The occurrence of double seismic planes beneath some island arcs can be explained in terms of an unbending of the descending lithosphere. Stress distributions in a representative cross section of the unbending lithosphere are calculated under reasonable physical conditions and physical properties of the lithosphere. The results show that various types of possible seismic zones appear in the descending lithosphere depending on the conditions mentioned above, and that the appearance of double seismic zones with a plane structure is quite sensitive to small changes in the physical conditions. The temperature at the double seismic zones in the descending lithosphere is estimated from the permitted temperature limit under which the double-plane structure of the seismic zone appears. The temperature of the dual seismic zone at a depth of about 180km ranges from 850° to 1, 000°C.
In addition to the pure unbending of the lithosphere, uniaxial extension along the descending direction of -1% is favored during the unbending for the appearance of a dual seismic zone. That is, double seismic zones having nearly the same thickness do not appear in the case of the unbending lithosphere with an additional compressional strain of more than 1% or with an additional extensional strain of more than 2% during unbending.

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Tectonic features of the Kyushu-Ryukyu arc have been investigated in detail, on the basis of seismicity and focal mechanism of shallow to intermediate-depth earthquakes with magnitudes around and greater than 5.0.
All shallow earthquakes in the Hyuga-nada region east of Kyushu are characterized by low-angled thrust faulting, which may be directly related to the underthrusting of the Philippine Sea plate beneath southwest Japan. Their frequent occurrence, dissected fault regions and somewhat large stress drop, in comparison with great earthquakes along the Nankai trough, may be attributed either to local stress concentration due to the contortion of the subducting plate resulting from southwestward warping of the trough, or to heterogeneous structure extending northward from the Kyushu-Palau ridge.
The shape of the Wadati-Benioff zone and the stress state within the descending lithosphere show marked differences between the northern and southern Ryukyu arcs bounded by the Tokara channel; the dip of the zone at depths below 100km reaches 70° in the northern arc, while it decreases to 40-50° in the southern section. Focal mechanisms of intermediate-depth earthquakes show down-dip tension and down-dip compression within the northern and southern parts of the lithosphere, respectively. Two interpretations may be possible of the above differential subduction, both of which are attributed to the difference in shear resistance to the subduction; one is due to a difference in the convergence plate velocities, and the other to that in the physical properties such as viscosity and density in the surrounding mantle in relation to volcanism. For lower resistance, the lithosphere will sink rather smoothly into the asthenosphere, which is dragged down by a gravitational pull with high dip angles and will be in a tensional state, whereas the subducting lithosphere will be subjected to compressional stress and have low dip angles if it receives higher resistance.

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The earthquake generating stress fields of the Western Rift Valley of Africa are discussed in detail from the data obtained by the seismological network of IRS, Zaire, and by the seismological stations in Africa. The present analysis is based on the method of composite mechanism solution using the initial motions of P-waves from earthquakes which occurred during the period from 1958 to 1970. The stress fields are analyzed for ten sub-regions divided on the ground of the characteristics of seismicity. Quite stable solutions are obtained for the sub-regions of Lake Kivu and its vicinities, which are located in the central part of the Western Rift Valley, and shows that the stress fields are of the normal faulting type. The direction of tension axis of each solution is generally perpendicular to the rift system or to the local faults. The perpendicularity is consistent with the results reported by several authors studying focal mechanisms of earthquakes occurring in the African Rifts and the ocean ridges. In contrast with this general conclusion, the strike-slip type of focal mechanism prevails in sub-regions of the southern part of the Western Rift Valley, and a heterogeneous stress field is suggested in the sub-region of the active volcanoes of Nyiragongo and Nyamuragira.

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A systems analysis of earthquake hazards in urban areas is carried out in relation to an issue of warning, in which the viewpoint from risk assessment is employed. Mass panic and systems panic are distinguished, and the latter is shown to be much more crucial than the former.
The diffusion of issued information is simulated for the Izu Oshima earthquake, 1978. It is concluded that dual information channels at mass media level and administrative level are dangerous (Chap. 2). In urban areas, separation of family and without in telephone lines are major sources of systems panic. The responses of individuals with respect to the timing of warnings are analyzed and simulated. It is concluded that several hours are required from the detection of abnormal signs to finish emergency actions (Chap. 2).
The "go home" behavior is the prevailing action for people in the downtown area of a large city. Mass evacuations from Sakae area in Nagoya City after an issue of warning are simulated. It is concluded that simultaneous rush to the metro from various areas should be avoided (Chap. 3).
A systems analysis of hazards is carried out from both demand and supply sides. Supply systems (water, electricity, city gas, etc.) and emergency service systems (fire fighting, mass evacuation, etc.) are evaluated based on fault net analysis. It is concluded that regional evacuation will not be effectual for any earthquake (Chap. 4).

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Analytical expressions of the quasi-static, surface deformations (displacement, strain and tilt) due to an inclined, rectangular fault in a viscoelastic half-space are obtained by applying a "correspondence principle" to the solutions of the associated elastic problem. The medium is assumed to be elastic dilatational and Maxwell deviatric, and the time dependence of a dislocation source is taken to be of a step function type.
From the analytical expressions, it is directly found that the viscoelastic part of the deformation field vanishes exactly for both an arbitrary slip on a horizontal fault plane and a dip-slip faulting on a vertical plane. In other cases, the viscoelastic part has a time dependence prescribed by a factor, 1-exp(-t/τ), where τ denotes the relaxation time determined from the Lame's elastic constants and the viscosity of the medium.
Patterns of the elastic and the viscoelastic parts of the deformation field are respectively shown for two representative fault models. As an example, postseismic vertical displacements associated with the Kanto earthquake of 1923 are computed by the fault model determined from the coseismic geodetic data, and compared with the observed crustal movements for the period of 1931-1950.

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