Seismic Strain Field On The Chinese Mainland And Numerical Model For Kinematics Of The Tibetan Plateau

The Chinese mainland is located in the southeastern part of the Eurasian Plate. The effect between the Indian Plate, Pacific Plate, Philippine Sea plates and Eurasian Plate and the deep dynamics in the Eurasian Plate construct the different active tectonics on the Chinese mainland. It also control the spatial pattern of continental strong earthquakes on the Chinese mainland. The hypothesis of active tectonic block was put forward in the project of "the mechanism and prediction of continental strong earthquakes" under the National Key Project of Science and Technology Development Programmer. The hypothesis was developed to study the mechanism of continental strong earthquakes and to predict the continental strong earthquakes on the Chinese mainland. Taking the hypothesis of active tectonic block as the academic basis, we use the different kinds of data to study the motion of active tectonic blocks and the crustal stress and strain fields on the Chinese continental. The data include the seismic data, geological data and geodetic data, et al. We study the present-day crustal movement and crustal stress and strain fields together with seismicity, and present the basic characteristics of the seismic strain field on the Chinese mainland. The domination of the motion and deformation state of active tectonic block to the whole strong earthquake activity level in the Chinese mainland is discussed too. At last, to discuss further the relation between the motion and deformation of active tectonic block and the seismicity, the numerical model of Tibet Plateau are constructed.Firstly, based on the distribution of global earthquakes and their kinematic and dynamic characteristics, the most active global-scale tectonics can be divided into three first-order tectonic system: the circum-Pacific deep subduction tectonic system, which is the region that oceanic lithosphere subduct under continental lithosphere; the mid-oceanic ridge tectonic system, where the joint tectonics of rift valleys and transform faults distribute widely; and the continent-continent shallow underthrusting tectonic system, where the different kinds of continental faults affect each other. We use the Harvard CMT catalogue, which provides various parameters of hypocenter, to discuss the focal faulting types, seismicities, and distribution of focal depths, et al. in different tectonic system. Classed by the focal faulting types, global earthquakes are mainly reverse faults and strike-slip faults. We can infer that the most directions of the principal compressive stress axis are near horizontal. In three global tectonic systems, the difference between focal ruptures indicates difference between the tectonic dynamics in them. The circum-Pacifictectonic system is the subduction zone of oceanic crust. The reverse faults illuminate the compressive stress state. The mid-oceanic ridge is accreting boundaries. The normal-fault ruptures indicate the tensive stress state in mid-oceanic regions. At the same time, focal types in continental tectonic system indicate the complicated tectonic stress state in it. Seismicities among three tectonic systems are also different. It shows the difference of tectonic activity, b-value in mid-oceanic ridge is large, and is relatively small in continental region. It shows the difference of material property and stress state. Considering the focal depth,earthquakes occur in continent and mid-oceanic ridge are mainly shallow ones. In circum-Pacific regions, more middle and deep earthquakes occur. Statistics results show that the focal depth of shallow earthquakes are usually large than their centroid depth. It indicate that the initial rupture easily occur in the deeper crust, and the ruptures mainly propagate upwards. These results provide meaningful constraint for the study of Chinese mainland.Secondly, we use the method of the Discontinue Deformation Analysis (DDA) on a spherical surface and the GPS survey results observed from 1999 to 2001 to calculate the movements and deformations of each active tectonic block. DDA not only present the displacement-deformation function that consider both the unitive motion of blocks and the continue deformation in blocks, but also consider the interaction between blocks in a mud-block system. It describe both the unitive motion and continue deformation of active tectonic blocks in the Chinese mainland, and it describe the pattern of kinematics and dynamics in the Chinese mainland better. The calculation results show that the movement and deformation of each active tectonic block on the Chinese mainland is different. Especially, the difference between blocks in western China and in eastern China is large. The movement and deformation of blocks in western China is stronger than that in eastern China, and the movement and deformation of blocks in one active tectonic block region are concordant to a certain extent at the same time. For example, the different motion and deformation of blocks in eastern China is small; the movement directions of blocks in Tibetan Plateau are dextra-rotating from south to north step to step. Then, for the boundaries of blocks all consist of active tectonic zones, and the results of boundaries' movement, calculated by using DDA on the GPS survey data, correspond to results of the geological survey. Since the active fault zones are centralized zones of tectonic activity, tectonic deformation, stress and strain etc, most earthquakes with M > 7.0 since 1988 on the Chinese continent occurred in the boundaries zone with larger slip rates or theboundaries zone with larger different maximum shear strain rate.Thirdly, taking the primary researches on the delineation, the motion and the interaction of active tectonic blocks on the Chinese mainland into consideration, and considering the homogeneity of seismicity in space, time, and magnitude in variation block regions, the dynamic evolution of geological tectonics and their correlation, and the differences of strain energy accumulation and release in deep crust, deformation process and geo-dynamics etc., the China continent is divided into 6 seismic sub-regions based on the division results of block regions. The seismic sub-regions are Northeastern China, North China, South China, Xiyu, Tibetan Plateau and Yunnan-Burma sub-regions. They are corresponding to the Chinese part of block regions, respectively. Based on the division, the mean stress and strain state of each sub-regions are calculated by using focal mechanism solutions of major earthquakes and historical strong earthquake data, combined with composite plane solution of small earthquakes. The results show the the major earthquake data are enough for studying of mean tectonic stress state in the crust of a large region. When the focal mechanism data are insufficient, considering the historical strong earthquakes, combine with composite fault plane solution of small earthquakes, the results of stress and strain in crust are more accurate and reliable. The maximum shear strain rates and earthquake strain release rate show the difference of crustal motion between east China and west China, and the crustal motion in west China are stronger than in east China. Comparing the maximum shear strain rates with earthquake strain release rate in seismic sub-regions, the maximal shear strain rates are linear correlated with seismicity level. Comparing the stress and strain state with strain states derived from GPS survey in each seismic sub-region, the two kinds of results are coherent. It shows the domination of the motion and deformation state to the whole strong earthquake activity level in Chinese mainland. It shows the interrelation between motion and deformation of active tectonic block and seismicity.Then, the problem that how to calculate the seismic strain field using different seismic parameters and complete historical earthquake catalogue is discussed in the paper. The statistic relationship between magnitude of surface wave Ms and scale seismic moment M0 in different region of China is presented using Harvard CMT catalogue and present-day earthquake catalogue of China. The statistical relations of Ms and M0 in different region of China are different. It may show the difference of seismicity between different active block regions. Then, we use the statistical relations of Ms and M0 andthe complete historical earthquake catalogue on the Chinese continental to deduce the distribution of seismic maximum shear strain rate field. The seismic strain filed (seismic maximum shear strain rate filed) is coincident with the GPS strain field. The distribution of strain field show that the Tibetan Plateau and its surrounding regions are the regions with a high-value of maximal shear strain rate. The distribution pattern indicates the domination of the northward motion of Indian Plate to crustal movement and seismicity on the Chinese continent. The strain rates in the Pamirs and Assam are the largest. It indicate that the two regions are the region with the most intense deformation and seismicity. The distribution of seismic apparent strain on the Chinese continent is also studied using the broadband seismic radiate energy catalogue, which released by NEIC, and the Harvard CMT catalogue. The apparent stains of earthquakes which located on the surrounding regions of Tibetan Plateau are higher, and the distribution of apparent strain relates with the intensity of crustal deformation.Through the above studies, we get the basic pattern of strain field on the Chinese continent, and discuss simply the relation between crustal movement and deformation and strong earthquakes. For The crustal motion and deformation in and near the Tibetan Plateau region are the most intensive around Chinese mainland, we choose the Tibetan Plateau active tectonic block region as the research region, and use the numerical method to simulate its kinematic characteristics. Taking the researches on active tectonic blocks and their boundary zones into consideration, an 2-dimension elastic finite element model of the Tibetan Plateau is constructed to explore hazards of the actual crustal motion and deformation on the Tibetan Plateau. The FEM model include the geological units in various grades, such as active tectonic blocks, active tectonic boundary zones and active fault zones, etc. The GPS survey results observed from 1999 to 2001 are used as boundary constraining condition to model the movement and deformation in the accumulating and releasing processes of crustal elastic strain energy. The simulation results show that the eastward movement of crust around longitude 90°E is the fastest. It indicate that the eastward movement of crust in Tibetan Plateau may be form 90°E. In the spatial span without great earthquake, the crust of Tibet Plateau move unitively, and the profile of interior deformation on the Tibet Plateau is continuous. The strain distribution on this Plateau is different among blocks and boundary zones. The deformation within block region is relatively small and its distribution is symmetric; the deformation on active boundary zones is large and its distribution is asymmetric relatively. Such a characteristic...