ACTIVE FAULTING AND GEOMORPHIC DEVELOPMENT OF COASTAL PLAINS IN THE INNER ARC ZONE OF NORTHEAST JAPAN

The Inner Arc Zone of Northeast Japan is characterized by a fold and thrust belt. In this zone, there are four coastal plains (Tsugaru, Noshiro, Shonai and Niigata). Each plain has coastline c. 30 km in length and the width is c. 20 km. The inner margin of plain bounded by hills composed of unconsolidated sedimentary rocks. N-S trending active faults and folds are recognized in and along the inner margin of the plains, and this indicates that these plains are under E-W compression. These plains have differences in their geomorphologic features, although they have a similar tectonic setting. 5e marine terraces are distributed on eastern and western margins in Tsugaru plain, and occupy most of the Noshiro plain, whereas the Shonai and Niigata plain have no marine terraces. The difference is caused by fault activities in each plain. In Tsugaru plain, the active fault runs between 5e marine terrace and the higher terrace or hills along the eastern margin of the plain. Vertical slip rate of this fault is 0.1-0.2 m/ka. The boundary is mainly caused by warping with 20-30 km wavelength. Existence of rather large fault, which is associated with forming of warp, is expected along opposite (east) side of hills. Two types of boundary faults are in Noshiro plain. One runs along inner margin of the plain and another boundsthe western margin of terrace. The former one is not so active (< 0.1 m/ka) at least in the late Pleistocene period. The latter is more active (>0.3 m/ka) and deforms marine and fluvial terraces significantly. Shonai plain is occupied by alluvial plain. The fault, which runs east margin of this plain, is revealed by trenching surveys and seismic reflection surveys. Tectonic landforms are characterized by anticlinal deformation with short wave length (< 1 km). Seismic reflection data shows that low-angle thrust makes a folded under the topographic boundary and surface deformations are related to secondary faults and folds. The inner margin of Niigata plain is bounded by narrow hilly zone. Most of active faults run along the east side of hills. These are west-dipping reverse faults and are related to uplift of hills. The slip rates of these faults are c. 0.2 m/ka in vertical. It seems that these reverse faulting occur on the planes of the normal fault during the tectonic setting was tension.