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.
Distributions, characteristics, and activities of offshore fault-related folds which comprise the southern part of the Eastern Boundary Fault Zone of Ishikari Lowland (EBFIL) are clarified by conducting a high-resolution multi-channel seismic reflection survey. To determine termination point of the fault zone, we placed seven survey lines perpendicular to the fault-related folds with a total line length of 107.5 km. We choose a compact water gun (15 cubic inch) as the seismic source to obtain high-resolution imagery of the Late Quaternary and investigate activity of the fault zone. Based on seismic facies, we distinguished packages of offshore sediment (unit 2, 3 and 4, in descending order), which separated by downlap surfaces and correlated to the Middle to the Late Pleistocene. They are progressively deformed by the active fault and compose series of fault-related folds (Fo1, Fo2 and Fo3). Traces of these folds are figured out from the seismic sections obtained by this study and existing surveys. Deformation related to these folds are clearly recognized in the topography of the unit 4 base. The reliefs of strata indicate that the Fo2 fold should have uplifted seafloor at a rate of 0.09-0.20 m/ky since 300 ka. The structures, spatial distributions and growth-rates of the folds suggest that Fo1 and Fo2 should compose series of anticline developing at the front of EBFIL. Besides, Fo2 fold would compose the southern termination of the fault zone. This study has successfully demonstrated that densely operated high-resolution seismic survey can figure out distribution and activity of offshore active faults in the Late Quaternary, even at the outside of shelf.
An ocean bottom seismographic (OBS) experiment was carried out to investigate the present‐day tectonics at the northern end of the Havre Trough, a backarc basin in the southwestern Pacific. Hypocenter distribution shows no seismicity within 50 km of the OBS array but two active regions outside the Trough; one is to the northeast of the array at a bathymetric high between the Lau Basin and Havre Trough; the other is to the southeast of the array beneath the landward slope of the Kermadec Trench. P wavespeed structure obtained from the refraction survey is characterized by a thinned island arc crust with a thickness of 9–10 km. Many volcanic intrusions shown in reflection profiles that support extension tectonics are covered by undisturbed sediments. These suggest little present activity related to rifting and/or spreading.
