ALBERT

Description: In the Part I of this paper, static surface displacements due to a strike-slip fault which takes place along a dipping boundary between two different media were investigated by applying the Mellin transform. The problem is extended to the case of a dip-slip fault and surface displacements are compared with those in a uniform semi-infinite medium with the same dislocation. Main conclusions are summarized as follows: The horizontal displacements are much affected than the vertical displacements by the dipping structure. The displacements on the surface of a soft dipping layer are amplified. For the dip-slip fault with dip-angle of 45º, for example, the maximum horizontal displacement, when the rigidity of the dipping layer is half of that of the lower medium, is about 1.6 times as large as that for the uniform medium. The position which gives the maximum displacement moves toward the dipdirection of the fault as the rigidity contrast increases. When comparing observed data with theoretical values derived under the assumption of uniform semi-infinite medium, one should pay attention to these differences. © 1975, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.

Description: Historical seismicity data since the year 1850 in Tohoku and Hokkaido, northern Japan, show a spatial and temporal correlation between intraplate seismicity in the land area and occurrence of large interplate earthquakes along the Japan and Kuril trenches. In the continental lithosphere adjacent landward to the rupture zones of recent large interplate earthquakes, almost all the earthquakes of magnitude 6.0 and above occurred during the periods from the 50 years before to the 10 years after the occurrence of the large interplate events. A simple statistical test shows that this correlation is significant at a 99.5% confidence level. During the period for the last 20 years, a group of intraplate earthquakes occurred in middle Tohoku with no large interplate earthquake seaward of their epicentral locations; this suggests a possibility of a large interplate event off the southern Sanriku coast in the near future. We examine other geophysical data to rate the possibility. All the examined data, i.e., historic and recent seismicity off the Sanriku, and crustal deformation revealed by the geodetic work, support the possibility of occurrence of a large event off the southern Sanriku in the near future. Source parameters are estimated for this expected event; they are conformable to those of recent large interplate events along the Japan and Kuril trenches. We can use the hypothesis on the correlation between intraplate seismicity and large interplate earthquakes as a criterion for forecasting the land area of high seismic risk for intraplate events as well as a criterion for forecasting a rupture zone of an impending large interplate earthquake. This may be of significance for instrumentation to detect various kind of phenomena associated with imminent intraplate and/or interplate earthquakes. © 1979, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.

Description: In the present tsunami warning system, the predominance of the approaching tsunami is estimated, taking account of the earthquake magnitude and focal depth. However the predominance of tsunami also depends on Other focal parameters, e.g. the rise time of the source time function, fault length etc. Accordingly it is quite natural that great tsunamis due to earthquakes with relatively small magnitudes have been sometimes observed. For the improvement of the tsunami warning system, it is necessary to develop the method to find the focal parameters before the tsunami wave arrivals and to make clear the dependence of the generation of tsunami on the above parameters. Effects of the focal parameters on the generation of tsunami and Rayleigh waves are investigated in a previous and in the present paper respectively. If the focal parameters are pre-estimated by the use of early arrival Rayleigh waves, the predominance of tsunami can be predicted. Although some of the focal parameters have fairly complex effects on both waves, it is shown that there is some possibility of the improvement of the present warning system by the use of tectonic features and body waves in addition to sub-oceanic Rayleigh wave observation. © 1976, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.

Description: An equation which represents the crack propagation is obtained by approximating the wave equation and the boundary conditions in the neighborhood of the dislocation surface. The problem of crack propagation with an unsteady finite rupture velocity is solved with the aid of this model and the computation is carried out by using the finite difference method. The proposed model takes the rupture propagation as the diifusive phenomena of the inhomogeneous initial stress field. It is clarified in this paper that the most important focal parameters for the dynamical process of fault motion are the effective stress (defined as the static frictional stress minus the sliding frictional stress) and the fracture strength (defined as the static frictional stress minus the initial stress). The former accelerates the fault motion and the latter decelerates it. All the rupture phenomena can be interpreted by the relative magnitude between these stresses. The importance of both stresses is not ascertained until the inhomogeneous initial stress field is introduced. © 1976, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.

Description: The partial differential equation, which represents approximately the propagation of a crack, obtained by Yamashita (1976) is solved numerically, and the dependence on the tectonic field of the source parameters is studied in detail. Main results are as follows. © 1977, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.

Description: This paper treats the effect of non-uniform initial stress in static crack problems. This paper is based on an earthquake source model proposed by Yamashita. Yamashita derived, from a dynamical approach, relations between static source parameters and the characteristics of the associated tectonic field. In the present paper, these relations are obtained by solving an equilibrium equation of dislocation distribution. Comparison of the two shows qualitatively good agreement. If earthquakes are caused by an accumulation of initial stress whose rate is negligibly low compared with rupture velocity, it is shown that a crack model in which dΔu(x)/dx has finite values at crack tips is the most appropriate where Δu(x) is the dislocation distribution. It is also shown that the value of stress drop computed on the assumption of non-uniform initial stress is, from a practical viewpoint, nearly the same as that computed with the aid of the expressions by Starr and Knopoff. © 1978, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.

Description: The focal process of deep-focus earthquakes is modelled physically using a viscous frictional law. Numerical experiments are conducted in which the equation of crack propagation derived by Yamashita (1976) and the heat conduction equation are solved simultaneously. In this way, the interaction between fault motion and thermal conduction is taken into consideration. When the frictional characteristics of a deep-focus earthquake are described by a transient creep equation, the sliding frictional stress initially increases rapidly as the dislocation continues. However, when the temperature on the dislocation surface becomes significantly higher than the initial temperature or exceeds the solidus temperature, the sliding frictional stress begins to decrease. Efficient fault motion depends on this decrease in sliding frictional stress, and also requires a large effective stress in order to overcome the resistance caused by the initial increase of sliding frictional stress. Effective stress is defined as the static frictional stress minus the sliding frictional stress. This paper shows that a large stress drop is caused by a large effective stress and therefore the fact that deep-focus earthquakes have large stress drops is fully explained by the model proposed in this paper. © 1978, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.

Description: Crack models in which nucleation can be treated are physically analysed by considering non-uniform initial stress or strain, and static and sliding frictional stress. The proposed models are, in the strict sence, applicable to physical phenomena near the foci of large earthquakes having bilateral rupture propagation. Crack models of two kinds are discussed in detail. In one model, rigidity, static and sliding frictional stress are assumed to be constant everywhere. In this case, the proposed model is applied to the 1961 Kitamino and the 1966 Parkfield earthquakes and the initial tectonic fields and frictional stress of these earthquakes are determined. In the other model, rigidity is assumed to be a function of position. This model yields the result that stress drop is directly proportional to effective stress, defined by the static frictional stress minus the sliding frictional stress, and the final stress is lower than the sliding frictional stress. In each case, the factors accelerating dislocation motion are a broadly accumulated initial stress and a small ratio of sliding to static frictional stress. This result is qualitatively the same as in a paper of Yamashita (1977). In Case II, it is shown that stress drop is not influenced by the spatial distribution of rigidity but strain drop is heavily influenced by that distribution. Accordingly a treatment like that of Case II is required when the value of stram drop is discussed. © 1977, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.