On 14 November, 2001, a great earthquake of Ms 8.1 occurred on the Kunlun fault in the Kusai Lake segment of the Kunlun Mountain, Qinghai Province in northwest China. The stresses measured at the sites near the Kunlun fault before and after the earthquake show a great drop after the earthquake. The values of the horizontal maximum principal stress at two sites are 12.9 Mpa and 6.8 Mpa before the earthquake. Measurement of the horizontal maximum principal stress at the same sites after the earthquake, showed values of 3.5 Mpa and 2.2 Mpa respectively. This is only about one‐third of that before the earthquake. Apparently stress that accumulated near the fault before the earthquake was released along with the rupturing of the surface. The direction of the maximum horizontal principal stress also varies before and after the earthquake. The direction is found to deviate from that of the regional structure stress field after the earthquake. Such stress changes offer a basis for predicting large earthquakes in the region.

No abstract

The Qinghai‐Tibet plateau is the most active in tectonic activity. In order to learn the recent state of stress, we have carried out stress measurements at different structural positions and depths and in different rocks by using piezomagnetic stress gauge overcoring method in the central Qinghai‐Tibet plateau. The results of stress measurements indicate that the direction of the maximum principal stress is generally in NE direction. This agrees with the result of the studies of geology, seismology, and geodesy. The values of the maximum principal stress are 3.6‐6.8MPa which are medium or slightly high compared with those obtained elsewhere in China from a viewpoint of regional stress analysis. But the stress measurement site at Amdo is an exception where the direction of the maximum principal stress is NWW and its value is 8.1MPa. This also indicates that the recent state of stress is complex near the plate suture zone. These results of stress measurements can provide important basic data for studies of active faults, geological hazards and engineering stability evaluation in the Qinghai‐Tibet railway construction area as well as for research of continental dynamics.

The study region is a strongly deformed frontal area of the Tibet plateau caused by the northeastward compression of Indian plate. The study on the present tectonic stress field in this region has important meaning for the research of continental dynamics. The values and directions of the ground stress at several sites of Lanzhou‐Maqu area are given in the paper using piezomagnetic stress gauge overcoring method. The measurement sites are situated in Ayishan, Dashui, Gahai, Ma'ai, and Qingshui. The previous stress data are also analyzed for systematically studying the tectonic stress field in the region of Lanzhou‐Maqu and its adjacent area. The result shows that the values of ground stress in depths of tens meters are medium as compared to that in other areas, and increase with depth, the increase rate differs in different tectonic units. The overall direction of maximum horizontal principal stress is northeast with a small variation in different tectonic units. The direction of maximum horizontal principal stress is nearly west‐east in Ordos block, NNW‐NE in Hexizoulang. The direction of maximum principal stress changes considerably in the southeast part of Qilanshan Mountains. The west Qinling Mountain is a transition zone of ground stress. The direction of maximum horizontal principal stress changes gradually from NE in the north to EW in the middle and SEE in the south part. The results agree with the strain field in this area from GPS observation.

This paper provides the magnitudes and directions of current stress measured at different sites in the Yangbajain‐Kangmar region in Tibet. In order to learn the current state of stress in Qinghai‐Tibet plateau and to meet the demands of Qinghai‐Tibet railway construction, we carried out in‐situ stress measurements in the region of Yangbajain‐Kangmar. The measurement sites were located at different tectonic positions, Yangbajain, Lhasa, Quxu and Kangmar, the test depth was between 11 m and 18 m, and piezomagnetic stress gauges were used. The results of stress measurement indicate that the direction of maximum principal stress is mainly NW‐NNW and the values of maximum principal stress is between 3.3 and 10.4 MPa, which are high or middle as compared with the values obtained elsewhere in China. But at Quxu site in the plate suture zone, the value of maximum principal stress is 2.3 MPa, which may indicate that the stress is partially released and the stress state is very complex in the plate suture zone.