Triboluminescence (TL) was observed during slow frictional sliding of quartz glass. The intensity of TL during stable sliding under constant normal stress was almost conlstant except at the beginning of the sliding. The intensity of TL increased exponentially as a function of normal stress. The variation of the intensity is discussed. TL with long duration observed for some kinds of rock samples fractured uniaxially was not observed.

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Geoidal undulation of short wavelengths and the distribution of free-air anomalies in the northwestern Pacific look alike. This resemblance is attribuable to the characteristic features of these two quantities. The spectra of areal distribution of both the free-air anomalies and the geoidal undulations have two peaks, one in relatively short and the other in long wavelengths.

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Lithospheric deformation at a subduction zone due to the collision and the consequent interaction between a trench and a seamount on the oceanic plate is examined using numerical simulation of the motion of viscous fluid. The results show that the slab-pull force at the trench cannot disappear even if a seamount collides against it. In the case of a small seamount the subduction motion continues, while in the case of a large seamount the motion of subduction terminates temporarily and another subduction takes place at the seaward edge of the seamount.

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The Urakawa-Oki earthquake (M=7.1) occurred offshore of Urakawa, Hokkaido, Japan, on March 21, 1982. In order to investigate the aftershock activity of this event, we deployed four ocean bottom seismographs (OBS's) off Urakawa. During an observation period from March 29 to April 6, the OBS network detected more than 4, 500 earthquakes. Magnitude range of these events is from -2.0 to 5.0.
Aftershock distribution of the Urakawa-Oki earthquake was determined on the basis of the OBS and land observations. In the analysis, we selected about 250 aftershocks with M≥2.0, because the seismic signals from these events were recorded clearly both by the OBS's and the land stations around the aftershock area. Hypocenter determination was carried out using a method of inversion analysis. Most of the aftershocks were located in a region of 42°00'-42°20'N and 142°25'-142°40'E. These events were distributed in a depth range of 10-30km. The characteristic dimensions of the aftershock area were estimated as 35km×25km. The events in the southern part(south of 42°15'N) were distributed on a plane with an area of 20-25km×10-15km dipping 20-30° northward. In the northern part of the aftershock area, we found another trend of aftershocks which inclined southward with a high angle of 60-70°. The events in this group were located on a plane with a relatively narrow area of 10km×10-15km. The aftershock distribution obtained in the present analysis suggests that the fracture mechanism of the Urakawa-Oki earthquake was very complicated.

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