Paleoseismology of deep‐sea faults based on marine surveys of northern Okushiri ridge in the Japan Sea

The eastern margin of the Japan Sea is an incipient convergent margin characterized by distributed active faults and large‐scale earthquakes. To evaluate the earthquake potential of the northern margin, the source area of the 1940 Shakotan–Hanto‐oki earthquake ( M 7.5) was reexamined, and the earthquake recurrence interval was estimated by combining bathymetric, seismic, and dive survey data with a tsunami simulation. Seismic profiling clarified that five anticlines have formed in and around this source area during the last 1.8 million years. The balanced cross section of one anticline supports that thrust faults extend to the base of the upper brittle crust at about 11 km depth and total slip is about 850 m. Dive surveys identified fresh collapses along the forelimbs of two anticlines. Inversion of observed tsunami waveforms to estimate the slip distribution suggests that these two anticlines were the sources of the 1940 earthquake. The other three anticlines may be seismic gaps. The dive surveys collected debris layers, possible records of paleoearthquakes, in muddy sediments. From the rate of muddy sediment deposition around the anticlines the minimum recurrence interval of earthquakes was estimated to be about 1100 years. From structural modeling the average interval was about 6400 years, assuming that fault slip started 1.8 million years ago and 3 m of slip occurred during each earthquake. Taking into account the above results and the instantaneous plate convergent rate between the Okhotsk and Amurian plates, the recurrence interval was inferred to be 1100 to a few thousand years.