Abstract：A MS6.4 earthquake occurred at Menyuan, Qinghai province, on January 21, 2016. In 1986, a MS6.4 earthquake happened in the same area with the same focal mechanism. This paper analyzes the regional crustal deformation by using data from the 10 continuous GPS stations and 74 campaign GPS stations in a 200 km range of these events. (1)Based on the velocity field of over ten year’s GPS data, the results of crustal strain rates and seismic moment accumulation rates of the Qilian-Haiyuan fault are significantly higher than other regions. In a 20 km×20 km range of the seismogenic area, the maximum and minimum principal strain rates are 21.5 nanostrain/a (in NW-SE direction, tension) and -46.6 nanostrain/a (in NE-SW direction, extrusion), the seismic moment accumulation rate is 17.4 N·m/a. The principal strain extrusion direction is consistent with the focal mechanism. (2) Based on the GPS continuous stations coordinate time series of 6 years before the event, the result shows the surface expansion at the seismogenic zone is in constant “non-linear” reduced trend since 2010. This means the seismogenic zone is in a state of shrinking. However, 2~3 months before the event, the surface expansion and maximum shear strain occurred with significant inverse trends. These abnormal changes of crustal deformation may reflect the nonlinear adjustment of the stress-strain accumulation when it is close to the critical seismogenic zone.
