Rate of displacement and recurrence time of earthquake of the submarine active fault off Tokai

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The objectives of this research program is 1)to establish the method of deep-sea active fault surveying system(integration of high resolution underwater positioning system + deeptow surveying system, and high resolution underwater positioning system + pinpoint sampling system from oceanfloor),and 2) to determine the rate of offset of the active faults off Tokai and the recurrence time of great earthquake using the method of deep-sea active fault surveying system. In the first two years, we spent our time to construct new survey method.In 2001, we applied our new method to the Enshu fault system located in Kumano Basin using Hakuho-maru. The Enshu fault system is characterized by dextral fault with vertical displacement and mudvokanoes are aligned on the fault. We can identify displacements on the mudvolcanoes induced by fault movement.As a first step, we performed remote sensing survey across the Enshu fault system using a high resolution underwater positioning system + deeptow surveyi … More ng system, and obtained high resolution bathymetric map with a 2m counter, backscattering image of ocean floor, and subbottom cross section across the Enshu Faults system. Next, we selected two position for sampling where the rate of displacement is likely to be calculated based on the remote sensing data, and performed piston core sampling using responders attached on piston corer.The operation to make the piston core approach to the intended sampling point with high accuracy requires a high ship navigation technique in case of no remote operation capability installed on the piston core. But the high ship navigation technique of the crew members of Hakuho-maru enable to take samples from the intended points.The results of core sample analysis revealed, we can not estimate the recurrence time of great earthquake, becausecorrelated layers are not identified hi the cores obtained from the two sites. So, we tried to estimate the recurrence time of great earthquake based on the event time of mud vokano eruption, assuming that the mud eruption corresponds with the earthquake event inducing the displacement of fault. Isotropic age determination revealed that the youngest age of the mud eruption event is 23KMa. So, we can not estimate the recurrence time of earthquake.We applied 3D pre-stack depth migration process to the 3D MCS data sets obtained from Kodaiba fault system in order to clarify the structural relationship between surface manifestation of the Kodaiba fault system and the deep seated decollement(seismogenic zone? ). The results of the process suggests that 1)the decollement is characterized by high reflectivity in a range of 5km from the point where the Kodaiba fault system merges with decollement(seismic nuclear? ), 2) the decollement is identified not continuously but intermittently, and 3)subducted ridge(the paleo-Zenisu ridge) is identified beneath the lower slope. Less