Late Holocene crustal uplift of Shikine Island, off the Pacific coast of central Japan, has been investigated. Shikine is a small island of about 3km (EW)×2.5km (NS), located on the northern tip of the Izu-Bonin arc, to the south of the Philippine Sea—Eurasian plate boundary of the Suruga and Sagami troughs. So far, historical coseismic uplift of the island, due to the 1498 or 1605 or 1703 great offshore earthquake, has been suspected based on raised fossil barnacles.
In the present study, six localities on the northern and eastern coast of the island, which are situated at the foot of a steep sea cliff, several tens of meters high, have been observed. At Loc. 1 fossil barnacles can be found in the range of 2.6-4.3m amsl (above mean sea level at Okada, Oshima Island).
Tetraclitella chinensis (sampling height, 3.2-3.6m amsl),
Megabalanus rosa (3.0-3.1m amsl), and
Tetraclitella chinensis (2.6m amsl) have been dated at 1980±130(GaK-9718), 1830±130(GaK-9717), and 1350±100(GaK-9720)
14Cy.B.P., respectively, showing a direct relationship between height and date. The
14C date of 2540±290y.B.P.(GaK-9721) for
Saccostrea echinata (2.6m amsl), however, is not in harmony with this tendency. This anomaly is probably due to subsequent reworking of the older shell. At this locality, the upper limit of living
Tetraclita squamosa Japonica bed indicates that the present high tide level is 1.0m amsl. At Loc. 2 fossil barnacles and shells can be found in the range of 3.9-4.2m amsl. Here, however, due to a small amount of sample number, only an unreliable
14C date of 1010±300y.B.P.(GaK-9717) has been obtained for
Tetraclita squamosa Japonica (4.2m amsl). At Loc. 3 fossil barnacles can be found in the range of 3.9-6.1m amsl. The
14C date of the barnacle
Tetraclitella chinensis which are scattered throughout 1.1m at section (3.9-5.1m amsl) is 1530±100y.B.P.(GaK-9722). The upper limits of both fossil and living barnacle beds at this locality are somewhat higher than at Loc. 1 or 2, probably due to the high energy of waves in a sea-cave. At Loc. 4, 5 and 6 no fossils are found, but clear raised coastal landforms can be seen. At Loc. 4 there is a raised notch, whose retreat point is 3.2m amsl. At Loc. 5 two steps of raised bench, 4.2m amsl and 2.6m amsl, exist. At Loc. 6 there is a raised bench of 3.7m amsl. Also around Loc. 1 and 2 benches at about 4m amsl can be recognized.
Judging from the above mentioned observations, Loc. 1 offers us the best evidence for the recent vertical movement of Shikine Island. As an evident discontinuity between the fossil bed and the living barnacles exists, it is certain that a sudden uplift took place after the formation of the lowest part of the fossil bed, that is ca. 1400y.B.P.
14C dates from Loc. 2 and 3 do not conflict with this estimation. The amount of uplift was approximately 3m. Emergence of the most prominent raised benches and sea-caves is probably correlated with that of fossil bed at Loc. 1. Another land uplift might be assumed, taking into consideration the existence of the lower and local bench at Loc. 5, or significantly older
14C dates from the upper part of fossil bed at Loc. 1. However, there is no enough data to confirm this possibility. Therefore, it is concluded that the distinct crustal uplift of Shikine Island is much older than the historical earthquakes of 1498 or 1605 or 1703.
If this uplift was due to an earthquake, the 684 Hakuho earthquake, a great interplate event of magnitude 8.4 off southwest Japan, might have been responsible for it. However, the possibility remains that an unknown earthquake near the island brought about the uplift. Another possibility is that the uplift might have been related to volcanic activity. All these presumptions, however, are still open to question. Further intensive a
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