Quaternary Crustal Movement in the Oga Peninsula

Abstract

In order to clarify Quaternary crustal movements in the Oga peninsula which belongs to the shallow earthquake zone along Japan Sea coast, the present writer examined crustal movements from regional distribution of raised shoreline, and compared them with co-earthquake crustal movement of Oga earthquake (1939, Mj=7.0).1) Raised shorelines on four marine terraces are 130-180m (TI), 70-125m (TII), 40=85m (TIII) and 25-55m (TIV) in altitude. Their maximum altitudes on each terrace are in the southwestern part of the peninsula and they decline northeast ward (Figs. 3, 4)2) Former shorlines resemble each other in deformantion pattern and more inclined on higher terraces (4.6×10-3: TII, 3.0×10-3: TIII, 1.9×10-3: TIV) and crustal movements have continued similar mode since the formation of TII.3) Fault movement (Fig. 6), tsunami analysis (Kishinoue, et al., 1939) and distribution of aftershocks (Fig. 5-B), is a thrust (45°E) with slight left-lateral component. This thrust is located along the western coast of the Oga peninsula (N60°E: Dip direction).4) Deformation of the raised shorelines can be explained as accumulation of co-earthquake crustal movements of Oga earthquake type. Northeastward inclination of raised shorelines of TIII is 2.2×10-3 and that of the co-earthquake deformation (1939) is 4×10-5. According to presumed interval of earthquake in this area (about 1600-2000 years: Usami, 1975), TIII is aged 11-12×104 years by rate of inclination and age is concordant with the age of Terrace (TIII) assumed by Ota (1968).5) Elevation of terraces, however, can not be explained only by accumulation of co-earthquake crustal movement. Long wave length crustal movement and eustatic movement may be superimposed on co-earthquake movement in this area (Fig. 7).