重力異常データのカラムインバージョンから見積もったモホ不連続面の形状

We 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

Possibility of temporal variations in earth tidal strain amplitudes associated with major earthquakes,

Earth tidal strains observed concurrently at two stations in the western Chubu region have been analyzed to see if there are any temporal variations of their amplitude in relation to major earthquakes in the region. The tidal strain records obtained at Kamitakara during 7 years and at Inuyama during 2 years are bandpass-filtered, and correlated with the corresponding theoretical tidal function predicted for a solid earth, using least squares mainly in the time domain, to estimate the tidal admittance over the periods. The admittances determined for the two-component Kamitakara strains appear to show a consistent and gradual increase during 10-11 months prior to the central Gifu earthquake of 1969 (M=6.6), with a maximum variation reaching about 15%, and after the earthquake they seem to decrease and recover their normal value a half year later. No such significant variations as in this period can be found over the entire period. The resolved amplitudes of the semidiurnal M2 and diurnal O1 waves show similar trends. The results from the Inuyama observations involve somewhat larger fluctuations, but appear to reach a peak 3-4 months before the earthquake and then decrease to the time of its occurrence. Secular strains and tilts observed at the above two stations also indicate appreciable changes during the corresponding periods. If these temporal variations of the tidal strain amplitudes are not spurious but real phenomena, they might be interpreted as a manifestation of some precursory changes in crustal rigidity around the seismic source region

71. Some Considerations on the Strain-Step associated with the Earthquake of the Central Part of Gifu Prefecture, September 9, 1969

A remarkable strain step was recorded by the strainmeter at the Inuyama Crustral Movement Observatory which is located at a distance of about 48km from the epicenter of the Gifu earthquake of September 9, 1989, with the magnitude of 6.6. At the same time a rapid increase of discharged water in the observational gallery was also observed. These phenomena were also observed at the time of the off Nemuro (Hokkaido) earthquake of August 12, 1969, with the magnitude of 7.8. In the case of the off Nemuro earthquake the epicentral distance was about 1200 km. These strain steps and the abnormal increase of discharged water at the time of earthquakes were taken into consideration for the analysis of the crustal deformation accompanied by earthquakes and principal strains obtained from the calculations are listed in Tables 1 and 2, respectively

Gravimetric connection between Tokyo and McMurdo Sound

A gravimetric connection from Tokyo (Japan) to McMurdo Sound (Antarctica) via Sydney (Australia) and Christchurch (New Zealand) was carried out by means of two LaCoste & Romberg gravimeters in October 1980 as part of international gravimetric connections around the Circum-Pacific zone. The gravimetric connection between Tokyo and McMurdo Sound could not be performed so precisely because scale values of the gravimeters employed could not be accurately determined. The gravity measurements in McMurdo Sound, however, were precisely performed using the two gravimeters in spite of low ambient temperatures. It was ascertained that the International Gravity Standardization Net 1971 (IGSN 71) is a homogeneous network inclusive of the gravity stations in McMurdo Sound

Dense Gravity Survey in the Kirishima Volcanoes and its Surrounding Calderas, Southern Kyushu, Japan

Dense gravity surveys were carried out in the area of the Kirishima volcanoes, the Kakuto, the Kobayashi basins and the Anraku region to reveal underground structures relevant to caldera formation. The Kirishima area (50×40 km2) provided more than 830 newly measured gravity data, from which a precise Bouguer anomaly map was drawn