Paleoseismic history of an active fault in a coastal area evaluated by arrayed drilling and ground penetrating radar survey : An example of the Funai Fault beneath the western part of Oita Plain, southwestern Japan

Beppu Bay, occupying the western portion of an arc-bisecting dextral fault system, is a tectonic depression that has existed since ca. 5 Ma. Funai Fault is a major normal fault along the southern margin of the tectonic basin. In order to estimate the activity of this fault, sedimentary facies, diatom assemblages, and radiocarbon ages of 17 terrestrial plant fragments and 4 marine carbonate samples were determined from seven drill cores. Based on the analysis, three sedimentary facies, namely delta front, delta plain, and artificial soil, were identified. These accumulation and defor-mation structures were imaged by ground penetrating radar profiling. Based on the results, the latest seismic event was identified at 800 to 400 cal BP based on the depositional ages and diatom assemblages from the uppermost mud layers in the delta plain sediment. The previous event was also identified at ca 2,100 cal BP on the basis of the vertical displacement of the lower mud layer in the delta front sediment. The recurrence interval was calculated at ca 1,700 years.