Continuous Holocene Submergence of Southern Sanriku Coast Consistent with the Coseismic Subsidence of the 2011 Tohoku-Oki Japan Earthquake Revealed from New Paleo-Geodetic Data

Abstract:

Time-dependent inconsistency of crustal movement is suggested in the Sanriku coast, northeast Japan. Coseismic subsidence up to 1.3 m of the 2011 M=9.0 Tohoku-oki earthquake and a century-long subsidence rate of 1 – 10 mm/yr apparently contradict to long-term uplift rate of 0.3-0.5 mm/yr estimated from a flight of Pleistocene marine terraces. Several possible scenarios may explain such inconsistency: 1) the 2011 event was typical, contributing long-term subsidence of the 10⁴yr time scale, 2) a significant interseismic contribution to long-term coastal uplift, or 3) a hidden huge event may occur to uplift the coastal region. To narrow the possible scenarios and reveal the history of vertical movement, we have been acquiring more paleo-geodetic data with better age constraints.

 This study detected continuous Holocene subsidence from a 40-m core data of the Rikuzen-takata, the southern Sanriku coast. On the basis of feature of core sediment, sedimentary facies was divided into braided river, tidal influenced environment, delta, and terrestrial marsh, from lower to upper, in ascending order. Age-depth curve was described based on twenty-five ¹⁴C ages. For the estimation of Holocene vertical movement, observed relative sea-level (RSL) was compared with theoretical RSL. RSL at 10 to 9.0 ka was estimated at the altitude of -30 to -27 m by using altitude of depositional surface of tidal deposits shown by both age-depth curve and molluscan shells in intertidal zone. Estimated RSL is lower than theoretical RSL without tectonic effect. This difference can be explained by Holocene tectonic subsidence. Moreover, detected subsidence is consistent with coseisimic subsidence of the 2011 event and a century-long submergence. Unlike the previous arguments of long-term uplift particularly in northern Sanriku, this agreement possibly suggests that the south Sanriku coast instead has been subsided by both the 2011 type coseismic and interseismic deformation on the 10⁴ yr time-scale.