- THE TOHOKU GEOGRAPHICAL ASSOCIATION
- 東北地理 (ISSN:03872777)
- vol.28, no.2, pp.111-120, 1976
Nagamachi-Rifu Dislocation Line runs in the NE-SW direction marking the boundary between Sendai Coastal Plain in the southeast and hilly land in the northwest. Along this dislocation line, deformation of terraces of the Hirose and Natori rivers have been noticed by many scholars. The present writers have investigated intensively the mode of deformation of the terraces and structure of Pliocene Sendai group, in order to clarify characteristics of Quaternary crustal movements along the dislocation line.<br>Main results are as follows:<br>1) The dislcoated terraces, i. e. Aobayama, Dainohara, Uwamachi, Nagamachi and Shimomachi Terraces in descending order, form an elevated zone stretching parallel to the dislocation line for 10km with width of 1km. On the northwestern margin of the zone, Dainenjiyama Faults (thrusts) are located shaping scarplets or gentle slopes and terrace deposits are often displaced on the faults. On the southeastern margin, a flexure structure associated with subsurface Nagamachi-Rifu Fault, which is presumed to be a thrust, forms conspicuous scarplets or steep slopes accompanying warped terrace deposits which submerge below alluvium.<br>2) These terraces are deformed with the greater rate on the older terrace in a same mode and the mode is also in harmony with that of Pliocene Sendai group. Average rates of vertical displacements on Dainenjiyama Faults and Nagamachi-Rifu Line are calculated to be 0.1+mm per year and 0.5mm per year respectively since formation of Aobayama Terrace III. Deformation rate of this terrace to that of uppermost Pliocene Dainenji bed is about 2 to 3.<br>3) Most of minor faults along the dislocation line are thrusts chiefly with dip-slip components and their strikes appear in the NE-SW direction, which coincide with characteristics of Dainenjiyama Faults and Shishiochi Fault located 1km to the west.<br>4) Judging from the deformation of Sendai Group with brittle shear fracture and unchangeable thickness of the group, commencement of crustal movements along Nagamachi-Rifu Line may be dated in Quaternary succeeding to the emergence of uppermost Pliocene Dainenji bed.<br>5) Maximum compressive principal stress axis, which is elucidated by the characteristics of major and minor faults along the dislocation line, lies in the NW-SE direction and coincides with the direction deduced from focal mechanisms of recent major earthquakes and microearthquakes analysis in the surrounding area.<br>6) This maximum stress direction pattern also concides with stress field inferred from characteristics of notable active faults in the vicinity. Some of historical great earthquakes have occurred along these faults indicating that these faults are still active under the same stress field.<br>Therefore it can be said that during Quaternary period continuous and accumulative crustal movements have been taking place along Nagamachi-Rifu Dislcoation Line under the NW-SE compressive stress field.