HR: 1340h
AN: T13C-1387    [Abstracts]
TI: Dive Surveys for Evaluation of Offshore Active Faults Along the Eastern Margin of Japan Sea
AU: * Okamura, Y
EM: okamura-y@aist.go.jp
AF: Active Fault Research Center, GSJ/AIST, Higashi, Tsukuba, 305-8567 Japan
AU: Satake, K
EM: kenji.satake@aist.go.jp
AF: Active Fault Research Center, GSJ/AIST, Higashi, Tsukuba, 305-8567 Japan
AU: Ikehara, K
EM: k-ikehara@aist.go.jp
AF: Institute for Geo-Resources and Environment, GSJ/AIST, Higashi, Tsukuba, 305-8567 Japan
AU: Takeuchi, A
EM: takeuchi@sci.toyama-u.ac.jp
AF: Faculty of Science, Toyama University, Gofuku 3190, Toyama, 930-8555 Japan
AB: The eastern margin of Japan Sea is an incipient subduction zone between the Amurian and Okhotuk Plates and is characterized by widely distributed active reverse faults. During the 20th century, four major earthquakes (M>7.5) occurred along the margin. The recurrence intervals of the earthqaukes are (probably) longer than several hundreds years, longer than the coverage of historical records. Turbidites have been used as records of paleoseismicity in the margin, and they provided recurrence intervals of earthquakes along several reverse fault zones. Turbidites, however, are not always distributed around reverse faults. In addition, turbidites in a core may be records of earthquakes along different faults around basins. Seafloor disruptions were widely observed by manned submersibles "Shinkai 2000" and "Shinkai 6500" in the source area of the 1993 Hokkaido-Nansei-Oki earthquake (e.g. Takeuchi et al., J. Geophys. Res., 103, 24109-24125, 1998). The dive surveys suggested that earthquakes have been recorded as seafloor disruptions, thus we have been conducting dive surveys to clarify paleoseisimicity of the active faults from visual observation of seafloor. Based on the observation of 15 dives in the margin, we defined two types of slopes which show different disruptions. The first type of slopes is composed of consolidated and highly fractured rocks. The slope is sporadically covered by unstable debris of sand to boulder sizes, and the basin floor on the foot of the slope contains debris layers less than a few centimeters in hemipelagic muddy sediments. We interpreted that the layers are records of ancient earthquakes. The other type of slopes is generally composed of semi-consolidated muddy sediments, and debris is not widely observed on slopes and basin floors. The more widespread disruptions are fissures and small-scale slope failures. We can identify recent ruptures along the fault but hardly determine ages of ruptures on this type of faults. We conclude that observation of seafloor using submersibles is useful to judge offshore faults to be active or not, however, it is not always easy to determine paleoseismicity of faults, particularly for the age. We will discuss possibility and limit of dive surveys of offshore active faults.
DE: 7221 Paleoseismology
DE: 8010 Fractures and faults
DE: 8102 Continental contractional orogenic belts
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
DE: 3045 Seafloor morphology and bottom photography
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting