| Subduction zones play an important role in the plate tectonics. Detailed studiesof the structure and dynamics of a subduction zone can help us better understand thefeatures of the subducting slab, arc magmatism, back-arc spreading andseismotectonics. In this work we study the North-Ryukyu subduction zone, theSouth-Kuril subduction zone and the Japan subduction zone, which have typicaltrench-arc-backarc systems and are formed by the strong interactions among thePacific plate, the Eurasian plate, the Phillipine Sea plate and the Okhotsk plate. Manylarge earthquakes, such as the2011Tohoku-oki earthquake (Mw9.0), and active arcvolcanoes exist in these Northwest-Pacific subduction zones. In the past two decades,many researchers have used seismic tomography methods to study the structure anddynamics of these subduction zones. The results obtained by these studies sheldimportant light on the three-dimensional (3-D) structure, magmatism andseismotectonics of these regions.However, the detailed3-D structure and dynamic processes of theNorthwest-Pacific subduction zones are still not very clear, especially in the forearcareas under the Pacific Ocean and back-arc areas beneath the marginal seas, becausefew ocean-bottom-seismometers (OBSs) are deployed in the forearc and backarc areas.Therefore, the suboceanic earthquakes cannot be located precisely with the routineprocedure of the land-based seismic network.To improve our understanding of the interplate megathrust earthquakes andbackarc magmatism in the Northwest-Pacific subduction zones, in this study we havedetermined, for the first time, high-resolution3-D structures of these subductionzones especially for the forearc and back-arc areas under the oceanic regions using alarge number of high-quality waveform and arrival-time data recorded by the dense Japanese seismic networks. We used the methods of Zhao et al.(1992,2007) for thetomographic imaging outside the seismic network.We obtained the following results,(1) high-resolution3-D seismic velocitystructure of the crust and upper mantle of the entire North-Ryukyu subduction zone;(2) high-resolution3-D seismic velocity structure and P-wave anisotropy of the crustand upper mantle of the entire South-Kuril subduction zone;(3) high-resolution3-Dseismic attenuation structure of the crust and upper mantle of the Japan subductionzone; and (4) source parameters including corner frequency, seismic moment, sourceradius and static stress drop of679earthquakes (Mw2.3-5.9) in the Japan subductionzone.Our results show that strong lateral heterogeneities and P-wave anisotropy existin the interplate megathrust zone under the forearc regions of the Northwest-Pacificsubduction zones. We find that large interplate earthquakes (M≥6.0) generallyoccurred in or around high-velocity (high-V) and high-Q patches in the megathrustzone. These high-V and high-Q patches are generally surrounded by significantlow-velocity (low-V) and low-Q anomalies. We think that the high-V and high-Qpatches in the megathrust zone probably represent strongly coupled areas, while thelow-V and low-Q anomalies may reflect the weakly coupled portions. In addition, wefind that the coseismic slip distributions of some great interplate earthquakes (M≥7.5)are not always limited in the high-V patches where the ruptures initiated. We thinkthat the rupture of an interplate earthquake can unimpededly pass through the low-Vand low-Q anomalies because of the weak interplate coupling there, and so result in agreat megathrust earthquake.We suggest that the high-V and high-Q patches in the megathrust zone mayrepresent asperities formed by subducted oceanic ridges, seamounts or othertopographic highs on the upper boundary of the subducting slab. In contrast, thelow-V and low-Q anomalies may result from subducted sediments and fluidsassociated with slab dehydration. The fluids in the megathrust zone may play an important role in the nucleation of megathrust earthquakes at the asperities, inaddition to the stress accumulation caused by the plate subduction. |
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