Complementary study of the Uemachi fault system in the Osaka Basin (2)
-Evaluation of the fault activity based on supplementary boring and re-interpretation of S-wave seismic reflection data-

Yuichi Sugiyama, Futoshi Nanayama, Kenichiro Miura, Takeshi Yoshikawa, Hiroshi Yokota, Masaki Suehiro, Masakazu Furutani,Yasuhiro Tochimoto, Koutarou Hirose, Yoshiharu Yokoyama, Naoko Kitada and Keiji Takemura

Five (three existing and two newly-extracted) continuously cored boreholes linked with a re-interpreted high-resolution S-wave seismic reflection profile restricted the timing of a Holocene activity of the Uemachi fault, Osaka City, within a period between 9,200 and 9,500 calibrated ¹⁴C years BP. The complementary study also revealed that a vertical displacement during the Holocene activity was more than 1.6 m but possibly less than 2.4 m around the Shin Yodo-gawa River. From existing deep boreholes and seismic reflection profiles, an average vertical slip rate in the past 3 My is estimated to be 0.3 m/ky. Based on the long-term slip rate, the recurrence interval is estimated to be approximately seven thousand years. The recurrence interval, however, may have been longer in Late Pleistocene and Holocene because a decline of the fault activity in those periods is suggested by a slowdown in subsidence rate of the downthrown side.

Fig. 1. Map showing the locations of Bor. 1.5 and Bor. 2.5 cored in the 2002 fiscal year and S-wave seismic reflection survey conducted in the 2000 fiscal year on the northern bank of the Shin Yodo-gawa River, Osaka City.	Fig. 2. Summarized correlation of the Shin Yodo-gawa cores.
Fig. 3. Reinterpreted depth-converted seismic reflection profile in an instantaneous-phase expression. See Fig. 1 for the location of the profile line.