Inter-plate and intraplate seismotectonic complex deduced from long-term and short-term records of vertical movements of the Sanriku coast on the Northeast Japan forearc

The 2011 off the Pacific coast of Tohoku Earthquake (Mw 9.0) was accompanied by wide crustal subsidence (max. 1.2 m) along the Saniku coast on the Northeast Japan forearc, about 150 km distant from the axis of Japan Trench. This fact led us to qualitatively and quantitatively reexamine the component of coseismic, post-seismic and inter-seismic crustal movements in cumulative long-term uplift of the coast on the forearc. We demonstrate a geodynamic diagram of vertical movements of the coast and refer to another possible intraplate earthquake off the coast, based on geomorphological method and subsurface core analysis. Mid-late Pleistocene marine terraces indicating the average uplift rate of 0.2-0.4 mm/yr are well developed along the northern part of the Sanriku coast. Holocene intermittently emergent shoreline topography is partially recognized at two levels, 4 m and 2 m in altitude. The 14C dates and lithofacies of geologic cores indicate the tendency of successive subsidence and the seeming subsidence rate of 3 mm/yr in Holocene. Recent tidal data show the faster subsidence rate of 5-9 mm/yr in the last 50 years. Furthermore, no historical large earthquakes with distinct coastal uplifts are documented in the last 1200 years. Such complex vertical movement of the Sanriku coast suggests that another unusual coseismic uplift different from the 2011's M9 earthquake occurred during continuous inter-seismic crustal subsidence, which was accompanied by vertical uplift of 5 m along the northern Sanriku coast. The seismic source fault is estimated be under a 150 km long flexural scarp, 20 km off the coast. The expected magnitude and recurrence interval of offshore earthquakes are more than M8 and 1500 years, respectively. This episodic co-seismic uplift by intraplate great earthquakes quantitatively excelled the inter-seismic subsidence by the drag of coupling of plates and the coseismic subsidence at gigantic plate boundary earthquakes, and the total plus balance in vertical movements has possibly produced the sequence of Pleistocene marine terraces corresponding to interglacial high sea-level stands.