Physics
Scientific paper
Feb 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997pepi..104...75n&link_type=abstract
Physics of the Earth and Planetary Interiors, v. 104, p. 75-89.
Physics
10
Scientific paper
Coda waves observed by local seismographic networks were analysed by using the inversion method presented by Nishigami (1991) to estimate spatial distribution of coda scatterers in the crust. First, deviations of the decay curve of observed coda envelope away from a reference curve were measured for lapse times greater than about 1.5-2.0 times the S-wave travel times. We assume this kind of deviation is mainly caused by the non-uniform distribution of scatterers in the crust. Next, the 3-D distribution of relative scattering strength in the crust was obtained by solving the observational equation, where single isotropic scattering and a spherical radiation from the source are assumed and the observational data is given by the deviation of coda envelope stated above. This method was applied to two volcanic regions and one active-fault region in the central part of Japan. In each analysis, the target region with dimension of 50-160 km in horizontal and 30-80 km in depth was divided into about 1500-2000 blocks with one side of 2-10 km, and 600-900 waveform traces from about 100-250 events observed at 7-38 stations were used. The lapse time of coda waves analysed was taken to be as short as possible to reduce the multiple scattering effect. It was, in the shortest case, about 3-13 s for the Ontake region. The result of inversion analyses showed several significant features. Strong scatterers were found at shallow part (0-20 km) of the crust around the Fukui earthquake fault. A low seismicity area between active faults showed a correlation with the weak scattering area. The detailed distribution of scatterers was also revealed around two active volcanoes, Mt. Ontake and Mt. Nikko-Shirane. The vertical distribution of strong scatterers just below the volcano from 0 to 7 km in depth and another horizontal one at a depth of 7 km were estimated in the Ontake region. An inclined distribution of scatterers was estimated at depths from 0 to 20 km just below Mt. Nikko-Shirane. The location of scatterers around these active volcanoes agreed well with that of the S-wave reflectors in the crust obtained by the NMO correction analysis. Frequency dependence of scatterer distribution was also examined for the Ontake region. The horizontal distribution of scatterers at 7 km depth showed a similar pattern for frequencies between 5 and 20 Hz, while other smaller-scale scatterers appeared to have frequency dependence.
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