Northern Honshu Research Articles

Collision structure in the upper crust beneath the southwestern foot of the Hidaka Mountains, Hokkaido, Japan as derived from explosion seismic observations

The P-wave velocity structure of the upper crust beneath a profile ranging from Niikappu to Samani in the southwestern foot of the Hidaka Mountains, Hokkaido, Japan was obtained through analysis of refraction and wide-angle reflection data. The mountains are characterized by high seismicity and a large gravity anomaly. The present profile crosses the source region of the 1982 Urakawa-oki earthquake ( M s 6.8). The length of the profile is 66 km striking northwest and southeast. Along the profile, 64 vertical geophones were set up and 5 shot points were chosen. For each shot, a 400–600 kg charge of dynamite was detonated. The studied area is composed of four major geological belts: Neogene sedimentary rocks, the Kamuikotan belt, the Yezo Group, and the Hidaka belt. The measurement line crosses these geological trend at an oblique angle. The structure obtained is characterized by remarkable velocity variations in the lateral direction and reflects the surface geological characteristics. A thin, high-velocity layer (HVL) was found between low-velocity materials in the central part of the profile, beneath the Kamuikotan Metamorphic Belt, at a depth ranging from 0.5 to 6 km, overthrusting toward the west on the low-velocity materials consisting of Neogene sedimentary rocks, and forming gentle folds. Outlines of the velocity structure of the Hidaka Mountains yielded by other studies have shown a large-scale overthrust structure associated with the collision of the Outer Kurile and the Outer Northern Honshu Arcs. The shallow velocity structure inferred by the present study showed a similar (although small scale) overthrust structure. The obtained structure shows that the composite tectonic force, comprising westward movement of the Outer Kurile Arc and northward movement of the Outer Northern Honshu Arc, plays an important role in the evolution of the tectonic features of the crust and upper mantle in a wide depth range beneath the Hidaka Mountains.

Fracture system related to geothermal reservoir based on core samples of slim holes. Example from the Uenotai geothermal field, northern Honshu, Japan

In order to clarify the role of fracture permeability in a reservoir, the properties and formation mechanism of fractures were investigated on core samples from the Uenotai geothermal field, Japan. The fractures were located and identified from the records of lost circulation while drilling. Productive fractures are classified into a dominant high-angle (average: 77°) group, and a less common low-angle (average: 28°) group. The high-angle fractures were formed as extensional hydrothermal fractures in response to fault-value behavior triggered by fluid pressure activation under lithostatic conditions. The low-angle fractures formed as extensional and/or extensional shear fractures under hydrostatic-lithostatic transitional conditions, with subhorizontal σ 1 stress. Both kinds of fractures were accompanied by hydrothermal mineral deposition.

Genetic differentiation among populations ofArabis serrata (Brassicaceae) along its geographic distribution

Levels and distribution of genetic variation were investigated in ten populations of the perennialArabis serrata distributed along a latitudinal gradient throughout Japan. Populations of this endemic species occupy predominantly three types of habitats: limestone and derived soils, volcanic and disturbed sites. Previous studies showed that plants ofA. serrata are differentiated in morphological and ecological traits under both natural conditions and common garden experiments suggesting genetic differentiation among populations. To assess the degree of genetic differentiation under different habitats ofA. serrata populations, we analyzed the isozyme genetic structure. Ten populations, located in mountains of central and northern Honshu and Hokkaido, were analyzed by starch gel enzyme electrophoresis. Fourteen loci of eight enzymes were resolved and six loci were monomorphic for all the populations. Populations sampled maintain low levels of genetic variation (P=0.16;H=0.05;A=1.16) compared to that maintained by other outcrossing seed plants. In some cases, few or no heterozygous individuals were detected, and consequently, mean observed heterozygosity was zero or near zero. Six (29%) of the fixation indices,F, estimated deviated significantly from Hardy Weinberg genotypic expectations indicating a deficiency of heterozygotes in most of the cases. The mean genetic identity (Nei'sI) between population pairs was 0.852 and indicates a moderate level of genetic differentiation among populations.Arabis serrata has most of its genetic variation distributed among rather than within populations. The among-population component of the total genetic diversity (G ST mean value) was 0.416, indicating genetic differentiation between populations. There groups of populations were recognized in an unrooted tree generated by the Neighbor-Joining method. These results suggest groups of populations differentiated regionally. Estimates of interpopulational gene flow (Nm) were very variable (range 0.049–3.718) with a meanNm=1.203 for all populations.

標識放流結果から推定した秋田沖漁場のトラフグ成魚の移動・回遊

To investigate the migration pattern of ocellate puffer Takifugu rubripes in the northern Japan Sea, seventy-two specimens of adult ocellate puffer were tagged and released on October 19, 1994 in the waters off Hachimori, Akita prefecture, Japan. Although most recoveries were reported within the Japan Sea from the east side of the Noto Peninsula to the northern tip of Honshu Island, some of them were reported from the Sanriku area that is on the Pacific side of northern Honshu Island. Major recovery was observed in spring (spawning season of the puffer) near Nanao Bay in the Noto Peninsula. There was no recapture at the area on the western side of the Noto Peninsula.These results suggest that most fishes of this stock in the northern Japan Sea migrate within the Japan Sea from the Noto Peninsula to the northern tip of Honshu Island and spawn in the Nanao Bay, while some migrate to the Pacific side of northern Honshu Island. They also indicate that the stock of the puffer in the northern Japan Sea is a different stock from that in the western part of the Noto Peninsula.

Reduced hypoosmoregulatory ability and alteration in gill chloride cell distribution in mature chum salmon ( Oncorhynchus keta ) migrating upstream for spawning

Osmoregulatory ability of mature chum salmon (Oncorhynchus keta) during spawning migration was examined by following the changes in gill Na+, K+-ATPase activity and in the distribution and morphology of chloride cells. Mature chum salmon caught in Otsuchi Bay, northern Honshu Island, Japan, died within 5 d in seawater (SW) in association with a marked increase in plasma osmolality, whereas the fish transferred to fresh water (FW) maintained plasma osmolality efficiently. Gill Na+, K+-ATPase activity decreased in both SW-maintained and FW-transferred fish. Well-developed chloride cells, identified by immunocytochemical staining specific for Na+, K+-ATPase, were present mainly in the filament epithelium of immature fish caught in the ocean. In mature fish caught in the bay, however, additional chloride cells were also found in the lamellar epithelium. The number of filament chloride cells decreased markedly in the mature fish both in SW and in FW, whereas the number of lamellar chloride cells was maintained. These results suggest that the loss of hypoosmoregulatory ability in mature chum salmon may be attributable to the decrease in filament chloride cells and associated decrease in gill Na+, K+-ATPase activity, and also that appearance of lamellar chloride cells may be preparatory to the forthcoming upstream migration.

Preferable Habitat Characteristics of Male Japanese Marsh Warblers Megalurus pryeri in Breeding Season at Hotoke-numa Reclaimed Area, Northern Honshu, Japan.

The Japanese Marsh Warbler Megalurus pryeri is an endangered grassland passerine species and their distribution is restricted to a few marsh lands in Japan. To collect information important to conservation of the species, habitat characteristics of male warblers in the breeding season was studied at Hotoke-numa reclaimed area, northern Honshu, Japan, in the summer of 1993. Vegetation structure was significantly different between areas available and unavailable for male warblers. Specifically, the density of sedges (Carex spp.) was higher, and the density of reeds (Phragmites communis) was lower, in available area. These results suggested that sedges may play an important role in breeding activities of Japanese Marsh Warblers and maintenance of such vegetation can be important for conservation of the species.

Interseismic horizontal deformation in northern Honshu and its relationship with the subduction of the Pacific Plate in the Japan Trench

We investigate the origin of the geodetically observed interseismic horizontal deformation in northern Honshu by comparing shear strain rates, principal strain rates, and velocity fields determined from geodetic data with those calculated from the elastic dislocation models involving interplate motion at the Japan trench. The agreement between the observed and predicted directions of the principal strain axes indicates that the geodetic strain field in northern Honshu is primarily elastic strain transmitted from the Japan trench. In order to match the strain rate tensors and velocity magnitudes obtained from the geodetic data, the dislocation model requires that 35% to 60% of the NUVEL1‐A Pacific‐North American plate motion is locked at the plate interface along the Japan trench. The down‐dip depth limit of the locked zone is inferred to be 55 km, which is consistent with the seismic data in the Japan trench.

Lithospheric Contributions to Arc Magmatism: Isotope Variations Along Strike in Volcanoes of Honshu, Japan

Major chemical exchange between the crust and mantle occurs in subduction zone environments, profoundly affecting the chemical evolution of Earth. The relative contributions of the subducting slab, mantle wedge, and arc lithosphere to the generation of island arc magmas, and ultimately new continental crust, are controversial. Isotopic data for lavas from a transect of volcanoes in a single arc segment of northern Honshu, Japan, have distinct variations coincident with changes in crustal lithology. These data imply that the relatively thin crustal lithosphere is an active geochemical filter for all traversing magmas and is responsible for significant modification of primary mantle melts.

Prolactin Kinetics during Freshwater Adaptation of Mature Chum Salmon, Oncorhynchus keta

Abstract When mature chum salmon (Oncorhynchus keta) caught in Otsuchi Bay (salinity 34 ppt), northern Honshu Island, Japan, were transferred directly to fresh water, they attained normal plasma ion levels within 24 hr. Plasma prolactin remained low in the fish kept in seawater. On transfer to fresh water, a significant increase in plasma prolactin concentration was seen only in females, but not in males. Metabolic clearance rate (MCR) and secretion rate of prolactin were calculated from its plasma levels after intra-arterial injection of chum salmon prolactin into chronically-cannulated fish. In both males and females, a significant increase in MCR was seen after transfer to fresh water, indicating that prolactin is involved in freshwater adaptation in both sexes. In females, the secretion rate increased significantly after 6 days in fresh water. No such change in the secretion rate was seen in males. It seems likely that prolactin secretion is affected by reproduction-related changes occurring in mature...

Integrated radiolarian and diatom biostratigraphy of the Neogene strata in the Tsugaru Peninsula, Aomori Prefecture, northern Honshu, Japan

Integrated radiolarian and diatom biostratigraphy of the Neogene strata in the Tsugaru Peninsula, Aomori Prefecture, northern Honshu, Japan

Intraplate deformation in the Japanese Islands: A kinematic study of intraplate deformation at a convergent plate margin

Relative motions within the deforming Japanese Islands with respect to the Sea of Japan are determined using earthquake records over the last 414 years, slip rates on Quaternary faults, and angular change rates obtained from triangulation in the last century. We use a least squares inversion method to match the strain rates on the surface of the Earth with continuous spline functions to recover the velocity gradient tensors. The directions of the principal strain axes obtained from seismic, geological, and geodetic data are in general agreement with each other, with the maximum shortening axis oriented in a WNW direction. Principal strain rate magnitudes obtained from the geodetic data are about 3–5 times larger than the principal strain rates obtained from the seismic and geological data. Intraplate deformation in southwestern Japan determined from the seismic data accommodates a velocity of5.5±2(l σ) mm/yr in a direction parallel to the Nankai trough, which is about 25% of the plate motion velocity component parallel to the Nankai trough between the Philippine Sea and Eurasian plates. In northern Honshu, the velocity vectors from the seismic data are nearly parallel to the plate motion vector at the Japan trench, and the intraplate deformation accommodates about 6% of the total Pacific‐Eurasian or North American plate motion velocity. The velocity vector at Kashima, northern Honshu, calculated from the geodetic data is about 9–12 mm/yr in a direction of N55°W‐N76°W, which is about half of the very long baseline interferometry velocity calculated in the Eurasian reference frame, suggesting that the Sea of Japan may not be rigidly attached to the assumed rigid Eurasian plate.

Rediscovery in Japan and host association of Salmincola carpionis (Copepoda: Lernaeopodidae), a parasite of wild and reared freshwater salmonids

The parasitic copepod Salmincola carpionis (Krøyer, 1837) was rediscovered in Japan, where this parasite occurred on species of the genus Salvelinus (whitespotted char, S. leucomaenis; brook trout, S. fontinalis; their natural hybrid; Dolly Varden, S. malma; Miyabe char, S. malma miyabei) and rarely on rainbow trout (Oncorhynchus mykiss). The copepods were found on salmonids from a river in central Honshu and on those reared at an aquarium and a fisheries experimental station in northern Honshu. The preferred attachment site was the buccal cavity but at high intensities copepods were a little more frequently found on other parts. Prevalence and mean intensity of infection increased with increased size and age of the fish. At a constant water temperature, there were no seasonal changes in prevalence or mean intensity of infection, which may have resulted from continuous egg production by the copepods. The copepods that were previously described as Salmincola faleulata (Wilson, 1908) and later identified as S. californiensis (Dana, 1852) from kokanee (O. nerka) from Hokkaido, Japan, are assigned to S. carpionis.

雨と風が湖色の季節変化を異常にした可能性について

雨と風が湖色の季節変化を異常にした可能性について

Petroleum Source Rocks from the Neogene Tsugaru Basin, Northern Honshu, Japan

The purpose of this study is to evaluate possible petroleum source rocks in the Neogene Tsugaru basin. Evaluation is based on three criteria: organic richness, organic maturity, and organic type. One hundred forty-one samples of mudstones from seven transects were analyzed. Samples from the middle Miocene Kodomari Formation and younger sections generally contain more than 0.5% total organic carbon and amorphous organic matter, but are thermally immature. Samples of the lower Miocene Odose and Isomatsu formations obtain maturities equivalent to the threshold of intense oil generation, but do not contain sufficient type or quality of organic matter to be considered an oil source. Although this study failed to identify an effective source rock, it does suggest that a possible effective source may exist off the west coast of Honshu and in the eastern part of the Tsugaru Peninsula. In these areas, organic maturation of the possibly organic-rich Neogene sediments may have proceeded further than in regions investigated in this study as a consequence of a Neogene sedimentary package that is more than 2000 m thick.

Carbon isotopic difference of saturated long-chain n-fatty acids between a terrestrial and a marine sediment.

Stable carbon isotopic compositions of individual n-fatty acids ranging from C20 to C30 in a terrestrial (riverine) and a marine (pelagic) sediment were determined using gas chromatography/isotope ratio monitoring mass spectrometry (GC/IRMS). These fatty acids in a terrestrial sediment at the Ohtsuchi river in northern Honshu Island, Japan, showed δ13C values (from -35 to -32‰ relative to PDB) lighter than that of the total organic carbon (TOC; -26.4‰) by about 6 to 9‰. The δ13C values from a marine sediment in the Pacific Ocean ca. 500 km off the coast of Honshu Island varied from -31 to -26‰, which were also lighter than that of the TOC (-20.5‰) by 6 to 9‰. The δ13C values of n-fatty acids ranging from C20 to C30 in the marine sediment were heavier by about 5 to 6‰ than those at the corresponding carbon numbers in the terrestrial one. Isotopically, the marine n-fatty acids were clearly distinguished from the terrestrial ones. These results suggest that long-chain n-fatty acids in marine sediment were not derived from land higher plant but from marine organisms, although long-chain n-fatty acids (≥C20) in marine sediments have often been used to assess terrestrial material contribution to the ocean.

Relation between the Low-Level Jet Stream and Immigration of the White-Backed Planthopper, Sogatella furcifera (Horvath), into Aomori Prefecture, Northern Honshu of Japan

Relation between the Low-Level Jet Stream and Immigration of the White-Backed Planthopper, <I>Sogatella furcifera</I> (Horvath), into Aomori Prefecture, Northern Honshu of Japan

Precise P and S wave velocity structures in the Kitakami Massif, Northern Honshu, Japan, from a seismic refraction experiment

The Kitakami massif, which is located in the eastern part of Northern Honshu, Japan, is composed of two geological units. The northern Kitakami terrane is characterized as a Jurassic accretionary complex, while the southern Kitakami terrane consists of pre‐Silurian basement and Silurian‐lower Cretaceous marine sediments. The boundary region of these two units, called the Hayachine tectonic belt (HTB), is composed of mafic to ultramafic rocks. The Kitakami massif experienced intense granitic intrusions in the Cretaceous. We present a detailed crustal structure model for the eastern part of the massif derived from an extensive seismic refraction experiment conducted on a 194‐km N‐S line. The uppermost crust is covered with a very thin (0.5–1 km) surface layer with a velocity of 3.1–5.4 km/s. The velocity structure below this layer shows remarkable lateral variation. In the northern Kitakami terrane the P wave velocity and Vp/Vs at the top of the basement are 5.85–5.95 km/s and 1.68–1.70, respectively. The seismic attenuation in this region is high (Qp = 150–200 and Qs = 70–100). In contrast, the uppermost crust in the southern Kitakami terrane is characterized by a high P wave velocity (6.05–6.15 km/s) and Vp/Vs (1.74–1.77). The Qp and Qs also show high values of 300–400 and 150–200, respectively. Such a structural difference persists to 14‐to 16‐km depth, at which the P wave velocity increases to 6.45 km/s. The low velocity and high attenuation in the northern Kitakami terrane represent a highly deformed structure of the accretionary complex. The high P wave velocity and Vp/Vs in the southern Kitakami terrane indicate the relatively mafic crustal composition, which may result from the fragment of the oceanic crust incorporated by the accretion process or the uplifting in the latest Jurassic‐early Cretaceous. A midcrustal interface determined from wide‐angle reflections shows an abrupt southward depth decrease from 25 to 20 km under the HTB. The P wave velocity and Vp/Vs between 14‐ and 16‐km depth and the midcrustal interface are 6.45–6.55 km/s and 1.74–1.78, respectively. The Moho depth under the northern Kitakami terrane decreases southward from 34 to 32 km. In the southern Kitakami terrane the Moho dips slightly southward. The P wave velocity and the Vp/Vs ratio in the lower crust are 6.9–7.0 km/s and 1.75–1.76, respectively. The P wave velocity in the uppermost mantle is not well resolved but is probably less than 7.7 km/s. The S wave velocity derived from relatively clear Sn is 4.35–4.40 km/s. Our results show that the HTB is a prominent structural boundary extending to the Moho. The crust of Kitakami massif was not homogenized by the Cretaceous granitic intrusions, and the original structural difference remains in the upper crust.

Materials for the rust flora of Japan VI

Ten rust species are reported with new information on range of distribution and host relations. Among these,Puccinia fagopyricola andUromyces junci were new to Japan, andAecidium araliae was newly found in northern Honshu. Additional collections were made ofBlastospora smilacis, Puccinia malvacearum andP. orbicula in northern Honshu. New hosts were added toColeosporium solidaginis, C. tussilaginis, Phragmidium miyakeanum andPuccinia caricis, and a new Japanese host toPuccinia malvacearum.

時間指標テフラ阿蘇３によって示唆される最終間氷期直後５ｄ期の海面低下

We have discovered a distal tephra with similar petrographic and chemical properties to the Aso-3 pyroclastic flow deposits of the Aso caldera origin at several localities of Honshu, Japan, and adjacent seas. It is concluded to be a fallout ash associated with the Aso-3 pyroclastic flow, The tephra, named Aso-3, is a cry tal vitric fine-grained ash, consisting of dacitic pumiceous and bubble-walled glass shards, and orthopyroxene and crynopyroxene as mafic phertocrysts. It is identified with the aid of combined petrographic and stratigraphic parameters : relatively high index of glass (n = 1.5131.519), low index of orthopyroxene (γ =1.7021.705) and dacitic glass composition (rich in alkali, K2O> Na2O, rich in TiO2, FeO* and CaO low abundance in light rare earth elements and characteristic low degree of angle in chondritenormalized pattern).This tephra was dated by several radiometric methods : 103±4.2 (FT), 110±3 (TL) and 123±6 (K-Ar) in ka. The stratigraphic position of this tephra is above the last interglacial culmination in Kyushu and Honshu, and below such widespread time-marker tephras as Toya, Ata, On-Pml, SK, etc. In addition, the tephra occurs in the marine isotope stage (MIS) 5d in the abyssal sediment in the northwest Pacific.The occurrence of this tephra in the Quaternary sequence in northern coastal areas of Miyazaki, south Kyushu, clearly indicates that the sea level was more than c. 70 m lower than the preceding high stand of sea level of the MIS 5e. At the coastal cliff of Kawaminami, the Aso-3 tephra occurs 21 m above the present sea level in the basal part of the valley-filling deposits (Tohriyamahama formation) which followed the prominent marine terrace, Sanzaibaru of the MIS 5e. Former shoreline of this stage was uplifted to the elevation of c. 90 m. This tephra was also recognized in the brackish to fresh water layer sandwiched between marine sediments, called Shibikawa formation in Oga Peninsula, northern Honshu. It clearly suggests significant lowering of sea level at the time of Aso-3 eruption as well.

Distribution of White Shark in Japanese Waters

The white shark was thought to be rare in Japanese waters except Okinawa, but the author doubted its rarity because of its coastal and temperate habitat and eurythermal nature. Therefore, records of large sharks were collected since 1992 and closely examined. Nineteen of sharks were the white shark Carcharodon carcharias, and such a high incidence in catches or sightings in only 18 months is surprising, because previous published records indicate for fewer numbers. Since many of the larger sharks are not landed because of their low commercial value, and since smaller specimens were not included in this survey, the white shark is not a rare species in Japan. The white shark is known to exist in waters of 7 to 27°C (mainly 15 to 22°C), and its eurythermal nature suggests that the species can remain all year round in the southern and middle parts of Japan. However, the white shark seems to be a seasonal visitor to the waters of northern Honshu and Hokkaido.