Crustal Movements in and around Shikoku, Southwest Japan Associated with Great Nankai Earthquakes
Crustal Movements in and around Shikoku, Southwest Japan Associated with Great Nankai Earthquakes
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日本西南部四国及其周边地区与南海大地震相关的地壳运动
DOI：
10.4294/zisin1948.49.3_361
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发表时间：
1996
期刊：
影响因子：
--
通讯作者：
Shozo Kimura
中科院分区：
文献类型：
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作者：
K. Okano;Shozo Kimura
我们对日本西南部四国及周边地区过去发生的南开大地震的地壳运动提出了新的看法。研究表明，这些运动不是由于菲律宾海板块的斜向俯冲，而是由于强烈的地震震动在东西向挤压应力状态下叠加到最上部的地壳。这一观点的依据如下：在1707、1854和1946年南开地震时，高知平原下沉，室户半岛和阿祖里半岛抬升，而旧文献表明，1605年南开地震时高池平原没有下沉，因此不能推断半岛抬升，因为它是基于1707年、1854年和1946年南开地震时出现的一组高知平原的下沉和木户半岛的抬升。这是由1605年(Keicho)地震由于海啸地震而没有伴随着地面强烈震动的原因来解释的。其次，由于隆升的半岛背斜轴为N-S方向，从应力方向的不一致性来看，很难将隆升归因于菲律宾海板块NW向的俯冲。另一方面，很容易解释这种抬升是由于强震增强了EW方向的应力所致。横跨室户半岛和白首里半岛的托萨湾以抑郁为特征。能够解释这种萧条的不是弹性回弹理论，而是我们的想法。由背斜(半岛)和宽阔凹陷(海湾)组成的弧前带沿日本岛弧交替排列，其起伏也不能用弹性回弹理论解释。此外，我们认为，目前在四国观测到的地表NW方向的压缩应变变化不是菲律宾海板NW方向斜俯冲的结果，而是恢复了1946年地震反向断裂导致的Muroto海角SE方向的同震运动(地表2-3m)引起的地表超位移。此外，本研究还成功地解释了1946年南开地震与目前在四国及邻近地区发生的地幔地震在P轴方向上的不一致，并考虑了沿日本岛弧方向的位移约束。
We propose a new idea for the crustal movements associated with past great Nankai earthquakes in Shikoku and the surrounding region, Southwest Japan. This study makes clear that the movements are not due to the oblique subduction of the Philippine sea plate, but due to superposition of strong seismic shaking to the uppermost crust in a compressional stress state in the E-W direction. The ground of this idea is as follows: at the time of the 1707, 1854 and 1946 Nankai earthquakes the Kochi plain subsided and the Muroto and Ashizuri peninsulas uplifted, whereas old documents show that the Kochi plain did not subside at the time of the 1605 Nankai earthquake and therefore no uplifting of the peninsulas is inferred because it is based on a set of subsidence of the Kochi plain and uplifting of the Muroto peninsula appearing at the time of the 1707, 1854 and 1946 Nankai earthquakes. This is explained by the reason why the 1605 (Keicho) earthquake was not accompanied with strong shaking of the ground owing to the tsunami earthquake. Next, because the uplifted peninsulas have anticline axes of the N-S direction, from unconsistency in stress direction it is difficult to attribute the uplifting to the subduction in the NW direction of the Philippine sea plate. On the other hand, it is easily explained that the uplifting was caused by the stress in the EW direction enhanced by strong seismic shaking. Tosa bay, which spreads between the Muroto and Ashizuri peninsulas, is characterized by depression. To be able to explain this depression is not by the elastic rebound theory, but by our idea. The undulation in the forearc zone composed of anticlines (peninsulas) and wide depressions (bays), which range alternately along the Japan island arc, can not also explained by the elastic rebound theory. In addition, we consider that the compressional strain variation in the NW direction of the ground surface observed at present in Shikoku does not result from the oblique subduction in the NW direction of the Philippine sea plate, but it is recovering the overdisplacement of the ground surface caused by the coseismic movement (2-3 m at the ground surface) in the SE direction of the Muroto promontory by reverse faulting of the 1946 earthquake. Moreover, this study shows that unconsistency in directions of P axes between the 1946 Nankai earthquake and mantle earthquakes presently occurring in Shikoku and its vicinity is succesfully explained by taking account of constraint of the displacement in the direction along the Japan island arc.