Deformed Structures Revealed by Deep and Shallow Seismic Reflection Surveys Around an Inferred Active Fault in the Southern Kyoto Basin, Japan

Kyoto is one of the most famous old cities, which attracts many tourists from all over the world. Although Kyoto has frequently been attacked by large earthquakes as written in many old archives, the area has been abnormally quiet since 1830, except small damages by the 1995 Hyogo-ken Nanbu earthquake. The Kyoto basin is surrounded by many faults. The southwestern end of the basin is limited by a lineament. It is not known whether the lineament is still active or not. To know the detailed geological structure around the lineament, we conducted deep and shallow seismic reflection surveys, collection of boring data and gravity survey in the southern Kyoto basin. Deep reflection surveys revealed that the pre-Neogene basement lies at about 700-800m in depth at the basin side, and becomes rapidly shallow toward the Yawata hillside. We found some faults around the lineament stated above. By correlation with the 800m deep boring KD-0 drilled by the Kyoto City, some marine clay beds of the Plio-Pleistocene Osaka Group, which is named Ma3, Ma4, Ma5, Ma6 and Ma9, can be identified on all seismic sections. Ages of these clay beds are dated very precisely, e.g. Ma3 is about 900ka and Ma9 is about 400ka. The vertical slip rate of the faults can be estimated using these ages as about 0.1m/ky between 400 and 900ka, and negligibly small before 900ka. Shallow high-resolution reflection surveys were conducted using SH-wave type land-streamer. The processing results have shown detailed shallow structures down to 30-40m deep and have revealed distribution of a sand-gravel layer that has N-value of about 30-40 and corresponds to a bearing layer for construction in this area. The sand-gravel layer has some deformed structures with displacements of a few meters, which suggests that the upper extensions of a fault found by deep seismic survey may cause these near-surface displacements. This indicates that the faults may be still active and may migrate toward the basin side.