岡山大学理学部地球科学教室Acta Medica Okayama1340-74142212015地震波の減衰がS 波偏向異方性に及ぼす影響17ENHitoshiOda10.18926/ESR/54115We investigate the effect of the seismic attenuation on the shear-wave polarization anisotropy by numerical experiments. For the experiments, the P-wave receiver functions are synthesized by assuming that a plane P wave is incident on an anisotropic multi-layer structure with anelastic properties. The synthetic receiver function shows smooth Ps phase waveforms with small amplitudes due to the attenuation of high-frequency spectral components, compared to that calculated for a multi-layer structure with perfect elasticity. From the shear-wave splitting of the Ps converted phases in the synthetic receiver functions, the S-wave polarization anisotropies for the anisotropic layers are measured as a function of propagation direction of the incident P wave. Consequently, we conclude that the seismic attenuation has a minor influence on the measurements of the S-wave polarization anisotropy.No potential conflict of interest relevant to this article was reported.岡山大学理学部地球科学教室Acta Medica Okayama1340-7414111994瀬戸内海沿岸で観測されたScSp相について1926ENMasakiNishiHitoshiOda10.18926/ESR/13955Observation of ScSp phase, P wave converted from ScS phase, has been made at a seismological observatory (SBK) located on the coast of the Seto Inland Sea, southwest Japan. The ScSp phases were found on the seismograms of earthqukes with long epicentral distances, but not observed for those with short epicentral distances. Such dependence of appearance of the ScSp phase on epientral distance is interpreted as being due to the Philippine Sea plate horizontally lying beneath the Shikoku and Chugoku districts.No potential conflict of interest relevant to this article was reported.岡山大学理学部地球科学教室Acta Medica Okayama1340-7414311996固有周波数のインバージョンによる地球内部構造の不均質性の推定 ―数値実験―3342ENSeijiOnishiHitoshiOda10.18926/ESR/13931An investigation was made on effect of lateral heterogenity of the earth's elasticity structure on the normal mode frequencies. The lateral heterogeneity is expressed by an expansion of spherical harmonic functions, Pιm(cosθ)sinmΦ and Pιm(cosθ)cosmΦ, up to the second order harmonics (ι≤2). Free oscillation freguencies of the heterogeneous earth were computed by the xyz algorithm. Further we derived an analytic expression of partial derivatives of eigenfrequency with respect to the expansion coefficients, and performed a numerical test to verify whether or not it is possible to estimate the heterogeneity of the earth's structure by the inversion of noemal mode frequencies.
When the earth is assumed to be elastically isotropic spheroid with short polar and long equatorial radii, frequency spectra affected by longitudinal heterogeneity terms, Pιm(cosθ)(sinmΦ, cosmΦ) (m≠ 0), for given values of ι and m coincide with each other, because these two terms describe the same heterogeneity when the earth is rotated by π/2m around rotation axis. In such a case, we cannot determine accurately the expansion coefficients of tha two heterogeneity terms by inversion of normal mode frequencies, whereas the coefficients of the latitudinal heterogeneity Pι0(cosθ) can be precisely determined. Therefore it is difficult to estimate of lateral heterogeneity of the earth's elasticity structure by the inversion of normal mode frequencies.No potential conflict of interest relevant to this article was reported.岡山大学理学部地球科学教室Acta Medica Okayama1340-7414411997西南日本の地殻及び上部マントルの三次元P波速度構造120ENTakashiTanakaHitoshiOda10.18926/ESR/13915Trevel-time data of the P wave were inverted to study three-dimensional structure of the P wave velocity in the upper mantle and crust beneath the Shikoku and Chugoku districts, southwest Japan. Low-velocity and high-velocity regions were found in the upper crust of the eastrn and western regions in the Shikoku district, respectively. The activity of crustal earthquakes is high in the eastern region. Moreover a significant high-velocity region exists in the uppermost mantle, where the activity of small earthquakes is very high. A low-velocity region with velocity of 7.0km/s was found just above the descending Philippine Sea plate with high velocity of 8.0 km/s. This region is continued to oceanic crust in the vicinity of the Nankai trough and its velocity is nearly equal to the P wave velocity (6.7 km/s) of oceanic crust. Thus the low velocity is interpreted as the oceanic crust. A high-velocity region is found below the depth of 50km in the Shikoku district. This region suggests the Philippine sea plate lying horizontally from the Shikoku districts to the ocast of the Seto Inland Sea of the Chugoku district. A low-velocity zone suggesting the volcanic front is found around the depth of 50km at the coast of the Japan Sea of the Chugoku distrit.No potential conflict of interest relevant to this article was reported.岡山大学理学部地球科学教室Acta Medica Okayama1340-7414712000S-wave velocity structure estimated from long-period coda waves1518ENHitoshiOda10.18926/ESR/13893Long-period coda waves were recorded on the vertical-component seismograms of aftershocks of the Hyogo-ken Nanbu earthquake, 1995. We identify the long-period coda waves as Rayleigh wave, because they appear after the S-arrival times and exhibit the normal dispersion that propagation velocity of the coda waves increases with an increase in period. By applying the moving window analysis to the coda waves from nine aftershocks, the group velocities are determined as a function of period within the range of 2 to 8 s. The group velocity dispersion data are inverted to investigate the S-wave velocity structure of the upper crust. The S-wave velocity structure is consistent with those obtained in previous studies using traval time analysis of bood waves.No potential conflict of interest relevant to this article was reported.岡山大学理学部地球科学教室Acta Medica Okayama1340-74141012003Elasticity of diopside CaMgSi(2)O(6) measured by means of the resonant sphere technique, RST1114ENIsaoSuzukiYasushiInouyeHitoshiOdaMasanaMorioka10.18926/ESR/13873Resonant frequencies for a single crystad diopside sphere are measured accurately and rhirteen elastic moduli are reluced by the least squares calculation, A set of mduli gives theoretical resonant frequencies close enough to the observed ones.No potential conflict of interest relevant to this article was reported.岡山大学理学部地球科学教室Acta Medica Okayama1340-74141112004Topography of the Moho discontinuity estimated by column inversion of gravity anomaly data15ENMarikoFukunagaHitoshiOdaRyuichiShichi10.18926/ESR/13861We present an inversion method, called "column inversion", to retrieve two-dimensional variation of the depth to the Moho discontinuity from Bouguer gravity anomaly data, assuming that a density jump between the lower crust and upper mantle is regionally invariant. The inversion method is applied to reveal the apatial undulation of the Moho discontinuity in the Chugoku and Shikoku districts, southwest Japan. The result shows that the Moho is deep in the land area and shallow in the sea area. The Moho is anomalously deep in the west regions of Shikoku and Chugoku districts, where the prominent negative gravity anomaly is observed. The two-dimensional variation of the Moho depth is quite similar to that determined by inversion of the first P-arrival time data. The column inversion of the Bouguer gravity anomaly data is proved to be useful for determining the fine structure of the Moho discontinuity undulation.No potential conflict of interest relevant to this article was reported.