The West off Fukuoka Earthquake which occurred on March 20th 2005, caused great concern for the Kego fault, because the Earthquake fault appeared on the northwestern extension of the Kego fault system. This paper reports on the Kego fault as seen by trenching studies atYakuin, which took advantage of construction of the subway Nanakuma-line in 1998.
To clarify the exact position and shape of the Kego fault, borehole data analysis was carried out along the Kego fault in Fukuoka city. The analysis clarified the existence of steep trough, which was formed by the Kego fault activity, which reached a depth of 85m to the bottom of the Quaternary deposits. Several borehole cores which drilled on parallel with Yakuin -trench revealed that the fault plain was almost vertical but reverse tilted near the subsurface.
The Kego fault was found 14 m below the ground surface and about 5 m west of the Yakuin-trench site. The fault cut the upper Pleistocene deposits almost vertically and slipped left with a N 30-40° W strike. By the detail observation it is concluded that there is no clear evidence of the fault was found in the Holocene deposits, Sumiyoshi formation. but more than 2 events of the fault were estimated from the disturbed upper Pleistocene horizons.
Shimoyama et al. (1999) reported that 2 fault events were found during the last 32,000 years at the Osanotrench, the southern part of the Kego fault, but they could not determine the age of the latest events. Yakuin-trench study concluded that the latest event occurred between 16,000 y. B. P. and 10,000 y. B. P. The mean return period of the Kego fault event was determined to be 15,500 years by the Osano-trench study (Table 2). From these data, it is calculated that the probability of the next fault event occurring during next 30 years is 0.4%. Fukuoka prefecture (1996) determined that the length of the Kego fault was 18.5 km, but this study revises the Kego fault length to 22.0 km, because of the northwestern extension was found in Hakata bay.