Tetsuzo Seno

A Model for the Motion of the Philippine Sea Plate Consistent With NUVEL-1 and Geological Data

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.

THE INSTANTANEOUS ROTATION VECTOR OF THE PHILIPPINE SEA PLATE RELATIVE TO THE EURASIAN PLATE

Abstract Using all the available fault-plane solutions of the inter-plate earthquakes along the northwestern and eastern boundaries of the Philippine Sea plate, the instantaneous rotation vector of the Philippine Sea plate relative to the Eurasian plate is determined (pole: 45.5°N, 150.2°E, angular velocity: 1.20 deg/m.y.). The solutions along the Taiwan—Philippine region are not used because of the tectonic complexity in this region. To overcome the difficulty in determining the rotation vector only from the northwestern boundary of the Philippine Sea plate, the general property that the three instantaneous rotation vectors, Pacific to Eurasia, Philippine Sea to Eurasia, and Pacific to Philippine Sea, are contained in the same plane, is used. The rotation vector of Pacific to Eurasia is assumed as in the model RM1 of Minster et al. (1974). The rotation vector of Pacific to Philippine Sea is determined at the same time. Using these results, historical earthquakes in the Sagami and Nankai trough region are studied on the basis of plate motion and fault models deduced from the geodetic data. In this region, the co-seismic slip is comparable to the rate of plate motion. The recent start of plate convergence (several million years ago) may be one reason for this strong coupling of the oceanic lithosphere with the continental one. The slip vectors at the triple junction which connects the Japan trench, Sagami trough, and Izu-Bonin trench, show that this junction is not stable and migrates to the northwest along the Sagami trough at the rate of 3 cm/yr. However, if the crustal extension exists behind the Izu-Bonin trench, it may stay stable. The slip vectors of the shallow earthquakes along the eastern coast of Luzon and the Philippine trench deviate from those computed from the rotation vector of the Philippine Sea plate relative to the Eurasian plate. To explain this, a minor plate which includes the entire Philippines is introduced on the evidence of seismicity and topography. The tectonic complexity in the Taiwan-Luzon region may be reduced to the collision of this block with Taiwan.

Can the Okhotsk Plate be discriminated from the North American plate

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 plates 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.

Effects of relative plate motion on the deep structure and penetration depth of slabs below the Izu-Bonin and Mariana island arcs

An increasing number of seismological studies indicate that slabs of subducted lithosphere penetrate the Earths 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 Earths 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.

Dehydration of serpentinized slab mantle: Seismic evidence from southwest Japan

The seismicity in the subducting Philippine Sea slab (PHS) beneath southwest Japan shows a variety of modes of occurrence. We try to explain this variety on the basis of dehydration embrittlement in the subducting oceanic crust and/or mantle. The PHS subducting along the Nankai Trough shows commonly a single narrow seismic zone shallower than 60 km, which may reflect dehydration embrittlement in the hydrated subducting oceanic crust only, implying the lack of hydrated slab mantle. The PHS beneath Kanto, however, shows a double seismic zone (Hori, 1997) in the mantle part. Here the serpentinized mantle wedge of the Izu-Bonin fore-arc is subducting, and the double zone can be explained by its dehydration. Beneath Kii Peninsula and Kyushu, seismic events within the slab mantle have also been detected. This indicates that the PHS mantle beneath these areas is also hydrated, which may have resulted from subduction of the serpentine stable in the Izu-Bonin back-arc area. Aqueous fluids released from the serpentinized mantle beneath Kii Peninsula may have initiated partial melting in the mantle wedge, as indicated by the presence of high 3He/4He ratios in the natural gasses and the shallow seismic swarms in this region (Wakita et al., 1987).

Sediment effect on tsunami generation of the 1896 Sanriku Tsunami Earthquake

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 Sanriku tsunami earthquake is quantified. We estimate the slip of the earthquake by numerically computing tsunamis and comparing their waveforms with those recorded at three tide gauges. The estimated slip for the model without the additional uplift is 10.4 m, and those with the additional uplift 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.

Rupture process of the Miyagi-Oki, Japan, earthquake of June 12, 1978

Abstract The faulting mechanism and multiple rupture process of the M = 7.4 Miyagi-Oki earthquake are studied using surface and body wave data from local and worldwide stations. The main results are as follows. (1) P-wave first motion data and radiation patterns of long-period surface waves indicate a predominantly thrust mechanism with strike N10° E, dip 20°W, and slip angle 76°. The seismic moment is 3.1 × 1027 dyne-cm. (2) Farfield SH waveforms and local seismograms suggest that the rupture occurred in two stages, being concordant with the two zones of aftershock activity revealed by the microearthquake network of Tohoku University. The upper and lower zones, located along the westward-dipping plate interface, are separated by a gap at a depth of 35 km and have dimensions of 37 × 34 and 24 × 34 km2, respectively. Rupture initiated at the southern end of the upper aftershock zone and propagated at N20°W subparallel to the trench axis. About 11 s later, the second shock, which was located 30 km landward (westward) of the first, initiated at the upper corner of the lower aftershock zone and propagated down-dip N80°W. Using Haskell modelling for this rupture process, synthetic seismograms were computed for teleseismic SH waves and nearfield body waves. Other parameters determined are: seismic moment M 0 = 1.7 × 10 27 dyne-cm, slip dislocation u = 1.9 m , Δσ = 95 bar, rupture velocity ν = 3.2 km s −1 , rise time τ = 2 s , for the first event; M 0 = 1.4 × 10 27 dyne-cm , u = 2.4 m , Δσ = 145 bar , for the second event; and time separation between the two shocks ΔT = 11 s. The above two-segment model does not explain well the sharp onsets of the body waves at near-source stations. An initial break of a small subsegment on the upper zone, which propagated down-dip, was hypothesized to explain the observed near-source seismograms. (3) The multiple rupture of the event and the absence of aftershocks between the two fault zones suggests that the frictional and/or sliding characteristics along the plate interface are not uniform. The rupture of the first event was arrested, presumably by a region of high fracture strength between the two zones. The fracture energy of the barrier was estimated to be 1010 erg cm−2. (4) The possible occurrence of a large earthquake has been noted for the region adjacent to and seaward of the area that ruptured during the 1978 event. The 1978 event does not appear to reduce the likelihood of occurrence of this expected earthquake.

Tectonic stress controls on ascent and emplacement of magmas

Abstract The tectonic stresses can significantly affect the propagation of a magma-filled crack. It has been pointed out that the rheological boundaries control the emplacement of magmas through the effect of stress. However, it has not been clarified how the role of rheological boundaries depends on the regional tectonic and thermal states. We have evaluated the role of rheological boundaries under various tectonic and thermal conditions and found that the level of magma emplacement may jump according to the changes in the tectonic force or the surface heat flow. The stress profiles were estimated by a simple model of lithospheric deformation. We employed a three-layer model of the lithosphere; the upper crust, the lower crust and the upper mantle have different rheological properties. A constant horizontal force is applied to the lithosphere, and the horizontal strain is assumed to be independent of depth. When realistic tectonic forces (>10 11 N/m) are applied, the rheological boundaries mainly control the emplacement of magma. The emplacement is expected at the MOHO, the upper–lower crust boundary, and the brittle–ductile boundary. For lower tectonic forces ( 11 N/m), the tectonic stress no longer plays an important role in the emplacement of magmas. When the tectonic stress controls the emplacement, the roles of rheological boundaries strongly depend on the surface heat flow. When the surface heat flow is relatively high (>80 mW/m 2 ), the stress in the mantle is quite low and the MOHO cannot trap ascending magmas. For relatively low heat flow ( 2 ), on the other hand, the MOHO acts as a magma trap, and the upper–lower crust boundary acts as a magma trap only when the magma supply rate is sufficiently high. Our results suggest that the emplacement depth can change responding to the change in the tectonic force and/or that in the surface heat flow. This may provide us a key to understand the relation between the evolution of a volcanic region and its tectonic and/or thermal history.

Velocity field of around the Sea of Okhotsk and Sea of Japan regions determined from a new continuous GPS network data

To investigate the current crustal movements in and around the Sea of Okhotsk and Sea of Japan regions, we have established a continuous GPS network. By the end of 1997, the network had been expanded to include 12 new stations. Data for the period from July 1995 to November 1997 were analyzed together with data from International GPS Service for Geodynamics (IGS) global stations. To fix the estimated coordinates to the terrestrial reference frame, the Tsukuba IGS station was assumed to be moving westward relative to the stable Eurasian continent at ∼2cm/yr according to Hekis[l996] estimate. We find that: (1) stations in the western margin of the Sea of Japan have eastward velocity vectors, (2) the pole position of the Okhotsk plate is located near Okha, which reconfirms the Okhotsk micro plate, (3) a plate boundary of the Okhotsk and Amurian plates between southen Sakhalin and Hokkaido is suggested.

Fractal asperities, invasion of barriers, and interplate earthquakes

I present a model to explain seismicity variations along consuming and transform fault plate boundaries. The basic assumptions of the model are: (1) plate boundary fault zones consist of asperities and barriers, which are defined as having negative and positive a-b values, respectively, of rate and state dependent friction laws, (2) circular-shaped asperities are distributed in a fractal manner, such that an asperity contains smaller asperities inside, (3) pore fluid pressure can be elevated almost to the lithostatic only in barriers (called invasion of barriers), and (4) a region whose barriers are invaded can rupture as an earthquake. Based on these assumptions, I re-estimate fault areas of interplate earthquakes along the San Andreas and near Japan. The derived relation between fault area and seismic moment for these earthquakes determines the fractal dimension of asperities to be 1.4, and nine smaller asperities are contained in a larger one of which the radius is 4.8 times those of the smaller ones. Various modes of invasion of barriers with a fractal distribution of asperities can explain the seismological phenomena such as variations of seismic coupling along plate boundaries, two types of earthquake families, and co-existence of the Gutenberg-Richter’s law and characteristic repeating earthquakes.