A symposium on the mechanical properties of rocks was held by the Volcanological Society of Japan. Several papers by active investigators in an experimental field were presented. This paper is one of them and describes not only a general discussion on the mechanical properties of rocks but some recent results of rock creep. The contents of this paper are as follows: (1) Introduction. (2) Influence factor on the mechanical properties of rocks. (3) Types of deformation. (4) Creep characteristics of rocks and their analyses. (5) Mechanism of rock deformation. (6) Natural phenomena in the earth associated with the mechanical properties of rocks.

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In this article, historical review of the experimental approach to the thermal dependence of elasticity of rocks and other broad characters of the subject matter are omitted. In general, change of elastic constants with temperature for single crystal and polycrystalline aggregate is different, and it is discussed by Ke’s model with grain boundary viscosity. Accordingly, it is impossible or at least very difficult to estimate the elastic parameters of polycrystalline aggregate at various temperatures, especially at high temperatures, based on the experimental values for the constituent single crystals. As for the low velocity layer in the earth’s crust proposed by Gutenberg, Hughes’ opinion that it is due to the preponderance of the velocity decrease by temperature is examined. Poisson’s ratio, calculated from Hughes’ measurements of V_p and V_s, shows the irregular change with temperature and pressure. It was concluded that the low velocity layer in the crust is not due to the effect of temperature, but due to the predomination of rocks having higher velocities in the upper crust. Low velocity zone in the upper mantle was examined from the behavior of Poisson’s ratio as a function of temperature adopting the materials of Gutenberg’s model. It was shown that there is some possibility to estimate the temperature distribution or the temperature gradient in the upper mantle by means of the comparison of the variation of Poisson’s ratio with depth in the earth to that with temperature. It is due to the fact that Poisson’s ratio is scarcely affected by pressure. Temperature distribution was drawn with rough assumption, and it seems interesting to see that temperature gradient becomes extraordinarily large at the upper part of the low velocity zone.

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§1. Introduction. - General view of rock deformation under high pressure. §2. Stress system. - Methods of compression and extension test under hydrostatic pressure. §3. Elastic modulus. - Variation of mean Young’s modulus with increasing pressure. Change of figure of curves of mean Poisson’s ratio-deforming stress relation with increasing pressure. Variation of elastic wave velosities associated with the change of elastic modulus under the large deformation. §4. Ductility. - Classification of rocks. Ductile property of carbonate rocks at high pressure. Non-ductile behavior of igneous rocks at high pressure. Effect of temperture on the ductility of carbonate rocks at high confining pressure. §5. Strength. - Variation of yield strength of carbonate rocks with congning pressure. Variation of crushing strength of igneous rocks with confining pressure. Empirical relation of strength-pressure of granite. Effect of temperature on the strength of carbonate rocks at high pressure. §6. Creep. - Relation between stress and creep rate. Estimation of creep limit of rocks under high pressure. §7. Experiments at high pressure and high temperature. - Stress-strain relation of carbonate and silicate rocks at 5k.b. and up to 800℃.

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In this paper, knowledge of the viscosity of volcanic rocks at 800 to 1400℃ is discussed whether there is any significance to be attached to the experimental results in order to obtain a better understanding of some matters related to the restoration of rocks to the viscosity possessed at or before the time of extrusion.

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Fracture of rock specimens is discussed from the geophysical point of view on the basis of the recent investigations by various authors. First, experimental results on the ultimate strength of rocks are summarized. The strength is appreciably influenced by the following factors: distribution of applied stress, confining pressure, temperature, rate of stress application, repeat of stress application, size of specimens, etc. In quantitative investigation of relations between these factors and the strength, remarkable fluctuation of the strength should be considered. Furthermore, some characteristics of fracturing of rocks are discussed : (1) time characteristics of fracture occurrence, (2) pattern of successive occurrence of local fractures in rock specimens, (3) magnitude-frequency relation of local fractures. Lastly, based on the above-mentioned results on the rock specimens and an information derived from geodetic measurements, the fracture strength of the eartn’s crust is estimated and the stress distribution in the crust is discussed, earthquake occurrence in the region being taken into account.

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This paper describes the conclusion of a symposium on the mechanical properties of rocks. Especially, the occurrence of earthquakes is discussed on the basis of the recent investigations of rock deformation. Moreover, the relationship between the experimental conditions for the deformation and velocity in rocks and the conditions of pressures and temperatures in the earth is described.

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