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D. M. McClung 《Natural Hazards》2014,72(2):1139-1158
Since human triggering is responsible for about 90 % of deaths from slab avalanches in Europe and North America, risk analysis is very important for skier triggering of avalanches. The depth to the weak layer and the slope angle are two key measureable quantities prior to dry slab avalanche release. Both are important in risk analysis. The probability of avalanche release dramatically increases as the slope angle increases above 25°. As the slab depth increases, the consequences increase rapidly if an avalanche releases. Simple risk analyses for skier triggering were done for both slope angle and slab depth. The slab depth analysis showed there is a range of about 0.6–1.0 m for which the risk of death is highest. For slope angles, the range with highest risk was shown to be 33°–45° within the known range (25°–55°) for skier-triggered avalanches. 相似文献
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This paper (Part II) constitutes the second of a two part series todefine the seven elements of avalanche forecasting. Part I contains the first four elements which are neededto present the human issues. This paper contains the last three elements which deal mostly with thephysical issues and their use in the decision-making process. Some basic rules of applied avalancheforecasting are included here, for the first time, to illustrate physically based principleswhich are used in applied avalanche forecasting and their link to data analysis and decisions.Since the seven elements of applied avalanche forecasting are strongly connected, the reader should consultPart I (this journal issue) as a prelude to the present paper. Part II contains sections about dataand information, scale issues in time and space, decision making and errors and physical rules ofapplied forecasting. Since all seven elements of applied avalanche forecasting are connected, Part II does not stand alone. 相似文献
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Natural Hazards - Field data from snow avalanche fracture lines for slope angle and slab depth (measured perpendicular to the weak layer) were analyzed for different triggering mechanisms. For... 相似文献
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Russell Bailie Jens Gutzmer Harald Strauss Eva Stüeken Craig McClung 《Mineralium Deposita》2010,45(5):481-496
Zn- and Cu-rich massive sulfide ores of volcanogenic origin [volcanogenic massive sulfide (VMS) deposits] occur as stratiform/stratabound
lenses of variable size hosted by gneisses, amphibolites, and schists of the Areachap Group, in the Northern Cape Province
of South Africa. The Areachap Group represents the highly deformed and metamorphosed remnants of a Mesoproterozoic volcanic
arc that was accreted onto the western margin of the Kaapvaal Craton during the ∼1.0–1.2 Ga Namaquan Orogeny. Sulfur isotope
data (δ34S) are presented for 57 sulfide separates and one barite sample from five massive sulfide occurrences in the Areachap Group.
Although sulfides from each site have distinct sulfur isotope values, all δ34S values fall within a very limited range (3.0‰ to 8.5‰). Barite has a δ34S value of 18.5‰, very different from that of associated sulfides. At one of the studied sites (Kantienpan), a distinct increase
in δ34S of sulfides is observed from the massive sulfide lens into the disseminated sulfides associated with a distinct footwall
alteration zone. Sulfide–sulfide and sulfide–barite mineral pairs which recrystallized together during amphibolite- and lower
granulite facies metamorphism are not in isotopic equilibrium. Sulfur isotope characteristics of sulfides and sulfates of
the Zn–Cu ores in the Areachap Group are, however, very similar to base metal sulfide accumulations associated with modern
volcanic arcs and unsedimented mid-ocean ridges. It is thus concluded that profound recrystallization and textural reconstitution
associated with high-grade regional metamorphism of the massive sulfide ores of the Areachap Group did not result in extensive
sulfur isotopic homogenization. This is similar to observations in other metamorphosed VMS deposit districts and confirms
that massive sulfide ores remain effectively a closed system for sulfur isotopes for both sulfides and sulfates during metamorphism. 相似文献
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Craig R. McClung Jens Gutzmer Nicolas J. Beukes Klaus Mezger Harald Strauss Ellen Gertloff 《Mineralium Deposita》2007,42(5):537-549
Stratiform and stratabound barite ± magnetite beds are intimately associated with the polymetallic Broken Hill-type (BHT)
massive sulfide deposits of the Aggeneys-Gamsberg Pb–Zn–Cu ± Ag–Ba district in the Northern Cape Province, South Africa. Barite
samples were collected and studied from four localities in the district. Although metamorphic water–rock interaction processes
have partially altered the chemical and to a lesser degree the isotopic composition of barite, samples identified as being
the least altered display distinctly different isotopic compositions that are thought to reflect different modes of origin.
All barite samples are marked by low concentrations of SrO (0.5 ± 0.2 wt%), highly radiogenic 87Sr/86Sr ratios, elevated δ
34S and δ
18O values compared to contemporaneous Mesoproterozoic seawater. Radiogenic 87Sr/86Sr signatures (0.7164 ± 0.0028) point to an evolved continental crustal source for Sr and Ba, while elevated δ
34S values (27.3 ± 4.9‰) indicate that contemporaneous seawater sulfate, modified by bacterial sulfate reduction, was the single
most important sulfur reservoir for barite deposition. Most importantly, δ
18O values suggest a lower temperature of formation for the Gamsberg deposit compared with the occurrences in the Aggeneys area,
i.e. Swartberg-Tank Hill and Big Syncline. The obvious differences in temperature of formation are in good agreement with
the Cu-rich, Ba-poor nature of the sulfide mineralization of the Aggeneys deposits vs the Cu-poor, Ba-rich character of the
Gamsberg deposit. In conjunction with this, isotopic and petrographic arguments favor a sub-seafloor replacement model for
the stratabound barite occurrences of the Aggeneys deposits, while at Gamsberg, deposition at the sediment–water interface
as a true sedimentary exhalite appears more likely. 相似文献
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D. M. McClung 《Natural Hazards》2011,59(3):1635-1645
In North America and Europe, most fatalities due to snow avalanches occur in the backcountry during recreational pursuits.
Of these, more than 90% of the fatal avalanches are triggered by the victims themselves. This pattern suggests that the primary
cause of avalanche fatalities for human-triggered avalanches is a failure in human perception. For the latter, people thought
that the state of stability or instability of the snow cover was different than it actually was. In this paper, the strength
and weight of evidence used to make decisions in backcountry travel are discussed from: (a) the perspective of the favored
hypothesis to proceed for good recreational enjoyment based on stability evaluation and (b) the null hypothesis based on an
assessment of instability. Based on the facts about snow slab avalanche release, it is argued that instability analysis is
the best framework for avalanche forecasting, whereas human action is most closely related to the favored hypothesis (stability
evaluation). Using scaling laws derived from: (a) fracture mechanics about the size of imperfections causing avalanches and
(b) avalanche dimensions, it is suggested that a snow slab could show stability over more than 99% of the total area. From
the concepts of Bayesian probability, it is shown that overconfidence about stability can arise when the weight of the likelihood
is high and the weight of prior is low. Similarly, underconfidence (excessive conservatism) often results when the weight
of the prior is high with little regard for the likelihood, which may be low. Overconfidence about stability is considered
to be a prime source of accidents. 相似文献
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J.P. Lacassie C.R. McClung R.H. Bailie J. Gutzmer J. Ruiz-Del-Solar 《Journal of Geochemical Exploration》2006,91(1-3):81-98
The Mesoproterozoic Bushmanland Group is situated in the central region of the 1000 to 1200 Ma Namaqualand Metamorphic Complex (NMC). The NMC comprises a belt of highly deformed medium- to high-grade metamorphic rocks to the west of the Archean Kaapvaal Craton of southern Africa. The Bushmanland Group, one of the many supracrustal sequences that make up the NMC, is a metavolcano-sedimentary succession that hosts economically significant concentrations of sillimanite and base-metal sulfide deposits. The present investigation was carried out to study the geochemistry of a large set of representative samples of psammo-pelitic schists from the Bushmanland Group, which includes data from three different schist units: Namies Schist Formation, Shaft Schist Formation and Ore Equivalent Schist. The objective was three-fold: to test the lateral correlatability of these schist units as determined by field relationships, to identify the geochemical signature of the schists and to test the validity of an Artificial Neural Network approach as an exploration tool. Two multidimensional datasets, respectively comprising 10 major and 18 trace elements, were constructed using selected published schist analyses. Both schist datasets were analyzed using self-organizing neural maps for visualizing and clustering high-dimensional geochemical data. Geochemical differences between the various schists were visualized using colored two-dimensional maps that can be visually and quantitatively interpreted. The results of this study confirm the lateral correlatability of the schist units evaluated in this communication. It was also found that each schist unit or portions of them represent a distinct geochemical signature that is related to true lithological variations. The results show that the Artificial Neural Network approach can be used as a powerful tool for regional mineral exploration in poly-deformed and metamorphosed terrains where identification of stratigraphic units through lateral correlation by means of fieldwork and petrography remains highly speculative. 相似文献
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