Similar earthquake activity in the northern part of the Ryukyu subduction zone observed by onshore and offshore seismic data
Yukihiro NakataniHiroshi YakiwaraShuichiro HiranoReiji KobayashiHiroki MiyamachiShigeru NakaoYusuke YamashitaKazunari UchidaTakeshi MatsushimaHiroshi ShimizuKazuo NakahigashiTomoaki YamadaHideji AbeMasanao Shinohara
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Seismic zone
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Earthquake prediction
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Ocean bottom
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Abstract The tectonic stress field of the Western Pacific Subduction Zone (WPSZ) has been affected by the interaction among the Euro‐Asian, Philippine and Pacific Plates, and the effect is manifested by the occurrence of huge earthquakes in this region. Here, using numerical simulation of the finite element method, attempts were made to evaluate the NW‐directed subduction of the Western Pacific Plate on the three secondary subduction zones, namely the Japan Subduction Zone, the Izu‐Bonin Subduction Zone and the Mariana Subduction Zone. In these zones, the structural geometry and viscoelastic property of the model were determined, as well as two main megathrust fault planes identified, by means of a comprehensive structural analysis of tomography and Crustal 2.0 data. This study shows that the tectonic stress field and earthquakes in the three secondary subduction zones have different distribution features under continuous NW‐directed subduction of the Western Pacific Plate; i.e. (1) the strong coupling area in the Japan Subduction Zone reflected subducted seamounts, ocean ridges or other topographic highs that control the frequent occurrence of historical large earthquakes and stress concentration; (2) in Izu‐Bonin Subduction Zone, there were fewer earthquakes compared with the Japan Subduction Zone, the reason for this kind of distribution is that the contact area and properties have been changed between the subducting Pacific Oceanic Plate and the overlying Philippine Oceanic Plate; and (3) earthquakes that happened in the Mariana Subduction Zone have complicated types, including thrust‐type, normal‐type and strike‐slip‐type earthquakes, which are triggered by the subducting and retreating of the Pacific Oceanic Plate, and an angular difference between the subducting direction of the Pacific Oceanic Plate and the strike of the Mariana Trench. In summary, formation of differences among the tectonic stress field and earthquake distribution of the three subduction zones are correlated not only to the geometrical structures of the major faults but also to the subducted seamounts and the strike directions of the trenches. Copyright © 2016 John Wiley & Sons, Ltd.
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Pacific Plate
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Abstract The southernmost part of the Ryukyu subduction, where the Philippine Sea Plate is subducting under the Eurasian Plate, is known to be a very seismically active region of transition from a north-dipping subduction along the Ryukyu subduction to an ~ SE–NW collision along the Taiwanese orogenic wedge. In this paper, we will focus on the Ryukyu forearc area close to Taiwan where the deformation is paroxysmal. In order to decipher the nature of the seismic deformation in this region, a three month passive experiment, combining 22 Ocean Bottom Seismometers and 51 onland stations, has been led. Starting from an a-priori heterogeneous model, we have obtained 801 well-located earthquake hypocenters, a precise P-wave tomography model and 14 focal mechanisms. The seismicity along the Ryukyu forearc is mainly located not only in the vicinity of the Interplate Seismogenic Zone (ISZ) but also within both the subducting PSP and the overriding plate. Seismicity within the upper-plate is essentially localized east of Nanao basin where NW–SE extension occurs, and northwest of the Hoping basin where strike-slip dominates. As revealed by both the P-wave velocity structure and the newly derived seismicity, we argue that a sub-vertical step offsetting the subducting PSP around 10 km may support the presence of a trench-parallel tear. The PSP also undergoes extension in its upper part that is probably caused by buckling and slab pull. The P-wave velocity structure reveals three other major features: (1) a continuity between the Central Range and the Ryukyu Arc with a shallower Moho (~ 30 km depth) between ~ 122.3°N and ~ 122.5°N along the Ryukyu Arc, (2) high P-wave velocities along the eastern side of the Central Range and, (3) two bodies with similar high crustal velocities (6.5–7.0 km/s) at 12–18 km depths, embedded within the Ryukyu arc basement, just north of Hoping Basin and north of the Nanao Basin.
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Eurasian Plate
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Earthquakes in shallow subduction zones account for the greatest part of seismic energy release in the Earth and often cause significant damage; in some cases they are accompanied by devastating tsuna
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