紀伊半島南部の完新世地殻変動

抄録

The southern part of Kii Peninsula faces to the Nankai Trough where the Philippine Sea Plate is subducted under the Eurasian Plate. In this region, we have experienced a lot of uplifts associated with great earthquakes in the historical period. The authors aim to clarify the mode of tectonic emergence in Kii Peninsula during the Holocene based on geomorphic and biological sea-level indicators.
The Nankaido earthquake (M=8.1) of 1946 is the most recent great earthquake along the Nankai Trough. The mode of tectonic movement before and after the Nankaido earthquake was known by the precise re-leveling measurement. Before the earthquake, the northern part of the peninsula had been uplifted and southern part had been subsided slowly. On the contrary, at the time of the earthquake, northern part was subsided, while the southern part was emerged abruptly. After the earthquake, tectonic movement turned into the similar mode as before the earthquake. YONEKURA (1968) estimated the residual uplift during one great earthquake cycle at Kushimoto as approximately 0.2-0.3 m. He also suggested that great earthquakes such as the Nankaido earthquake of 1946 had recurred at an interval about 110 years, and the residual uplifts during earthquakes and intervening period had been accumulated through the late Pleistocene, forming emerged marine terraces.
In Kii Peninsula, evidence for former sea levels is recognized as notches, wave cut benches and the calcareous remains of attaching organisms living in the tidal zone. Based on the recognition of highly concentrated zone in the vertical distribution of these former sea level indicators, six former sea levels are distinguished in the southern part of Kii Peninsula : (I) 5.8 m, (II) 4.1 m, (III) 3.3 m, (IV) 2.8 m, (V) 2.0 m and (VI) 0.8 m above the present mean sea level. These former sea levels are dated : (I) 6, 000-5, 500 yr BP, (II) 5, 000-4, 000 yr BP, (III) 3, 800-2, 600 yr BP, (IV) 2, 400-2, 000 yr BP, (V) 1, 800-800 yr BP and (VI) 600-200 yr BP by the radiocarbon method. We may regard the cause of drastic drops of sea levels as coseismic uplifts named event 6 to event 1 in chronological order. Event 1 to event 6 must have occurred at 200-0 yr BP, 800-600 yr BP, 2, 000-1, 800 yr BP, 2, 600-2, 400 yr BP, 4, 000-3, 800 yr BP and 5, 500-5, 000 yr BP respectively. It can be said that these earthquakes have recurred at an irregular interval of more than several hundred years during the late Holocene, and the residual uplift during each earthquake cycle might be significantly larger than the value of 0.2-0.3 m suggested by YONEKURA (1968).
Earthquakes such as the Nankaido earthquake of 1946 might be inter-plate type, and the residual uplifts during these earthquake cycles could not be accumulated from the view point of the elastic rebound theory. By the way, the value of 0.2-0.3 m as the residual uplift associated with Nankaido earthquake of 1946 type estimated by YONEKURA (1968) is unreliable because the precise re-leveling measurement was not done throughout a whole interval of earthquakes. The earthquakes reconstructed in this paper, i.e. event 1 to event 6 are not of inter-plate type, but of intra-plate type which have resulted in the cumulative uplifts of Kii Peninsula.
The patterns of uplifts caused by the events can be classified into two types : (1) amounts of uplift of event 6, event 4 and event 2 are larger in the southwestern part of the peninsula, whereas (2) those of event 5 and event 3 are larger in the southeastern part. It is difficult to distinguish the amount of uplift associated with event 1 from the present residual uplift of the Nankaido earthquake of 1946.