I re-analyzed the seismic profiles on west of Akita city, which were surveyed by Hydrographic Department of Japan in 1982. Buried active faults which deformed alluvium were recognized on the area. These faults are distributed in the same strike in echelon within a few kilometers. They are regarded as segments of one fault system (south of Oga peninsular fault, tentative name). The fault system is about 20 kilometers in length. Average rate of the fault displacement is estimated at 0.9 to 1.0 m/ky., based on the assumption that the basement of alluvium is 18,000 year before the present. Assuming that the acoustic reflectors in alluvium indicate event sediments by the past earthquakes, estimated recurrence interval of the fault is two thousand and a few hundred years, and the latest event was occurred about two thousand and a few hundred years ago. The magnitude of the earthquake is estimated to be M=7.0 based on the method proposed by Matsuda(1975).
The Geological Survey of Japan has been collecting offshore single channel seismic profiling data around Japan for the purpose of compilation of marine geological maps. The maps are generally based on the interpretation of the on board profiles plotted on a graphic recorder. The seismic data has also been recorded on analogue tapes. The author tried to improve the quality of the analogue seismic data by digitizing and signal processing. Digital band-pass filter is very effective to reduce noise, AGC can make clear weak reflections. Spike deconvolution filter shortens reflection waves and contributes to distinguish close reflections. Predictive deconvolution filter can reduces multiple reflections in the shallow area less than 150 m deep, but the filter may also reduces real reflection. Diffraction patterns from dipping strata and faults can be fairly removed by F-K migration. The size and vertical exaggeration of profiles can be easily changed as the interpreter likes. These processed single channel seismic profiles are useful to clarify geologic structures inclined less than 15-20°, and shallower than 1.0 s below sea bottom in the water depths less than 3,000 m. The processed profiles make possible more detailed and reliable geologic interpretation than onboard profiles, although the processing can not show geologic structures which are not observed on the onboard profiles. Single channel seismic data recorded on analog tapes which have been believed to be useless can be improved by digitizing and processing, and the improved profiles can be useful for the study of marine active faults.
Recently, high resolution seismic survey techniques have become necessary in many actual geological and geophysical investigations. Therefore , we are developing a new type of deep-towed system that can install many electronic components and different types of acoustic sensors at the same time. This paper reports our new digital seismic profiler unit that is currently developed and shows the result of our experiment off Omaezaki, Japan. We started to develop a deep-towed seismic profiling system in the second half of 1980's. Quick development of electronics and computers has lead us to down-sizing and smaller weight for a seismic unit. Major and minor improvements were made for the unit by conducting several offshore experiments in the last decade. We show here a seismic record taken in 1997. Significantly higher resolution seismic profile was obtained compared with the data by a surface-towed streamer cable system we got previously. The new profiler system has high enough capabilities suitable for real geological and geophysical surveys.