The Problem With the Plate Boundary in the Sea of Japan

The boundary between the North American and Eurasian plates in NE Asia is 'shifty.' It is sometimes drawn through the Sea of Japan, sometimes through the Hokkaido axial zone, or not at all. The boundary may or may not extend through central Japan along the Itoigawa-Shizuoka Tectonic Line (ISTL). How to explain this uncertainty? Geodetic models suggest that NE Japan moves independently from Eurasia, but the inferred plate boundary cannot be constrained geologically or seismologically. Available published data provides other constraints. A series of destructive (M>7) intraplate earthquakes in the Japan Sea with E-thrusting focal mechanisms was thought to be evidence of nascent subduction, but the 1983 Nihonkai-Chubu (M7.7) earthquake, was shown to have occurred on the boundary between thick oceanic crust and island-arc crust of central Japan. These earthquakes appear to result from reactivation of inherited structures that formed during back-arc opening, rather than from subduction. Furthermore, seismic images show no crustal discontinuities that could be interpreted as a plate boundary. As for the ISTL, available geological and seismological evidence indicates that the southern ISTL marks a W-dipping deformation boundary related to the collision of the Izu-Bonin arc with SW Japan (15 Ma). Although indented by the collision, the geological structure/basement of SW Japan are continuous across the ISTL, making it an unlikely plate boundary. The northern ISTL has a different origin, as an E-dipping boundary fault for a Neogene rift basin. The fault has been inactive in the late Quaternary and, again, does not constitute a major geological discontinuity in the basement. Hence, the problem: geodetic models imply a plate boundary between Japan and Eurasia, but published geological and seismological evidence does not support placing it in the Japan Sea or at the ISTL. If, as studies show, almost half of the convergence between North America and Eurasia is taken up in Hokkaido and across N Japan, the small amount of remaining convergence may be difficult to distinguish given the large elastic response in the upper plate (N Honshu) after the 2011 Tohoku-oki (M9.0) earthquake, and strong coupling at the megathrust. To draw such a plate boundary on tectonic maps implies a degree of certainty about its location which is unfounded.