Holocene Marine Terraces and Holocene Tectonic Movements on the Nishi-Tsugaru Coast, Northeast Japan

Abstract

Nishi-tsugaru Coast is located in the lnner Arc of the Northeast Japan Arc. More than six levels of the Pleistocene marine terraces have developed on the coast. This paper describes height distribution of Holocene marine terraces including emerged benches and discusses Holocene tectonic movements on the Nishi-tsugaru Coast. The results are summarized as follows.1) Four levels of Holocene shorelines are observed on the Nishi-tsugaru Coast and are named LI, LII, BI, and BII in descending order. LI and LII are marine terraces and BI and BII are emerged benches. The lowest paleoshoreline BII has emerged during the earthquakes that occured around lwadate, southern part of this coast in 1704 and off the Cape of Ohdose, northern part of it in 1793.2) The formative age of the highest terrace LI is ca 6, 000yr B. P. by 14C dating. It's height ranges 5-8m a. s. l.. Maximun uplift rate reaching to 1.3mm/yr indicates that Nishi-tsugaru Coast is one of the most active region in the Honshu Island.3) The most rapidly uplifted Holocene terraces are distributed in the areas where Pleistocene marine terraces have been uplifted exceedingly. These areas coincide with the places where the coast traverses Mts. Masugata in the north and Mts. Shiragami in the south. The height distribution pattern of each Holocene paleoshoreline is very similar to that of the lowest bench emerged at the earthquakes in 18th century. Development of Holocene marine terraces in the Nishi-tsugaru Coast is attributed to intermittent upheaval of Mts. Masugata and Mts. Shiragami accompanied by earthquakes through Pleistocene.