Aseismic negative dislocation model and deformation analysis of crustal horizontal movement during 1999--2001 in the northeast margin of Qinghai-Xizang block

Through numerical simulation for GPS data, aseismic negative dislocation model for crustal horizontal movement during 1999~2001 in the northeast margin of Qinghai-Xizang block is presented, combined with the spatial distribution of apparent strain field in this area, the characteristics of motion and deformation of active blocks and their boundary faults, together with the place and intensity of strain accumulation are analyzed. It is shown that: a) 9 active blocks appeared totally clockwise motion from eastward by north to eastward by south. Obvious sinistral strike-slip and NE-NEE relative compressive motion between the blocks separated by Qilianshan-Haiyuan fault zone was discovered; b) 20 fault segments (most of them showed compression) locked the relative motion between blocks to varying degrees, among the total, the mid-east segment of Qilianshan fault (containing the place where it meets Riyueshan-Lajishan fault) and the place where it meets Haiyuan fault and Zhuanglanghe fault, more favored accumulation of strain. Moreover, the region where Riyueshan-Lajishan fault meets north boundary of Qaidam block may have strain accumulation to some degree. c) Obtained magnitude of block velocities and locking of their boundaries were less than relevant results for observation in the period of 1993~1999.

Inversion for negative dislocation on elastic block boundaries in the northeastern margin of Qinghai-Xizang block and prediction for strong earthquake location

On the basis of the velocity field results of horizontal crustal movement obtained from GPS measurements during the periods of 1993-1999, 1999-2001 and 2001-2003 in the northeastern margin of Qinghai-Xizang block, and by the inversion of negative dislocation model for the elastic block boundaries, we provide in this paper a qualitative analysis and quantitative description for the difference of motion and deformation between the tectonic blocks and their boundary faults, time-space distribution of tectonic strain field, and locations with highly accumulated strain energy and correlative intensity. Furthermore, taking the regional tectonics and block strain into full consideration, we investigate the common features of background precursors relating to location prediction for M greater than or equal 6 earthquakes.

Characteristics of Recent Horizontal Crustal Movement and Tectonic Deformation in the Northwest China Region

Making use of observation data of GPS in the Northwest China region and infrared distancemeasurements crossing the Qilian-Longshoushan fault zone up to 2004, aided by the least square collocation and inversion of negative dislocation model for the boundaries of elastic blocks and the singular force-source, the dynamic evolution features of deformation and strain fields before and after the Ms = 8. 1 earthquake on the west of Kunlun Mountains Pass, especially the recent tectonic deformation and stress field status three years after this earthquake are studied. The possible regions or segments of active blocks and their boundaries reflecting accumulation background of high strain energy of producing earthquakes over middle magnitude, are obtained, as well as the potential epicenter. The results show that after shortterm relaxation and adjustment in the northern margin of Qinghai-Xizang （Tibet） block after the Ms = 8. 1 earthquake, the main control action of background field of northeastward pushing of Indian plate is now recovering. Moreover, the following regions are found to have the background of high strain energy accumulation. They are the middle segment of the northern Tianshan fault zone and its meeting region with the western segment, the middle and western segments of southern Tianshan fault zone and the meeting region with Western Kunlun fault zone, the middle segment of Altun fault, the middle-eastern segment of Qilianshan fault zone and its meeting region with Haiyuan fault, the meeting region of northern margin fault of west Qinling Range and the southeastward expanding line of Zhuanglanghe fault; The Linze and Haiynan areas also see accumulation of strain energy to some degree.

Regional Tectonic Deformation Background and Medium- and Short-Term Precursors to the Minle-Shandan Earthquakes1

Using GPS observations of horizontal movement from 2001 to 2003 and the cross-fault mobile short-levelling data of 1988～2003, and with the aid of the improved negative dislocation model and the time-varying curve of strain intensity ratio of fault deformation, the regional tectonic deformation background and medium- and short-term precursors related to the preparation of the Minle-Shandan earthquakes of M S6.1 and M S5.8 on October 25, 2003 are investigated. The results reveal that, under the background of the wide-range deformation adjustment, short-term relaxation and recovery caused by the Kunlun Mountains earthquake of M S8.1, the hypocenters of the earthquakes are located on the north edge of the shear stress enhancement zone between the compressional locked segments of block boundary fault, a place which may represent an accelerated strain accumulation. An obvious anomaly of strain intensity ratio appeared in short-levelling measurements crossing over the fault at the Shihuiyaokou site, the closest to the epicenters, 3 months before the occurrence of the earthquakes. In addition, the variation in number of anomalies from 10-odd days to months before the earthquakes in the entire monitoring area and the anomaly concentration and local enhancement relative to near source in the 3 months before the earthquakes are regarded to be precursors to the two events.