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SUMMARY We determined P‐ and S‐wave tomography and P‐wave anisotropic structure of the Alaska subduction zone using 259 283 P‐ and 73 817 S‐wave arrival times from 7268 local shallow and intermediate‐depth earthquakes recorded by more than 400 seismic stations. The results show strong velocity heterogeneities in the crust and upper mantle. Low‐velocity anomalies are revealed in the mantle wedge with significant along‐arc variations under the active volcanoes. In the mantle wedge, the low‐velocity zone extends down to 100–150 km depth under the backarc. The results indicate that H2O and fluids brought downwards by the subducting Pacific slab are released to the mantle wedge by dehydration and they are subsequently transported to the surface by the upwelling flow in the mantle wedge. Significant P‐wave anisotropic anomalies are revealed under Alaska. The predominant fast velocity direction (FVD) is trench‐parallel in the shallow part of the mantle wedge (<90 km depth) and in the subslab mantle, whereas the FVD is trench‐normal within the subducting Pacific slab. The trench‐parallel FVDs in the mantle wedge and subslab mantle may be caused by 3‐D mantle flow that is induced by the complex geometry and strong curvature of the Pacific slab under Alaska. The flat and oblique subduction of the Pacific slab may play a key role in forming the trench‐parallel FVD under the slab. The trench‐normal FVD in the subducting Pacific slab may reflect the original fossil anisotropy when the Pacific Plate was produced at the mid‐ocean ridge.

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The spatial analysis was conducted to analyze the positions of earthquakes hypocenters in the transit zone of the upper mantle and the focal mechanisms of the strongest earthquakes in the subduction slabs of the Okhotsk Sea segment of the Kuril-Kamchatka island arc and the Japan Sea segment of the Japanese island arc. It revealed a significant difference in the morphology of these slabs, as well as in the positions of the earthquake hypocenters relative to the active and stagnating parts of the slabs and the forces that caused the earthquakes. Based on the seismic data presented in the article, it is confirmed that there are two types of subduction of the oceanic lithospheric plates in the mantle. The article discusses relationships between the subduction and various geological processes at the upperlower mantle boundary. It considers possible causes (including those related to phase transitions) of deep-focus earthquakes, in case of which splitting of the oceanic lithospheric plates takes place at depths near the upper-lower mantle boundary. Subduction of the oceanic lithospheric plates and their splitting predetermine a possibility for the crustal elements to penetrate into the lower mantle and deeper into the D″ layer, wherein new plumes arise and transport the deep magma together with the recycled substance into the crust. Deep-focus earthquakes are a necessary link in the mechanism providing for the recycling of chemical elements in the crust -mantle -D″ layer system and thus leading to the formation of a wide range of mineral deposits.

Seismic anisotropy records past and present tectonic deformations and provides important constraints for understanding the structure and dynamics of the Earth's interior. In this work, we use tremendous amounts of high‐quality P wave arrival times from local and regional earthquakes to determine a high‐resolution tomographic model of 3‐D P wave azimuthal anisotropy down to 1,000‐km depth beneath East Asia. Our results show that trench‐parallel fast‐velocity directions (FVDs) are visible in the shallow portion of the subducting Pacific slab (<80 km), whereas the deeper portion of the Pacific slab mainly exhibits trench‐normal FVDs, except for the stagnant slab in the mantle transition zone (MTZ) where obvious NE‐SW FVDs are revealed. The FVDs in the subslab mantle change from a subduction‐parallel trend at depths of 80–400 km to a subduction‐normal trend in the MTZ. Large‐scale low‐velocity anomalies are revealed beneath the Philippine Sea plate where the FVD is NE‐SW. The FVDs along the Izu‐Bonin arc and in a slab gap exhibit a striking anticlockwise toroidal trend. All these features may reflect complex 3‐D flows in the mantle wedge due to tearing and dehydration processes of the subducting Pacific slab. The subducting Pacific slab is split at ~300‐km depth under the Bonin arc and then penetrates into the lower mantle, whereas under East Asia the Pacific slab becomes stagnant in the MTZ and reaches the North‐South Gravity Lineament in China. The intraplate volcanoes in East Asia are caused by hot and wet upwelling flows in the big mantle wedge above the stagnant Pacific slab.

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