern Tibetan plateau, because of the northeastward intensive insertion of the Asam corner plus the hampering of the quasi-rigid South China block, the highly plastic crustal material of this region rotates clockwise about the East Himalayan syntaxis and slides toward southeast.④The Tarim block plays an important role in the present-day crustal deformation of the whole great triangular seismic region in central Asia. Due to its accommodation and transform, the horizontal motions in this region exhibit a divergence feature in NNE to NE directions. Within the Tarim basin, velocities are relatively uniform implying little deformation of its interior.⑤The Tian Shan area also experiences strong compressive deformation only inferior to the Tibetan plateau. Across the Tian Shan, GPS velocities become rapidly from 15-20mm/a to 1-5mm/a.⑥In the central and western Mongolia and the Baikal region, GPS velocities are generally 20-5mm/a, without obvious regularity. It means that these regions have relatively weak deformation of the crust.⑦As the Pamir region lacks sufficient GPS sites, its differential crustal motions cannot be determined directly. Considering the GPS velocity fields surrounding this region, it is speculated that it may strongly rotate under the hampering of the rigid Tarim block.4. The continuous GPS strain field in the great triangular seismic region in central Asia derived from preferred spatial interpolation. Adopting the 2D high tension spline (τ=0.95) interpolation algorithm, this work made interpolation uniformly to the irregular GPS velocity field with a 0.5°×0.5°grid. Then the regular grid strain rate with 1°×1°grid was calculated by 9 velocity vectors of the boundaries and interiors of each grid, yielding situations of crustal deformation of each area in the great triangular seismic region of central Asia quantitatively and intuitively.①The whole Himalayan arc-shaped terrain is subject to intensive compression and shortening along the convergence direction of the India plate and Eurasia plate with typical compression and shortening strain rate of 30-60×10-9/a, locally reaching 80-90×10-9/a. Such deformation in the western Himalaya is more intensive than that of the eastern Himalaya. Meanwhile, it also experiences slight lateral extension in the direction roughly normal to the plate convergence.②In the Lhasa terrain, the strain rate of compression and shortening in the plate convergence direction declines to20-30×10-9/a. While in the middle its lateral extension is very notable with representative values 20-30×10-9/a, and locally as high as 40-60×10-9/a, which is consistent with widespread NS trending normal faults in this region.③The typical strain rate of mid-west of the Qiangtang block is approximate NS compression and EW extension. The value is about 15-25nanostrain/a.While in the eastern Tibetan plateau, the strain field exhibits obvious nearly NS extension with a rate 20-30 nanostrain/a.④No obvious strain is seen in the Sichuan basin and its north, with the value below 10 nanostrain/a. The Kunlun block and Qaidam block have a uniform strain rate, mainly in a NE-SW compression and NW-SE extension. The strain rate values are 15-25nanostrain/a for compression and 10-15nanostrain/a for extension, respectively.⑤The strain in the Chuandian region southeast of the Tibetan plateau is strong and complex with variable directions. Nevertheless, it is a clear pattern that the approximately EW extension and NS compression characterize this region.⑥In the Tien Shan region, NS compressive deformation is dominant. But the compressive shortening is not uniform, where the maximum strain rate value is about 30nanostrain/a⑦In the Baikal region. NW-SE extensional strain exists in the whole zone. The largest strain rate is 40nanostrain/a. In the eastern Baikal exists some NNW and NNE compression, with the maximum value 10nanostrain/a. Analysis of plane dilatation strain of Baikal shows that it is a whole expansion.5. Division of deformed crustal blocks and average strain analysis for the great triangular seismic region in central Asia. This region comprises the Tibet-Himalaya compression area, Tien Shan compressive orogen, Baikal extensional area and Chuandian shear area. Based on previous studies, according to characteristics of crustal deformation, this work subdivides the whole region into five types of deformed blocks, i.e. quasi-rigid block, purely extensional block, purely compressive block, extension-dominant block, and compression-dominant block. Consequently, the great triangular seismic region in central Asia comprises the following blocks:East Tarim, West Tarim, Ordos, Alasan, South China, Sayan, Amur, Zhungeer, Tien Shan, Qilian, Bayan Hara, Qiangtang, Lhasa, South Yunnan and Chuandian. Among them, the Ordos block, Tarim block, and Alasan block are rigid massifs.6. Fitting and interpretation of the present-day GPS velocity field in the great triangular seismic region in central Asia based on joint usage of the fault dislocation model in half-space, Euler rotation model of the quasi-rigid block and strain-rotation model of the elastic block. This work simplifies and synthesizes the major active faults in the great triangular seismic region in central Asia, builds up the 3D fault segment geometries of each fault in half infinite elastic space, and gives an necessary prior information of fault movements. Constrained with 1233 GPS velocity vectors in the great triangular seismic region, it adjusts the prior slip rate of each fault segment within reasonable ranges, then makes the best fit of GPS velocity vectors with a model of half infinite elastic space containing deep fault dislocations as well as inversion of slip rates of all the fault segments. At the same time, it adopts the rigid block rotation model and elastic block strain and rotation model to describe the rest residuals which can not be well explained.7. Relationship between seismic activity and crustal deformation in the great triangular seismic region. This work analyzes the relationships among spatial-temporal distributions of earthquakes, the field of accumulative seismic moments, field of plane expansion rates and field of maximum shear strains. And it compares the spatial corresponding relationship between the accumulation rates of seismic moments and release of seismic energy. The result shows that although there exists some spatial correlation among the strain rate field, planes expansion rate field, shear strain rate field and earthquakes in the study region, it is of a complicated relationship. On the other hand, the correlation between the seismic energy release and accumulation rate of seismic moments is not obvious. The most possible reason for this result is that only quake data of 100 years are used for calculation of seismic energy release. Compared with the average recurrence intervals of intraplate earthquakes, such a time span is too short to describe the distribution of seismic energy release in a complete seismic cycle.