Seismic return motion

Possibilities of backswing of fault displacement are investigated on the basis of a simple crack model. Although it has been suggested that the backswing would not occur in real earthquakes because of friction, there is evidence that some earthquakes might have caused return motion “locally” on the fault surface. This paper attempts to derive conditions for such local return motion. Spatial extension of the crack beyond the stress-filled region into the stress-deficient region gives a favorable site for the return motion, the site being just outside the stress-filled region. In our model the return motion occurs at this site when the frictional stress is less than about 30% of the regional stress in the stress-filled region. The overall return motion, on the other hand, would not occur unless the frictional stress is less than 10% of the regional stress.     

Sulphide and oxide minerals from the Ohmine granitic rocks in Kii Peninsula,central Japan,and their primary paragenetic relations

In the Ohmine granitic rocks of Kii Peninsula, central Japan, ilmenite, rutile, pyrrhotite, pyrite and chalcopyrite commonly occur, but no magnetite is present. Their primary paragenetic relations were confirmed by examining their mode of occurrences as inclusion species in major silicate minerals and the phase relations in the Cu-Fe-S system, though their parageneses changed in cooling of the rocks. This makes it possible to estimate the fugacities of oxygen and sulfur in the granitic rocks based upon their paragenetic relations, and to discuss the nature of volatiles in the granitic rocks. Significant subsolidus reactions for sulfide minerals continued until the rocks cooled below 300° C and took place in two types of mineral grains, in groundmass and as inclusions. The removal of sulfur and copper through the silicate crystals was too easy to preserve the chemistry of sulfides included in silicates.     

Tectonoelectricity

Anomalous earth-current changes in association with an earthquake occurrence have often been reported since the 19th century. Changes reported in classical as well as recent literature are reviewed, although it is hard to say that any physically clear relationship between earth-current changes and earthquakes has been established. Even though a strong correlation between precursory earthcurrent changes and earthquakes has often been reported in the U.S.S.R. and the People's Republic of China in recent years, it appears that a general rule governing the earthquake-related earth-current changes is still very much unknown, partly because of contamination by artificial and natural noise.In contrast to the difficulty in understanding earth-current changes, monitoring of earth-resistivity seems to provide a much clearer means for interpreting geoelectric phenomena in association with tectonic processes in the Earth. Soviet and American work showed that the resistivity decreases to a considerable extent in the focal region prior to an earthquake. Such a change seems to be closely correlated to dilatancy generation in the Earth's crust. This finding doubtless provides an important means for earthquake prediction.An ususual, highly sensitive resistivity variometer, developed by the writer, disclosed that a resistivity change takes place at a teleseismic distance when an earthquake of large and medium magnitude occurs. Many of the resistivity changes thus observed are preceded by a premonitory change, which is supposed to reflect the preliminary rupture at a focal region.     

Study of the structure of silica film by infrared spectroscopy and electron diffraction analyses

Amorphous silicon oxide films have been studied on the basis of electron diffraction (ED) analyses and infrared (IR) spectroscopy in order to elucidate the relationship between the structures. After the heat treatment of the film in air at 300 and 500°C, the ED pattern showed halo rings, and the IR spectra clearly changed. Intensity analysis of the ED pattern provided evidence for the structural change of the amorphous film. It was concluded that the spectral changes in the ranges of 9.2–10.2, 12.5–13.5 and 19.5–22.5 μm were the result of phase transitions of the microcrystallites of α-cristobalite to β-cristobalite, and α- or β-quartz. Astrophysical implications have been discussed.     

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.     

Structure of the strong tidal jet in the naruto strait

In order to clarify the structure of the strong tidal current at the Naruto Strait in the Seto Inland Sea of Japan, the sea-level values were observed in the strait and the current measurements were made with an Acoustic Doppler Current Profiler (ADCP).The tidal volume transports for M₂ and S₂ tides were about 74×10³ and 26×10³ m³ sec^–1, respectively. The horizontal profile of the velocity at the phase of the strong tidal current compares favorably with a theoretical profile of the two-dimensional steady turbulent jet except for the side parts of the profile. Moreover, the entrainment rate of the surrounding water into the strong tidal jet was estimated from the difference of mass flux between two cross-sections at the strait, the entrainment rate and entrainment constant for both the northward and southward flows being about 1.3–2.5×10^–4m^–1 and about 0.03–0.05, respectively.     

The partition of minor transition metals between manganese oxides and seawater

The behavior of transition metals (Co, Ni, Cu and Zn) on precipitation of manganese oxides from seawater pumped up from a well at the site of the Marine Science Museum, Tokai University, and collected from several tanks along the water supply system to an aquarium was investigated. The distribution coefficients of cobalt and nickel between manganese oxides and seawater were different at different sampling points along the water supply system. At sampling points with high rates of manganese oxide precipitation, the distribution coefficients were about one order of magnitude smaller than those at the points with low precipitation rates, while there were no remarkable differences in the distribution coefficients of copper and zinc among the sampling points. The distribution coefficients of minor transition metals, with the exception of copper, observed at the points with low precipitation rates were comparable to those measured experimentally using manganese dioxides.     

The rate of oxidation of ferrous iron in seawater and the partition of elements between iron oxides and seawater

At the Minamichita Beach Land (Mihama-cho, Aichi, Japan), seawater is pumped up from underground and is supplied to aquaria. The underground seawater containsca. 2 ppm of Fe (II), 0.1 ppm of Mn (II) and a little dissolved oxygen. Iron oxide is formed in the seawater when aerated. The oxidation rate of Fe (II) was measured to be 1.4×10¹⁴ mol^–3 l ³ min^–1, which is comparable to the lowest values in the literature. The slow rate of Fe (II) oxidation obtained here can be attributed to the presence of organically bound iron in the seawater. The distribution coefficient of cations between seawater and iron oxide phase was in the order of Cu>Ni>Co>Cd>Mn, which is consistent with that predicted from their hydrolysis constants. The adsorption affinity sequence of oxyanions was phosphate >vanadate> molybdate. The difference in phosphate from the prediction of the adsorption theory was attributed to the formation of ferriphosphate on the oxide surface. On the basis of these data, the limitation and usefulness in the application of the distribution coefficients to marine environments were discussed.     

Errors in sampling procedure for the determination of dissolved oxygen by Winkler method

Sources of systematic error in the sampling procedure by Winkler method were examined. In the laboratory experiment, the dissolution of atmospheric oxygen into oxygenfree sea water during sampling amounts to 0.022 ml/l, and the effect of dissolved oxygen in sea water remaining in oxygen bottle is 0.019 ml/l. The effect of reagents added amounts to 0.017 ml/l. In the sampling procedures aboard, the effect of dissolution became bigger, and the table is presented for the correction to the reported value of dissolved oxygen measured by the “Manual of Oceanographic Observation”. The corrected value of dissolved oxygen in the routine analysis may not be correct within 0.02 ml/l.