Abstract: Using the mobile GNSS observations of 1999—2007, 2007—1999, and continuous GNSS observations since 2010.5 and their processing results, horizontal deformation field and its changes before the Minxian—Zhangxian M_S6.6 earthquake have been analyzed. We found that (1) the Minxian—Zhangxian M_S6.6 earthquake occurred in the transition area ofcrust movement, in which movement magnitude is smaller and horizontal deformation is relatively weak; (2) the maximum principal strain of this earthquake zone in 1999—2007 is transition zone from positive to negative strain, the minimum principal strain in 2009—2011 also is transition zone from positive to negative strain, which is relatively large in 1999—2007; (3) the maximum shear strain of this earthquake zone is relatively small, and also is transformation zone for the left and right rotary; (4) the result of the continuous station shows that the earthquake happened in a almost pure thrust tectonic regime, the coseismic deformation is small, its scope is in the range of 10 kilometers from the epicenter; (5) there are no anormaly found in short-time GNSS observations before the earthquake, but there is anabnormaldeformation deficit in trend.

Key words: The northeastern edge of Qinghai—Tibetan plateau, horizontal deformation, Deformation anomaly, Minxian—Zhangxian earthquake