HOLOCENE CRUSTAL MOVEMENT IN MUROTO PENINSULA SOUTHWEST JAPAN

Many great earthquakes have occurred in historical period along the Nankai Trough where Phillipine Sea Plate subsides under Eurasia Plate. Muroto Peninsula jutting out to the Nankai Trough, has experienced coseismic uplift appeared at the great earthquakes. The author aims in this paper to clarify coseismic crustal movement during the Holocene based on geomorphological and biological sea-level indicators in Muroto Peninsula.<br> Nankaido earthquake (M=8.1) occurred in December 26, 1946 is the latest one among these great earthquakes. In Muroto Peninsula, seismic crustal movement was clarified by the precise re-leveling before and after this earthquake. Before the earthquake, the northern part of the peninsula was uplifted and the southern part was subsided slowly. On the contrary, at the time of the earthquake, while the northern part was subsided, southern part was uplifted abruptly. Crustal movement after the earthquake showed the similar mode as before the earthquake. Yoshikawa et al. (1964) considered that the amount of residual uplift at Cape Muroto was 0.25m per one great earthquake such as Nankaido earthquake type recurred at an interval of 110 years, and the residual uplift had been accumulated through the late Pleistocene.<br> In Muroto Peninsula, the evidence for former sea levels is recognized as the marine terraces, notches, wave cut benches, potholes and the calcareous remains of attaching organisms living in tidal zone. Based on the certification of high concentrating zone in the vertical distribution of these former sea level indicators, six former sea levels are distinguished at Cape Muroto, i.e. I:11.0m, II:8.7m, III:6.6m, IV:6.0m, V:3.7m, and VI:1.3m in height. The former sea levels for each preceding height are aged as I : 6, 000-5, 000 y. B. P., II:4, 000-2, 700 y. B. P., III:2, 600-2, 200 y. B. P., IV:2, 000-1, 100 y. B. P., V:1, 000-800 y. B. P., and VI:700-200y. B. P., by means of radiocarbon dating. The earthquakes caused these abrupt drops of sea levels are named event 1 to event 6 according to the new age in chronological order. Event 1 to event 6 probably occurred at 200-0 y. B. P., 800-700 y. B. P., 1, 100-1, 000y. B. P., 2, 200-2, 000y. B. P., 2, 700-2, 600y. B. P., and S, 000-4, 000y. B. P. respective-ly. Figure 7 indicates that abrupt seismic uplift has repeated at an irregular interval of several hundred years during the Holocene, and the amount of residual uplift per one earthquake was bigger than that of Nankaido earthquake in 1946.<br> Earthquakes of Nankaido type are the inter-plate earthquakes, and the uplift with these earthquakes can not be accumulated from the view of elastic rebound theory. The value of 0.25m for the residual uplift with Nankaido earthquake type estimted by Yoshikawa et at. (1964) (as mentioned above), is unreliable because the precise re-leveling was not done throughout an interval of earthquakes, and its value should be decreased to about Om. Earthquakes reconstructed in this paper, i.e. event 1 to event 6 are not inter-plate earthquakes, but intra-plate earthquakes which have resulted in the cumulative uplift (Shimazaki, 1980) of Muroto Peninsula. The uplift pattern caused by each event is distinctive. While amounts of uplift of event 6 and event 4 are bigger in the southwestern part of the peninsula, those of event 5, event 3, and event 2 are bigger in the southeastern part. It is difficult to distinguish the amount of uplift with event 1 from resudial uplift by Nankaido earthquake. As observed around Cape Oyama, Higashino River and Irugi River, local uplifts occurred at event 1 to event 3 and event 5. The cause of these local uplifts could bee due to repeated displacements of active faults within Muroto Peninsula (Fig. 11).