Southern Kuril Research Articles

Stagnant slabs in the upper and lower mantle transition region

We made a region‐by‐region examination of subducted slab images along the circum‐Pacific for some of the recent global mantle tomographic models, specifically for two high‐resolution P velocity models and two long‐wavelength S velocity models. We extracted the slab images that are most consistent among different models. We found that subducted slabs tend to be subhorizontally deflected or flattened in the upper and lower mantle transition region, the depth range of which corresponds roughly to the Bullen transition region (400–1000 km). The deflected or flattened slabs reside at different depths, either above or across the 660‐km discontinuity as in Chile Andes, Aleutian, Southern Kurile, Japan, and Izu‐Bonin; slightly below the discontinuity as in Northern Kurile, Mariana, and Philippine; or well below it as in Peru Andes, Java, and Tonga‐Kermadec. There is little indication for most of these slabs to continue “directly” to greater depths well beyond the transition region. Mantle downflow associated with present slab subduction appears to be blocked strongly to turn into predominantly horizontal flow in the transition region. Recent global tomographic models show also a group of lithospheric slabs deeply sinking through the lower mantle, typically the presumed Farallon slab beneath North and Central America and the presumed Indian (Tethys) slab beneath Himalaya and the Bay of Bengal. These remnant slabs are not connected to the surface plates or to the presently subducting slabs and appear to sink independently from the latter. The presence of these deeply sinking slabs implies that the pre‐Eocene subduction occurred in much the same way as in the present day to accumulate slab bodies in the transition region and that the consequent unstable downflow occurred extensively through the transition region in the Eocene epoch to detach many of the surface plates from the subducted slabs at depths and hence to cause the reorganization of global plate motion.

Ancient DNA Analysis of Brown Bear (Ursus arctos) Remains from the Archeological Site of Rebun Island, Hokkaido, Japan

Ancient DNA was analyzed from skull remains of 12 brown bears (Ursus arctos) excavated from the archeological site of the Okhotsk Culture on Rebun Island of Hokkaido, where no natural populations of brown bears currently occur, in order to trace their original habitats. The Okhotsk Culture developed around southern coastal regions of the Okhotsk Sea (southern Sakhalin, Rebun and Rishiri Islands, northern and eastern Hokkaido, and southern Kuril Islands) during 6–11th centuries, A.D. The ancient people of those days are considered to have involved brown bears for traditional ceremonies and rituals. From the skull remains, partial fragments (approximately 250–360 base pairs) of the mitochondrial DNA (mtDNA) control region were successfully sequenced. Compared with sequence data of modern brown bears of the Hokkaido main land, ancient mtDNAs of Rebun Island were phylogenetically classified into either of two lineages of modern mtDNA: the north-central Hokkaido lineage and southern Hokkaido lineage. The southern Hokkaido lineage was identified from three juvenile (less than one year old) ancient bears, while the north-central Hokkaido lineage was mainly from adults (more than three years old). Our findings demonstrated that juvenile ancient bears of Rebun Island were originated from southern Hokkaido, which was an outside area of the Okhotsk Culture and belonged to the Epi-Jomon Culture with a close relation to a northern part of the Tohoku district. The molecular phylogeographic study on ancient and modern brown bears provides an insight to further understanding zooarcheology and ancient people's cultures around Hokkaido.

Icelinus pietschi sp. nov. and a rare species, Sigmistes smithi , from the southern Kuril Archipelago (Scorpaeniformes: Cottidae)

A new cottid, Icelinus pietschi sp. nov., and a rare species, Sigmistes smithi Schultz, were collected from intertidal zones in the southern Kuril Archipelago. Icelinus pietschi, represented by five specimens (34.8–42.2 mm SL) from Iturup Island, is distinguished from other species of Icelinus by having the dorsal scale rows terminating before the posteriormost base of the second dorsal fin, spines absent on the dorsal surface of the head, a single common opening of the terminal pores of the mandibular canal, a cirrus on the base of the nasal spine, and a slightly bifid uppermost preopercular spine without denticles dorsally. Six examples (36.8–42.9 mm SL) from Urup Island of Sigmistes smithi, previously known only rarely from the middle of the Aleutian Archipelago, constitute the first record of the species from the western North Pacific.

Radiocarbon Reservoir Correction Ages in the Peter the Great Gulf, Sea of Japan, and Eastern Coast of the Kunashir, Southern Kuriles (Northwestern Pacific)

The radiocarbon reservoir age correction values (R) for the Russian Far East are estimated as 370 ± 26 yr for the northwestern Sea of Japan, and 711 ± 46 yr for the southern Kurile Islands.

Three Years on a Path to Nowhere: The Hashimoto Initiative in Russian-Japanese Relations

FT/ he end of the cold war produced strong expectations, both in Russia and Japan, for final normalization of their relations and the promotion of extensive co-operation; nevertheless, the abrogation of Yeltsin's visit to Tokyo in August 1992 has basically undermined that possibility. Although he went toJapan in 1993, Yeltsin's visit did not produce much, leaving the relationship at a stalemate. Since then, both nations have looked for a new paradigm without much success. What looked like a move to break the ice was the intensification ofJapanese activities under PrimeMinister Ryutaro Hashimoto (1996-1998), his new Eurasian doctrine in particular. Proclaimed in July 1997, it was largely evaluated as a radical breakthrough in the Russian-Japanese dialogue.' Hashimoto called for a revision ofJapan's Russian policy and pledged to base future relations on three principles: mutual trust, mutual benefit and a long-term to create a solid foundation for the twenty-first century. Two major points encouraged a positive Russian response: a statement that the territorial dispute over the Northern Territories (Southern Kurils) should be discussed calmly, based on a long-term perspective and a call for expanded co-operation in the development of energy resources in Siberia and the Russian Far East.2 This statement paved the way for two no neckties Russian-Japanese summits in 1997-1998 and for the official visit to Moscow by Hashimoto's successor, Keizo Obuchi, in 1998, which was responded to by the visit of Yeltsin's successor, Vladimir Putin, to Tokyo in 2000. A formal goal to reach

How to Win Friends and Influence People: Japanese Economic Aid Linkage and the Kurile Islands

For more than fifty years the dispute over the southern Kurile Islands has soured Japanese-Russian relations. The fate of these four small islands has great emotional and legal significance for Japan, despite the Kuriles' relatively limited economic and security importance. Japan firmly believes that the islands were seized illegally by the the Soviet Union in 1945. Postwar arrangements called for Japan to give up all conquered territories. Yet, Tokyo argues, while the rest of the Kuriles had long been disputed between the two countries, the southern islands had always legally been part of Japan--and thus should not have been taken away. As part of the core Japanese territory, their loss also resonated with the Japanese people in a much deeper way than did the loss of more distant colonial territories such as Sakhalin, Korea, or Taiwan. Therefore, since 1945 regaining those islands has been a key national objective for Japan. Japan is often held up as one of the clearest examples of what Richard Rosecrance called a trading state, that is, a state whose influence in the world is based mainly on its wealth.' Hence it is not surprising that Japan has long pinned its hopes for recovering the disputed islands on its economic influence. Clearly Japan had an economic advantage over the the Soviet Union-and now has one over Russia. Yet how can Japan best use its economic strength to buy back the disputed territory? Two possible strategies can be discerned. Until the early 1990s Japan refused to grant economic favors to Moscow unless it agreed to return the islands. By the

Interannual Dynamics of the Trophic Structure of Zooplankton of the Southern Kuril Region

Based on the data on feeding ecology of common species of zooplankton, the main trophic groups of plankton of the southern Kuril region are identified. These are phytophages, zoophages, and omnivores. The interannual dynamics of trophic characteristics of the plankton community are examined. Predatory plankton significantly dominated in 1992–1994; however, in 1995–1996 its biomass decreased markedly. The biomass dynamics of predatory and nonpredatory zooplankton, as well as changes in the structure of nonpredatory zoolankton, suggest structural changes in the planktonic community.

The influence of plate motions on three‐dimensional back arc mantle flow and shear wave splitting

Both the polarization direction of the fast shear waves and the types of deformation within overriding plates vary between the back arc basins of western Pacific subduction zones. The goal of this study is to test the possibility that motions of the overriding plates may control the patterns of seismic anisotropy and therefore the observed shear wave splitting. We calculated three‐dimensional models of viscous asthenospheric flow driven by the motions of the subducting slab and overriding plates. Shear wave splitting was calculated for polymineralic anisotropy within the back arc mantle wedge assuming that the anisotropy was created by flow‐induced strain. Predicted splitting may differ substantially depending on whether anisotropy is computed directly using polycrystalline plasticity models or is based on the orientation of finite strain. A parameter study shows that both finite strain and textural anisotropy developed within three‐dimensional, plate‐coupled asthenospheric flow models are very heterogeneous when back arc shearing occurs within the overriding plate. Therefore predicted shear wave splitting varies strongly as a function of plate motion boundary conditions and with ray path through the back arc asthenosphere. Flow models incorporating plate motion boundary conditions for the Tonga, southern Kuril, and eastern Aleutian subduction zones produce splitting parameters consistent with observations from each region. Trench‐parallel flow generated by small variations in the dip of the subducting plate may be important in explaining observed fast directions of anisotropy sampled within the innermost corner of the mantle wedge.

The three-dimensional shear velocity structure of the mantle from the inversion of body, surface and higher-mode waveforms

SUMMARY We present a 3-D model of shear heterogeneity (Vsh) in the whole mantle, derived from the inversion of hand-picked body, surface and higher-mode waveforms. The forward and inverse problems are formulated using the non-linear asymptotic coupling theory, the zeroth-order asymptotic expansion of a Born seismogram computed by normal mode summation and including coupling across mode branches. We invert iteratively for mantle heterogeneity and for centroid moment tensors until convergence. Our model, SAW24B16, is parametrized laterally in spherical harmonics up to degree 24 and radially in 16 cubic b-splines with knots spaced to reflect the data sampling with depth. The power spectrum of the model is dominated by heterogeneity in the boundary layers at low degrees: degrees 5 and 6 near the surface and degree 2 near the core–mantle boundary. The rest of the lower mantle is dominated by degree 3. We find significant heterogeneity up to spherical harmonic degree 24 in the transition zone and the uppermost part of the lower mantle. Our model displays three slow domains extending continuously between uppermost and lowermost mantle, one originating under Africa and two in the Pacific. The results of our modelling also suggest an interaction near the surface between the anomalies in the Pacific and the network of mid-ocean ridges in the East Pacific, and between the African anomaly and the low-velocity zone in the Red Sea, the East African Rift, the Mid-Indian Ridge and the Mid-Atlantic Ridge. These anomalies appear to ascend into the upper mantle without altering their shape near the 670 km discontinuity. Fast anomalies accumulate near the 670 km discontinuity under the Southern Kurile, the Japan trench and the Izu arc, with little or no penetration into the lower mantle, in agreement with results from a variety of regional studies. Cross-sections through subduction zones such as the Marianas, Tonga–Kermadec, Java, the palaeosubduction zone beneath North America and the Peru–Chile trench show continuous fast structures penetrating into the lower mantle, with the depth of penetration varying from region to region. The distribution of both fast and slow anomalies near 670 km thus suggests that the discontinuity does not act as a strong barrier to upgoing and downgoing flow between the upper and the lower mantle.

Strain partitioning and interplate friction in oblique subduction zones: Constraints provided by experimental modeling

Physical modeling of oblique subduction is performed to study the mechanism of strain partitioning. The model is two‐layer and includes the elasto‐plastic lithosphere (the overriding and subducting plates) and the low‐viscosity liquid asthenosphere. The subduction is driven by a push force from a piston and a pull force when the density contrast Δρ between the subducting plate and the asthenosphere is positive. We vary both Δρ and the interplate friction (frictional stresses). Slip partitioning is obtained only in the models with high interplate friction and only when the overriding plate contains a weak zone. This zone in the models corresponds either to locally thinned lithosphere or to cut (fault). The horizontal, trench‐normal component of the interplate friction force |Fph| can be comparable with the absolute value of the horizontal component of the nonhydrostatic interplate pressure force |Fph | in the subduction zone. Ffh is always negative (compression), while Fph can be either negative (compressional subduction regime) or positive (extensional regime). High friction, which promotes partitioning, can completely cancel the extensional (suction) force Fph. Back arc tension and strike‐slip faulting appear thus as conflicting processes, although they can coexist in the same subduction zone, depending on the relative values of relevant forces. It appears that high friction can exist only in compressional subduction zones where partitioning should develop more easily. This conclusion is supported by the comparison of two oblique subduction zones, having similar geometry: the compressional southern Kurile zone (strong partitioning) and extensional southern Ryukyu zone (no lithospheric‐scale partitioning). Other factors controlling the strain partitioning are the length of the oblique subduction zone, the boundary conditions at the transverse limits of the forearc sliver, and of course, the obliquity of subduction.

The distribution of major biochemical components in the waters of the Kuril Islands

In this paper, data are presented on the distribution of dissolved and particulate carbohydrates, proteins, lipids, and nucleic acids (NAs) in the waters of the Kuril Islands on both the Sea of Okhotsk and Pacific Ocean sides. By analyzing discrepancies in the absolute and relative concentrations of these substances, it is revealed that the waters of the Sea of Okhotsk have a strong bioproductivity. Changes in the absolute and relative conventrations of the major biochemical components of dissolved organic matter (DOM) and particulate organic matter (POM) are shown to result from the biological activity of auto- and heterotrophs of shelf and deep-sea ecosystems. Carbohydrates and proteins prove to be the major biochemical components of DOM and POM, respectively. Given high primary productivity, the carbohydrate content in POM is comparable to that of protein. In the southern Kuril area, the high concentration of dissolved and particulate NAs (which exceeds that of dissolved protein) and the high content of dissolved lipids are both strongly associated with local fish and zooplankton distribution. In the shelf and pelagic waters, biological factors dominate the horizontal and vertical distribution patterns of the major biochemical components of DOM and POM. The hydrological regime only affects these patterns in the straits of the Kuril Islands. For the first time, the biochemical composition of DOM and POM in the sea surface microlayer (SML) is characterized. In this microlayer, the concentrations of major biochemical components of DOM and POM are, to a great extent, higher than in the underlying layer.

Sources of Tsunami and Tsunamigenic Earthquakes in Subduction Zones

—We classified tsunamigenic earthquakes in subduction zones into three types earth quakes at the plate interface (typical interplate events), earthquakes at the outer rise, within the subducting slab or overlying crust (intraplate events), and "tsunami earthquakes" that generate considerably larger tsunamis than expected from seismic waves. The depth range of a typical interplate earthquake source is 10–40km, controlled by temperature and other geological parameters. The slip distribution varies both with depth and along-strike. Recent examples show very different temporal change of slip distribution in the Aleutians and the Japan trench. The tsunamigenic coseismic slip of the 1957 Aleutian earthquake was concentrated on an asperity located in the western half of an aftershock zone 1200km long. This asperity ruptured again in the 1986 Andreanof Islands and 1996 Delarof Islands earthquakes. By contrast, the source of the 1994 Sanriku-oki earthquake corresponds to the low slip region of the previous interplate event, the 1968 Tokachi-oki earthquake. Tsunamis from intraplate earthquakes within the subducting slab can be at least as large as those from interplate earthquakes; tsunami hazard assessments must include such events. Similarity in macroseismic data from two southern Kuril earthquakes illustrates difficulty in distinguishing interplate and slab events on the basis of historical data such as felt reports and tsunami heights. Most moment release of tsunami earthquakes occurs in a narrow region near the trench, and the concentrated slip is responsible for the large tsunami. Numerical modeling of the 1996 Peru earthquake confirms this model, which has been proposed for other tsunami earthquakes, including 1896 Sanriku, 1946 Aleutian and 1992 Nicaragua.

Two New Species of the Genus Stavsolus Ricker from the Asian Far East (Plecoptera: Perlodidae)

Stavsolus manchuricus sp.n. from Southern Primorye and China, and Stavsolus ainu sp.n. from Sakhalin, Southern Kuril Islands and Hokkaido, both previously identified as Stavsolus japonicus (Okamoto), are described, illustrated, and compared with the closely related S. japonicus. For better distinction a supplementary description of S. japonicus is given.

Lines in the Snow: Imagining the Russo-Japanese Frontier

J N that web of intersecting tragedies which we call Second World one of the most curious conflicts was surely the three-week war between the Soviet Union andJapan, which broke out on 8 August 1945, and ended several days afterJapan's official surrender to the allied powers. Perhaps the shortest of the many wars within the War, it has created the most prolonged and intractable search for peace. Today, more that fifty years after the event, the Japanese and Russian governments are still locked in unresolved discussions about a peace treaty which they now, under terms of the 1997 'Yeltsin-Hashimoto Plan, hope to sign sometime before the end of the year 2000. Their main stumbling block has been conflict over the Russo-Japanese frontier: specifically over Japan's claims to the islands of Shikotan, Kunashir, Iturup and the Habomai group, seized by Soviet troops (along with the rest of the Kurile island chain and the southern half of Sakhalin) during those two weeks of fighting in August 1945; a few rocky dots in the ocean with a total population of about twenty thousand. As in many border disputes, even the place names are bones of contention. The Russian government calls these islands the Southern Kuriles, while the Japanese state denies that they are part of the Kurile archipelago, and refers to the region as the Northern Territories (Hopp6 Ry6do) .1 What fascinates me about this story is not so much the glacially slow progress of international diplomacy, but rather the nature of the frontier itself: the arbitrary, moveable line which separates Russia from Japan, and which now runs between Hokkaido to the south and the islands of Habomai, Kunashir and Sakhalin to the north. Crossing one part of this line in the summer of 1996, on the rusting ferry which connects the northernmost Japanese city of Wakkanai with the Russian island of Sakhalin, I sat next to a man who has lived all his life in a house overlooking Wakkanai harbour. Every clear day he looks out of his bedroom window at the dark line of the Sakhalin coast on the horizon. But for the first thirty years of his life it was as inaccessible as the moon. Now he was going to set foot on it for the first time. This is not simply a border between nation and nation. Across the water, as guidebooks inform the small trickle ofJapanese tourists who make this

Outstanding student paper awards

The Seismology Section presented the following outstanding student paper awards at the AGU 1997 Fall Meeting in San Francisco, California, on last December.Matthew J. Fouch presented a paper entitled “Shear Wave Splitting, Continental Roots, and Patterns of Asthenospheric Flow.” Matt received a B.A. in geology from Pomona College in Claremont, Calif., in 1993, and an Sc.M. in geophysics from Brown University in Providence, R.I., in 1996. He currently is completing a Ph.D. in geophysics at Brown under the supervision of Karen M. Fischer. In previous research at Brown, Matt examined seismic anisotropy and mantle flow in the southern Kuril, Japanese, Izu‐Bonin, and Mariana subduction zones through modeling of observed shear wave splitting. More recently, his research has focused on constraining the structure and deformation of the sub‐continental mantle using a combination of seismic observations and numerical methods. In this work, Matt has developed numerical models of predicted shear wave splitting from asthenospheric flow around a continental root. Using these models, he has estimated the possible lithospheric and asthenospheric contributions to seismic anisotropy for eastern North America.

Variation of transition zone high‐velocity anomalies and depression of 660 km discontinuity associated with subduction zones from the southern Kuriles to Izu‐Bonin and Ryukyu

The seismic structure of the transition zone beneath the northwestern Pacific ocean is studied by comparing regional distance seismic waveforms with synthetic seismograms. By modeling waveforms we incorporate the travel times of secondary waves that are not included in traveltime data used in most P wave tomography experiments. These secondary waves are highly sensitive to the velocity above the 660 km discontinuity and thus add information on the seismic structure of the transition zone independent of that used in many tomography experiments. We used just two one‐dimensional models in the waveform comparison. The iasp91(slightly modified in the shallower depths as aiasp) is used as a global average structure, and model M3.11 is used as representative of transition zone structure near a deep subducting slab. Model M3.11 is faster than iasp91 from 525 to 660 km depth but also has a 30 km deeper “660” km discontinuity. Our results show strong variations in the transition zone structure beneath the northwestern Pacific. Data that sample beneath the northwestern Japanese subduction zone and the southern Kuriles are well fit by model M3.11. The deep structure beneath the Bohai Sea, to the west of Korea, also appears to show slab‐related velocity anomalies in the transition zone. In contrast, the seismic data sampling beneath the Yellow Sea to the East China Sea are better modeled using the global average model iasp91. Data that sampled beneath the Philippine Sea showed evidence for anomalously high velocity in the transition zone, but there is little apparent change in the depth of the 660 km discontinuity there. The overall results are in agreement with past work indicating fairly broad regions where subducting plate is lying flat or piling up within the transition zone, although we find that the region where this occurs, and thus the total volume of slab within the upper mantle, is considerably less than that seen in past studies. The unusual structure beneath the Philippine plate may be due to the relatively recent descent of slab beneath this region. The slab itself is cold, and thus anomalously fast, but the deeper mantle may not have cooled enough for the 660 km discontinuity to have deepened.

Anisotropy and Flow in Pacific Subduction Zone Back-arcs

—We have obtained constraints on the strength and orientation of anisotropy in the mantle beneath the Tonga, southern Kuril, Japan, and Izu-Bonin subduction zones using shear-wave splitting in S phases from local earthquakes and in teleseismic core phases such as SKS. The observed splitting in all four subduction zones is consistent with a model in which the lower transition zone (520–660 km) and lower mantle are isotropic, and in which significant anisotropy occurs in the back-arc upper mantle. The upper transition zone (410–520 km) beneath the southern Kurils appears to contain weak anisotropy. The observed fast directions indicate that the geometry of back-arc strain in the upper mantle varies systematically across the western Pacific rim. Beneath Izu-Bonin and Tonga, fast directions are aligned with the azimuth of subducting Pacific plate motion and are parallel or sub-parallel to overriding plate extension. However, fast directions beneath the Japan Sea, western Honshu, and Sakhalin Island are highly oblique to subducting plate motion and parallel to present or past overriding plate shearing. Models of back-arc mantle flow that are driven by viscous coupling to local plate motions can reproduce the splitting observed in Tonga and Izu-Bonin, but further three-dimensional flow modeling is required to ascertain whether viscous plate coupling can explain the splitting observed in the southern Kurils and Japan. The fast directions in the southern Kurils and Japan may require strain in the back-arc mantle that is driven by regional or global patterns of mantle flow.

Source, composition and distribution of the fluid in the Kurile mantle wedge: Constraints from across-arc variations of B/Nb and B isotopes

B and Nb concentrations and B isotope composition were determined for lavas covering the entire length of the Kurile arc. B/Nb and δ 11B of these lavas show clear across-arc variations in which both are highest at the trench side and continuously decrease as the slab depth increases. B/Nb in the volcanic front lavas (8–25) is significantly higher than those of MORB and OIB, indicating that the mantle wedge is extensively metasomatized by B-rich fluid derived from the subducting slab. The frontal δ 11B values are also out of the range of MORB and OIB, and systematic difference exists between Northern Kurile (NK: +5.4±0.5‰) and Central and Southern Kurile (CSK: +4.3±0.3‰). However, these two parameters approach or coincide with mantle values at the most back-arc side. The δ 11B and 87Sr/ 86Sr linearly correlate with Nb/B, suggesting control by simple mixing of two isotopically homogeneous components; that is, slab-derived fluid and mantle wedge. The estimated δ 11B and 87Sr/ 86Sr of the fluid are +7.0±0.6‰ and 0.70326±0.00005 for NK, and +6.3±0.3‰ and >0.7033 for CSK, respectively. These fluid compositions are consistent with a source constituted by >95% altered oceanic crust and <5% oceanic sediment, with the slight difference between NK and CSK resulting from the different lithologies of subducting sediments. The mantle wedge beneath the Kurile arc has B, Sr and Nd isotope compositions identical with MORB but shows significantly higher B/Nb. The above trends bear a close resemblance to those observed in the Izu arc, strongly suggesting their universality in the arc system. The across-arc decline of B/Nb observed both in Kurile and Izu implies that the fluid flux from the slab continuously decreases as the slab depth increases. However, this relates only to a final stage of slab dehydration since most of the volatile components are lost at the fore-arc region. In contrast to B, K is uniformly enriched across the arc. This requires control by a K-bearing mineral phase either in the slab (phengite) or the mantle wedge (phlogopite), although such a mineral is responsible for only a small part of the whole fluid transport.

Deep azimuthal seismic anisotropy in the southern Kurile and Japan subduction zones

Shear wave splitting parameters of local and teleseismic S waves from intermediate and deep earthquakes in the southern Kurile and Japan subduction zones are combined with splitting parameters obtained from SKS and SKKS waves to determine depth variation in shear wave splitting both above and below the earthquake. Local S wave splitting results measured at station MAJO (Matsushiro, Japan) show fast directions, from NNE to NE, parallel to the extension directions measured from Quaternary fault and geodetic data. The inferred finite strain field from shear wave splitting is consistent with the closing of the Sea of Japan. At YSS (Yuzhno Sakhalinsk, Russia) the shear wave splitting parameters obtained from local S waves are approximately oriented N‐S and appear to be controlled by deformation of the upper plate rather than the subducted slab. SKS phases recorded at YSS, located on the southern tip of Sakhalin Island, show an approximately N‐S oriented fast direction and lag time of 1.3±0.3 s. Local S phases recorded at station YSS yield fast directions similar to the SKS results and the magnitude of splitting varies systematically with depth. These results combine to indicate that very little splitting occurs in the asthenosphere below the southern Kurile slab at about 450 km depth. S waves from deep earthquakes beneath MDJ (Mudanjiang, Heilongjiang Province, China) show ∼1.0 s of lag time, while SKKS results show ∼1.6 s of lag time. One SKS phase, however, shows similar lag times as those observed by the split local S waves. This difference in shear wave splitting lag times indicates significant variation in azimuthal anisotropy beneath a minimum depth of 350 km. This inference is consistent with the source‐side splitting of ∼0.8 s lag time observed from deep teleseismic S waves which traverse the lower parts of the upper mantle and the upper mantle/lower mantle boundary. A plausible explanation for the presence of deep seismic anisotropy is that shear wave splitting is occurring in the metastable olivine in the flattened and broadened southern Japan slab. Another explanation for these observations is the presence of an anisotropic layer composed primarily of highly anisotropic β‐spinel at the base of the 410‐km discontinuity.

Modification of the Oyashio Water in the Hokkaido and Tohoku areas

Changes of the Oyashio Water properties are examined along the Oyashio northeast of Honshu, Japan. CTD data from simultaneous observations made by multiple ships in September 1989 are used. The Oyashio Water is modified by layering and vertical mixing with warm, saline waters flowing out from the Okhotsk Sea and the Japan Sea at densities lower than 130 cl/ton (26.7 σ δ). At densities greater than 130 cl/ton, the properties of Oyashio Water do not change significantly from the southeast of Iturup Island to the coast of Tohoku district at 39°20′N. The large and abrupt modification of Oyashio Water occurs within 400 km of the coast of Tohoku district, i.e. at the westernmost part of the Oyashio Front at about 39°N, by mixing with Kuroshio Water. By this modification, a salinity minimum is formed at densities around 26.8 6e corresponding to North Pacific Intermediate Water, which is the salinity minimum water broadly located in the subtropical region. The modified Oyashio Water flows northeastward along the Oyashio Front and joins the offshore flow of the Oyashio, which is separated from the main flow of the Oyashio southeast of the southern Kuril Islands carrying the typical western subarctic water.