Characteristics of present-day active strain field of Chinese mainland

On the basis of the GPS data obtained from repeated measurements carried out in 2004 and 2007,the horizontal principal strain of the Chinese mainland is calculated,which shows that the direction of principal compressive strain axis of each subplate is basically consistent with the P-axis of focal mechanism solution and the principal compressive stress axis acquired by geological method.It indicates that the crustal tectonic stress field is relatively stable in regions in a long time.The principal compressive stress axes of Qinghai-Tibet and Xinjiang subplates in the western part of Chinese mainland direct to NS and NNE-SSW,which are controlled by the force from the col-lision of the Eurasia Plate and India Plate.The principal compressive strain axes of Heilongjiang and North China subplates in the eastern part direct to ENE-WSW,which shows that they are subject to the force from the collision and underthrust of the Eurasia Plate to the North America and Pacific plates.At the same time,they are also af-fected by the lateral force from Qinghai-Tibet and Xinjiang subplates.The principal compressive strain axis of South China plate is WNW-ESE,which reflects that it is affected by the force from the collision of Philippine Sea Plate and Eurasia Plate and it is also subject to the lateral force from Qinghai-Tibet subplate.It is apparent from the comparison between the principal compressive strain axes in the periods of 2004～2007 and 2001～2004 that the acting directions of principal compressive stress of subplates in both periods are basically consistent.However,there is certain difference between their directional concentrations of principal compressive stress axes.The sur-face strain rates of different tectonic units in both periods indicate that the events predominating by compressive variation decrease,while the events predominating by tensile change increase.

On the basis of the GPS data obtained from repeated measurements carried out in the period of 2004 to 2007, the horizontal principal strain in Chinese mainland is calculated. The result shows that the direction of principal compressive strain axis of each sub-plate is generally consistent with the P-axis of earthquake focal mechanism solutions and the direction of principal compressive stress axis deduced from geological analysis. This means that the crustal tectonic stress field is relatively stable in a region and for a long period. The principal compressive stress axes of the Qinghai-Xizang and Xinjiang sublates in western China trend in N-S and NNE-SSW, respectively, which are thought to be controlled by the collision between India plate and Eurasia plate. The principal compressive strain axes of the Heilongjiang and North China subplates in eastern China orientate in ENE-WSW, displaying the effect of collision and underthrust of the Eurasia plate to the North America and Pacific plate, and also showing the lateral action applied by the Qinghai-Xizang and Xinjiang sub-plates. The principal compressive strain axis of the South China sub-plate is in the direction of WNW-ESE, reflecting the effect of collision between Philippine Sea plate and Eurasia plate, and also the effect of lateral action from the Qinghai-Xizang sub-plate. It is apparent from the comparison between the trends of principal compressive strain axes in the periods of 2004--2007 and 2001--2004 that the directions of principal compressive stress of the sub-plates in both periods are basically consistent, but there is certain difference in the degree of directional dominance of the stress axes. The plane strain rates of different tectonic units in both periods show a tendency that the events predominated by compressive variation decrease, while the events predominated by tensile change increase.