A plate model to account for the genetic relation between great volcanic eruptions and earthquakes which occurred along the Sagami trough branching off from Japan trench, and along the Nankai trough off the Chubu district is proposed on the basis of the newly compiled neotectonic map. The northwestern Philippine Sea plate is bounded by the Nankai trough to the west and by the Sagami trough to the north. The Nankai trough is regarded as an active subduction zone in recent geologic time. So the Philippine Sea plate moves northwestwards in parallel with the Sagami trough. Consequently, the Sagami trough is regarded as a transform fault of trench-trench type which joints the Nankai trough and Japan trench. On a cross section of the northwestern Philippine Sea plate cut parallel to the Sagami trough, the Nankai trough, Oshima Volcano, Miyakejima Volcano, and the Izu-Bonin trench stand side by side from northwest to southeast. Large cracks for rising magma are expected to locate below these volcanoes, thereby dividing the plate into three segments (P1, P2 and P3 from east to west in Fig. 7) in the cross-section. P1 is pushed by a northwestward motion of the Pacific plate, therefore, at the first stage, the eruption of Miyakejima Volcano may occur owing to squeezing of magma beneath the volcano in response to the stress generated by P1, and eventually, the bottom floor of the summit crater of Oshima Volcano may start to rise up. Secondly, the bottom floor of the summit crater may come up owing to the successive compressive stress generated by P2 together with P1, and then a great and/or large eruption of Oshima Volcano may occur. Thirdly, the Philippine Sea plate moves further northwestwards and then a great earthquake may occur to release the strain accumulated along the Sagami trough. Since such a continuous process is repeated several times, the Philippine Sea plate may go down beneath the Nankai trough where a greater earthquake may inevitably occur as the result. In fact, epicenters of recorded great earthquakes of M≥7.5 are located in a limited area along the Sagami trough and in the just inner parts of the Nankai trough. Evidently, greater earthquakes of M≥8.0 have mainly occurred in the latter province. The plate model can well be applied to explain the genetic relation of the recorded eruptions and earthquakes in the studied region. Added to the facts, recent vertical crustal movement of the southern Kanto district also supports the model inferred from these events. So that, mutual relationship among eruption, earthquake and recent crustal movement may be an excellent clue to predict the coming great earthquake. Further detailed discussion is summarized as follows. 1) It is concluded with full confidence that great and/or large eruptions of Oshima Volcano in the last 200 years have occurred in 2 to 14 years after the eruptions of Miyakejima Volcano. The large eruption of Miyakejima Volcano occurred in 1962, from which that of Oshima Volcano will ensue in the next time. Added to the fact, the proper activity of Oshima Volcano must have started itself since 1972, based on seismicity of volcanic earthquakes in and around Oshima Volcano. 2) Judging from that the bottom floor of the summit crater of Oshima Volcano is regarded as risen up since around 1960, and that the strain has been accumulated based on the observation of the recent crustal vertical movement at Aburatsubo in the Miura Peninsula and the bottom floor may soon come to the peak position as shown in Fig. 11. The coming great and/or large eruption of Oshima Volcano might be expected to start from around a last half of 1970's by all above accounts.
The altitudes of the coastal terraces, which are presumed to be formed about 30, 000 years B. P., are remarkably different at each locality. In the regions such as Scandinavia and Antarctica, which were covered by ice sheets during the Würm glaciation maximum, the altitudes of the “30, 000 B. P. terraces” are relatively high. This is probably due to the isostatic uplift with the reccession of the glaciers during the Post-glaciation. In the “crustal active zones”, such as the Circum-Pacific and East Africa, the terraces discussed here are situated at elevations from lower than-80 M to higher than +50 M above the present sea level namely, the variation of their altitudes is great. This great variation is believed to have been caused by the violence of the block-movement during the late Quaternary. In the “stable continents”, though the sea level of 30, 000 years B. P. lie below the present one at the mouth areas of the Mississippi and the Senegal rivers, they reached above the present one in regions such as the east coast of North America, India and the west coast of France. From these observations, it is summarized that the altitudes of the “30, 000 B. P. terraces” are high in the upheaval-areas, low in the subsidence-areas and intermediate in the stable areas, corresponding to the tendency of the crustal movement suggested from a geologic-geomorphologic point of view. Though these terraces have the great variation in the altitudes as above mentioned, most of them are higher than-30 M. Additionally, there are not a few localities indicating that these terraces were formed by the transgression. The sea level-change curves based on these facts from some areas show the rise of the sea level at about 30, 000 years B. P. Therefore, it is considered that the sea level was globally high at this time and that the “30, 000 B. P. terraces” will be found probably at many other coastal areas in addition to the reported ones. On the other hand, it has been recognized recently by studies of the paleotemperature that the relatively warm interval occurred about 30, 000 years B. P. in many areas of the world. Moreover, it has been reported that the glaciers considerably receded during this warm interval in many continents. These facts mean that the Würm interstade of about 30, 000 years B. P. was a global phenomenon and that the high sea level of that time can be explained generally by the glacio-eustatic theory, though the exact quantitative relationship between the sea level and climate still remains questionable.