2008 Volume 2008 Issue 29 Pages 59-70
The Umi fault was first described by Ikeda et al. (2004) as an active fault'of about 9km in length. Fukuoka Prefectural Government organized a research committee for examination and investigation of the Umi fault in 2005. The present study is based on air-photograph analysis, geological and topographical field surveys, soundreflection surveys in the marine areas, drilling and trenching, and tephrochronological studies. Some characters of the Umi fault are clarified through this intensive study.
The Umi fault extends father north and south than was previously known, but it does not extend as far north as Hakata Bay and the Genkainada Sea. The length of the fault is 17km at least. The average vertical slip rate in the late Quaternary is estimated to be 0.02-0.03m/kyr (Activity Rank C in Matsuda 1975). This slip rate of the Umi fault is lower than those of the neighboring Kego and Nishiyama faults. The last activity of the fault was confirmed to be younger than 4300y. B. P. by the trenching study at Yamaura.
The average recurrence interval of the activity is estimated at 29 kyr. This estimation is based on the assumption that the observed slip at the Yamaura trench (0.6m) occurred in one earthquake event and that the slip at Uenohara (1.9m), which is observed on middle terrace surface, formed immediately after the Aso-4 pyroclastic flow, is a result of repeated earthquake events of the same size slip.
