Evaluating Earthquake Potential Caused by the Active Kego Fault Beneath the Largest Urban Area in Southwest Japan Based on the Paleoseismological Surveys

Three surface-rupturing events were identified on the Kego fault in Southwest Japan, located on southeastern extension of aftershock area for the 2005 M7.0 (JMA) west off Fukuoka earthquake. We conducted trench and boring excavation surveys to evaluate the earthquake potential caused by the fault. The last event on the southern part of the Kego fault occurred about 4,300 cal yBP or younger and the penultimate one occurred about 9,000 cal yBP, and the third one occurred about 15,000 cal yBP. The date of the last event, together with mean recurrence interval roughly estimated as 5,000 years implies a high probability (more than 3 percent) of the fault to produce a large earthquake in the next 100 years. On March 20th 2005, M7.0 (JMA) west off Fukuoka earthquake occurred at northern Kyushu, Southwest Japan. Based on the calculation of the Coulomb stress change on the Kego fault, the earthquake probability for the next 30 years on the fault is raised up to about 7 percent due to the stress transferred from the earthquake. The Kego fault is a NW-SE-trending left-lateral strike-slip fault extending 22 km beneath the Fukuoka City area, which has the largest population in the southwest Japan. We excavated two trenches and eighteen borings on the fault in order to know the exact elapsed time since the latest surface-rupturing earthquake on the Kego fault and its recurrence interval, which are critical factors to determine the probability. The most valuable information came from the Kamiori site in the southern part and we estimated three events (earthquakes) as below; 1) 7,500 cal yBP (probably 4,300 cal yBP) or younger, 2) between 9,520 cal yBP and 8,760 cal yBP and 3) about 15,000 cal yBP.