Displacement rate of the Uemachi Fault and basement block movements of the Osaka Plain, Southwest Japan

Abstract

The Osaka sedimentary basin is a tectonic subsidence basin. The basement rocks of the Osaka Plain in the northern part of the Osaka sedimentary basin has been broken into two major blocks called West and East Osaka blocks by the Uemachi Fault trending north to south. The Osaka Plain consists of 1, 000-2, 000 m thick Quaternary sediments. Tectonic movements of the basement blocks are estimated by detailed study of the geological section of the Osaka Plain. This section is drawn by the reviewing and compiling deep drilling cores, borehole logs and depth sections of the seismic reflection surveys. The thickness of sediments and the difference of distribution levels of marine clay as time markers indicate the tectonic movements of these basement blocks and the Uemachi Fault. The two basement blocks have been tilting separately, but both have continuously subsided as one basin since ca. 1.2 Ma. The northern part of this basin has subsided from ca. 1.2 Ma to 0.4 Ma with the subsiding rate of 0.7-0.5 m/ky. And then, the subsidence rate has moderated to 0.5-0.2 m/ky.