Geological studies suggest that megathrust earthquakes, such as the 2011 earthquake off the Pacific coast of Tohoku, have occurred repeatedly along the Japan Trench with a recurrence interval of approximately 1000 years. It is therefore important to examine the tectonic crustal movements of the Japanese Islands based on the observed relative sea level (RSL) changes, which may be a societal demand after the 2011 earthquake off the Pacific coast of Tohoku.
In tectonically active Japanese Islands, observed RSL changes during the late Quaternary are caused by change of ocean volume, spatially and temporally non-uniform tectonic crustal movement associated with the plate subductions and seismic activity and glacial isostatic adjustment (GIA) of the Earth in response to the redistribution of ice and water loads. It is possible to infer average rates of tectonic crustal movement along the Japanese coastlines on three typical timescales of ~50 yr, ~6 kyr (kilo-year) and ~125 kyr based on tide gauge and Holocene RSL observations and the altitudes of marine terraces formed at the last interglacial (LIG) phase at ~125 kyr BP.
The rates on a timescale of ~50 yr are derived from tide gauge data, thermosteric sea-level changes due to thermal expansion of the oceans and predictions due to the GIA processes for the last deglaciation and also recent melting of the mountain glaciers and Greenland and Antarctic ice sheets (Nakada et al. 2013). Those for ~6 kyr are inferred from many Holocene observations and the predictions by GIA modelling, considering uncertainties for temporal changes in eustatic sea-level for the mid- to late-Holocene. Those for ~1254 kyr are also inferred from many marine terraces formed at the LIG. Many marine terraces for the LIG are observed at altitudes higher than 40 m at sites facing the Pacific Ocean. However, terraces with an altitude higher than 10 m have not been reported at sites along the seismically inactive west coast of Kyushu. In fact, there are few active faults in this area. Thus, the long-term crustal deformation inferred from the altitude of LIG terraces appears to be very different along the various Japanese coastlines.
Our studies based on these observations show that the rates of vertical tectonic crustal movements on a timescale of ~6 kyr are consistent with the rates for ~125 kyr and GIA-predictions in many sites, but inconsistent with those for ~50 yr in most sites except for a few sites, indicating that the rates on a timescale of ~50 yr are not representative of the tectonic crustal movements for timescales longer than ~6 kyr in most sites along the Japanese coastlines. However, the inferred rates on these timescales may be useful in discussing the recurrence of megathrust earthquake with its interval of ~ 1 kyr such as the 2011 earthquake off the Pacific coast of Tohoku. Please see the following papers: