Active volcanoes of an island arc are situated in a specific belt, which is called a volcanic belt and parallels to an oceanic trench associated with the island arc. The trenchside border of a volcanic belt is named a volcanic front, as volcanoes are clustered rather near the border on the trench side than at the axis of the belt (Figs. 1 and 2).
Mantle earthquakes of an island arc show a systematic distribution of the foci on a plane dipping away from an oceanic trench, the plane being called a seismic plane.
Mantle earthquakes are closely related in space to volcanoes of the island arc. The depths of the seismic plane underneath the volcanic belt are limited to a range from 150 to 250km. On the other hand, basic differences in chemical composition among the basaltic primary magmas reflect differences in the depth at which the magmas were generated, instead of successive stages in the differentiation of magma. The depths of the seismic plane appear to be responsible for the magma-generation depths, as the systematic regional variation in chemical composition of the primary magmas shows a close correlation to the regional variation in depth of the seismic plane. Therefore, the generation of mantle earthquakes and the generation of primary magmas would have a common origin (Fig. 3).
Complementary relationships in distribution between hypocenters of mantle earthquakes and the magma-generation zone are stressed on in this paper and are summarized as follows:
1. The hypocenters of mantle earthquakes are abundant in the frontal area of the volcanic front, where no volcanoes are present. To the contrary, they are rare underneath the volcanic belt. The relationship is clearly given in a map of Japan (Figs. 4 and 5).
2. Ratio of total numbers of mantle earthquakes at depths from 175 to 275km to total numbers of those from 75 to 125km is distinctly lower in island arcs than in non-islandarc regions. The relationship is given in a histogram and a table. Under the island arcs the hypocenters are relatively scarce at depths from 175 to 275km, where magmas would be generated (Fig. 7 and Table 1).
3. Along the belt at the back of a volcanic front, either the bulk of the belt is occupied by many of volcanoes as is the case in Japan, or some sections of the belt are occupied by a series of volcanoes as is the case in the Pacific coast region of South America. In the latter case, nonvolcanic sections of the belt at the back of an extension of the volcanic front display higher seismicity in the mantle.
Thus the greater part of epicenters of mantle earthquakes shows a complementary distribution to loci of volcanoes in island arcs. It would imply an exclusive relation between the generation of mantle earthquakes and the generation of primary magmas. Therefore, it seems very likely that the source of energy is common with the each other and the difference of generating whether shocks or magmas is attributed to a condition of circumstance, for instance, the temperature of the particular place.
