Kiyoshi Seya

Thermal expansion of fayalite, Fe2SiO4

Thermal expansion of single-crystal fayalite has been measured by a dilatometric method at temperatures between 25 °C and 850 °C. The results show the presence of anomalous expansion in the b axis, which is correlated to the anomalous variation of elastic moduli with temperature. Grüneisens parameter is 1.10 and the thermal Debye temperature is 565 K, which is close to the acoustic Debye temperature of 511 K.

Subducting oceanic crust on the Philippine Sea plate in Southwest Japan

Abstract It is well-known that subcrustal earthquakes occur in Southwest Japan owing to subduction of the Philippine Sea (PHS) plate. Observing the earthquakes at a single station in the Chugoku district, we frequently find a distinct pair of later P and S phases after weak initial P and S phases, respectively. These later phases were observed only for earthquakes with particular source-receiver geometries; the events in the west Seto Inland sea, epicentral distances from 120 km to 240 km, and the source-receiver direction almost parallel to the strike of the Nankai trough. The feature of initial and later phases was summarized as follows: (1) The later phases are observed for subcrustal earthquakes occurring at depths between 40 km and 60 km, but not for those below the depth, (2) “Single-station” apparent velocities of initial P and S phases are 8.0 km/s and 4.9 km/s, and they are somewhat higher than ordinary velocities of the uppermost mantle. (3) Single-station apparent velocities of later P and S phases are, on the other hand, 6.6 km/s and 3.8 km/s, and they are comparable to the velocities of the continental lower crust and the main layer of oceanic crust. These features could not be explained unless the subcrustal earthquakes take place within the low-velocity oceanic crust overlying the high-velocity oceanic plate. This suggests that the oceanic crust subducts beneath the west Seto Inland sea as a part of the PHS plate.

Thermal expansion of single-crystal tephroite

The thermal expansion of tephroite (Mn2SiO4) at temperatures between 25 and 850°C has been determined by a dilatometric technique. The analysis of data in terms of Grüneisens theory yields the Grüneisens parameter γ=1.04, and the pressure derivative of rigidity (∂G/∂P)=0.7.

Application of the Dugdale crack model to the stress-strain relations and bulk moduli of a granite

Variations of dynamic and static hulk moduli and compressional strain of a rock sample of granite in response to hydrostatic pressure at room temperature are summarized as follows: 1. (1) “ nonlinear” increase of compressional strain becomes “linear” at σ0 (border pressure, for this rock, σ0 = 1.36 kbar and is the pressure which divides the compression curve in two parts: the crack-active part and the crack-closed part). 2. (2) static bulk modulus obtained from compressional strain data ϵ drastically increases and levels off at pressures higher than the border pressure σ0, 3. (3) pressure dependence of dynamic bulk modulus obtained from acoustic measurements has a trend similar to that of the static one, and 4. (4) border pressure σ0 from (3) is just equal to that from (1) and (2). To interpret these phenomena and understand the underlying reason for these variations, two crack models were examined, i.e., an elliptical crack model with constant aspect ratio and the Dugdale crack model. The difference in closing processes for the two models results in different changes in effective bulk modulus and compressional strain due to the applied pressure. If the total number of cracks cannot change with increasing pressure, the drastic increase in bulk modulus and nonlinear part of the stress-strain curve at low pressure can be attributed to reduction of crack width or crack length. It is shown that the Dugdale crack model explains better the observed results. Theoretical calculations show that the crack density of the rock sample is 0.42 (dimensionless) and that the yield stress of 2.09 kbar acts in the proximity of tips of Dugdale cracks. These values seem reasonable when we consider the values of the porosity of 0.16% and the border pressure of 1.36 kbar.

Effect of occurrence rate of acoustic emissions on their statistical behavior

The parameterm in Ishimoto-Iidas relation was investigated for acoustic emissions (AEs) occurring in rock samples under uniaxial compression. In the experiment, we found: 1) The large AEs are counted without serious error but the number of small AEs is systematically underestimated at high AE rates, 2) the frequency distribution of maximum AE amplitudes becomes nonlinear in logarithmic scale with increasing AE rate, and 3) there exists a strong negative correlation betweenm-value and AE rate. The miscount of small AEs was interpreted as due to overlap of the large and small AEs. We call the miscount “masking effect”. A statistical analysis based on the masking effect showed that them-value decreases more effectively as the AE rate increases, and thus the masking effect is a possible origin both for the nonlinear frequency distribution of maximum AE amplitudes and for the negative correlation ofm-value with AE rate. We emphasize that one should be careful of the masking effect to examine correctly the change, ofm-value. In order to eliminate the masking effect, AEs should be measured by a measurement system with low sensitivity. Even if the masking effect is eliminated, them-value decreases before the main fracture of a rock sample. Them-value is a key parameter to predict the main fracture.