We investigate angular velocity vectors of the Philippine Sea (PH) plate relative to the adjacent major plates, Eurasia (EU) and Pacific (PA), and the smaller Caroline (CR) plate. Earthquake slip vector data along the Philippine Sea plate boundary are inverted, subject to the constraint that EU‐PA motion equals that predicted by the global relative plate model NUVEL‐1. The resulting solution fails to satisfy geological constraints along the Caroline‐Pacific boundary: convergence along the Mussau Trench and divergence along the Sorol Trough. We then seek solutions satisfying both the CR‐PA boundary conditions and the Philippine Sea slip vector data, by adjusting the PA‐PH and EU‐PH best fitting poles within their error ellipses. We also consider northern Honshu to be part of the North American plate and impose the constraint that the Philippine Sea plate subducts beneath northern Honshu along the Sagami Trough in a NNW‐NW direction. Of the solutions satisfying these conditions, we select the best EU‐PH as 48.2°N, 157.0°E, 1.09°/m.y., corresponding to a pole far from Japan and south of Kamchatka, and PA‐PH, 1.2°N, 134.2°E, 1.00°/m.y. Predicted NA‐PH and EU‐PH convergence rates in central Honshu are consistent with estimated seismic slip rates. Previous estimates of the EU‐PH pole close to central Honshu are inconsistent with extension within the Bonin backarc implied by earthquake slip vectors and NNW‐NW convergence of the Bonin forearc at the Sagami Trough.

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The plate geometry in northeast Asia has been a long‐standing question, with a major issue being whether the Sea of Okhotsk and northern Japanese islands are better regarded as part of the North American plate or as a separate Okhotsk plate. This question has been difficult to resolve, because earthquake slip vectors along the Kuril and Japan trenches are consistent with either Pacific‐North America or Pacific‐Okhotsk plate motion. To circumvent this difficulty, we also use slip vectors of earthquakes along Sakhalin Island and the eastern margin of the Japan Sea and compare them to the predicted Eurasia‐Okhotsk and Eurasia‐North America motions. For a model with a separate Okhotsk plate, we invert 10 Eurasia‐Okhotsk and 255 Pacific‐Okhotsk slip vectors with Pacific‐North America and Eurasia‐North America NUVEL‐1 data. Alternatively, for a model without an Okhotsk plate, those Eurasia‐Okhotsk and Pacific‐Okhotsk data are regarded as Eurasia‐North America and Pacific‐North America data, respectively. The model with an Okhotsk plate fits the data better than one in which this region is treated as part of the North American plate. Because the improved fit exceeds that expected purely from the additional plate, the data indicate that the Okhotsk plate can be resolved from the North American plate. The motions on the Okhotsk plate's boundaries predicted by the best fitting Euler vectors are generally consistent with the recent tectonics. The Eurasia‐Okhotsk pole is located at northernmost Sakhalin Island and predicts right‐lateral strike slip motion on the NNE striking fault plane of the May 27, 1995, Neftegorsk earthquake, consistent with the centroid moment tensor focal mechanism and the surface faulting. Along the northern boundary of the Okhotsk plate, the North America‐Okhotsk Euler vector predicts left‐lateral strike slip, consistent with the observed focal mechanisms. On the NW boundary of the Okhotsk plate, the Eurasia‐Okhotsk Euler vector predicts E‐W extension, discordant with the limited focal mechanisms and geological data. This misfit may imply that another plate is necessary west of the Magadan region in southeast Siberia, but this possibility is hard to confirm without further data, such as might be obtained from space‐based geodesy.

[1] Dehydration embrittlement of metamorphosed oceanic crust and mantle in the subducting slab may be responsible for the occurrence of intermediate-depth earthquakes. We explore the possibility that this hypothesis can explain the morphology of the double seismic zones observed in northeast Japan, southwest Japan, northeast Taiwan, northern Chile, Cape Mendocino, and eastern Aleutians. We calculate transient temperature structures of slabs based on geologically estimated subduction histories of these regions. We then determine dehydration loci of metamorphosed oceanic crust and serpentinized mantle using experimentally derived phase diagrams. The depth range of the dehydration loci of metamorphosed oceanic crust and serpentine is dependent on slab age. The dehydration loci of serpentine produce a double-layered structure. Because the upper dehydration loci of serpentine are mostly located in the wedge mantle above the slab, we regard the upper plane seismicity representing dehydration embrittlement in the oceanic crust, and we fix the slab geometry so that the upper plane seismicity is just below the upper surface of the slab. We find that the lower plane seismicity is located at the lower dehydration loci of serpentine, which indicates that the morphology of the double seismic zones is consistent with the dehydration embrittlement.INDEX TERMS: 7218 Seismology: Lithosphere and upper mantle; 7209 Seismology: Earthquake dynamics and mechanics; 7220 Seismology: Oceanic crust; KEYWORDS: double seismic zone, dehydration embrittlement, serpentine, oceanic crust, subducting plate Citation: Yamasaki, T., and T. Seno, Double seismic zone and dehydration embrittlement of the subducting slab,