Vol. 45 No. 2 SCIENCE IN CHINA (Series D) February 2002 Determination of Euler parameters of Philippine Sea plate and the inferences ZANG Shaoxian (臧绍先) 1 , CHEN Qiyong (陈起永) 1* , NING Jieyuan (宁杰远) 1 , SHEN Zhengkang (沈正康) 2 & LIU Yonggang (刘永刚) 1 1 Department of Geophysics, Peking University, Beijing 100871, China; 2 Department of Earth Sciences, University of California, Los Angeles, USA Correspondence should be addressed to Zang Shaoxian (email: sxzang@pku.edu.cn) Received July 20, 2001 Abstract Euler vectors of 12 plates, including Philippine Sea plate (PH), relative to a randomly fixed Pacific plate(PA) were determined by inverting the 1122 data from NUVEL-1 global plate motion model, earthquake slip vectors along Philippine Sea plate boundary, and GPS observed velocities. Euler vectors of Philippine Sea plate relative to adjacent plates are also gained. Our results are well consistent with observed data and can satisfy the geological and geophysical constraints along the Caroline(CR)-PH and PA-CR boundaries. Deformation of Philippine Sea plate is also discussed by using the plate motion Euler parameters. Keywords: Philippine Sea plate, earthquake slip vectors, intraplate deformation, relative plate motion, plate motion parameters. Euler parameters are the basic parameters describing the plate motion. The Philippine Sea plate, proposed as a distinct plate at the early stage of plate motion theory, plays a special role in the tectonics history of west Pacific and east Eurasia continent. Containing mainly convergent subduction boundaries and lacking of accreting boundary, which is useful to determine relative motion rates, and transform boundary, which can determine relative motion azimuth more accurately than earthquake slip vectors, along the boundaries make it very hard to determine Euler vector of Philippine Sea plate. It is a difficulty and also a focus of plate tectonics. Before the availability of accurate Geodesy data, only slip vectors of thrust earthquakes along the subducted boundary of Philippine Sea plate can be used to constrain its motion, but there exist at least three factors that cause uncertainties when using these earthquake slip vectors. First, the back-arc spreading and the deformation of overriding plate, such as the spreading of Mariana Trough and strike slip fault such as Philippine fault within overriding plate, make the azimuth of earthquake slip vectors differ from that predicted by plate motion. Second, Some small colliding zones around the Philippine Sea plate make earthquake focal mechanism more complicated. Finally, it is uncertain whether the north boundary of Philippine Sea plate is adjacent to Eurasia plate or North America plate, or even a distinct Okhotsk plate. These problems must be carefully analyzed before determining the Euler vector of Philippine Sea plate with intraplate earthquake * Now at Department of Geological Sciences, Northwestern University, Evanston, IL 60208, USA