Block Structure Bounded by Active Strike-slip Faults in the Northern Part of Kinki District, Southwest Japan

Abstract

Geological and geophysical surveys were carried out to discover the hidden active faults and to estimate the dimensions of fractured zone in the northern part of Kinki district, southwest Japan. Combining the geological and geophysical methods was effective to these purposes. In the geological survey, the attention was paid to the distribution of artesian springs except for the lineaments and the outcrops. In the geophysical survey, ELF-magnetotelluric and radioactive (γ-ray) methods were principal. Gravity method was applied to the sedimentary basins. The surveyed areas were distributed widely in the expected traces of major active faults beyond their terminals. The results show that the major active strike-slip faults, such as the Yamasaki, the Jumantsuji, the Mitoke, the Yabu-Yagi and the Yamada faults, extend and continue each other through the undiscovered and fault-hidden areas. The newly discovered fracture named the Ute fault is considered to be a tectonic boundary between the Kinki and the Chugoku districts. The resistivity structures of the known active faults, such as the Hanaori, the Mikata and the Yanagase faults, are also confirmed. After these surveys, the pattern of active strike-slip faults suggests that the conjugate set of major strike-slip faults makes domains of land-mass called tectonic block. The Tamba block, which is enclosed by the chain of the Ute, the Yamasaki, the Jumantsuji, the Arima-Takatsuki, the Mitoke and the Yabu-Yagi faults, is a typical example. In the central part of Kinki district, the reverse type of active north-south striking faults is a common feature. The Miyakata fault, which is an example of this type parallel to the Ikoma fault, has a vertical-slip of 200 m or more estimated from the gravity measurements. This fault could be assumed to be a part of the block boundary. After comparison of major active faults, seven tectonic blocks are tentatively recognized in the region to the north of the Median Tectonic Line (MTL). The historical large earthquakes, of magnitude above 5.5, have occurred at and near the block boundaries. Two active chains of block boundaries through historical ages could be recognized as convergence lines of the stress field. One extends from the Mikata-Hanaori fault towards the MTL through the Obaku fault and the Ikoma or the Miyakata fault. The other stretches from the Yanagase fault to the Ise-Bay fault through the Yoro fault. These active chains are situated in both outsides of the blocks composed of a granitic basement. The blocks between these chains are quiet in the intrablock micro-seismicity, but the exterior blocks composed of old and hard sedimentary basement are active.