排序方式： 共有26条查询结果，搜索用时 15 毫秒
11.
Benchmarks installed on the upper eastern flank of Mt. Etna in 1982 have subsided continually since then, with the rate of subsidence twice accelerating prior to eruptions. The first of these eruptions was in December 1985, and the second in September 1989. This pattern of accelerating downslope movement has also been observed prior to landslides, and recent work applying knowledge of the failure of materials has shown that analysis of the inverse rate of these movements can be used to predict the time of failure. Post-eruption analyses of geodetic or seismic data from volcanoes has shown that in several cases, this approach could have been used to forecast eruptions weeks in advance. Applying the same principles to the accelerating subsidence on Mt. Etna's eastern flank prior to the eruptions of 1985 and 1989 shows that rough estimates of eruption dates could have been obtained several months in advance. These observations also suggest a speculative eruption-triggering mechanism involving an interplay between slope creep deformation and extensional weakening over the zone of intrusion. 相似文献
12.
Dynamic aspects of the long runout Ontake-san debris avalanche are evaluated by a comparison of several models. An unsteady numerical model assumes two-dimensional flow of an incompressible biviscous or Newtonian fluid, represented as a continuum with a free surface. Internal deformation of the flowing mass is considered, as well as boundary resistances. Thus flow thinning and deposit shape as well as flow kinematics may be modeled. Parameters are adjusted to match observed runout, with additional constraints on velocity and emplacement time. With abundant constraints for Ontake-san, from careful field investigations by Japanese research teams, our analysis indicates that a substantial decrease in flow resistance occurred as a function of displacement. Constant-property models that match runout tend to overestimate the peak velocities and to underestimate the emplacement times. A staged increase in mobility in both constant volume and variable volume models leads to results consistent with field data. Runout in a channel overflow area was also modeled. Qualitatively similar results have been obtained by other researchers using simple sliding block models with empirical parameters, a slide block model with rational parameter selection, a modified flood simulation, and a multi-element frictional slide model. The relative merits of these models are compared.
The field mechanisms associated with this mobility increase with displacement are poorly understood, but the question is now identified as a target for future research at debris avalanche sites, and some plausible mechanisms are considered. The main reason probably involves the entrainment of river water and saturated sediment, leading to enhanced efficiency of fluid pressure mechanisms with undrained shear; in addition, progressive shearing reduced the mean particle size and angularity, and the cohesion and friction (and apparent viscosity) of avalanche debris near the wetted perimeter. Hydroplaning — the shearing of water films and slurries — may have occurred locally. 相似文献
13.
A study of emplacement temperatures was carried out for the largest of the 22 November 1994 nuée ardente deposits at Merapi Volcano, based mainly on the response of plastic and woody materials subjected to the hot pyroclastic current and the deposits, and to some extent on eyewitness observations. The study emphasizes the Turgo–Kaliurang area in the distal part of the area affected by the nuée ardente, where nearly 100 casualties occurred. The term nuée ardente as used here includes channeled block-and-ash flows, and associated ash-clouds of surge and fallout origins. The emplacement temperature of the 8 m thick channeled block-and-ash deposit was relatively high, 550°C, based mainly on eyewitness reports of visual thermal radiance. Emplacement temperatures for ash-cloud deposits a few cm thick were deduced from polymer objects collected at Turgo and Kaliurang. Most polymers do not display a sharp melting range, but polyethylene terephthalate used in water bottles melts between 245 and 265°C, and parts of the bottles that had been deformed during fabrication molding turn a milky color at 200°C. The experimental evidence suggests that deposits in the Turgo area briefly achieved a maximum temperature near 300°C, whereas those near Kaliurang were <200°C. Maximum ash deposit temperatures occurred in fallout with a local source in the channeled block-and-ash flow of the Boyong river valley; the surge deposit was cooler (180°C) due to entrainment of cool air and soils, and tree singe-zone temperatures were around 100°C. 相似文献
14.
F. Lavigne J. C. Thouret B. Voight H. Suwa A. Sumaryono 《Journal of Volcanology and Geothermal Research》2000,100(1-4)
Merapi volcano, in Central Java, is one of the most active volcanoes in the world. At least 23 of the 61 reported eruptions since the mid-1500s have produced source deposits for lahars. The combined lahar deposits cover about 286 km2 on the flanks and the surrounding piedmonts of the volcano. At Merapi, lahars are commonly rain-triggered by rainfalls having an average intensity of about 40 mm in 2 h. Most occur during the rainy season from November to April, and have average velocities of 5–7 m/s at 1000 m in elevation. A wide range of facies may be generated from a single flow, which may transform downvalley from debris flow to hyperconcentrated streamflow.Because of the high frequency and magnitude of the lahar events, lahar-related hazards are high below about 450–600 m elevation in each of the 13 rivers which drain the volcano. Hazard-zone maps for lahar were produced by Pardyanto et al. (Volcanic hazard map, Merapi volcano, Central Java (1/100,000). Geol. Surv. of Indonesia, Bandung, II, 4, 1978) and the Japanese–Indonesian Cooperation Agency (Master plan for land conservation and volcanic debris control in the area of Mt Merapi, Jakarta, 1980), but these maps are of a very small scale to meet modern zoning requirements. More recently, a few large-scale maps (1/10,000- and 1/2000-scale) and risk assessments have been completed for a few critical river systems. 相似文献
15.
16.
B. Voight 《Engineering Geology》1969,3(4):335-339
HAST (1967) recently concluded that the results of his stress measurement work confirm the validity of the contraction hypothesis for the evolution of the earth. This conclusion may not be warranted. A critique and discussion of alternatives that involve continental separation and sea floor spreading are presented in this paper. 相似文献
17.
I. M. Watson C. Oppenheimer B. Voight P. W. Francis A. Clarke J. Stix A. Miller D. M. Pyle M. R. Burton S. R. Young G. Norton S. Loughlin B. Darroux MVO Staff 《Journal of Volcanology and Geothermal Research》2000,98(1-4)
We examine the correlations between SO2 emission rate, seismicity and ground deformation in the month prior to the 25 June 1997 dome collapse of the Soufriere Hills Volcano, Montserrat. During this period, the volcano exhibited a pattern of cyclic inflation and deflation with an 8–14 h period. We find that SO2 emission rates, measured by COSPEC, correlate with the amplitude of these tilt cycles, and that higher rates of SO2 emission were associated with stronger ground deformation and enhanced hybrid seismicity. Within tilt cycles, degassing peaks coincide with maximum deformation gradients. Increases in the amount of gas in the magma conduit feeding the dome, probably due to increases in volatile content of ascending magma volume can account for the observed increases in tilt amplitude, hybrid seismicity and SO2 emission rate. 相似文献
18.
B. Voight K. D. Young D. Hidayat Subandrio M. A. Purbawinata A. Ratdomopurbo Suharna Panut D. S. Sayudi R. LaHusen J. Marso T. L. Murray M. Dejean M. Iguchi K. Ishihara 《Journal of Volcanology and Geothermal Research》2000,100(1-4)
Following the eruption of January 1992, episodes of lava dome growth accompanied by generation of dome-collapse nuées ardentes occurred in 1994–1998. In addition, nuées ardentes were generated by fountain-collapse in January 1997, and the 1998 events also suggest an explosive component. Significant tilt and seismic precursors on varying time scales preceded these events. Deformation about the summit has been detected by electronic tiltmeters since November 1992, with inflation corresponding generally to lava dome growth, and deflation (or decreased inflation) corresponding to loss of dome mass. Strong short-term (days to weeks) accelerations in tilt rate and seismicity occurred prior to the major nuées ardentes episodes, apart from those of 22 November 1994 which were preceded by steadily increasing tilt for over 200 days but lacked short-term precursors. Because of the combination of populated hazardous areas and the lack of an issued warning, about 100 casualties occurred in 1994. In contrast, the strong precursors in 1997 and 1998 provided advance warning to observatory scientists, enabled the stepped raising of alert levels, and aided hazard management. As a result of these factors, but also the fortunate fact that the large nuées ardentes did not quite descend into populated areas, no casualties occurred. The nuée ardente episode of 1994 is interpreted as purely due to gravitational collapse, whereas those of 1997 and 1998 were influenced by gas-pressurization of the lava dome. 相似文献
19.
Summary The elastic response (i.e., displacement fields, strain and stress distributions) has been determined by structural matrix analysis for a homogeneous rock layer subjected to two-dimensional distributions of vertical displacement applied along a basal boundary. The results obtained compare favorably with biharmonic function solutions obtained bySanford [42]2) and the authors. The analysis was then extended: the effects of different magnitudes of superposed gravitationally-induced stresses, release of a constrained side boundary, and introduction of pronounced heterogeneity were each quantitatively evaluated. Each of these changed conditions effected important alterations in response characteristics, e.g., in configuration of stress trajectories, predicted locations and configurations of initial fracture surfaces, shear stress distributions, and displacement fields.These results are cited as an example of the utility of finite-element analysis (originally developed for evaluation of aeronautical structural components) for solutions of boundary value problems of a geological/geophysical nature. The breadth of specific physical problems succeptible to evaluation by this method is large and includes (i) behavior of rocks under static/dynamic loading, (ii) seismic response, (iii) heat and fluid flow, and (iv) distribution of potential. Most of the fundamental problems in geophysics, structural geology, geohydrology, geomorphology, glaciology, and engineering geology involve the above categories either directly or otherwise. The finite-element approach to these problems if bound to have enormous significance, for unlike classical mechanics, it can be readily adapted to solution of systems characterized by non-linear, anisotropic, heterogeneous material properties, of any exterior configuration, containing structural discontinuities, and subjected to any viable combinations of load/displacement boundary conditions. 相似文献
20.
Investigation of well-exposed volcaniclastic deposits of Shiveluch volcano indicates that large-scale failures have occurred
at least eight times in its history: approximately 10,000, 5700, 3700, 2600, 1600, 1000, 600 14C BP and 1964 AD. The volcano was stable during the Late Pleistocene, when a large cone was formed (Old Shiveluch), and became
unstable in the Holocene when repetitive collapses of a portion of the edifice (Young Shiveluch) generated debris avalanches.
The transition in stability was connected with a change in composition of the erupting magma (increased SiO2 from ca. 55–56% to 60–62%) that resulted in an abrupt increase of viscosity and the production of lava domes. Each failure
was triggered by a disturbance of the volcanic edifice related to the ascent of a new batch of viscous magma. The failures
occurred before magma intruded into the upper part of the edifice, suggesting that the trigger mechanism was indirectly associated
with magma and involved shaking by a moderate to large volcanic earthquake and/or enhancement of edifice pore pressure due
to pressurised juvenile gas. The failures typically included: (a) a retrogressive landslide involving backward rotation of
slide blocks; (b) fragmentation of the leading blocks and their transformation into a debris avalanche, while the trailing
slide blocks decelerate and soon come to rest; and (c) long-distance runout of the avalanche as a transient wave of debris
with yield strength that glides on a thin weak layer of mixed facies developed at the avalanche base. All the failures of
Young Shiveluch were immediately followed by explosive eruptions that developed along a similar pattern. The slope failure
was the first event, followed by a plinian eruption accompanied by partial fountain collapse and the emplacement of pumice
flows. In several cases the slope failure depressurised the hydrothermal system to cause phreatic explosions that preceded
the magmatic eruption. The collapse-induced plinian eruptions were moderate-sized and ordinary events in the history of the
volcano. No evidence for directed blasts was found associated with any of the slope failures.
Received: 28 June 1998 / Accepted: 28 March 1999 相似文献