西南日本外帯の地形と地震性地殻変動 (第四紀テクトニクス特集)

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The Pacific coast of Southwest Japan has been attacked by violent earthquakes accompanied by remarkable crustal deformation at intervals of 100 to 150 years in the historical period. At the most recent great earthquake in 1946, promontories protruding south into the Pacific Ocean were upheaved by about one meter, being tilted northwards, and inland mountainous regions were subsided. The mode of recent crustal deformation, including seismic one, of Shikoku has been revealed with precise levellings, as shown in Figs. 1-3.In the southern part of Shikoku runs a hinge line of the recent crustal deformation, which was subsided at the seismic time and upheaved in the inter-seismic periods. The coastal areas south of the hinge line were tilted southwards in the inter-seismic periods and remarkably northwards at the seismic time, while the mountainous region north of the line was quite reversely deformed.In the vicinity of Muroto Promontory, the southeastern tip of Shikoku, a characteristic process of post-seismic crustal deformation was clarified with precise levellings carried out seven times for six years after the great earthquake in 1946 (Okada et al., 1953). Immediately after the earthquake, Muroto Promontory was rapidly tilted southwards, and then the rate of southward tilting exponentially decreased to become as constant as in the pre-seismic period. The post-seismic crustal deformation (Fig. 3) is nearly reverse to the seismic one (Fig. 2) and is different in its mode from the pre-seismic one (Fig. 1), while similar minor features are found in their mode. It is, therefore, inferred that the post-seismic crustal deformation was chiefly caused by seismic after-effect and the pre-seismic crustal deformation resulted from secular tectonic movement and decelerated seismic after-effect.From this inference secular crustal deformation of Shikoku was tentatively estimated, by subtracting the post-seismic vertical displacement from the triplicated pre-seismic one. Although there is no definite reason for triplicating the latter, the estimated secular crustal deformation (Fig. 6) is nearly concordant with the patterns of geomorphological and geological structure of the mountains, and is negatively correlated with Bouguer's anomalies of gravity (Geogr. Surv. Inst., 1966).Resultant crustal deformation of Shikoku in the seismic and inter-seismic periods was also obtained from results of precise levellings (Fig. 7), assuming that great earthquakes accompanying crustal deformation of similar mode have occurred at intervals of about 120 years. Coastal terraces on the south coast descend northward and their heights have a positive correlation with the resultant deformation (Fig. 9). Topographic features of various types caused by subsidence are found along the hinge line, which is inferred to have been subsided as a result of the seismic and inter-seismic crustal deformation. Heights of the mountains north of the hinge line show positive correlations with the resultant deformation, except in the central part of the western Shikoku Mountains, and coefficients of regression are larger in higher mountains than in lower ones (Figs. 10 and 11). In the coastal areas south of the hinge line, however, correlation between heights of the mountains and the resultant deformation are negative (Figs. 9 and 11). This means that the mountains have been upheaved by tectonic movement of similar mode to the recent crustal deformation including seismic one, which probably dates from before the formation of coastal terraces and at least after the evolution of the lower mountains.A quite similar relation between the recent crustal deformation and the geomorphic features is also found in Kii Peninsula, east of Shikoku.
日本第四紀学会の論文

西南日本外帯の地形と地震性地殻変動 (第四紀テクトニクス特集)

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