South Of Tokyo Research Articles

Endangered wild populations of endemic Calanthe orchids on an isolated Japanese island tested for viruses

Leaf samples of endangered endemic orchid Calanthe izu-insularis Ohwi & Satomi collected at the time of full-bloom from two natural habitats on Mikurajima Island on (33°50′N, 139°37′E), ~200 km south of Tokyo, Japan, were examined for nine species of virus. Cucumber mosaic virus (CMV) was detected by reverse transcriptase–polymerase chain reaction (RT–PCR) and hybridisation in 4 of the 104 leaf samples from different plants of C. izu-insularis. Five plants were removed from their natural habitat and cultivated in a greenhouse. Concentrations of detectable CMV in these plants increased and CMV was detected in all five plants after 2 weeks of cultivation. However, after an additional 10 weeks of cultivation, CMV was not detected in any of the plants. Thus, it seems possible that almost all of the plants of this species in their natural habitat might harbour CMV at concentrations that are lower than the limit of detection by RT–PCR and hybridisation. The importance of these results for the conservation in situ is discussed.

An 18-Year Exposure Test of Thermal-Sprayed Zn, Al, and Zn-Al Coatings in Marine Environment

Abstract The thermal spray committee of the Japan Association of Corrosion Control (JACC) has been conducting a marine corrosion test of thermal-sprayed Zn, Al, and Zn-Al coatings since 1986. The coating was applied to twelve steel pipes by arc- and flame-spraying to varied thicknesses and subjected to various post-spray treatments. The samples were set vertically into seawater at a port located approximately 80 km south of Tokyo. The corrosion performance of these coatings has been inspected annually by recording their appearance and thickness at the sea-air, splash, and tidal zones. No significant changes were observed for the first several years of exposure. After 7 years, however, Zn coatings with and without sealing started to suffer degradation in the immersed portion. Contrary to this, most of the Al and Zn-Al coatings still exhibit superb corrosion performance. In order to place the test in a proper perspective, a special team of about 20 members was formed to conduct a survey on corrosion prevent...

Distribution of Frankia, ectomycorrhizal fungi, and bacteria in soil after the eruption of Miyake-Jima (Izu Islands, Japan) in 2000

Miyake-Jima, a round island of about 8km in diameter, is located about 200km south of Tokyo, Japan (34°08′ N, 139°53′ E). In July 2000, the central cone of this island collapsed and a new eruption started with the falling of volcanic ash and SO2 emission. In October 2001, we measured the distribution of bacteria, and Frankia, a symbiotic nitrogen-fixing actinomycete that forms root nodules, and ectomycorrhizal fungi associated with alder in newly deposited ash and its underlying soil by cultivation of Alnus sieboldiana seedlings in these soils. There were fewer bacteria growing on the nutrient broth agar and fewer bacteria growing on 100-fold diluted nutrient broth agar in newly deposited volcanic ash than there were growing in the buried old soil. In four out of five sites, little or no nodulation was observed in newly deposited ash; abundant root nodules were formed in the underlying soil. Ectomycorrhizae formed in the seedlings cultivated in the underlying soil. In May 2003, the distribution of Frankia and ectomycorrhizal fungi in soil at different depths (up to 160cm) was also investigated. Frankia and ectomycorrhizal fungi were relatively abundant in surface soil layers.

Crystallization of microlites and degassing during magma ascent: Constraints on the fluid mechanical behavior of magma during the Tenjo Eruption on Kozu Island, Japan

Bubble and crystal textures provide information with regard to the kinetics of the vesiculation and crystallization processes. They also provide insights into the fluid mechanical behavior of magma in a conduit. We performed textural (bubble and crystal) and compositional analyses of pyroclasts that were obtained from the Tenjo pyroclastic flow, which resulted on account of the eruption in 838 A.D. on Kozu Island, about 200 km south of Tokyo, Japan. Pyroclasts in one flow unit (300∼2,060 kg/m3; average density 1330 kg/m3) can be classified into three types on the basis of vesicle textures. Type I pyroclasts have small isolated spherical bubbles with higher vesicularities (67–77 vol.%) and number density (10.8–11.7 log m−3). Type II pyroclasts have vesicularities similar to type I (61–69 vol.%), but most bubbles exhibit evidences of bubble coalescence, and lower number densities than type I (8.9–9.5 log m−3). Type III pyroclasts contain highly deformed bubbles with lower vesicularities (16–34 vol.%) and number densities (8.2–9.0 log m−3). The microlite volume fraction (DRE converted) also changes consistently across type I, type II, and type III as 0.06, 0.08, and 0.10–0.15, respectively. However, the number density of the microlites remains nearly invariant in all the pyroclast types. These facts indicate that the variation in the microlite volume fraction is controlled not by the number density (i.e., nucleation process), but by the size (i.e., growth process); the growth history of each type of microlite was different. Water content determinations show that the three types of pumices have similar H2O contents (2.6±0.2 wt%). This fact implies that all three types were quenched at nearly the same depth (35±5 MPa, assuming that the magma was water-saturated) in the conduit. If the crystal sizes are limited only by growth time, a variation in this parameter can be related to the residence time, which is attributed to the flow heterogeneity in the conduit. By assuming a laminar Poiseuille-type flow, these textural observations can be explained by the difference in ascent velocity and shearing motion across the conduit, which in turn results in the differences in growth times of crystals, degrees of deformation, and bubble coalescence. Consequently, for crystals in the inner part of the conduit, the crystal growth time from nucleation to quenching is shorter than that near the conduit wall. The vesicle texture variation of bubbles in types I, II, and III results from the difference in the deformation history, implying that the effect of degassing occurred primarily towards the conduit wall.

Increase in nitrate and chloride deposition in east Asia due to increased sulfate associated with the eruption of Miyakejima Volcano

The eruption of Miyakejima Volcano, 180 km south of Tokyo, Japan, since July 2000 has resulted in the emission of large amounts of sulfur dioxide. The volumes of sulfur dioxide emitted were vast, equivalent to half the anthropogenic emission from China on annual average. Short‐time aerosol sampling, conducted at the Happo Ridge observatory in the central mountainous region of Japan, has revealed that the abundant volcanic sulfate resulted in the expulsion of nonvolcanic nitrate and chloride in aerosols into the gas phase. As the deposition velocities of gaseous nitrate and chloride are much higher than those in aerosols, their deposition should be accelerated. This additional deposition is referred to as “indirect acidification” in this study, corresponding to “direct acidification” by the deposition of volcanic sulfate. This indirect acidification is estimated quantitatively using a regional‐scale Eulerian aerosol transport model and is found to be generally less pronounced than direct acidification, although the indirect effect was 2.1 greater than the direct effect over the North Pacific Ocean in winter. The minimum contribution of indirect acidification is found to be 7%, which is still not negligible. Indirect acidification may also occur in the case of general air pollution, accelerating the acid deposition of nitrate and chloride in sulfate‐rich contaminated air masses. It therefore appears to be important in general to consider indirect acidification in the discussion of environmental acidification by sulfur oxides.

Long-term geodetic measurements of large scale deformation at Iwo-jima caldera, Japan

Iwo-jima is a volcanic caldera located 1250 km south of Tokyo, and is part of the Izu–Ogasawara arc. An extremely high uplift rate (averaging 0.25 m/a over several hundred years) characterizes the volcanic activity of Iwo-jima. This deformation is believed to be due to post caldera uplift. To investigate the mechanisms contributing to the high uplift rate, we have repeatedly conducted leveling and trilateration surveys from 1976 to 1995, and GPS surveys from 1996 to 2002. These observations have detected two patterns of continuous crustal deformation: concentric subsidence around Motoyama at the center of the Iwo-jima caldera and uplift surrounding the subsiding area. During the period from 1977 to 1995, subsidence at Motoyama reached 0.54 m and uplift in the southwestern part of the island exceeded 3 m. In addition, the island has experienced broad, episodic uplift, which amounted to more than 1 m during volcanic unrest both in 1982 and 2001. Horizontal displacements around Motoyama exhibit contraction due to the concentric subsidence, indicating the presence of a deflating source at depth. Vertical deformation dominates the displacement field during periods of broad episodic uplift, suggesting a deeper and/or more extensive source. Focusing the Motoyama area, we modeled the subsidence source simultaneously with the broad episodic uplift, which is approximated by deformation with a planar gradient, for 14 times between 1976 and 2002, using a grid search combined with a least-squares method. By assuming the subsidence source could be located at any grid point, we applied the least-squares method to calculate the intensity of subsidence source and the parameters for the uplift with a planar gradient and then selected the best-fitting source location. The best-fitting subsidence source geometries for 1998–2000 and 2000–2002 (the two time periods with GPS results) are horizontal, squared-shaped sills with side lengths of 4 and 5 km, respectively. Adopting the horizontal sill with a side length of 4 km as the deflating source, we applied this model to the 12 additional measurement epochs. Stable solutions for the source were obtained for 8 of the 12 periods. The results indicate that the deflating source is located beneath the Motoyama area with a depth ranging from 0.1 to 2.4 km over time. The average rate of sill closing from 1976 to 2002 was 0.46 ± 0.32 m/a. Our analysis also quantified the episodic broad uplift, though the available data do not permit modeling of the source of the continuous uplift surrounding the subsidence area. Considering the density structure in the caldera, the continuous subsidence and uplift may be explained by movement of volcanic fluids (probably geothermal) to balance the vertical load of the volcano, while the episodic uplift appears to be related to magmatic unrest.

A possible case of SES selectivity at Kozu-shima Island, Japan

Geoelectric potential difference monitoring has been conducted in Kozu-shima Island about 170 km south of Tokyo. During the monitoring period (14 May 1997–30 November 1999), there were 21 anomalous geoelectric changes that could have been VAN-type seismic electric signals (SES) and 26 M>3 earthquakes within about 20 km. Possible correspondence between the geoelectric changes and earthquakes has been examined. There seems to be some systematic tendency regarding the dependence of appearance/non-appearance of changes and their polarity on epicentral location. This observation, however, is preliminary and admittedly inconclusive because of small number of cases and non-uniqueness in signal-earthquake correlation.

Submarine silicic caldera at the front of the Izu-Bonin arc, Japan: Voluminous seafloor eruptions of rhyolite pumice

Myojin Knoll caldera, a submarine rhyolitic center 400 km south of Tokyo, is one of nine silicic calderas along the northern 600 km of the Izu-Bonin(-Ogasawara) arc and the first anywhere to receive detailed, submersible-based study. The caldera, slightly smaller than the Crater Lake structure in Oregon, is 6 × 7 km in diameter; its inner walls are 500–900 m high, and it has a remarkably flat floor at 1400 m below sea level (mbsl). The caldera collapse volume is ∼18 km 3 , suggesting that more than 40 km 3 of pumiceous tephra may have been erupted at the time the caldera formed. Precaldera seafloor eruptions built a broad volcanic edifice consisting of overlapping composite volcanoes made of rhyolitic lavas, shallow intrusions, and a variety of volcaniclastic deposits—including thick accumulations of rhyolitic pumice erupted at 900– 500 mbsl. The caldera-forming eruption produced a 150–200 m deposit of nonwelded, fines-depleted pumice that resembles a colossal layer of popcorn at the top of the caldera wall. Freshly erupted pumice behaved as “sinkers” or “floaters,” depending on the environment in which it cooled. The pumice clasts deposited proximally and exposed in the caldera wall were likely quenched in eruption columns that remained below sea level. This pumice ingested seawater and sank as gases filling its vesicles cooled, particularly as steam in its vesicles condensed to liquid water. Some eruption columns may have broken through the sea surface and entered the air, especially during vigorous phases of the caldera-forming eruption. These pumices had the opportunity to ingest air as they cooled, becoming floaters as they fell back to the sea; these could have been carried distally on the sea surface by the combined effects of ocean currents and wind. The age of the caldera is unknown, but it may be as young as several thousand years. Its magmatic system at depth retains sufficient heat to sustain an actively growing intracaldera Kuroko- type polymetallic sulfide deposit, rich in gold and silver and topped by chimneys emitting fluids as hot as 278 °C. Sufficient time has elapsed, however, for a 250-m-high postcaldera dome to grow on the caldera floor and for the caldera rim to be deeply scalloped by slumping.

三宅島火山の形成史

Miyakejima Volcano is located 200 km south of Tokyo, Japan, and is one of the active volcanoes situated on the Izu-Mariana Arc. The main cone of Miyakejima has two nested calderas : outer Kuwanokitaira Caldera, 4 km in diameter, and inner Hatchodaira Caldera, 1.8 km by 1.6 km across. The central cone, Oyama, grew in the Hatchodaira Caldera.A short recurrence time of about 22 year-period and continuous crustal inflation in the recent years suggested that the volcano was proceeding to the next eruption. Activity began on June, 26, 2000, resulting in subsidence to form and the forming of a new caldera 1.6 km in diameter. The new Hatchodaira Caldera almost overlaps the site and size of the former Hatchodaira Caldera. This paper presents the volcano-history of Miyakejima during the last 10000 years.Based on the eruption style, together with whole-rock bulk chemistry, the development history during the last 10000 years is divided into four stages. These are 1) the Ofunato Stage of 10000-4000 y.B.P., 2) the Tsubota Stage of 4000-2500 y.B.P., 3) the Oyama stage, from 2500y.B.P. to the early 15th century, and 4) the Shinmio stage, 1469 AD to the present. Since 1085 AD, at least 14 eruptions are documented. The last four eruptions occurred in 1874, 1940, 1962, and 1983.The Ofunato stage is characterized by porphyritic basalts. These lavas and pyroclastics contributed the growth of the main cone and filling up of the Kuwanokitaira Caldera. The Tsubota Stage, reopened after a 3000-year repose, is distinguished by andesitic products from lateral and central eruptions. The Oyama Stage began with the formation of the Hatchodaira Caldera, which resulted from the discharge of ca. 0.37km3 scoria, explosion breccia, and accretionary lapilli. Subsequent products from central and lateral eruptions filled the caldera. It is noteworthy that phreato-magmatic eruptions from the central vent prevail over dry magmatic eruptions. Overflows of lavas from the rim of the Hatchodaira Caldera occurred in 9th century. At the Shinmio stage, 12 documented eruptions, without exception, took place from lateral fissures, with some accompanied by central eruption.The variation of Mg# (=Mg/ (Mg+Fe) × 100) versus erupted age show an abrupt increase of the ratio 2500, 1300, and 500 y.B.P. with a mild decrease. This pattern suggests that relatively undifferentiated magma was supplied to the magma plumbling system underneath the Miyakejima Volcano, which was slowly differentiating.

Changes in paleoseismic studies of active faults in Japan since the 1995 Kobe earthquake

This paper discusses recent progress of onshore active faults studies in Japan, especially since the 1995 destructive Kobe earthquake, when the number of trenching studies, which are essential for the reconstruction of onshore paleoearthquakes, has rapidly increased. The timing and repeat interval of paleoearthquakes are here reviewed for the Miura Peninsula, south of Tokyo and the Awaji Island and Kobe-Osaka area, in central Japan, where trenching have been carried out very intensively in the last few years.

Bistellettadines A and B: Two Bioactive Dimeric Stellettadines from a Marine Sponge Stelletta sp.1

We recently have reported isolation of stellettazole A (1), an antibacterial homosesquiterpene amide of 1-guanidinopropyl-3-methylhistamine, from a Stelletta sponge collected from Shikine-jima island, 200 km south of Tokyo.2 Interestingly, it inhibited Ca2+/calmodulin-dependent phosphodiesterase. Therefore, we continued to examine the sponge extract, which resulted in the isolation of two dimeric stellettadines, bistellettadines A (2) and B (3). Stellettadine A (4) is a sesquiterpene amide of 1,4-diguanidinobutane possessing larval metamorphosis-inducing activity in an ascidian, which we had isolated from another Stelletta sponge.3 In this paper, we describe isolation, structure determination, and biological activities of these new metabolites.

A kuroko-type polymetallic sulfide deposit in a submarine silicic caldera

Manned submersible studies have delineated a large and actively growing Kuroko-type volcanogenic massive sulfide deposit 400 kilometers south of Tokyo in Myojin Knoll submarine caldera. The sulfide body is located on the caldera floor at a depth of 1210 to 1360 meters, has an area of 400 by 400 by 30 meters, and is notably rich in gold and silver. The discovery of a large Kuroko-type polymetallic sulfide deposit in this arc-front caldera raises the possibility that the numerous unexplored submarine silicic calderas elsewhere might have similar deposits.

Excavation Study of the Kitatake Active Fault at Nagasawa, Yokosuka City, Kanagawa Prefecture, Japan

The Kitatake fault is the major fault of Kitatake fault zone, WNW-ESE striking across the Miura Peninsula, south of Tokyo. The Kitatake fault zone is almost parallel to Kinugasa fault zone (north) and to Takeyama fault zone (south). All of these fault zones are active and show right-lateral slips inferred from geomorphic features. The eastern half of the Kitatake fault delineates the boundary between Zushi Formation (Late Miocene) in the north and Hayama Group (EarlyMiddle Miocene) in the south and shows south-facing scarps probably owing to the resistant Zushi Formation.We carried out observations in 1994 and dating in 1995 with three trenches, one being 9 meters deep, on the unconsolidated Holocene sediments along the Kitatake fault. We obtained 36 dates from 33 samples.The latest episodic slip along the fault seems to have ocurred at about 1, 440 yBP (14C), that is A.D. 600650. The ratio of the horizontal component with the vertical one may be about 2.04.2. The amount of the latest episodic slip is not certain, but might be in the order of meters.Another episodic slip prior to the latest one probably occurred at least before 3, 070 yBP ( 14C).

Submarine cable OBS using a retired submarine telecommunication cable: GeO-TOC program

In order to study the Earth's structure and subduction zone tectonics, seismic data from the oceanic region are extremely important. The present seismograph distribution in the oceanic region, however, provides a very poor coverage. To improve this poor seismic coverage, a cable OBS system using a retired submarine telecommunication cable is proposed. The GeO-TOC cable runs from Ninomiya, Japan, to Guam through the Izu-Bonin forearc and the Marina Trough. The total length of the cable is 2659 km. An OBS, IZU, using the GeO-TOC cable, was successfully installed at the landward slope of the Izu-Bonin Trench in January 1997. The IZU OBS is located approximately 400 km south of Tokyo. The installation method is similar to repair work on submarine cables. The IZU OBS is equipped with three accelerometers, a hydrophone, a quartz pressure gauge, and a quartz precision thermometer with a few temperature sensors to monitor overheating of the internal electronics. After installation, the voltage increase is 90 V when the current is maintained at a constant 370 mA. Data from accelerometers are digitized by 24-bit A/D converters and sent to Ninomiya at 9600 bps for each component. Hydrophone data are sent to Ninomiya as analog signals using the AM (Amplitude Modulation) method for safety reasons. Hydrophone data are digitized at the shore station. Other slow-rate data are multiplexed and sent to the shore at 9600 bps. The instrument can be controlled by a shore computer. All data will be transmitted from Ninomiya to Tokyo and combined with other existing seismic data.

Tephra dispersal from Myojinsho, Japan, during its shallow submarine eruption of 1952-1953

A new and detailed bathymetric map of the Myojinsho shallow submarine volcano provides a framework to interpret the physical volcanology of its 1952–1953 eruption, especially how the silicic pyroclasts, both primary and reworked, enlarged the volcano and were dispersed into the surrounding marine environment. Myojinsho, 420 km south of Tokyo along the Izu–Ogasawara arc, was the site of approximately 1000 phreatomagmatic explosions during the 12.5-month eruption. These explosions shattered growing dacite domes, producing dense clasts that immediately sank into the sea; minor amounts of pumice floated on the sea surface after some of these events. The Myojinsho cone has slopes of almost precisely 21° in the depth range 300–700 m.We interpret this to be the result of angle-of-repose deposition of submarine pyroclastic gravity flows that traveled downslope in all directions. Many of these gravity flows resulted from explosions and associated dome collapse, but others were likely triggered by the remobilization of debris temporarily deposited on the summit and steep upper slopes of the cone. Tephra was repeatedly carried into air in subaerial eruption columns and fell into the sea within 1–2 km of the volcano's summit, entering water as deep as 400 m. Because the fall velocity of single particles decreased by a factor of ∼30 in passing from air into the sea, we expect that the upper part of the water column was repeatedly choked with hyperconcentrations of fallout tephra. Gravitational instabilities within these tephra-choked regions could have formed vertical density currents that descended at velocities greater than those of the individual particles they contained. Upon reaching the sea floor, many of these currents probably continued to move downslope along Myojinsho's submarine slopes. Fine tephra was elutriated from the rubbly summit of the volcano by upwelling plumes of heated seawater that persisted for the entire duration of the eruption. Ocean currents carried this tephra to distal areas, where it presumably forms a pyroclastic component of deep-sea sediment.

The First Breeding Success in the Short-tailed Albatross Colony Restoration Project on Tori-shima, Using Decoys and Vocal Lure

Only around 180 breeding pairs of the endangered Short-tailed Albatross Diomedea albatrus remain. The overwhelming majority of these, about 170 pairs, nest on Tori-shima, in the Izu Islands (30°28'N, 140°18'E), an uninhabited island about 600 kilometers south of Tokyo. The remaining pairs nest in the Senkaku Islands. Tsubame-zaki, the traditional breeding site on Tori-shima, is located on a steep slope composed of loose volcanic gravel. This poor environment is thought to be responsible for the low breeding success rates observed in recent years. The project described here was designed to improve breeding success by relocating the colony to Hatsune-zaki on the other side of the island, a more favorable breeding site with gentler slopes.A preliminary experiment was conducted near the traditional colony at Tsubame-zaki in November 1991, using 10 elaborate life-like decoys and tape recorded albatross calls. Result showed that the Short-tailed Albatrosses are attracted by these decoys and calls. A second experiment was conducted for four days in April 1992 at Hatsune-zaki, using 16 decoys and tape recorded calls. Nine individuals were observed to fly close over the study site. In November 1992, 41 decoys were set up at Hatsune-zaki for the 1992-93 breeding season, with no vocal lure. In March 1993, 9 decoys were added, and a long-term playback device, driven by solar batteries, was installed as a vocal lure. The number of decoys was increased steadily, reaching a total of 69 in November 1994. Every year from 1993, observations at the study site were conducted during daylight hours for a period of about six days either in January, February or March. Five individuals were observed to land in March 1993, and 5, 29, and 41 individuals were observed to land in 1994, 1995, and 1996 respectively. The total time of stay per unit observation time (stay index) increased from 0.31 in 1993 to 0.86 in 1994, 1.64 in 1995, and 2.37 in 1996. Stay index thus increased 7.6 times during this four year period. Percentage of landings in the area of decoys to all the landings at Hatsune-zaki was 78.9% in 1993, but declined to 23.1% in 1994, 8.3% in 1995, and 31.4% in 1996, with more landings occurring in the area below the decoys. It was suggested that changes in the vocal lure apparatus and directions of speakers made in 1993 were responsible for this switch in landing area. When landing in the area of decoys, birds more often landed near, or approached on foot, adult-type decoys than sub-adult types. Also, landings tended to concentrate in the area where the volume of play-back sounds was high. Most of the birds landing at the site were immature and sub-adult. Only four adults out of a total 41 landings were observed in 1996. Four to six year old birds accounted for 70% of all the landings in 1995. Courtship displays by two individuals were observed in 1993, and in 1994 an individual was observed to stay overnight at the study site. Courtship displays were again observed several times in 1994; and two individuals were observed to remain at a fixed spot for a long time in 1995. In November 1995 a six year old male and a five year old female bred at the site, producing a single egg which was hatched successfully. This young left the island on 10 June 1996. In November 1996, three pairs built nests, two of which produced eggs. One of the two successful pairs was the pair that bred in 1995. Three of the four breeding birds were of known age, and had been born in 1990 or 1991. Ages at first breeding for these three birds were 4 years 11 months (female), 5 years 11 months (male), and 6 years 11 months (male). Individuals that were hatched at Tsubame-zaki in 1990 were first observed at Hatsune-zaki in January 1994. These observations suggest that it takes two to three years for an individual to start breeding after the first arrival at the breeding site.

硫黄島における海岸地形の変動特性

Iwo Jima is a small volcanic island located in the Western Pacific about 1, 250 km south of Tokyo. The island, with an area about 23 km2, is mostly rimmed by sandy beaches exceeding 200 m in width. No ports or shore protection works have been constructed along the coast of this island since it became a territory of Japan. Further, the beach is always subjected to rough seas because it is fully exposed to the open sea. Therefore, a natural sandy beach has been preserved on this island, where we can observe the coastal geomorphology formed by ocean waves. It should be noted however, that there exists quite an unusual upheaval on this island. Since 1982, the author has been conducting various surveys along the entire coast of Iwo Jima to investigate both crustal movements and variations of the coastal geomorphology of this island. In this paper, cross-levelling data are analyzed to check variations of coastal geomorphology on Iwo Jima. Here, cross-levelling has been done at fifty measuring sections distributed along the entire beach of the island at intervals of roughly 300 m. The surveys have been performed 17 times so far since 1982 at each of the fifty sections, resulting in 850 data points. In the analysis, the following five physical terms have been chosen as the terms representing the features of the coastal geomorphology of the island : (1) Variations of the shoreline in a certain period, Δ Lx ; (2) Foreshore slope, Ifs ; (3) Variations of foreshore area in a certain period, Δ Sfs ; (4) Backshore slope, Ibs ; and (5) Foreshore height, Zfs.Analyses have been done on the data of each physical term to investigate both time-course and local variations. The following is a summary of the findings.(a) Shoreline has advanced roughly 50 m on average over the island during the past 14 years since 1982, and is still advancing at almost the same rate.(b) Foreshore slope is stable at every coast, keeping values which range from roughly 0.15 to 0.22.(c) Foreshore area has been increased roughly 170 m2 on average over the island during the past 14 years since 1982, and is still increasing at almost the same rate.(d) Backshore slope is stable at every coast, keeping values which range from roughly 0.08 to 0.18.(e) Foreshore height ranges from 1.6 m to 4.2 m, although it varies with time and location.

Evidence for subsurface magma bodies beneath Izu-Oshima volcano inferred from a seismic scattering analysis and possible interpretation of the magma plumbing system of the 1986 eruptive activity

Observed seismic waveforms were analyzed to find the distribution of seismic scatterers beneath Izu-Oshima volcano in an island located about 100 km south of Tokyo, Japan, using seismic waveforms generated by earthquakes, whose epicentral distances are from several tens to about a thousand kilometers. The method employed in the study was developed based on a prestack migration of shot gathers and is composed of the following procedure: (1) static correction, (2) the estimation of waveforms incident into a volume of interest, i.e., subsurface structure of the volcano, and (3) an inverse procedure using the developed migration scheme. The location of scatterers and the order of scattering became clear with the present analysis. The petrological implications and results from geoelectromagnetic studies by other researchers agree with the proposed model in which the existence of main and submagma reservoirs beneath the volcano were indicated. These results are interpreted in terms of a model on the magma migration path during the 1986 eruptive activity of the volcano. Seismic scattering analysis played a significant and indispensable role in this study to give a direct view of a possible underground plumbing system, and is considered a promising method for revealing the unknown structure of volcanoes in the future.

Electromagnetic Monitoring of Miyake-jima Volcano, Izu-Bonn Arc, Japan: A Preliminary Report

Miyake-jima Island, about 150 km south of Tokyo in Izu-Bonin Arc, is one of the most active basalt volcanoes in Japan. Big eruptions took place in 1940, 1962 and 1983. In this volcano, magma ascends towards a depth of a few km below the summit without any significant earthquakes or deformation, then gives rise to flank fissure eruptions because of the blockaded vent just beneath the summit crater. Hence eruption forecasts are very difficult to make with mechanical methods (i.e., seismic and deformation measurements) alone. We have developed an electromagnetic monitoring system of the volcano that combines magnetic, resistivity and electric field (SP) measurements. We expect that magma injection and the hydrothermal materials dispatched from it will result in thermal demagnetization, resistivity change and SP variations together with the electrokinetic-magnetic effect. Since October, 1995, we have continuously operated eight well distributed proton magnetometers over the island as well as two SP measurement systems on the NE and SW fissure zones. SP surveys brought to light distinct anomalies, which strongly suggest a close relation to the eruption mechanism. They are a positive anomaly up to 700 mV centered around the summit, and two negative ones amounting to -250 mV on the north and -100 mV on the southwestern mountainside. These anomalies can originate from a common mechanism: Rainwater penetrates from fissure zones along fractures toward the center of the volcano, a few km deep, where it is warmed by the heat supplied from deep-seated magma to rise through the summit vent. The down flow makes the negative, while the upwelling the positive SP anomalies, respectively. Miyake-jima island is located near the path of the Kuroshio, the most dominant ocean current in the western Pacific. A large magnetic variation amounting to several nT was observed to result from the meander of the flow path. This phenomenon produces serious EM noise and complicates monitoring for volcanic activity. It is crucial to investigate the characteristics of motionally-induced EM fields and to properly eliminate their effects.

Bolometric direct dark matter search at the University of Tokyo

A direct dark matter search experiment is now under preparation at the University of Tokyo. An array of LiF bolometers with a total mass of roughly one kilogram is to be used aiming at a direct detection of spin-dependently interacting supersymmetric neutralinos. A dedicated low-radioactivity dilution refrigerator for the bolometer has been constructed along with a small on-line helium requefier to help the detector operation in the underground laboratory where liquid helium supply is not always available. A long term test operation of a bolometer at the University of Tokyo proved the stability of the detector for more than 45 days. The detector will be installed in the Nokogiri-yama underground laboratory of the Institute for Cosmic Ray Research of the University of Tokyo. The laboratory is 200 m w.e. deep and located about 60 km to the south of Tokyo.