CONSTRAINTS ON INTERSEISMIC DEFORMATION AT JAPAN TRENCH FROM VLBI DATA

We use space geodetic data from very long baseline interferometry to estimate the velocity relative to the plates of two sites near Tokyo. We use these results and elastic models of interseismic deformation to put constraints on the slip rate along the main thrust at the Japan subduction zone. The sites at Kashima and Tsukuba lie 150 and 200 km, respectively, west of the axis of the Japan trench, where the Pacific plate subducts at approximately 97 mm/yr. The two sites also lie 160 km from the Sagami trough, where the Philippine plate subducts obliquely at approximately 32 mm/yr. Relative to the North American plate Kashima moves at 13+/-2 mm/yr toward N46+/-4-degrees-W and Tsukuba moves at 9+/-3 mm/yr toward N42+/-19-degrees-W. These observed velocities are intermediate between the velocities of the Pacific and Eurasian plates, consistent with the overriding plate being the Eurasian plate. In this case the observed velocities reflect the sum of (a) permanent west-northwest shortening in northern Honshu and (b) elastic deformation due to locking the main thrust fault at Japan trench. But the observed velocities are also part way between the velocities of the Philippine and North American plates, and they may result from Philippine-North American deformation. Nevertheless, the observed velocities limit the locked segment of the main thrust at Japan trench to no more than 27 km vertically or 100 km along dip. Thus, the main Pacific plate thrust fault is not strongly coupled, part of it must creep, and it probably does not generate great earthquakes.