关键词: 琉球海沟, 马尼拉海沟, 构造分段, 最大潜在地震

Abstract:

After the Fukushima nuclear accident caused by the “3·11” earthquake tsunami in Japan, whether the coastal nuclear power stations in China are liable to similar earthquake tsunami impact has been widely concerned by the whole society. According to the previous results of earthquake tsunami impact assessment conducted by professional departments on coastal nuclear power plants, China's coastal areas do not have the conditions for the occurrence of large-scale earthquake tsunami, but in order to fully learn from the experience and lessons of the Fukushima nuclear accident caused by Japan's “3·11” earthquake tsunami, definite conclusions have been drawn on the offshore tsunami and its impact on nuclear power plants in the early assessment work of potential tsunami impact of coastal nuclear power stations in China, combined with the structural background, historical seismic data and tsunami impact analysis. However, whether the earthquakes in the Ryukyu trench, Manila trench and other areas can generate tsunami has not been systematically considered. Therefore, in this paper, the seismogenic capacity of the Ryukyu trench and Manila trench is evaluated based on the seismotectonic background and relevant seismic source parameters.
Both Ryukyu and Manila trench belong to the west Pacific plate subduction zone, while the Japan's “3·11”earthquake is also located in the west Pacific plate subduction zone. Therefore, whether the former has the same tectonic background and conditions as the “3·11” earthquake does is the key factor to assess whether the Ryukyu trench and Manila trench have the same potential for M9 earthquake. Based on the analysis of a large number of data, this paper evaluates the tectonic background, segmentation characteristics and maximum potential earthquake generating capacity of the Ryukyu trench and the Manila trench. The Ryukyu trench and Manila trench are located in the west of the Philippine Sea plate. There are also subduction zones distributing in the east of the Philippine Sea plate from Izu-Ogasawara trench, Mariana trench to Yap Palau-Ayu trench. Since the Ryukyu trench-Manila trench subduction zones are not in the direct contact zone between the Pacific plate and the Eurasian plate, the plate tectonic setting is obviously different from the low-angle subduction zone where the Japan's March 11 earthquake locates. From the perspective of tectonic system, the Ryukyu trench belongs to the subduction tectonic system of trench-island arc-back arc basin. The island arc and trench are retreating eastward, showing the characteristics of weak coupling. The overall scale of the Manila trench is small, and affected by the “slab window” in the subduction slab formed by the ancient spreading ridge, the length of these two trench zones is much smaller than that of the subduction zones where M_W≥9 earthquakes have occurred.
Based on the comprehensive analysis of the differences in trench structure, earthquake data and etc., the Ryukyu trench can be divided into 6 rupture segments, and the section of the Manila trench concerned in this study can also be divided into 6 rupture segments. At the same time, the possibility of combined rupture of the rupture segment is considered from a conservative standpoint. The rupture segments RL5 and RL6 of the Ryukyu trench, RM2 and RM3 of the Manila trench all have the possibility of combined rupture, and rupture segments RM4, RM5 and RM6 also have the possibility of combined rupture. To sum up, the comprehensive estimation result of the maximum potential earthquake in the subduction zone is magnitude 8.5 in the Ryukyu trench and magnitude 8.8 in the Manila trench.

Key words: Ryukyu Trench, Manila Trench, structural segmentation, maximum potential earthquake