Climatic and environmental changes at southeastern coast of Lake Biwa over past 3000 years, inferred from borehole temperature data

In order to infer past climatic change in central Japan, we measured temperatures in a borehole at the Karasuma site, on the southeastern coast of Lake Biwa, and reconstructed sediment surface temperature history during the last 3000 years. The reconstructed temperature history shows apparent Medieval Warm Period, Little Ice Age, and contemporary temperature warming. However, the large amplitude of the temperature changes up to 4-5 K cannot be explained by past climatic change only, suggesting that there was some other cause of the larger amplitude temperature changes. The onsets of temperature decrease in the late 12th century a.d. and temperature increase in the mid 17th century a.d. appear to coincide with occurrences of two destructive earthquakes (1185 and 1662 a.d.) that caused water level changes of Lake Biwa. It suggests that the reconstructed sediment surface temperature history reflects the environmental change due to tectonically induced water level changes of the lake. If the annual mean of the ground surface temperature was higher than that of the bottom water temperature in a shallow part of the lake, which is consistent with the present-day data, the large amplitude of the sediment surface temperature change may be attributed to a combined effect of past climatic and environmental changes. Thus, we suggest that the borehole temperature at the Karasuma site preserves information not only on past climate changes but also on environmental changes due to tectonically induced water level changes.     

Crustal structure and transform margin development south of Svalbard based on ocean bottom seismometer data

The Barents Sea is located in the northwestern corner of the Eurasian continent, where the crustal terrain was assembled in the Caledonian orogeny during Late Ordovician and Silurian times. The western Barents Sea margin developed primarily as a transform margin during the early Tertiary. In the northwestern part south of Svalbard, multichannel reflection seismic lines have poor resolution below the Permian sequence, and the early post-orogenic development is not well known here. In 1998, an ocean bottom seismometer (OBS) survey was collected southwest to southeast of the Svalbard archipelago. One profile was shot across the continental transform margin south of Svalbard, which is presented here. P-wave modeling of the OBS profile indicates a Caledonian suture in the continental basement south of Svalbard, also proposed previously based on a deep seismic reflection line coincident with the OBS profile. The suture zone is associated with a small crustal root and westward dipping mantle reflectivity, and it marks a boundary between two different crystalline basement terrains. The western terrain has low (6.2–6.45 km s⁻¹) P-wave velocities, while the eastern has higher (6.3–6.9 km s⁻¹) velocities. Gravity modeling agrees with this, as an increased density is needed in the eastern block. The S-wave data predict a quartz-rich lithology compatible with felsic gneiss to granite within and west of the suture zone, and an intermediate lithological composition to the east. A geological model assuming westward dipping Caledonian subduction and collision can explain the missing lower crust in the western block by subduction erosion of the lower crust, as well as the observed structuring. Due to the transform margin setting, the tectonic thinning of the continental block during opening of the Norwegian-Greenland Sea is restricted to the outer 35 km of the continental block, and the continent–ocean boundary (COB) can be located to within 5 km in our data. Distinct from the outer high commonly observed on transform margins, the upper part of the continental crust at the margin is dominated by two large, rotated down-faulted blocks with throws of 2–3 km on each fault, apparently formed during the transform margin development. Analysis of the gravity field shows that these faults probably merge to one single fault to the south of our profile, and that the downfaulting dominates the whole margin segment from Spitsbergen to Bjørnøya. South of Bjørnøya, the faulting leaves the continental margin to terminate as a graben 75 km south of the island. Adjacent to the continental margin, there is no clear oceanic layer 2 seismic signature. However, the top basement velocity of 6.55 km s⁻¹ is significantly lower than the high (7 km s⁻¹) velocity reported earlier from expanding spread profiles (ESPs), and we interpret the velocity structure of the oceanic crust to be a result of a development induced by the 7–8-km-thick sedimentary overburden.     

A new hybrid geoid model for Japan, GSIGEO2000

 The latest gravimetric geoid model for Japan, JGEOID2000, was successfully combined with the nationwide net of GPS at benchmarks, yielding a new hybrid geoid model for Japan, GSIGEO2000. The least-squares collocation (LSC) method was applied as an interpolation for fitting JGEOID2000 to the GPS/leveling geoid undulations. The GPS/leveling geoid undulation data were reanalyzed in advance, in terms of three-dimensional positions from GPS and orthometric heights from leveling. The new hybrid geoid model is, therefore, compatible with the new Japanese geodetic reference frame. GSIGEO2000 was evaluated internally and independently and the precision was estimated at 4 cm throughout nearly the whole region. Received: 15 October 2001 / Accepted: 27 March 2002 Acknowledgments. Messrs. Toshio Kunimi and Tadashi Saito at the Third Geodetic Division of the Geographical Survey Institute (GSI) mainly carried out the computations of most of the updated leveled heights. With regard to the reanalysis of GPS data, the discussions with Messrs. Yuki Hatanaka and Shoichi Matsumura of GSI were of great help in building the analysis strategy. Messrs. Kazuyuki Tanaka and Hiromi Shigematsu collaborated in the preparatory stages of GPS data computation. The authors' thanks are extended to these colleagues. Some plots were made by GMT software (Wessel and Smith 1991). Correspondence to: Y. Kuroishi     

Bonding character of SiO2 stishovite under high pressures up to 30 Gpa

 The charge density and bond character of the rutile-type structure of SiO₂ (stishovite) under compression to 30 GPa were investigated by X-ray diffraction study using synchrotron radiation and AgKα rotating anode X-ray generator through a newly devised diamond-anvil cell. The valence electron density was determined by least-squares refinement including the κ parameter and the electron population in the X-ray atomic scattering parameters. The oxygen κ-parameter of SiO₂ is 0.94 under ambient conditions and 1.11 at 29.1 GPa and the silicon valence changes from +2.12(8) at ambient pressure to +2.26(15) at 29.1 GPa. These values indicate that the electron distributions are more localized with increasing pressure. The difference Fourier map shows the deformation of the valence electron distribution and the bonding electron population in residual electron densities. The bonding electron observed from the X-ray diffraction study is interpreted by molecular orbital calculations. The deformation of SiO₆octahedra and the bonding electron density of stishovite structures are elucidated from the overlapping electron orbits. The O–O distances of shared and unshared edge of SiO₆ octahedra change with the cation ionicity. The repulsive force between the two cations in the adjacent octahedron makes its shared edge shorter. The pressure changes of the apical and equatorial Si–O interatomic distances are explained by the electron density of state (DOS) of Si and electron configuration. Received: 7 January 2002 / Accepted: 6 May 2002     

A tidal front in winter influenced by river discharge

Water quality of Osaka Bay is greatly influenced by freshwater discharge from rivers, to the effect of salinity playing a major role in forming the stratification. The tidal front is expected to appear in such an area even in winter considering the theory of the formation of a tidal front. From the field observation, we recognized a tidal front in winter for the first time in Osaka Bay. The critical value of a parameter log(H/U ³) for generation of a tidal front is obtained as a function of river discharge and cooling effect through sea surface. Differences of nutrients (NH₄–N, PO₄–P) concentration across the front are not clear, probably because they are utilized by phytoplankton easily. But in (NO₂–N)+(NO₃–N), DIN, PP and T–P, there is a discontinuity structure across the front, even if the absolute value of concentration difference is small.     

Apparent velocity measurements on an oceanic lithosphere

A direct measurement of apparent velocities for oceanic paths was made with an array of sensitive ocean bottom seismographs. The measurement was performed by recording waves from shallow earthquakes which occurred in the area close to trench axes and which were accurately located by the land seismological network in Japan. The range of epicentral distances is from 500 to 1,800 km.The observed P travel times are less than those in the Jeffreys-Bullen tables by 6–10 s for the range of distances.Since the dimension of the OBS array is about 400 km, the apparent velocities are determined quite precisely and show little dependence on the epicentral distances. The average value of the apparent velocities for the range 500–1,700 km is 8.64 ± 0.13 km/s.An offset of travel times, which is thought to be associated with a low-velocity layer underneath the oceanic lithosphere, has been observed.These results indicate that a high-velocity layer with a velocity of 8.6 km/s exists in the lower part of the oceanic lithosphere. Beneath the 8.6-km/s layer there is a thin low-velocity layer under which the velocity of the P wave is again 8.6 km/s.     

Effect of Mine Water on the Stability of Underground Coal Mine Roadways

The basic objectives of mine roadways are to provide sufficient cross sections to accommodate equipment, transport, personnel travel and ventilation. However, many roadways become damaged to the extent of needing maintenance, generally dinting, and in some cases requiring re-ripping. Strata conditions play important effects on stability of roadways and other mining activities. Weak rocks cause excessive roadway closures, and water softens some rocks and worsens the closure problem. Therefore, study on effect of mine water on the stability of underground coal mine roadways is important for underground coal mine development. A scientific discussion on the effect of water on the stability of roadways is given on the basis of results obtained by means of field investigation and laboratory tests. Based on analysis, rock in saturated condition has only between 0.19 to 0.49 of its compressive strength and 0.17 to 0.59 of its tensile strength in dry condition. Among the coal measures rock in research area, water has the most obvious effect on the strength reduction of shale. The slaking behavior of shale has also been the worst among the other. Field investigation of roadway driven in shale shown vertical closure in the wet and dry condition area reached 40–60 and 5–15 cm respectively, in 30 days after the drivage. Among the measures, drainage is considered to be the most economical and simplest method to reduce the water content in the rocks or the rock masses.