Hachijo Island Research Articles

Magmatic dike resonances inferred from very-long-period seismic signals.

Very-long-period (VLP) signals showing simple decaying harmonic oscillations with periods near 10 seconds and lasting for about 300 seconds were observed in association with an earthquake swarm that occurred beneath Hachijo Island, Japan. Results from the source-mechanism analysis and waveform simulation based on a fluid-filled crack model consistently point to the resonance of a dike filled with a basaltic magma as the source of the VLP signals. Thus, VLP signals can be used to probe the state of the fluid and dynamic processes within a volcanic system.

古文書にみられる八丈島の食生活に関する一考察―動物性食品について―

It is reported that the islanders of Hachijo have had a longevity of life that dates back to ancient times. As a matter of fact, according to an ancient document from the modern ages, the people of Hachijo island tended to live on a kind of dumplings refrerred as Hengo.These dumplings were made a mixture of refined grains and Hengo. They were also known to collect and eat Ashitaba leaves which were found in the mountains and also in the fields. From the sea, they were able to catch fish and shellfish and they also collected seaweed.We have studied in great detail the different kinds of food types consumed by these people. This included extensive research into the various kinds of animal food they were known to eat.The following is a short summary of the findings of our research: It was noted that the animal foods they tended to eat were fish, shellfish, sea turtles and seals as well as various kinds of birds and so on. During times of great famine however, it was reported that they were known to also consume beef.

レザーファーンのまだら黄化症発生に及ぼす遮光率と土壌水分の影響およびその防止指針

Leaves of leather fern are used as a foliage plant and Mottled Yellowing Syndrome (MYS) has been occurring on leaves cultivated in protected greenhouses at Hachijo Island since the 1990s. Experiments with different shading levels and soil moisture levels were carried out to develop cultivation methods that will prevent the occurrence of MYS. Relationships between the greenhouse environment and the occurrence of MYS were examined and the following results were obtained.The temperature of air, the canopy surface, and soil in the greenhouses were affected not only by shading levels but also by the color of the shading net. Air and canopy surface temperature decreased with an increase of shading rate with blue nets, however, temperatures under black nets were quite high due to the high surface temperature of the nets, which caused a high occurrence of MYS.Numbers of leaves decreased with an increase of shading level, but the leaf size increased. Chlorophyll content of the leaves increased with shading levels too.An optimum greenhouse environment to increase leaf size, leaf number, and chlorophyll content was obtained by a using a blue net with a 55% shading rate and the soil water potential controlled to less than 0.016MPa.MYS occurrence is increased by high temperature stress of the plant rather than high solar radiation in greenhouses. Also, MYS occurrence was decreased by frequent irrigation.MYS occurrence (%) can be estimated by degree days over 34°C of maximum air temperature for well-irrigated greenhouses, and by 1.5 times the degree days over 32°C for less-irrigated greenhouses. Farmers can manage the greenhouse environment to reduce MYS damage by applying these results.Greenhouses devoted to leather fern cultivation should be shaded (around half the amount of sunlight compared to outside) and should be controlled to keep good ventilation and soil moisture by frequent irrigation, which provides a desirable canopy surface-, air- and soil temperature, resulting in reduction of MYS damage and improvement in productivity and quality.

A Research on the Dietary Habits in Hachijo island: Daily Meals in Hachijo-Hikki

A Research on the Dietary Habits in Hachijo island: Daily Meals in Hachijo-Hikki

レザーファーンまだら黄化症発生ハウスと非発生ハウスにおける微気象環境の違い

Leather fern is a major horticultural plant in Hachijo Island, Tokyo and has been grown in protected cultivation. The visual damage of leather fern leaves called “Mottled Yellowing Syndrome (MYS)” has occurred on the leaves cultivated in greenhouses since 1990's. Appearance of MYS on the leaves reduces their market value.In order to elucidate the cause of the occurrence of MYS, we investigated the environments of leather fern greenhouse by field survey and interviews with greenhouse owners. Furthermore, we measured the micrometeorology of both a greenhouse with MYS (MYS house) and a greenhouse without MYS (non-MYS house) in August 1997.From field investigation and measurements, the following results were revealed.(1) MYS occurred in greenhouses with low shading material and low ventilation area, which were enclosed by thick windbreaks or surrounding hill slopes.(2) MYS occurred in poorly irrigated greenhouses with inadequate water supply and longer drought period.(3) There were obvious micrometeorological differences between the MYS house and the non-MYS house. Light transmittances of MYS and non-MYS houses were 44% and 17%, and the ventilation-coefficient of the MYS house was 1.9 times higher than that of the non-MYS house. Air temperature and relative humidity inside the MYS house were 3.3°C and 9% higher than outside, while those in the non-MYS house were 1.2°C higher and 3% lower than outside. Soil water potentials of MYS and non-MYS houses were 0.003 and 0.023MPa, respectively.(4) MYS of leather fern occurred in greenhouses under conditions of high air- soil- temperature, high evaporation, and low soil water contents.(5) The low humidity around the plants could bring about water stress on the plant; thus not only the high temperature stress but also water stress or both could cause MYS.

Stratigraphy and Eruption Ages of Deposits at the Southeast Side of Nishiyama Volcano, Hachijo Island during the Last 2, 500 Years

Hachijo Island, located on the volcanic front of the Izu-Ogasawara (Bonin) arc, consists of two stratovolcanoes : Nishiyama and Higashiyama. Nishiyama (Alt. 854.3 m) is a scarcely dissected cone called “Hachijo-fuji”. Nishiyama began its volcanic activities about 10, 000 years ago. Many lateral volcanoes exist around Nishiyama, and one of them, Kandoyama (Alt. 194.9 m) is a tuff cone which was formed by a phreatomagmatic eruption. This study discusses the stratigraphy of deposits, and the eruptive types and the eruption ages of Nishiyama after the formation of Kandoyama. The results are summarized as follows.1) This study names four air-fall pyroclastic materials “Ny1-4”, which erupted from Nishiyama after the pyroclastic surge eruption of Kandoyama (ca. 2, 500 y.BP). All of them are scoriaceous.2) Ny2 erupted from a lateral volcano on the west of Kandoyama. This scoria cone is composed of agglutinate.3) Ny3 erupted from the summit crater of Nishiyama ca. 1, 100 y.BP. Ny3 was widely distributed on the southeast flank of Nishiyama, but it did not reach the foot of Higashiyama. Prior to the eruption of Ny3, a large amount of lava flowed down the southeast flank of Nishiyama.4) Ny4 erupted from a crater row along the southeast side of Nishiyama. From the description found in archives, it is clear that the eruption of Ny4 corresponds to A.D. 1605 (Keicho 10th) eruption. Ny4 deposited around the vents as agglutinate, and air-fall materials were widely distributed on the southeast side of Nishiyama. However, the total volume of Ny4 was the same or less than that of the Miyakejima 1983 eruption and the Izu-Oshima 1986 eruption.5) There are descriptions in the archives of at least four eruptions and about ten occurrences of earthquakes and tsunami in and around Hachijo Island. The fissures and tsunami deposits which followed historic earthquakes were found at the Yaene site located on the south side of Nishiyama. The pyroclastic materials from Nishiyama date these past earthquakes.

Pictorial 2: Stratigraphy and Eruption Ages of Deposits at the Southeast Side of Nishiyama Volcano, Hachijo Island during the Last 2, 500 Years

Pictorial 2: Stratigraphy and Eruption Ages of Deposits at the Southeast Side of Nishiyama Volcano, Hachijo Island during the Last 2, 500 Years

Gyalidea pacifica (Lichenized Ascomycotina, Solorinellaceae), a New Gyalectoid Lichen from Japan

Gyalidea pacifica Harada & Vezda is described as new from Hachijo Island of the Izu Islands, Japan. It was found growing on non-calcareous stones. The lichen genus Gyalidea in the family Solorinellaceae is characterized by its crustose thallus; coccoid green phycobiont; brownish apothecia; non-amyloid hymenium; almost simple and septate paraphyses; and hyaline, muriform (or transversely septate) spores. Gyalidea is represented worldwide by 26 previously described species, occurring on foliicolous, saxicolous, terricolous, and corticolous substrates (Etayo & V~zda 1994; Kalb & Hafellner 1992; VWzda & Poelt 1991). Hitherto, only G. japonica Harada & V~zda (1991), which was found on non-calcareous pebbles in Shizuoka-ken, has been recorded from Japan. Recently, however, the senior author discovered a second species, occurring on non-calcareous stones, among specimens collected on Hachijo Island of the Izu Islands, about 300 km south of Tokyo. It is newly described here as Gyalidea pacifica Harada & Vrzda sp. nov.

B31 14C 年代測定値からみた過去 10,000 年間の八丈島, 西山火山の噴火史

B31 14C 年代測定値からみた過去 10,000 年間の八丈島, 西山火山の噴火史

黒潮と日本人の遭遇史 （第三部） 追補論考

Since ancient times the South China Sea used to be the international access sea to China for envoy and trade ships from southern and central Asia and Europe. Searching ancient drifting records about wrecked foreigners who reached Japan, the author has found several illustrations that show transferred driftings from northeastward or eastward winds and currents in the South China Sea to the Kuroshio. They are examined in the light of modern oceanography and meteorology knowledge. These transferred driftings must have contributed to direct introduction of various foreign cultures to local places in Japan. Next, three typical routes of Kentoshi, the Japanese envoy to China in the Tang Dynasty, are illustrated together with circles of the visible horizon from the top of main islands in the East China Sea, and driftings of wrecked ships of the Kentoshi in 753-754 are examined by considering winds and the Kuroshio stream. In November, 748 Ganjin, Chinese Buddhist naturalized as a Japanese archbishop later, failed to reach Japan and drifted to Hainan Island by a violent wind. The 14 days recorded as the drifting period is too short to carry the boat to the Island even if the wind was very strong. The earliest mentions of the Kuroshio stream by the Japanese in provincial maps of Ryukyu (Bakufu, 1646-47, 1702) and of the term Kurose Gawa in Mori's map (1752) are examined. Regarding the problem how people in the Edo period (1603-1867) understood the Kuroshio, three topics are discussed. (1) Toward the end of the 18th century, a strange tale of red tides was believed in Hachijo Island, to carry wrecked boats to the boundless ocean. It is similar to contemporary legends of the Kuroshio, regarded as a terminal current of no return. (2) The view of fragmental currents might interrupt the recognition of the Kuroshio as a long continuous current in the Edo period. The view seems to be latent in current charts by Fuglister (1955) and in Sailing Directory for the Southeast Coast of Honshu (Japan Hydrographic Department, 1949). (3) Toward the end of the Edo period a ship's journal frankly recognized difficulties in perceiving of the Kuroshio stream. This demonstrative attitude shows the Japanese at that time slowly reaching a modernized understanding of the Kuroshio. A chronological table for the history of encounters with the Kuroshio is compiled.

G29 14C 年代測定値からみた八丈島、東山・西山火山の活動史

G29 14C 年代測定値からみた八丈島、東山・西山火山の活動史

Seismic radiation by magma injection: An anomalous seismic event near Tori Shima, Japan

The earthquake with a bodywave magnitude mb=5.5, which occurred near Tori Shima, Japan, on June 13, 1984 (origin time: 0229:25.3 UT, 31.448°N, 140.036°E, depth of 10 km, mb =5.5, MS=5.5) is anomalous because it generated tsunamis which are disproportionately large for the magnitude of the earthquake. At Hachijo Island, 150 km from the epicenter, tsunamis were visually observed with peak‐to‐peak amplitude of 130 to 150 cm. Long‐period seismic radiation is also anomalous. Love waves are almost absent, and Rayleigh waves are radiated with equal amplitude and phase in all directions. A simple double‐couple model cannot explain these observations. With the assumption of no net volume change at the source, these data can be best explained with a compensated linear vector dipole (CLVD) with the principal tensional dipole in the vertical direction. The scalar moment of this dipole is 4×1024 dyn cm. Moment tensor inversions of long‐period body waves and surface waves yield an almost identical solution. This CLVD source can be interpreted as horizontal fluid injection. The location of the event is in the Smith depression which is one of the nascent back arc basins just behind the Bonin arc. These basins are filled with thick sediments, and numerous young volcanoes are found near this site. Magmatic injection is most likely to occur in this tectonic environment. However, the time scale of the seismic event seems too short for magma injection to occur. A more likely mechanism involves water‐magma interaction. The injection may be viewed as hydrofracturing driven by supercritical water heated by injected magma. The estimated volume of injected water is about 0.018 km3 and that of basaltic magma is about 10% of this. This type of deformation is more efficient for tsunami generation than faulting with the same scalar moment.

14C-ages of Fossil Wood in the Mudflow Deposit at Higashiyama Volcano, Hachijo Island

¹⁴C-ages of Fossil Wood in the Mudflow Deposit at Higashiyama Volcano, Hachijo Island

GPS Measurements in the Tokai Region and Izu Hachijo Island (March 1989-June 1992)

GPS Measurements in the Tokai Region and Izu Hachijo Island (March 1989-June 1992)

Eight new cytotoxic metabolites closely related to onnamide A from two marine sponges of the genus Theonella

New cytotoxic compounds 1– 7 were isolated along with onnamide A ( 9) from a marine sponge Theonella sp. collected at Hachijo Island, whereas another Theonella sponge yielded 8. The structures of these compounds were determined by interpretation of the NMR spectral data as well as by comparison of spectral data with those of 9. Six compounds ( 1– 6) were highly cytotoxic against the P388 cell line.

Phytoplankton pigment distributions in regional upwelling around the Izu Peninsula detected by Coastal Zone Color Scanner on May 1982

Phytoplankton pigment (chlorophyll a + pheopigments) distributions in a regional upwelling around the Izu Peninsula obtained by the Coastal Zone Color Scanner (CZCS) on May 23, 1982, were compared with ship-observed pigment and satellite sea-surface-temperature distributions. Pigment concentrations detected by the CZCS were positively correlated with the ship-observed pigment concentrations. However, they were about factor of 5 smaller when atmospheric correction parameters known for typical oceanic and land aerosol were used and when the parameters were estimated with the “clear water algorithm”. When the atmospheric correction parameters were adjusted so that a pigment concentration derived by CZCS was equivalent to a concentration obtained by the ship at a coincide location, the pattern and magnitudes of the CZCS-derived pigment distributions showed remarkable agreement with ship-observed pigment distributions. Thus, the normal atmospheric correction algorithm may not be suitable for waters around Japan, and the development of better atmospheric correction methods combined with more verification programs is required. The pigment distributions showed patterns that were similar to those observed in sea-surface-temperature distributions. Cold water showed higher pigment concentrations, and warm water showed lower pigment concentrations. The Kuroshio, which can be identified by generally warm, low pigment water, showed a large meander which flowed offshore at Shiono-misaki, looped back onshore from Hachijo Island to Omaezaki and then flowed northeast along the Izu and Boso Peninsulas. Locally upwelled water along the Izu Peninsula was seen clearly in the sea-surface-temperature and CZCS pigment distributions as a region of cold water and high pigment concentrations. Cold upwelled waters were also found at the eastern side of the Izu Islands, but pigment concentrations in these waters was not always high. This difference in the two upwelling regions may be caused by different physical and biological interactions.

Production and turnover rates of fatty acids in marine particulate matter through phytoplankton photosynthesis

A combined 13C and gas chromatography-mass spectrometry ( 13C-GC-MS) method was applied for the determination of the fatty acid composition of photosynthetic products of phytoplankton. The accuracy of the estimation of 13C abundance by GC-MS was ascertained by analyzing 13C-enriched authentic specimens. The production rates of 17 fatty acids was determined in the regional upwelling and non-upwelling area around Hachijo Island, Japan. Although 14:0, 16:0 and 16:1 were the main components of the photosynthetically produced fatty acids at both stations, a marked difference in the proportion of 16:1 was found between the two stations, probably reflecting the difference in the dominant phytoplankton species. The ratio of unsaturated fatty acids to saturated fatty acids in the photosynthetic products was high in the upwelling area. The calculated turnover rate of each fatty acid through phytoplankton photosynthesis varied widely. Polyunsaturated fatty acids showed higher turnover rates, whereas low turnover rates were noticed in odd carbon number acids (iso 15:0, anteiso 15:0, 15:0 and 17:0) and 18:1 (n—7), which have been reported to originate from bacteria. The weighted average values of the turnover rates of all fatty acids determined in this study were 0.021 and 0.087 h −1 for the non-upwelling and the regional upwelling area, respectively.

Eruptive History of the Higashiyama Volcano, Hachijo Island during the Last 22,000 Years

Eruptive History of the Higashiyama Volcano, Hachijo Island during the Last 22,000 Years

八丈島におけるオオクロバエとケブカクロバエの標識再捕獲による季節的移動の確認

Mark and recapture experiments of blow flies, Calliphora nigribarbis Vollenhoven and Aldrichina grahami (Aldrich), were attempted on Hachijo Island, Tokyo, in the autumn of 1983 and the spring of 1984 to investigate their seasonal migration. One female C. nigribarbis released at Station (St.) M (altitude 700m) in the moss forest of Mt. Hachijo-Fuji (854m) at the end of October 1983 was recaptured at St. 1 (50m) in the town at the end of November. At the end of March 1984,a total of 841 flies of both sexes of C. nigribarbis and A. grahami were marked and released at St. T (50m). One male and one female C. nigribarbis and one male A. grahami were recaptured at St. G (600m), St. 3 (400m) and St. 2 (200m), respectively, at the end of April. One female C. nigribarbis released on 24-25 May 1984 at St. T was trapped at St. 4 (600m) on 29 May 1984. The result of mark and recapture experiments confirmed the possibility that adults emerged in the town migrate from the lowland to the moss forest of Mt. Hachijo-Fuji before summer. Some of them most probably return to the lowland in autumn.

伊豆諸島・利島 (としま) におけるつつが虫病の初報告例および第2例

It has been known that tsutsugamushi disease, so-called "Shichito-fever", is widely spread among the Izu Islands, Tokyo Metropolis. The cases were reported in Oshima Island, Niijima Island, Shikine Island, Kozu Island, Miyake Island, Mikura Island, and Hachijo Island previously, although no case has been reported in Toshima Island. In this paper, we report the first two cases of tsutsugamushi disease found in Toshima Island in December 1988 and December 1989. The first case was a 73 year-old male and the second case was a 83 year-old female, respectively. Fever, erythema and eschar were observed in both cases, while lymphadenopathy and hepatosplenomegaly were not detected. After tetracycline was administered, the fever immediately went down and erythema gradually disappeared in both cases. Specific immunofluorescence tests demonstrated that IgG antibody titers rose against Karp, Gilliam and Kato strains, and that IgM antibody titers rose only against Gilliam strain in both cases. Therefore, Gilliam-like strain of Rickettsia tsutsugamushi may play an important role in Toshima Island.