Offshore extension of the Kakuda-Yahiko fault and its fault-propagating fold are investigated based on combination of high-resolution seismic surveys and sedimentary analysis to evaluate their Holocene activity. High-resolution multi-channel seismic surveys with the boomer sonar (13 lines) and high-frequency seismic survey with the chirp sonar (3 lines) ran across the fault line and its extension, and provided us sharp imageries of stratigraphic succession formed after the last lowstand. We can clearly recognize progressive deformation at the eastern-wing of the fault-propagating fold in the observed succession. To determine actual ages of the seismic strata, we measured radiocarbon dates from sedimentary cores obtained by 2 offshore borings and 2 vibro-coring surveys. Based on deformation of strata between 10.8 ka and 4.7 ka, averaged vertical slip rate was figured out to be 2.1 m/ky. On the other hand, we investigated temporal changes of accumulation rates at both the upthrown and the downthrown sides of the fault. Dynamic changes of the accumulation rates indicate that there were approximately 3 events at least during the last 10.8 ky and the averaged vertical slip rate was 3.0 m/ky during the period. Another focus of our study is on the latest activity of the fault. On the fault-propagating fold, we distinguished the uppermost strata younger than 0.9 ka without any deformation from these older than 2.1 ka with obvious fault deformation based on the profile obtained by the chirp-sonar seismic survey. These observations affirm that the latest event should have occurred between 0.9 ka and 2.1 ka. Furthermore, structural analysis on the deformed strata of 2.1 ka indicates that throw of the fault at the latest “single” activity was ca. 7.5 m. This study successfully demonstrated that set of parameters about the active fault including average slip rate, activity history, timing of the latest event, and associate surface displacement per activity of the fault could be elaborated from series of offshore active fault surveys.
Elongated trough-like depressions were discovered on the shelf break, southwest off the Kyushu. Each depression is 200−250 meters in width, hundreds meters to 2 kilometers in length and 10−20 meters in relative depth. The depressions are discontinuously aligned for 18 kilometers parallel to the strike of the shelf slope. The shelf and upper shelf slope consist of a deposit of the Pleistocene Ito pyroclastic flow in origin. The distribution of the depressions and geological information indicate that the depressions are not any of erosional features by tidal currents or gravitational flows, but “crown crack”, which is a kind of tensional cracks on the top of a submarine slide body during the initial stage of sliding.
