Large-scale slope failure and active erosion occurring in the southwest Ryukyu fore-arc area
Large-scale slope failure and active erosion occurring in the southwest Ryukyu fore-arc area
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DOI：
10.5194/nhess-1-203-2001
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发表时间：
2001-01-01
影响因子：
4.6
通讯作者：
Ono, T.
中科院分区：
文献类型：
--
作者：
Matsumoto, T.;Kimura, M.;Ono, T.
台湾岛以东的西南琉球地区是菲律宾海板块和欧亚板块之间的一条弧形边界。该地区的地形特征有：(1)在石垣岛附近有一个大型圆形剧场，恰好位于1771年海啸地震的估计震中上；(2)圆形剧场以北有许多深海峡谷；(3)15-20公里宽的弧前盆地；(4)海沟轴线上15-20公里宽的平面；(5)位于弧前区的东西向半地块等。最近的几次地形测量探险揭示了这一点。1992年，ShINKAI6500在石垣岛以南120公里处的一个支线上的弧前区进行了潜水调查。该遗址的主要特点是由一系列陡峭的斜坡和悬崖组成的崎岖地形。由于沉积物本身的重量，表面的一部分被侵蚀，因此基准层暴露出来。由于目前的表面滑动和侵蚀，潜水过程中现场覆盖着悬浮颗粒。1997年，ROV Kaiko对同一地点进行了重新勘测，证实该地点发生了持续的斜坡坍塌。1997年，Kaiko探险队观察到的另一个例子是石垣岛以南80公里处向南倾斜的斜坡上有一个大规模的泥块。这显然来源于石垣岛以南扇形矿床南缘陡坡的浅部，地形特征表明整个研究区为N-S或NE-西南向张应力。从这个意义上说，该地区两块板块之间的相对运动是倾斜于板块边界的。因此，板块边界的向海迁移可能是由于两种不同岩石圈结构交界处的重力不稳定所致。1998年5月3日在石垣岛东南偏南250公里的菲律宾海板块发生的最近一次地震的机制证明了这一点。
The southwestern Ryukyu area east of Taiwan Island is an arcuate boundary between Philippine Sea Plate and Eurasian Plate. The topographic features in the area are characterised by (1) a large-scale amphitheatre off Ishigaki Island, just on the estimated epicentre of the tsunamigenic earthquake in 1771, (2) lots of deep sea canyons located north of the amphitheatre, (3) 15-20 km wide fore-arc basin, (4) 15-20 km wide flat plane in the axial area of the trench, (5) E-W trending half grabens located on the fore-arc area, etc., which were revealed by several recent topographic survey expeditions.The diving survey by SHINKAI6500 in the fore-arc area on a spur located 120 km south of Ishigaki Island was carried out in 1992. The site is characterised dominantly by rough topography consisting of a series of steep slopes and escarpments. A part of the surface is eroded due to the weight of the sediment itself and consequently the basement layer is exposed. The site was covered with suspended particles during the diving, due to the present surface sliding and erosion. The same site was re-surveyed in 1997 by ROV KAIKO, which confirmed the continuous slope failure taking place in the site. Another example that was observed by KAIKO expedition in 1997 is a large-scale mud block on the southward dipping slope 80 km south of Ishigaki Island. This is apparently derived from the shallower part of the steep slope on the southern edge of the fan deposit south of Ishigaki Island.The topographic features suggest N-S or NE-SW tensional stress over the whole study area. In this sense, the relative motion between the two plates in this area is oblique to the plate boundary. So, the seaward migration of the plate boundary may occur due to the gravitational instability at the boundary of the two different lithospheric structures. This is evidenced by a lack of accretionary sediment on the fore-arc and the mechanism of a recent earthquake which occurred on 3 May 1998 in the Philippine Sea Plate 250 km SSE of Ishigaki Island.