Basaltic pyroclastic flows of Fuji volcano, Japan: characteristics of the deposits and their origin     

T. Yamamoto  A. Takada  Y. Ishizuka  N. Miyaji  Y. Tajima 《Bulletin of Volcanology》2005,67(7):622-633

Fuji volcano is the largest active volcano in Japan, and consists of Ko-Fuji and Shin-Fuji volcanoes. Although basaltic in composition, small-volume pyroclastic flows have been repeatedly generated during the Younger stage of Shin-Fuji volcano. Deposits of those pyroclastic flows have been identified along multiple drainage valleys on the western flanks between 1,300 and 2,000 m a.s.l., and have been stratigraphically divided into the Shin-Fuji Younger pyroclastic flows (SYP) 1 to 4. Downstream debris flow deposits are found which contain abundant material derived from the pyroclastic flow deposits. The new¹⁴C ages for SYP1 to SYP4 are 3.2, 3.0, 2.9, and 2.5 ka, respectively, and correspond to a period where explosive summit eruptions generated many scoria fall deposits mostly toward the east. The SYP1 to SYP4 deposits consist of two facies: the massive facies is about 2 m thick and contains basaltic bombs of less than 50 cm in size, scoria lapilli, and fresh lithic basalt fragments supported in an ash matrix; the surge facies is represented by beds 1 to 15 cm thick, consisting mainly of ash with minor amount of fine lapilli. The bombs and scoria are 15 to 30% in volume within the massive facies. The ashes within the SYP deposits consist largely of comminuted basalt lithics and crystals that are derived from the Middle-stage lava flows exposed at the western flanks. SYP1 to SYP4 were only dispersed down the western flanks. The reason for this one-sided distribution is the asymmetric topography of the edifice; the western slopes of the volcano are the steepest (over 34 degrees). Most pyroclastic materials cannot rest stably on the slopes steeper than 33 degrees. Therefore, ejecta from the explosive summit eruptions that fell on the steep slopes tumbled down the slopes and were remobilized as high-temperature granular flows. These flows consisted of large pyroclastics and moved as granular avalanches along the valley bottom. Furthermore, the avalanching flows increased in volume by abrasion from the edifice and generated abundant ashes by the collision of clasts. The large amount of the fine material was presumably available within the transport system as the basal avalanches propagated below the angle of repose. Taking the typical kinetic friction coefficient of small pyroclastic flows, such flows could descend the western flanks where scattered houses are below 1,000 m a.s.l. A similar type of pyroclastic flow could result if explosive summit eruptions occur in the future.Editorial responsibility: R Cioni  相似文献   

Lateral Variations of Lg Coda Q in Southern Mexico     

J. Yamamoto  L. Quintanar  R.B. Herrmann  C. Fuentes 《Pure and Applied Geophysics》1997,149(3):575-599

—Broad band digital three-component data recorded at UNM, a GEOSCOPE station, were used to estimate Lg coda Q for 34 medium size (3.9 ≤m _b ≤ 6.3) earthquakes with travel paths laying in different geological provinces of southern Mexico in an effort to establish the possible existence of geological structures acting as wave guides and/or travel paths of low attenuation between the Pacific coast and the Valley of Mexico. The stacked spectral ratio method proposed by XIE and NUTTLI (1988) was chosen for computing the coda Q. The variation range of Q ₀ (Q at 1?Hz) and the frequency dependence parameter η estimates averaged on the frequency interval of 0.5 to 2?Hz for the regions and the three components considered are: i) Guerrero region 173 ≤Q¯ ₀≤ 182 and 0.6 ≤Q¯ ₀ ≤ 0.7, ii) Oaxaca region 183 ≤Q¯ ₀ ≤ 198 and 0.6 ≤Q¯ ₀ ≤ 0.8, iii) Michoacan-Jalisco region 187 ≤Q¯ ₀ ≤ 204 and 0.7 ≤Q¯ ₀ ≤ 0.8 and iv) eastern portion of the Transmexican Volcanic Belt (TMVB) 313 ≤Q ₀≤ 335 and η = 0.9. ¶The results show a very high coda Q for the TMVB as compared to other regions of southern Mexico. This unexpected result is difficult to reconcile with the geophysical characteristics of the TMVB, e.g., low seismicity, high volcanic activity and high heat flow typical of a highly attenuating (low Q) region. Visual inspection of seismograms indicates that for earthquakes with seismic waves traveling along the TMVB, the amplitude decay of Lg coda is anomalously slow as compared to other earthquakes in southern Mexico. Thus, it seems that the high Q value found does not entirely reflect the attenuation characteristics of the TMVB but it is probably contaminated by a wave-guide effect. This phenomenon produces an enhancement in the time duration of the Lg wave trains travelling along this geological structure. This result is important to establish the role played by the transmission medium in the extremely long duration of ground motion observed during the September 19, 1985 Michoacan earthquake. ¶The overall spatial distribution of coda Q values indicates that events with focus in the Michoacan-Jalisco and Oaxaca regions yield slightly higher values than those from Guerrero. This feature is more pronounced for the horizontal component of coda Q. A slight dependence of average coda Q ^?1 on earthquake focal depth is observed in the frequency range of 0.2 to 1.0?Hz approximately on the horizontal component. Deeper (h > 50?km) events yield lower values of Q ^?1 than shallower events. For frequencies higher than 1.0?Hz no clear dependence of Q ^?1 on focal depth is observed. However, due to the estimates uncertainties this result is not clearly established.  相似文献   

Late cretaceous to early paleogene paleomagnetic results from Sikhote Alin, far eastern Russia: implications for deformation of East Asia   总被引：1，自引：0，他引：1  

Yo-ichiro Otofuji  Takaaki Matsuda  Tetsumaru Itaya  Takeshi Shibata  Michiko Matsumoto  Takahiro Yamamoto  Chiyo Morimoto  Ruslan G. Kulinich  Petr S. Zimin  Anatoly P. Matunin  Vladimir G. Sakhno  Katsuhiro Kimura 《Earth and Planetary Science Letters》1995,130(1-4):95-108

Welded tuffs in the Bogopol and Sijanov groups were sampled at 27 sites from 12 caldera formations in the Sikhote Alin mountain range around Kavalerovo (44.3°N, 135.0°E) for chronological and paleomagnetic studies. KAr age dates show that the welded tuffs erupted between 66 Ma and 46 Ma. All sites yield reliable paleomagnetic directions, with unblocking temperatures higher than 560°C. The high-temperature component at 12 sites and the medium-temperature component at 3 sites in the Bogopol Group show reversed polarity (D = 193.7°, I = −57.6°,₉₅ = 8.1°). The high-temperature component at 11 sites in the Sijanov Group showed both reversed and normal polarities and its mean direction reveals no detectable deflection from north (D = −2.9°, I = 59.6°,₉₅ = 11.2°). The combined paleomagnetic direction of the two groups yields a paleomagnetic pole of 250.5°E, 84.1°N (A₉₅ = 8.8°), which falls near Cretaceous paleomagnetic poles from Outer Mongolia, Inner Mongolia, the North China Block and the South China Block. The Sikhote Alin area appears not to have been subjected to detectable motion with respect to East Asia since about 50 Ma. This implies that the Sikhote Alin area behaved as an integral part of East Asia during the opening of the Japan Sea at about 15 Ma. However, significant separation between the paleomagnetic poles of East Asia and Europe during the Jurassic-Paleogene implies a major relative movement between these two blocks since the Paleogene.  相似文献   

Elasticity and anharmonicity of potassium chloride at high temperature     

Shigeru Yamamoto  Orson L. Anderson 《Physics and Chemistry of Minerals》1987,14(4):332-340

High temperature elasticity of single crystal potassium chloride has been studied by the Rectangular Parallelepiped Resonance (RPR) method up to 870 K (? 3.8 times the Debye temperature, ?). The elastic stiffness moduli, C ₁₁ and C ₄₄, decrease linearly with temperature while C ₁₂ increases slightly with temperature. The RPR method is particularly suited to measurements of elasticity at high temperatures, since no glues (which decompose at high temperature) are used to connect the transducers to a specimen. As a consequence, the measured spectrum closely approximates the theoretical spectrum of a specimen freely suspended in space with no external contact. The present elasticity data permits the investigation of the thermodynamic properties of potassium chloride far above the Debye temperature when used together with the previous zero-pressure data on thermal expansivity and heat capacity. The equation of state of potassium chloride is virtually unaffected by anharmonicity, even at T/?=3.8. One result is that the thermal pressure for KCl above the Debye temperature linearly increases with temperature. There is also small dependence on volume, in contrast to NaCl where there is no dependence on volume.  相似文献   

Full-correlation analysis of turbulent scattering layers in the mesosphere observed by the MU radar     

Mamoru Yamamoto  Toru Sato  Toshitaka Tsuda  Shoichiro Fukao  Susumu Kato 《Pure and Applied Geophysics》1989,130(2-3):605-616

We have applied a full-correlation analysis technique to the echo power fluctuations observed by the MU radar (35°N, 136°E), and analyzed the horizontal structure of the scattering pattern in the mesosphere as well as their horizontal motions. The velocity of the scattering pattern did not agree with the background wind velocity, but was associated with the horizontal propagating direction of a saturated inertia gravity wave identified in the wind field. The length of the long axis of the characteristic ellipse of the scattering pattern was approximately 50 km, and the direction was almost perpendicular to the propagating direction of the wave. The correlation time of the scattering pattern was approximately 700 s, which is much longer than the lifetime of the isolated turbulence itself. This implies that the observed scattering pattern is associated with a region where the saturated inertia gravity wave generates turbulence.  相似文献   

Diurnal variations of the planetary boundary layer observed with anL-band clear-air doppler radar     

Hiroyuki Hashiguchi  Manabu D. Yamanaka  Toshitaka Tsuda  Mamoru Yamamoto  Takuji Nakamura  Tatsuhiro Adachi  Shoichiro Fukao  Toru Sato  Daniel L. Tobing 《Boundary-Layer Meteorology》1995,74(4):419-424

Based on continuous observations of the planetary boundary layer (PBL) with anL-band (1357.5 MHz) boundary-layer radar (BLR) at a hilly location in Japan, we have discovered that on clear days, a thin enhanced echo layer corresponding to the top of the PBL (or mixed layer) appeared at about 500 m height in the morning and ascended to about 1500 m in the afternoon. Strong upward velocities were observed below the echo layer (or inside the PBL), reaching 1500 m in the afternoon.  相似文献   

Verification test of the mean event size model for moderate earthquakes in the Kanto region, central Japan     

M. Imoto  N. Yamamoto 《Tectonophysics》2006,417(1-2):131

In the Kanto region, there have been reports of decreasing b-values prior to earthquakes (M ≥ 5.5). The change of b-values is defined as the difference between the long- and short-term average earthquake magnitudes. A hazard function for moderately large earthquakes has also been proposed. This model was based on earthquakes that occurred between 1982 and 1999, and its effectiveness is measured retrospectively. In order to evaluate the effectiveness of the model through the use of independent data, the verification test is based on earthquakes observed after January 2000. Through the end of 2004, the only event where there was a decrease in mean event size was observed on June 3, 2000. This decrease resulted in a log-likelihood for the proposed model 1.3 units larger than that of the Poisson model, supporting the validity of the proposed model. Without accumulating further examples, we attempted to improve the verification test by expanding the study volume and by lowering the target cutoff magnitude in order to overcome the small sample size. When two other targets from the expanded volume were added, the difference in the log-likelihood (ΔlnL) increased to 3.6. In this case, the information rate per event was about 1.2, larger than that of the model period. Lowering the cutoff magnitude increased ΔlnL to 2.5. These extended tests led to higher confidence in the model with the larger ΔlnL value than did the primary test. From the viewpoint of the per-event information rate, each case involving targets with a magnitude 5.5 and larger resulted in better performance than in the model period.  相似文献   

⁴⁰Ar/³⁹Ar ages and palaeomagnetism of transitionally magnetized volcanic rocks in the Society Islands, French Polynesia: Raiatea excursion in the upper-Gauss Chron     

Y. Yamamoto  O. Ishizuka  M. Sudo  K. Uto 《Geophysical Journal International》2007,169(1):41-59

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Progressive metamorphism of the Taitao ophiolite; evidence for axial and off-axis hydrothermal alterations     

Takazo Shibuya  Tsuyoshi Komiya  Ryo Anma  Tsutomu Ota  Soichi Omori  Yoshiaki Kon  Shinji Yamamoto  Shigenori Maruyama 《Lithos》2007,98(1-4):233-260

We estimated metamorphic conditions for the  6 Ma Taitao ophiolite, associated with the Chile triple junction. The metamorphic grade of the ophiolite, estimated from secondary matrix minerals, changes stratigraphically downwards from the zeolite facies, through the prehnite–actinolite facies, greenschist facies and the greenschist–amphibolite transition, to the amphibolite facies. The metamorphic facies series corresponds to the low-pressure type. The metamorphic zone boundaries are subparallel to the internal lithological boundaries of the ophiolite, indicating that the metamorphism was due to axial hydrothermal alteration at a mid-ocean ridge.

Mineral assemblages and the compositions of veins systematically change from quartz-dominated, through epidote-dominated, to prehnite-dominated with increasing depth. Temperatures estimated from the vein assemblages range from  230 °C in the volcanic unit to  380 °C at the bottom of the gabbro unit, systematically  200 °C lower than estimates from the adjoining matrix minerals. The late development of veins and the systematically lower temperatures suggest that the vein-forming alteration was due to off-axis hydrothermal alteration.

Comparison between the Taitao ophiolite with its mid-ocean ridge (MOR) affinity, and other ophiolites and MOR crusts, suggests that the Taitao ophiolite has many hydrothermal alteration features similar to those of MOR crusts. This is consistent with the tectonic history that the Taitao ophiolite was formed at the South Chile ridge system near the South American continent (Anma, R., Armstrong, R., Danhara, T., Orihashi, Y. and Iwano, H., 2006. Zircon sensitive high mass-resolution ion microprobe U–Pb and fission-track ages for gabbros and sheeted dykes of the Taitao ophiolite, Southern Chile, and their tectonic implications. The Island Arc, 15(1): 130–142).  相似文献   

Geostatistical Evaluation of the Mechanical Properties of Rock Mass for TBM Tunnelling by Seismic Reflection Method     

K. Aoki  Y. Mito  T. Yamamoto  S. Shirasagi 《Rock Mechanics and Rock Engineering》2007,40(6):591-602

Summary. The evaluation of the rock mass mechanical properties by the seismic reflection method and TBM driving is proposed for TBM tunnelling. The relationship between the reflection number derived from the three-dimensional seismic reflection method and the rock strength index (RSI) derived from TBM driving data is examined, and the methodology of conversion from the reflection number to the RSI is proposed. Furthermore a geostatistical prediction methodology to provide a three-dimensional geotechnical profile ahead of the tunnel face is proposed. The performance of this prediction method is verified by actual field data.  相似文献