Kinematics of the M87 Jet in the Collimation Zone: Gradual Acceleration and Velocity Stratification by Park, Jongho, Hada, Kazuhiro, Kino, Motoki, Nakamura, Masanori, Hodgson, Jeffrey, Ro, Hyunwook, Cui, Yuzhu, Asada, Keiichi, Algaba, Juan Carlos, Sawada-Satoh, Satoko, Lee, Sang-Sung, Cho, Ilje, Shen, Zhiqiang, Jiang, Wu, Trippe, Sascha, Niinuma, Kotaro, Sohn, Bong Won, Jung, Taehyun, Zhao, Guang-Yao, Wajima, Kiyoaki, Tazaki, Fumie, Honma, Mareki, An, Tao, Akiyama, Kazunori, Byun, Do-Young, Kim, Jongsoo, Zhang, Yingkang, Cheng, Xiaopeng, Kobayashi, Hideyuki, Shibata, Katsunori M., Lee, Jee Won, Roh, Duk-Gyoo, Oh, Se-Jin, Yeom, Jae-Hwan, Jung, Dong-Kyu, Oh, Chungsik, Kim, Hyo-Ryoung, Hwang, Ju-Yeon, Hagiwara, Yoshiaki