A study for prediction of great earthquakes in and around Sagami Bay

Abstract

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.