Interplate coupling off northeastern Japan before the 2011 Tohoku‐oki earthquake, inferred from seafloor geodetic data

Abstract

AbstractSeafloor crustal movements off Miyagi and Fukushima Prefectures, northeastern Japan, observed for about 9 years before the 2011 Tohoku‐oki earthquake (M9.0) with the GPS/acoustic geodetic technique, revealed the spatiotemporal variation of interplate coupling in this region. Off Miyagi Prefecture, temporal variations of the crustal movements are characterized as a strain accumulation‐release process in relation to the 2005 Miyagi‐oki earthquake (M7.2). The intraplate velocity since December 2006, when the strain accumulation seemed to have restarted, is 4–5 cm/yr relative to the North American plate, which is significantly lower than that before the 2005 event, 6–7 cm/yr. This suggests that interplate coupling off Miyagi Prefecture since around 2007 is weaker than that before the 2005 event. On the other hand, off Fukushima Prefecture, a westward crustal movement at a constant rate of about 2 cm/yr relative to the North American plate has been observed for the interseismic period from 2002 to 2008. The interplate coupling off Fukushima Prefecture is significantly weak compared with that off Miyagi Prefecture. This is consistent with the observed coseismic displacements associated with the 2011 event, which are much smaller off Fukushima Prefecture than off Miyagi Prefecture.