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Southeast Asia is surrounded by subduction zones resulting from the interactions of several lithospheric plates. Its evolution has been also influenced by active tectonics due to the Indo‐Asian collision in the Cenozoic. In this study, we use a large number of arrival‐time data of local and regional earthquakes to determine 3‐D P wave tomography and azimuthal anisotropy in the mantle beneath SE Asia. High‐velocity (high‐V) anomalies representing the subducting slabs are clearly visible in the upper mantle and the mantle transition zone (MTZ). Low‐velocity (low‐V) zones with trench‐normal anisotropy are revealed in the uppermost mantle, which indicate back‐arc spreading or secondary mantle‐wedge flow induced by the slab subduction. In contrast, trench‐parallel anisotropy dominates in the deep upper mantle and reflects structures either in the subducting slab or in the upper mantle surrounding the slab. The trench‐parallel anisotropy is also significant in the lower MTZ, which may contribute to shear wave splitting observations. A low‐V body extending down to the lower mantle is visible under the Hainan volcano far away from the plate boundaries, suggesting that Hainan is a hot spot fed by a lower‐mantle plume. The low‐V body under Hainan is connected with low‐V zones in the upper mantle under SE Tibet and Vietnam. Our P wave anisotropy results reflect significant mantle flow existing in the asthenosphere from SE Tibet to Hainan and further southwestward to Vietnam. The present study, especially the 3‐D P wave anisotropy results, provides important new insight into mantle dynamics in SE Asia.

S U M M A R YWe determined P-and S-wave tomography and P-wave anisotropic structure under the Honshu arc from the Japan Trench to the backarc area under the Japan Sea using 310 749 P-and 150 563 S-wave arrivals from 4655 local earthquakes recorded by 982 seismograph stations. Arrival times from 1451 suboceanic earthquakes relocated with sP depth phases enable us to determine the structures under the Pacific Ocean and Japan Sea, which expand the study region from the land area to the whole arc from the Japan Trench to the Japan Sea with a width of more than 500 km. The results show strong heterogeneities above the subducting Pacific slab under the Pacific Ocean and most large thrust-type earthquakes occurred in the high-velocity areas where the Pacific slab and the overriding continental plate may be strongly coupled. Low-velocity (low-V ) zones are imaged in the mantle wedge with significant along-arc variations under the volcanic front. The mantle-wedge low-V zone extends westwards under the Japan Sea and it is connected with the subducting Pacific slab at depths of 150-200 km under the backarc.The results indicate that the H 2 O and fluids brought downwards by the subducting Pacific slab are released into the mantle wedge by dehydration and are subsequently transported to the surface by the upwelling flow in the mantle wedge. Significant P-wave anisotropic anomalies are revealed under the Honshu arc. The predominant fast velocity direction (FVD) is E-W in the mantle wedge while it is N-S in the subducting Pacific slab. The anisotropy in the mantle wedge is the result of deformation caused by the subduction of the Pacific plate and the induced mantle-wedge convection, while the FVD pattern in the middle of the mantle wedge argues for the 3-D mantle flow or the specific alignment of the olivine in the partial-melting mantle. The N-S (trench-parallel) FVD in the subducting Pacific slab represents either the original fossil anisotropy when the Pacific plate formed or the trench-parallel crystallographic and shaped preferred orientation in the subducting slab due to the slab bending. The present results shed new light on the structural heterogeneities and seismic anisotropy under the Honshu arc, which may improve our understanding of the dynamic processes of subduction zones.

We present high‐resolution 3‐D images of P wave velocity (Vp), azimuthal anisotropy (AAN), and radial anisotropy (RAN) down to 900‐km depth beneath Alaska obtained by inverting a large number of high‐quality arrival time data from local earthquakes and teleseismic events simultaneously. Our results show that the high‐Vp Pacific slab has subducted down to 450‐ to 500‐km depths. A prominent slab gap is revealed at depths of 65–120 km near the Wrangell volcanic field, which is likely a slab tear acting as a channel that provides ascending mantle materials to generate magmas feeding the surface volcanoes. In the back‐arc mantle wedge near the eastern slab edge, the AAN exhibits trench‐parallel fast‐velocity directions (FVDs), which may reflect along‐strike mantle flow. The FVDs in the subducting Pacific slab are nearly east‐west, which may indicate fossil anisotropy formed at the mid‐ocean ridge. A negative RAN is revealed within the subducting slab, which may be caused by the fast plate subduction with a steep dip angle. Trench‐normal FVDs of the AAN are revealed in the mantle below the Pacific slab, which may reflect mantle flow entrained by the subducting slab. A positive RAN is revealed in the mantle beneath the Yakutat slab, indicating that its shallow subduction flattens the mantle flow below the slab to be subhorizontal. Along‐strike FVDs of the AAN around the eastern slab edge may indicate the edge‐induced toroidal mantle flow.

[1] To study the anisotropic structure beneath northeast (NE) Japan, we made 4366 shear wave splitting measurements using high-quality seismograms of many earthquakes occurring in the crust and the subducting Pacific slab. Our results provide important new information on the S wave anisotropy in the upper crust, lower crust, mantle wedge, and subducting Pacific slab. In the upper crust, the anisotropy is mainly caused by the stress-aligned fluid-saturated microcracks. The measured delay times (DTs) increase to 0.10 s at 10-11 km depth; the fast velocity directions (FVDs) are parallel to either the tectonic stress or the strike of active faults. The maximum DTs for the low-frequency earthquakes near the Moho are 0.15-0.17 s, suggesting strong anisotropy at the base of the crust or in the uppermost mantle. The measurements for the intermediate-depth earthquakes in the Pacific slab show dominant E-W (trench-normal) FVDs in the back-arc area and N-S (trench-parallel) FVDs in the fore-arc area. The trench-normal FVDs in the back-arc area are caused by the corner flow in the mantle wedge as a result of the subduction of the Pacific plate. The maximum DTs for the slab earthquakes reach 0.30-0.32 s at 100 km depth, but only half of the total DTs are produced in the mantle wedge. The small DTs in the mantle wedge may result from an isotropic or weak anisotropic zone in the middle of the mantle wedge. In the fore arc, the dominant trench-parallel FVDs for the slab earthquakes are consistent with those for the upper crust earthquakes, and ∼80% of the total DTs can be accounted for by the anisotropy in the crust. In the subducting Pacific slab, the trench-parallel FVDs may reflect either the original fossil anisotropy in the Pacific plate when the plate was produced in the mid-ocean ridge or the preferred orientations of the crystals and cracks in the upper part of the subducting slab.Components: 8500 words, 15 figures.

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