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Volume 41 (1995)
- Issue 4 Pages 357-
- Issue 3 Pages 227-
- Issue 2 Pages 117-
- Issue 1 Pages 1-
Volume 41, Issue 1
Displaying 1-6 of 6 articles from this issue
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Yoichi Fukuda1995 Volume 41 Issue 1 Pages 1-16
Published: March 25, 1995
Released on J-STAGE: July 05, 2011
JOURNAL FREE ACCESSThis paper reviews the precise determination of a geoid in and around Japan. Recent developments in space technology have provided new observations of the Earth's gravity field. The figure of a marine geoid has been determined from satellite altimetry, and the combination of GPS, SLR and VLBI with conventional leveling survey has provided a centimeter geoid for the land area. An important problem today is to obtain a geoid by combining all these heterogeneous gravity field data. The Least Squares Collocation (LSC) method has great advantage for solving the problem. LSC is an approximation method for harmonic functions in a reproducing kernel Hilbert space, and the philosophy behind it is quite different from the Stokes integral. A practical application of the method to the problem is given, and recent progress of geoid studies, especially those related to GPS/Leveling and sea surface dynamic topography, is discussed.View full abstractDownload PDF (2174K) -
Masao Komazawa1995 Volume 41 Issue 1 Pages 17-45
Published: March 25, 1995
Released on J-STAGE: July 05, 2011
JOURNAL FREE ACCESSA detailed processing of gravity data and a compilation of preexisting data were conducted in and around the Aso volcanic area. The density structure was then examined in detail. To remove the skin effects of surface layers (pyroclastic flows or lake deposits) and to estimate the deeper structure of the caldera, a gravity data processing technique CVUR (comparison of variance of upward-continuation residual) was proposed. The density of the surface layer in the Aso volcanic area was estimated to be 2.2-2.3 g/cm3. The Aso caldera was revealed to have a piston-cylinder type structure rather than a funnel-shaped structure (upside-down cone) model with a single low anomaly as previously believed. Newly-compiled Bouguer gravity anomalies with densely-distributed data ;.show that five local minor Bouguer gravity lows exist in the Aso caldera to produce a major gravity low zone distributed as a symbol “<” shape. The major gravity low has a steep gradient inside the caldera rim. The central areas of the minor low anomalies have relatively flat bottoms, like a pot or a bucket. Residuals of gravity anomalies corresponding to deep and shallow structure were detected by a band-pass filter designed with two upward-continuation filters. A local minimum of about -4 mGal, which was considered to be related to the deep structure appeared along the northern slope of the central cones, but not in the central cones. To explain this local minimum, a cylindrical structural model with a lower density contrast of 0.15 g/cm 3 was proposed, and the model was compared with the seismic attenuation model. In order to study the residual anomalies related to shallow structures, a three-dimensional structural analysis was performed using density-varying models in consideration of the compaction of lake deposits and pyroclastics. It was found that the gravity basement was subsided inside the caldera rim with a steep gravity gradient, and local minor depressions of the basement were found to correspond to five local flat-bottomed low anomalies. These results also suggest that the Aso caldera belongs to the “piston-cylinder type (Valles-type)” of having large depressions or collapses, rather than the “funnel-type” of caldera derived from large volcanic eruptions.View full abstractDownload PDF (3873K) -
Hiromichi Tsuji1995 Volume 41 Issue 1 Pages 47-73
Published: March 25, 1995
Released on J-STAGE: September 07, 2010
JOURNAL FREE ACCESSGlobal Positioning System (GPS) data collected with P-codeless receivers at four regional tracking sites in Japan are processed using GIPSY software with precise ephemerides of GPS satellites. Daily solutions of site coordinates from July 1992 to September 1994 yield relative velocities between tracking sites on the North American plate (NA), the Philippine Sea plate (PH), and the Eurasian plate (EU). The precision of site coordinates with respect to the reference site at Tsukuba (TSU) reaches 5-25 mm for 1, 000 km baselines. A time series analysis of site coordinates leads to the following geodetic results and tectonic implications. Coseismic movement is detected at the Shin totsukawa site (STK) located about 200 km east-north-east of the epicenter of the Southwestern off Hokkaido earthquake in July 12, 1993. The horizontal displacement estimated with GPS is 3 cm west and 1 cm south, which is consistent with that estimated from a fault model. Assuming the velocity of TSU with respect to NA measured by Very Long Baseline Interferometry (VLBI), velocities obtained with GPS are compared to the latest plate motion models. They show good agreement at Chichijima (CCJ) on PH and at Kanoya (KNY) on EU. A small residual velocity at CCJ suggests that the back arcspreading of Izu-Bonin arc at the west of CCJ may not be significant. KNY moves to the south-south-east about 0.5 cm/year faster than the model. This is consistent with an eastward movement of Shanghai on EU revealed by VLBI.View full abstractDownload PDF (3792K) -
Tadahiro Sato, Kazuo Shibuya, Yoshiaki Tamura, Masaki Kanao, Masatsugu ...1995 Volume 41 Issue 1 Pages 75-89
Published: March 25, 1995
Released on J-STAGE: September 07, 2010
JOURNAL FREE ACCESSSince March 22 of 1993, observations of Earth tides and Earth's free oscillations with a superconducting gravimeter (SCG) were continuing at Syowa Station, Antarctica (69.0°S, 39.5°E). Combined with the other SCG data from the global network, we expect to detect signals related with a fluid core resonance, core undertones in the fluid core, the Slichter mode of the inner core, etc. and to elucidate nature of long-period tides. Parallel observations were also continuing using a LaCoste & Romberg D gravimeter which was equipped with an electrostatic feedback amplifier and the same analog filter that were used in the SCG observations. Preliminary analyses of 1 year tidal data (March 22, 1993-March 21, 1994) generally support the previously obtained diurnal and semidiurnal tidal gravimetric factors, and confirm 10% larger M2 5 factor than the theoretical value by Dehant-Wahr. Subtraction of both short- and long-period tides from the original signals resulted in a fairly small (0.06 ugal/day: 1 μgal=10-8 m/s2) trends including instrumental drift. An rms noise level of the data obtained through the MODE filter (MODE data) is estimated typically as 5-20 ugal (1 μgal=10-11 m/s2). The recorded MODE data include high quality free oscillation signals from the Hokkaido-Nansei-Oki Earthquake (Ms =8.2) on July 12, and the earthquake of South of Mariana Island (Ms=8.2) on August 8, 1993.View full abstractDownload PDF (3395K) -
N. Ananga, R. Coleman, C. Rizos1995 Volume 41 Issue 1 Pages 91-97
Published: March 25, 1995
Released on J-STAGE: March 01, 2011
JOURNAL FREE ACCESSThe aim of this paper is to determine the stability of tide gauge bench marks (including trigonometric stations) along the coast of Southern Australia. A series of bench marks were monitored using repeated Global Positioning System observations of South Australia. Their displacements were determined using three-dimensional network adjustment. Seventeen bench marks including twelve tide gauge sites were involved in the adjustment with two observation periods. The baseline lengths of the sites ranged from 178 km to 365 km. Individual day solutions were initially determined. These solutions were combined together and adjusted simultaneously imposing extended free net constraints on the parameters. The relative vertical heights of the bench marks are estimated, and the analysis indicated that the precision of the height differences were in the range of ±4.2 cm.View full abstractDownload PDF (590K) -
Hui Li, Shuzo Takemoto, Takeyasu Yamamoto, Shigeaki Otsuka1995 Volume 41 Issue 1 Pages 99-109
Published: March 25, 1995
Released on J-STAGE: September 07, 2010
JOURNAL FREE ACCESSThe effect of fluid-core resonance on strain tides has a different feature from that of gravity and tilt tides. We first studied the method to verify the fluid-core dynamic effect on strain tides and then calculated its spatial and azimuthal distributions varying with the station latitude and baseline orientation. The effect theoretically estimated for the laser strainmeter at the Rokko-Takao station, Kobe, Japan can reach -20% and+80% for K1 and Ψ1 constituents, respectively. Next, we analyzed two year observational data from March 1989 to February 1991 obtained from the laser strainmeter at Rokko-Takao by applying Venedikov's harmonic analysis method. Comparing the results obtained by harmonic analysis with those theoretically expected, precision of analytical results is good enough to be used in searching for the fluid-core dynamic effect. As a result, frequency dependence of straintidal factors obtaine from the laser strainmeter at Rokko-Takao is fairly well consistent with that theoretically predicted, except for the S1 constituent which would be disturbed by meteorological effects.View full abstractDownload PDF (1088K)
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