Pyroclastic flows from the 1991 eruption of Unzen volcano,Japan   总被引：1，自引：0，他引：1  

Takahiro Yamamoto  Shinji Takarada  Shigeru Suto 《Bulletin of Volcanology》1993,55(3):166-175

Pyroclastic flows from Unzen were generated by gravitational collapse of the growing lava dome. As soon as the parental lobe failed at the edge of the dome, spontaneous shattering of lava occurred and induced a gravity flow of blocks and finer debris. The flows had a overhanging, tongue-like head and cone- or rollershaped vortices expanding outward and upward. Most of the flows traveled from 1 to 3 km, but some flows reached more than 4 km, burning houses and killing people in the evacuated zone of Kita-kamikoba on the eastern foot of the volcano. The velocities of the flows ranged from 15 to 25 m/s on the gentle middle flank. Observations of the flows and their deposits suggest that they consisted of a dense basal avalanche and an overlying turbulent ash cloud. The basal avalanche swept down a topographic low and formed to tongue-like lobe having well-defined levees; it is presumed to have moved as a non-Newtonian fluid. The measured velocities and runout distances of the flows can be matched to a Bingham model for the basal avalanche by the addition of turbulent resistance. The rheologic model parameters for the 29 May flow are as follows: the density is 1300 kg/m³, the yield strength is 850 Pa, the viscosity is 90 Pa s, and the thickness of the avalanche is 2 m. The ash cloud is interpreted as a turbulent mixing layer above the basal avalanche. The buoyant portions of the cloud produced ash-fall deposits, whereas the dense portions moved as a surge separated from the parental avalanche. The ash-cloud surges formed a wide devastated zone covered by very thin debris. The initial velocities of the 3 June surges, when they detached from avalanches, are determined by the runout distance and the angle of the energy-line slope. A comparison between the estimated velocities of the 3 June avalanches and the surges indicates that the surges that extended steep slopes along the avalanche path, detached directly from the turbulent heads of the avalanches. The over-running surge that reached Kita-Kamikoba had an estimated velocity higher than that of the avalanche; this farther-travelled surge is presumed to have been generated by collapse of a rising ash-cloud plume.  相似文献   

Automatic analysis of geological structure from dip-strike data   总被引：1，自引：0，他引：1  

Kaichiro Yamamoto  Niichi Nishiwaki-Nakajima 《Mathematical Geology》1993,25(7):819-832

We present a technique to analyze dip-strike data to estimate a geologic structure in a previous study, in which some technical and geological problems are left unsolved. In this study, we have solved some geological problems which are to take the lateral discontinuity of the structure into account and use levels of bedding plane as input data. And also, we tried to extend the technique to more general problems that the structure consists of vertically different geologic units. We show the technique revised and some artificial and actual examples analyzed to check its validity and applicability.  相似文献   

Analytical expressions for three different modes in harmonic motion of sliding structures     

Masashi Iura  Kunihito Matsui  Iku Kosaka 《地震工程与结构动力学》1992,21(9):757-769

The problem of response of sliding structures subjected to harmonic support motions is considered. The periodic motions, consisting of the three different modes, stick-stick, stick-slip and slip-slip, are a significant part of the responses to harmonic excitations. Assuming the periodicity of the motion, the condition of the initiation of the slip-slip motion is derived. Then analytical expressions for the occurrence of the periodic motions are obtained without integrating the equations of motion. The accuracy of the analytical expressions is confirmed by comparison with numerical results herein obtained. It is observed that the numerical results are sensitive to the starting and ending times of slip motions.  相似文献   

Microfracture processes in the breakdown zone during dynamic shear rupture inferred from laboratory observation of near-fault high-frequency strong motion     

Naoyuki Kato  Kiyohiko Yamamoto  Tomowo Hirasawa 《Pure and Applied Geophysics》1994,142(3-4):713-734

High-frequency velocities are measured during stick-slip motion in the immediate vicinity of a fault in a granite sample to reveal the microscopic process taking place in the breakdown zone defined in the slip-weakening model. It is found that 1) the onset time of the observed strong motion approximately coincides with the local rupture onset time, 2) the observed near-fault high-frequency strong-motion duration is approximately proportional to the local breakdown time, and 3) the power spectra of strong motions exhibit significant amplitudes at frequencies above the value off _max, wheref _max is a cut-off frequency relevant to rupturing the breakdown zone. These observations suggest that the high-frequency motion would be due to the incoherent brittle microfracture whose characteristic scale is much shorter than the breakdown zone size. We present a stochastic fault model to synthesize the near-fault high-frequency velocity waveforms. In the model, a number of small circular subfaults are distributed randomly on the fault and the rupture onset time of an individual subfault is assumed to be random. The main features of the observed velocity waveforms are well explained by this numerical modeling. It is concluded that approximately half of the total energy of high-frequency elastic waves observed at a point is radiated from the propagating breakdown zone. We emphasize the importance of the observation of near-fault high-frequency strong motions for large shallow earthquakes.  相似文献   

Upper limit of heaving pressure derived by pore-water pressure measurements of partially frozen soil     

Tsutomu Takashi  Takahiro Ohrai  Hideo Yamamoto  Jun Okamoto 《Engineering Geology》1981,18(1-4):245-257

Experiments were conducted to estimate heaving pressures of saturated soil partially frozen in a closed system. Temperatures at both ends of a specimen were kept constant, i.e., positive at the top and negative at the bottom. When the overburden pressure P was maintained at a constant value, the pore-water pressure P_w, which showed a certain value before freezing, decreased gradually as freezing progressed, finally attaining a specific value, whereafter the specimen ceased taking water into it. The pressure difference between P and P_w, at this stage was defined as the upper limit of heaving pressure σ_u, which evidently depended on the temperature θ_c of the cooling end, in accordance with the relation: σ_u = −11.4 θ_c (kg/cm²)

It corresponds to the modified Clausius-Clapeyron's formula, which gives the freezingpoint depression of an ice—water system, where the pressure acting on the ice differs from that on the water. This is the same as the value obtained by Radd and Oertle (1973). It is considered, however, that, when θ_c lowers, the value of θ_u reaches finally a constant value smaller than the one obtained by the above equation. Denoted by σ_{u max}, it was defined as a maximum heaving pressure. The value of σ_{u max} depended on soil type.  相似文献   

On the ionospheric reflection of downcoming whistler waves including the ground effect     

Masashi Hayakawa 《Pure and Applied Geophysics》1974,112(3):513-517

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Redox front formation in an uplifting sedimentary rock sequence: An analogue for redox-controlling processes in the geosphere around deep geological repositories for radioactive waste     

H. Yoshida  R. Metcalfe  K. Yamamoto  Y. Murakami  D. Hoshii  A. Kanekiyo  T. Naganuma  T. Hayashi 《Applied Geochemistry》2008,23(8):2364-2381

Subsurface redox fronts control the mobilization and fixation of many trace elements, including potential pollutants such as certain radionuclides. Any safety assessment for a deep geological repository for radioactive wastes needs to take into account adequately the long-term redox processes in the geosphere surrounding the repository. To build confidence in understanding these processes, a redox front in a reduced siliceous sedimentary rock distributed in an uplifting area in Japan has been studied in detail. Geochemical analyses show increased concentrations of Fe and trace elements, including rare earth elements (REEs), at the redox front, even though concentrations of reduced rock matrix constituents show little change. Detailed SEM observations revealed that fossilized microorganisms composed of amorphous granules made exclusively of Fe and Si occur in the rock’s pore space. Microbial 16S rDNA analysis suggests that there is presently a zonation of different bacterial groups within the redox band, and bacterial zonation played an important role in the concentration of Fe-oxyhydroxides at the redox front. These water–rock–microbe interactions can be considered analogous to the processes occurring in the redox fronts that would develop around geological repositories for radioactive waste. Once formed, the Fe-oxyhydroxides within such a front would be preserved even after reducing conditions resume following repository closure.  相似文献   

Sequential processes in a landslide hazard at a slate quarry in Okayama,Japan   总被引：1，自引：0，他引：1  

Hiroshi Suwa  T. Mizuno  S. Suzuki  Y. Yamamoto  K. Ito 《Natural Hazards》2008,45(2):321-331

The 12 March 2001 landslide at a slate quarry in Okayama, Japan killed three workers. Composite studies based on field surveys of the landslide slope, interviews with local residents and quarry workers, and inspections of hydrological and seismological data have been used to clarify the causes of this slide and its movements. The results indicate that the landslide was enabled firstly by the steepness of the slope, which had been undercut by river; secondly, the structure was that of a dip-slope that was prone to deep-seated slides along bedding planes; thirdly, numerous joints and faults were present. Surprisingly, rainfall, earthquakes, and explosions do not appear to have played any role in the triggering of this slide. The interviews demonstrated that the frequency of precursory failures increased over a period of several hours before the 12 March 2001 landslide. Inspection of the seismograph records and the eyewitness evidence both indicate that the main part of the landslide consisted of two phases of slope failure within 23 s. After the slide, the frequency of the failures gradually decreased with time over a period of several days. Three new terms are proposed for landslides: foreslide, mainslide, and afterslide, following the terms foreshock, main shock, and aftershock used in seismology.  相似文献   

Factors influencing maintenance and decline of a diatom bloom in the Yatsushiro Sea,Japan     

Goh Onitsuka  Tomoyuki Shikata  Saho Kitatsuji  Kazuo Abe  Takeshi Yamamoto  Hiroki Ochiai  Hitoshi Matsuo 《Journal of Oceanography》2016,72(4):617-627

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Sr–Nd isotopic and chemical characteristics of the silicic magma reservoir of the Aira pyroclastic eruption, southern Kyushu, Japan     

Yoji Arakawa  Mie Kurosawa  Kaori Takahashi  Yoji Kobayashi  Masashi Tsukui  Hiroshi Amakawa 《Journal of Volcanology and Geothermal Research》1998,80(3-4)

Sr and Nd isotope and geochemical investigations were performed on a remarkably homogeneous, high-silica rhyolite magma reservoir of the Aira pyroclastic eruption (22,000 years ago), southern Kyushu, Japan. The Aira caldera was formed by this eruption with four flow units (Osumi pumice fall, Tsumaya pryoclastic flow, Kamewarizaka breccia and Ito pyroclastic flow). Quite narrow chemical compositions (e.g., 74.0–76.5 wt% of SiO₂) and Sr and Nd isotopic values (⁸⁷Sr/⁸⁶Sr=0.70584–0.70599 and _Nd=−5.62 to −4.10) were detected for silicic pumices from the four units, with the exception of minor amounts of dark pumices in the units. The high Sr isotope ratios (0.7065–0.7076) for the dark pumices clearly suggest a different origin from the silicic pumices. Andesite to basalt lavas in pre-caldera (0.37–0.93 Ma) and post-caldera (historical) eruptions show lower ⁸⁷Sr/⁸⁶Sr (0.70465–0.70540) and higher _Nd (−1.03 to +0.96) values than those of the Aira silicic and dark pumices. Both andesites of pre- and post-caldera stages are very similar in major- and trace-element characteristics and isotope ratios, suggesting that the both andesites had a same source and experienced the same process of magma generation (magma mixing between basaltic and dacitic magmas). Elemental and isotopic signatures deny direct genetic relationships between the Aira pumices and pre- and post-caldera lavas. Relatively upper levels of crust (middle–upper crust) are assumed to have been involved for magma generation for the Aira silicic and dark pumices. The Aira silicic magma was derived by partial melting of a separate crust which had homogeneous chemistry and limited isotope compositions, while the magma for the Aira dark pumice was generated by AFC mixing process between the basement sedimentary rocks and basaltic parental magma, or by partial melting of crustal materials which underlay the basement sediments. The silicic magma did not occupy an upper part of a large magma body with strong compositional zonation, but formed an independent magma body within the crust. The input and mixing of the magma for dark pumices to the base of the Aira silicic magma reservoir might trigger the eruptions in the upper part of the magma body and could produce a slight Sr isotope gradient in the reservoir. An extremely high thermal structure within the crust, which was caused by the uprise and accumulation of the basaltic magma, is presumed to have formed the large volume of silicic magma of the Aira stage.  相似文献