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121.
Masato Iguchi Hiroshi Yakiwara Takeshi Tameguri Muhamad Hendrasto Jun-ichi Hirabayashi 《Journal of Volcanology and Geothermal Research》2008
A common sequence of phenomena associated with volcanic explosions is extracted based on seismic and ground deformation observations at 3 active volcanoes in Japan and Indonesia. Macroscopic inflation-related ground deformations are detected prior to individual explosions, while deflations are observed during eruptions. Precursory inflation occurs 5 min to several hours before eruption at the Sakurajima volcano, but just 1–2 min at Suwanosejima and 3–30 min at the Semeru volcano. The sequence commences with minor contraction, which is detected by extensometers 1.5 min before eruption at Sakurajima, as a dilatant first motion of the explosion earthquakes 0.2–0.3 s before surface explosions at Suwanosejima, and as downward tilt 4–5 s prior to eruption at the Semeru volcano. The sequence is detected for explosive eruptions with > 0.1 μrad tilt change at Sakurajima, 90% at Suwanosejima and 75% at Semeru volcanoes. It is inferred that the minor contraction is caused by a volume and pressure decrease due to the release of gas from a pocket at the top of the conduit as the gas pressure exceeds the strength of the confining plug. The subsequent violent expansion may be triggered by sudden outgassing of the water-saturated magma induced by the decrease in confining pressure. 相似文献
122.
In order to clarify the time relation of the expansion of a gas pocket and failure of its overlying plug of lava during Vulcanian
eruptions, infrasound records and video images of the Vulcanian eruption that occurred at Sakurajima volcano on January 2,
2007 were analyzed with respect to their origin times. Weak (≤3 Pa) and slowly increasing air pressure preceded the impulsive
compression phase by 0.25–0.32 s, and a longer-period rarefaction phase of infrasound waves was recognized at all microphone
stations. The velocity of the compression phase was assumed to be supersonic (ca. 400 m/s) up to 850 m above the crater bottom
from other recent explosions. On the other hand, the propagation velocity of the preceding weak signal was regarded to be
similar to the air sound velocity because the lack of impulsiveness is unlikely to be related to the main compression phase.
Therefore, the estimated origin time of the main compression phase was delayed by 0.5–0.7 s from the preceding phase. The
origin time of the preceding phase coincided with the onset of the isotropic expansion process of the pressurized gas pocket,
which was obtained by the waveform inversion of the explosion earthquake. In contrast, the origin time of the main impulsive
phase coincided with the time when the expansion rate reached its peak. This observation suggests that the volumetric increase
of the gas pocket caused swelling of the surface of the crater bottom and its subsequent failure. When the expansion velocity
exceeded a threshold level, the main impulsive compression phase radiated with a high velocity by the sudden releases of the
pressurized gases. The volumetric change at the source was estimated to be 280–560 m3 from the preceding phase of the infrasound. This volume change indicates that the vertical displacement of the swelling ground
was on the order of 1.0 m, assuming the radius of the lava plug was ca. 10 m. 相似文献
123.
Abstract We report a new chondrite that fell in Hashima City in central Japan sometime during the period 1868–1912. The chondrite weighs 1110.64 g and exhibits distinct chondritic structure. Chondrules occupy 24 vol% of the stone and consist of olivine (average Fa17,8), low-Ca pyroxene (average Fs15,8 Wo0.9), devitrified glass and lesser amounts of oligoclase (ca. Ab80Or4), kamacite, taenite, troilite and chromian spinel. Matrix occupying 76 vol% of the stone consists of olivine, low-Ca pyroxene, kamacite, taenite, troilite, cryptocrystalline minerals and lesser amounts of chromian spinel and chlorapatite. Matrix minerals have the same compositions as those in chondrules. Mineral chemistry, bulk chemistry and magnetic properties indicate that Hashima is an H-group chondrite. Well-defined chondrules, scarcely recrystallized oligoclase and relatively small variations of olivine and low-Ca pyroxene compositions indicate that Hashima is of petrologic type 4. 相似文献
124.
Taketo Shimano Takeshi Nishimura Nobuyuki Chiga Yoshinobu Shibasaki Masato Iguchi Daisuke Miki Akihiko Yokoo 《Bulletin of Volcanology》2013,75(12):1-7
We develop an automatic system for the sampling of ash fall particles, to be used for continuous monitoring of magma ascent and eruptive dynamics at active volcanoes. The system consists of a sampling apparatus and cameras to monitor surface phenomena during eruptions. The Sampling Apparatus for Time Series Unmanned Monitoring of Ash (SATSUMA-I and SATSUMA-II) is less than 10 kg in weight and works automatically for more than a month with a 10-kg lead battery to obtain a total of 30 to 36 samples in one cycle of operation. The time range covered in one cycle varies from less than an hour to several months, depending on the aims of observation, allowing researchers to target minute-scale fluctuations in a single eruptive event, as well as daily to weekly trends in persistent volcanic activity. The latest version, SATSUMA-II, also enables control of sampling parameters remotely by e-mail commands. Durability of the apparatus is high: our prototypes worked for several months, in rainy and typhoon seasons, at windy and humid locations, and under strong sunlight. We have been successful in collecting ash samples emitted from Showa crater almost everyday for more than 4 years (2008–2012) at Sakurajima volcano in southwest Japan. 相似文献
125.
126.
Effect of local nonlinearity in cohesionless soil on optimal radius minimizing fixed-head pile bending by inertial and kinematic interactions 总被引:1,自引:1,他引:0
Masato Saitoh 《Acta Geotechnica》2010,5(4):273-286
This study presents the effects of a local nonlinearity in cohesionless soil upon the optimal radius minimizing the bending
strains of a vertical, cylindrical fixed-head pile embedded in a layered soil stratum in a soil–pile–structure system where
the kinematic interaction dominates. The seismic deformation method (SDM) with discretized numerical models is applied since
the SDM is a static numerical method that can easily consider realistic conditions of layered soil strata and the nonlinearity
of the soil. In the numerical models, the local nonlinearity of the soil in the vicinity of the pile is represented by subgrade
springs having bi-linear skeleton curves with a simple hysteretic loop. Various amplitudes of the lateral displacements of
the soil and the lateral forces at the head of the pile are considered as numerical parameters. The results of parametric
analyses reveal the presence of an optimal pile radius that locally minimizes the bending strains of the piles under strong
nonlinearity of the soil, and the optimal pile radius tends to increase as the degree of nonlinearity increases. Criteria
are presented for predicting the increment in the optimal radius of soil–pile–structure systems under strong nonlinearity
in the soil. 相似文献
127.
Abstract— Hydrogen‐isotopic compositions of carbonate and maskelynite in Allan Hills (ALH) 84001 were measured by secondary ion mass spectrometry (SIMS). the δd values of both minerals show considerable deviation. The deviation seems to be caused by addition of varying amounts of terrestrial water in the case of carbonate. In the case of maskelynite, H is heterogeneously distributed and the deviation in δD values seems to be due to mixing of this indigenous heavy H with isotopically normal H present in the SIMS chamber. The indigenous δD value in ALH 84001 seems to be ~2000%‰. Carbonate rather than maskelynite seems to be the main carrier of H in ALH 84001. Because ALH 84001 is ~4 Ga old, the H‐isotopic composition suggests that a large fraction of the initial martian atmosphere had already escaped by 4 Ga. 相似文献
128.
Toru Kobari Hiroyasu Akamatsu Masato Minowa Toshihiro Ichikawa Kazuo Iseki Ryuji Fukuda Masataka Higashi 《Journal of Oceanography》2010,66(5):673-684
Seasonal changes in the shape and size composition of fecal pellets were investigated with sediment trap samples from 50 and
150 m in Kagoshima Bay to evaluate how the mesozooplankton community affects fecal pellet flux. Deep vertical mixing was evident
in March, and thermal stratification was developed above 50 m in June, August and November. Chlorophyll a, suspended particulate organic carbon (POC) and copepod abundance were uniform throughout the water column during the seasonal
mixing and concentrated above 50 m in the stratified seasons. Calanoids were the most predominant copepods in March and poecilostomatoids
composed more than 45% of the copepod community in June, August and November. Fecal pellet fluxes at 50 and 150 m were the
highest in March, nearly half of POC flux. The relative contribution declined considerably in the other months, especially
for less than 4% of POC flux in August. The decline was corresponded to the predominance of cyclopoids and poecilostomatoids.
Cylindrical pellets dominated the fecal matters at both depths throughout the study period, while larger cylindrical pellets
nearly disappeared at 150 m in June, August and November. Copepod incubation revealed that cylindrical and oval pellets were
egested by calanoids and the other copepods, respectively. We suggest that cylindrical fecal pellets produced by calanoid
copepods contribute to feces flux but the predominance of poecilostomatoids and/or cyclopoids decreases feces flux via the
increase of oval pellets and fragmentation of larger cylindrical pellets. 相似文献