| The western Pacific region is the most typical and most active subduction zone on Earth, and so it has been one of the most studied regions by many geoscientists since the advent of plate tectonics. In this study, we have investigated the morphology and seismic velocity structure of the subducting Pacific slab under Kamchatka and Japan Islands.We determined a 3-D P-wave velocity structure of the mantle down to 700 km depth under the Kamchatka peninsula by applying teleseismic tomography to 678 P-wave arrival times recorded by 16 seismic stations. The results show two significant structural features. One is the high-velocity subducting Pacific slab, which is visible in the upper mantle and extends below the 660-km discontinuity under southern Kamchatka, while it shortens toward the north and terminates near the Aleatian-Kamchatka junction. The other is a low-velocity anomaly interpreted as the asthenospheric flow, which is imaged beneath northern Kamchatka and under the junction. Two isolated high-velocity anomalies are imaged in and below the mantle transition zone, which are interpreted as the Pacific slab detached about 2 Ma ago and the Komandorsky lithosphere subducted about 10 Ma ago. Combining with many previous results, we conclude that the slab loss occurring under northern Kamchatka may be caused by slab-edge pinch-off by the asthenospheric flow. In addition, the subducted Meiji seamounts may have played an important role in the detachment of the Pacific slab, which cause the Pacific plate to subduct under Kamchatka with a lower dip angle near the junction.Although many studies have been made to image the subducting Pacific slab in and around the Japan Islands, details of the slab structure (such as the slab thickness, the relation between seismic velocity and depth variation, the subducting oceanic crust and the metastable olivine wedge) are still unclear. In this study, we have addressed these issues by adopting a forward-modeling approach with a 3-D ray-tracing technique and using arrival times from teleseismic, local and regional events recorded by the seismic network on the Japan Islands. Firstly, we use 333 teleseismic events and find that the average thickness of the Pacific slab beneath Japan is 85 km. Secondly, we use 3283 local and regional earthquakes with focal depths greater than 40 km to study the vertical distribution of velocity anomaly in the slab. Our result shows that the amplitude of velocity perturbation decreases with depth, which is related to the variation in temperature. Thirdly, we use the local and regional events with focal depths from 40 to 300 km to study the subducting oceanic crust beneath Northeast Japan and Hokkaido. Our results display that the oceanic crust extends down to 110 km depth under both regions, the average thickness of the oceanic crust is 7.5 and 5 km, and the velocity perturbation in the oceanic crust relative to the 1-D model is 1% and -3%, respectively. These results can be interpreted that the oceanic crust has dehydrated and metamorphosed gradually because of the increasing temperature and pressure with depth. After analyzing the relationship between the hypocenters and the oceanic crust, we consider that the earthquakes near the upper slab boundary are caused by the dehydration embrittlement of the oceanic crust. Finally, we use 23 deep earthquakes to investigate the metastable olivine wedge within the subducting Pacific slab beneath the Japan Sea and Izu-Bonin region. The results indicate that a low-velocity anomaly (-3%) indeed exists within the slab below 400 km depth, which is interpreted as the metastable olivine wedge. After careful earthquake relocation, we find that most deep earthquakes occurred within the wedge, suggesting that deep earthquakes may be caused by the phase transformational faulting.
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