フォッサマグナ地域の磁気的構造とキュリー点深度

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Extensive magnetic surveys, which have been undertaken in the Islands of Japan and surrounding regions, have revealed marked magnetic anomalies in various regions including the Fossa Magna region. Some of the anomalies are likely to be closely related to tectonics. Magnetic data also provide valuable information on the temperature distribution in the crust through the determination of an isotherm corresponding to the so-called Curie point ; crustal rocks lose their magnetization at temperatures higher than the Curie point. In this paper, magnetic anomalies are presented with special reference to tectonics in the Fossa Magna region as well as the Curie point depth distribution there. In the Southern Fossa Magna region, a regional magnetic anomaly extends as shown in Fig. 1; a positive anomaly is located near the northern-most edge of the Philippine Sea plate and a negative anomaly extends behind the Sagami and the Suruga troughs. This anomaly can be interpreted as indicating the subduction of the oceanic Philippine Sea plate, the top of which is likely to be highly magnetized. Then the configuration of subducting plate is inferred from block models which can account for the observed magnetic anomalies, as clearly shown in Fig. 2 for some profiles. The depth to the subducting plate thus inferred seems to be somewhat shallower than that derived from the distribution of hypocenters of microearthquakes. Recently, a complicated configuration including fracture and flexure has been proposed and hence a threedimensional interpretation seems to be required to cope in a realistic way with such a complicated structure, whereas in block models a rather two-dimensional structure is implicitly assumed. In view of this defect, the discrepancy is not meaningful and we may conclude that the block models largely represent the depth to the subducting Philippine Sea plate. The Itoigawa-Shizuoka tectonic line turned out to be located at a steep gradient of Curie point depth, which is deeper in the west (see Fig. 4). This fact implies that the Itoigawa-Shizuoka line is in fact a tectonic boundary. Some tectonophysicists claim that the eastern margin of the Japan Sea is a plate boundary. As seen in Fig. 3, the Curie point depth is generally shallow there, whereas it is fairly deep at the Japan trench where the Pacific plate sinks. However, this fact would not contradict with the new hypothesis if we take into account the age of seafloor of the Japan Sea (about 20 Ma). The Curie point depth data also provide useful information on the magnitude of a possible largest earthquake. On the one hand, earthquakes are not generated at high temperature because of ductile behavior of rocks. On the other hand, the earthquake magnitude is determined by the area of rupture zone. Hence the magnitude should not be large in areas where the Curie point depth is shallow. This relation can be quantified and then the zoning of the magnitude for the largest earthquake was derived as shown in Fig. 5. The magnitude of 7.7 for the earthquake which occurred at the eastern margin of the Japan Sea exceeds the upper bound if the region is not supposed to be a plate boundary. This difficulty is removed if the relation for interplate earthquakes is applied in this region, supporting the new hypothesis that the eastern margin of the Japan Sea is a plate boundary. The electrical conductivity is also expected to provide important information on tectonic processes. Although high conductivity zones have been found in the crust beneath the Fossa Magna region, their relation to tectonics is not clear at present. In particular, the high conductivity zone beneath the Northern Fossa Magna (see Fig. 6) extends in a direction rather perpendicular to the Itoigawa-Shizuoka line. No clear conclusion seems to be derived until a systematic study is undertaken in the entire Fossa Magna region.
社団法人東京地学協会の論文

フォッサマグナ地域の磁気的構造とキュリー点深度

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