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tokens: We acknowledge the referee for valuable comments that have improved the paper a lot. M.L.G. thanks M. Lister and G.-Y Zhao for helpful comments on the model fitting method to the VLBA data. This work is supported by the National Natural Science Foundation of China (No. 11273050); the 973 Program 2015CB857100; Key Laboratory of Radio Astronomy, Chinese Academy of Sciences; and the program of the Light in China’s Western Region (Grant No. YBXM-2014-02). N.M. is funded by an ASI fellowship under contract number I/005/11/0. This research has made use of data from the MOJAVE database that is maintained by the MOJAVE team (Lister et al. 2009 ). This research has made use of data from the University of Michigan Radio Astronomy Observatory which has been supported by the University of Michigan and by a series of grants from the National Science Foundation, most recently AST-0607523.