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,

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An increasing number of seismological studies indicate that slabs of subducted lithosphere penetrate the Earth's lower mantle below some island arcs but are deflected, or, rather, laid down, in the transition zone below others. Recent numerical simulations of mantle flow also advocate a hybrid form of mantle convection, with intermittent layering. We present a multi-disciplinary analysis of slab morphology and mantle dynamics in which we account explicitly for the history of subduction below specific island arcs in an attempt to understand what controls lateral variations in slab morphology and penetration depth. Central in our discussion are the Izu-Bonin and Mariana subduction zones. We argue that the differences in the tectonic evolution of these subduction zones--in particular the amount and rate of trench migration--can explain why the slab of subducted oceanic lithosphere seems to be (at least temporarily) stagnant in the Earth's transition zone below the Izu-Bonin arc but penetrates into the lower mantle below the Mariana arc. We briefly speculate on the applicability of our model of the temporal and spatial evolution of slab morphology to other subduction zones. Although further investigation is necessary, our tentative model shows the potential for interpreting seismic images of slab structure by accounting for the plate-tectonic history of the subduction zones involved. We therefore hope that the ideas outlined here will stimulate and direct new research initiatives.

The amount of shallow seismic activity in subduction zones varies greatly from region to region. We quantify this seismicity by calculating seismic moment release rates and seismic slip rates for 24 subduction zones. To calculate the moment release, we sum the seismic moment for all interplate thrust‐type events with surface wave magnitude Ms≥7.0, occurring from 1904 to 1980. We present a time history plot of the seismic moment release for each subduction zone; these exhibit the differences in the seismic release patterns. For subduction zones where the time window of our study is not representative, the total moment release is corrected using information on repeat times. The moment release rates are compared with various subduction parameters in order to determine which factors influence the degree of coupling. These parameters include the age of the subducting lithosphere, absolute velocities of the upper and subducting plates, convergence velocity, and length, maximum depth, and dip of the Wadati‐Benioff zone. The moment release rate decreases as the age of the subducting lithosphere increases, when the zones belonging to a single subducting plate are considered. This age versus moment release relation is consistent for the zones in which the Pacific, Cocos, Philippine Sea, and Indian plates are subducting. The moment release rates for the subduction zones in which the Pacific plate is subducting are much higher than for zones of other plates with similar age. The age versus moment release relation holds among the zones which belong to one subducting plate; however, zones with similar ages but belonging to different plates do not have the same moment release rates. This suggests that within a single plate the age is the dominating factor affecting the strength of seismic coupling but that each plate as a whole has a characteristic moment release budget. Zones with retreating upper plates tend to have lower moment release rates. The moment release rate does not increase with convergence velocity; no simple relationship was found between these two parameters. The moment release rate depends most clearly on the age of the subducting lithosphere and the absolute velocity of the upper plate. These are the two independent subduction zone parameters among the variables considered. The other variables depend on these two parameters.

Abstract. The 1896 Sanriku earthquake was one of the most devastating tsunami earthquakes, which generated an anomalously larger tsunami than expected from its seismic waves. Previous studies indicate that the earthquake occurred beneath the accretionary wedge near the trench axis. It was pointed out recently that sediments near a toe of an inner trench slope with a large horizontal movement due to the earthquake might have caused an additional uplift. In this paper, the effect of the additional uplift to tsunami generation of the 1896 Sanfiku tsunami earthquake is quantified. We estimate the slip of the earthquake by numerically computing tsunamis and comparing their waveforms with those recorded at three fide gauges. The estimated slip for the model without the additional uplift is 10.4 m, and those with the additional uplit• are 5.9-6.7 m. This indicates that the additional uplift of the sediments near the trench has a large effect on the tsunami generation.