2018 Volume 127 Issue 6 Pages 735-757
Marine terraces on the northern Sanriku Coast are mapped, verifying their emergent times with a tephra analysis to provide accurate information on long-term coastal uplift along the Sanriku Coast. The results contradict the conventional view of century-long subsidence and coseismic subsidence associated with the 2011 Tohoku-oki earthquake. Eight visible tephra layers are found to cover the Taneichi surfaces reported as Marine Isotope Stage (MIS) 5 marine terraces in previous research on the northern Sanriku Coast, and these correlate with To-H, To-Of, To-G, To-Ok2, To-AP, To-CP, Toya, and ZP2 tephra layers previously identified at the Kamikita Plain based on petrographic properties. Consequently, because the Toya tephra (ca. 112-115 ka) covers marine sand and/or gravel beds composing the terraces, they are confirmed to be MIS 5e marine terraces. The paleo-shoreline of the MIS 5e marine terraces is inferred, considering topographic profiles and aeolian deposits covering terrace surfaces, and their heights are measured with high-resolution digital elevation models to be ca. 40 m above the present sea level at the Kamikita Plain and ca. 20-30 m at the northern Sanriku Coast. In addition, uplift rates from the Kamikita Plain to the northern Sanriku Coast are estimated to be 0.3 mm/yr and 0.1-0.2 mm/yr, respectively, tending to gradually decrease toward the south, and may be subject to subsidence further south along the southern Sanriku Coast.