| Budget Amount *help |
¥3,500,000 (Direct Cost: ¥3,500,000)
Fiscal Year 2000: ¥1,500,000 (Direct Cost: ¥1,500,000)
Fiscal Year 1999: ¥2,000,000 (Direct Cost: ¥2,000,000)
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| Research Abstract |
Recent seismic exploration in Japan is carried out in complex geological structure area. For instance the area is the exploration in and around active tectonic area such as active fault area and volcanic area. It can not neglect an effect of scattered wave by randomly distributed scatterers, which is difficult to identify by usual techniques in seismic reflection. In this study, we investigated effects of scattered energy by crustal inhomogeneity on records obtained in seismic reflection experiments. The study has carried out in two major approaches. One is detection of scattered waves in seismic reflection experiment. Another is investigation for characteristics of inhomogeneity as spatial distribution of scatterer and strength of inhomogeneity.
An seismic reflection and refraction experiment has carried out in the Senya faults area. Displacement had appeared with reverse faulting on Rikuu Earthquake in 1896 (M7.2). By means of P-wave shallow seismic reflection survey across the outcro
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p of the earthquake fault and geological survey, New four faults are estimated in this area. These faults appear in the migration section down to four hundred meters depth. These five faults are reverse faults with east dip. Folded structure appear in the east of the earthquake fault. However, intermittend plane reflectors are observed in the west of the earthquake fault. The active fault is still migrating westward in this study area. In this experiment, strong reflected phases appear by earthquake faults. This suggests inhomogeneity is strong on the fault plane and generate scattered wave. The strength of reflected phase is depend on reflection point. This also suggests that the fault has strong inhomogeneity and spatial variation of inhomogeneity.
We discuss the single scattering model by using seismic array data. By analyzing observed seismic array records, we found that coda energy level for slant-stacked record is smaller than that for single station record as expected from the single scattering model. The energy level of slant-stacked waveform is dependent on stacking direction. The difference between envelopes stacked in the direction of hypocenter and in the opposite one attributes to the area of scattering shell reflected by geometrical relationship among source, array, and lapse time. The energy level variation for different slant-stacking directions suggests that horizontally aligned scatterers predominate over vertical ones. The present analysis is probably useful to detect scattering anisotropy.
Temporal variation of the scatterer distribution has been detected around the focal area of M6.1 earthquake, northeastern Japan, by applying semblance analysis to seismic array data. The variation was found in slant-stacked waveforms of two explosions detonated just above the focal area one month before and two months after the M6.1 earthquake. According to geodetic observations, a pressure source that might be magma reservoir exists beneath this region and its location changed during the observation period. Slant-stacked waveforms are slightly different between the two explosions. The difference suggests that a scatterer contributing to the coda part is shifted toward the east and to the deeper depth only after three months. In this region, seismic and volcanic activity was very active during the interval of the two explosions. Volcanic activity of Mt. Iwate just north of the M6.1 earthquake, which started several months before, continued during the period. It can be considered that scatterer change relates to seismic and/or volcanic activity in the studied area. Less
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