Determination of the Amurian Plate Motion

Dongping Wei and Tetsuzo Seno

Earthquake Research Institute, University of Tokyo, Japan

Graduate School, Academia Sinica, Beijing, P> R> China

in Mantle Dynamics and Plate Interactions in East Asia, edited by Martin Flower ,GeoDynamics Series., AGU, in press, 1998

The Amurian microplate (AM), discriminated from the Eurasian plate (EU), has been suggested to exist in the area including part of north China, Mongolia and Siberia based on seismicity, recent tectonics, and earthquake focal mechanisms. The postulated Amurian plate is in contact with the Eurasian plate (EU), the Okhotsk plate (OK), and the Philippine Sea plate (PH) at its northern, eastern, and southeastern boundaries, respectively. Farther to the east and northeast, the Pacific plate (PA) and the North American plate (NA) border these plates. We determined the motion of the Amurian plate with respect to the surrounding major plates using, as relative plate motion data, the slip vectors of earthquakes that occurred along these boundaries and the NUVEL-1 data along the NA-PA and EU-NA boundaries. For the AM-EU-OK-PH-PA-NA 6-plate system, using the Monte-Carlo box method, we simultaneously computed all the closure-enforced Euler vectors. The Euler vectors obtained for the Amurian plate are: AM-EU (60.42N, 123.25E, 0.025deg/m.y.), AM-OK (53.20N, 141.95E, 0.476deg/m.y.), and AM-PH (50.84N, 158.13E, 1.204deg/m.y.). The AM-EU pole is located to the northeast of the Baikal Rift and to the northwest of the Stanovoy Range. The results of this study imply that the Baikal Rift is extending at a rate of 0.4-0.7 mm/yr, consistent with the geologically estimated rates. The AM-OK Euler vector gives 6-15 mm/yr for the convergence rate in the eastern margin of the Japan Sea and the AM-PH Euler vector gives 51-66 mm/yr for the convergence rate along the Nankai Trough, both of which are larger than the previous estimates without AM because of the eastward motion of the Amurian plate with respect to the Eurasian plate.

Figure 1(a)

Seismicity in east Asia. Earthquake epicenters are from ISC during the period 1964-1987 (mb>4.2, Depth<60 km).

Figure 2

Earthquake slip vector directions along the
EU-AM, AM-OK, OK-PA, PH-PA, and PH-AM boundaries. These data and the other NUVEL-1 NA-PA and NA-EU data are used to derive the best-fitting Euler vectors of OK-AM-EU-PH-PA-NA 6-plate system. The dashed line is the southern AM-EU boundary (see the text for details).

Figure 3 A Sketch map of the Monte-Carlo box method used in this study. The Closure-enforced best-fittingEuler vectors are obtained by moving the k-dimensional boxfrom a high chisquare point to a low chisquare point along the chisquare surface. At each time, k parameters are selected, and theminimum point within the box is searched.

Figure 4

Relative velocities predicted by the Euler vectors obtained in this study. Azimuths are indicated by arrows and rates are shown in a unit of milimeters per year. Arrows around the Okhotsk plate indicate the relative motion directions of OK with respect to other plates. Arrows along EU-AM boundary indicate the motions of AM with respect to EU, and arrows along the AM-PH boundary indicate the motions of PH with respect to AM.