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991.
The fissure swarm of the Askja volcanic system along the divergent plate boundary of N Iceland 总被引:1,自引:1,他引:0
Ásta Rut Hjartardóttir Páll Einarsson Haraldur Sigurdsson 《Bulletin of Volcanology》2009,71(9):961-975
Divergent plate boundaries, such as the one crossing Iceland, are characterized by a high density of subparallel volcanic fissures and tectonic fractures, collectively termed rift zones, or fissure swarms when extending from a specific volcano. Volcanic fissures and tectonic fractures in the fissure swarms are formed during rifting events, when magma intrudes fractures to form dikes and even feeds fissure eruptions. We mapped volcanic fissures and tectonic fractures in a part of the divergent plate boundary in northern Iceland. The study area is ~1,800 km2, located within and north of the Askja central volcano. The style of fractures changes with distance from Askja. Close to Askja the swarm is dominated by eruptive fissures. The proportion of tectonic fractures gets larger with distance from Askja. This may indicate that magma pressure is generally higher in dikes close to Askja than farther away from it. Volcanic fissures and tectonic fractures are either oriented away from or concentric with the 3–4 identified calderas in Askja. The average azimuth of fissures and fractures in the area deviates significantly from the azimuth perpendicular to the direction of plate velocity. As this deviation decreases gradually northward, we suggest that the effect of the triple junction of the North American, Eurasian and the Hreppar microplate is a likely cause for this deviation. Shallow, tectonic earthquakes in the vicinity of Askja are often located in a relatively unfractured area between the fissure swarms of Askja and Kverkfjöll. These earthquakes are associated with strike-slip faulting according to fault plane solutions. We suggest that the latest magma intrusions into either the Askja or the Kverkfjöll fissure swarms rotated the maximum stress axis from being vertical to horizontal, causing the formation of strike-slip faults instead of the dilatational fractures related to the fissure swarms. The activity in different parts of the Askja fissure swarm is uneven in time and switches between subswarms, as shown by a fissure swarm that is exposed in an early Holocene lava NW of Herðubreið but disappears under a younger (3500–4500 BP) lava flow. We suggest that the location of inflation centres in Askja central volcano controls into which part of the Askja fissure swarm a dike propagates. The size and amount of fractures in the Kollóttadyngja lava shield decrease with increasing elevation. We suggest that this occurred as the depth to the propagating dike(s) was greater under central Kollóttadyngja than under its flanks, due to topography. 相似文献
992.
Víctor Leiva Antonio Sanhueza José M. Angulo 《Stochastic Environmental Research and Risk Assessment (SERRA)》2009,23(3):299-307
In this article, we develop a new model based on the Birnbaum-Saunders distribution that results to be both useful and practical
for environmental sciences. The density, distribution and hazard functions, moments and properties of this new model are presented.
A graphical analysis of the density is also provided. Furthermore, we estimate parameters, propose asymptotic inference and
discuss influence diagnostics by using likelihood methods for the new distribution. An illustrative example with real data
related to water quality indicates the adequacy on the new distribution. 相似文献
993.
R. Sigbjörnsson J. Th. Snæbjörnsson S. M. Higgins B. Halldórsson S. Ólafsson 《Bulletin of Earthquake Engineering》2009,7(1):113-126
This preliminary study aims to investigate a M
w
6.3 earthquake that occurred in South Iceland on Thursday 29 May 2008 at 15:45 UTC. The epicentre was in the Olfus District
between the towns of Selfoss and Hveragerdi. This study examines the data recorded and the damage observed immediately after
the event. Horizontal accelerations of up to 80%g were recorded in the epicentral region and there is visual evidence that
the vertical acceleration exceeded 1 g. The PGA data is compared to a ground motion estimation model developed for the South
Iceland earthquakes in June 2000. In general the basic properties of this event are found to be similar to the characteristics
of the South Iceland earthquakes in June 2000. The duration of strong-motion is short and the intensity attenuates rapidly
with increasing distance. The earthquake action resisted by buildings in the near fault area is inspected through evaluation
of elastic as well as inelastic response spectra. The vast majority of structures seemed to withstand the strong-motion fairly
competently and without significant visual damage due firstly to the low-rise, predominantly reinforced concrete or timber,
style of buildings. Secondly, the short duration of strong-motion contributed to the endurance of structures. 相似文献
994.
Hernando Tavera Isabel Bernal Fleur O. Strasser Maria C. Arango-Gaviria John E. Alarcón Julian J. Bommer 《Bulletin of Earthquake Engineering》2009,7(1):71-111
A Mw 7.9 earthquake event occurred on 15 August 2007 off the coast of central Peru, 60 km west of the city of Pisco. This event
is associated with subduction processes at the interface of the Nazca and South American plates, and was characterised by
a complex source mechanism involving rupture on two main asperities, with unilateral rupture propagation to the southeast.
The rupture process is clearly reflected in the ground motions recorded during this event, which include two separate episodes
of strong shaking. The event triggered 18 accelerographic stations; the recordings are examined in terms of their characteristics
and compared to the predictions of ground-motion prediction equations for subduction environments, using the maximum-likelihood-based
method of Scherbaum et al. (Bull Seismol Soc Am 94(6):2164–2185, 2004). Additionally, macroseismic observations and damage
patterns are examined and discussed in the light of local construction practices, drawing on field observations gathered during
the post-earthquake reconnaissance missions. 相似文献
995.
Shear effects on hollow section piers under seismic actions: experimental and numerical analysis 总被引:1,自引:0,他引:1
Raimundo Delgado Pedro Delgado Nelson Vila Pouca António Arêde Patrício Rocha Aníbal Costa 《Bulletin of Earthquake Engineering》2009,7(2):377-389
Shear effects are often a very important issue on the seismic behaviour of piers, particularly for hollow section bridge piers.
In fact, for this type of piers the cyclic response is similar to that of a structural wall in which both the transverse reinforcement
ratio and the detailing can play an important role on its performance, even likely to be determinant in terms of the failure
mechanism. On the other hand, codes and design guidelines are usually very conservative concerning shear capacity in order
to avoid any shear failure mechanism likely to trigger well known catastrophic consequences. Therefore, research studies on
this topic are still needed for a better understanding of pier cyclic shear response and also for improvement of the performance
under seismic actions. Pursuing this general objective, this paper partially reports on an experimental/numerical campaign
carried out on 1:4 reduced scale bridge piers in order to highlight and investigate shear-type problems. Within the scope
of this paper, two specimens types were selected having equal rectangular hollow section (900 × 450 mm2, 75 mm thick) but different transverse reinforcement detailing, namely one with a single stirrup per wall (representative
of typical bridge construction without seismic design requirements) and another with multiple stirrups, according to Eurocode
8 provisions. Numerical simulations of the experimental results were also conducted aiming at contributing for complete and
consistent interpretations of experimental results. Detailed modelling was performed allowing for realistic simulations of
the non linear behaviour, particularly suitable when a significant shear component is involved. Therefore, the numerical strategy
was based on a detailed 3D FEM discretization using a two-scalar variable damage model for the concrete constitutive law and
a suitable cyclic behaviour law for steel bars represented by truss elements. Results have shown that shear deformation and
failure modes are well simulated, while providing detailed insight concerning concrete damage pattern and distribution of
yielding on the transverse and longitudinal reinforcement. 相似文献
996.
We present a new development in the asymmetric continuum theory with the shear oscillations (twist motions) and independent
spin; these motions (displacement velocities and spin) can be shifted in phase to describe the independent rebound processes.
Our approach provides an extension of the asymmetric continuum theory by including the microfragmentation processes with a
double transport which may appear in an advanced fracture process under very high load. The related nonlinear equations, leading
to soliton solutions, are derived. 相似文献
997.
Paola Tuccimei Michele Soligo Joaquin Ginés Angel Ginés Joan Fornós Jan Kramers Igor Maria Villa 《地球表面变化过程与地形》2010,35(7):782-790
Phreatic overgrowths on speleothems (POS) are carbonate formations deposited at the water table of caves in unique karstic coastal settings having morphologies that can be directly related to sea level at the time of formation. The U‐Th ages of calcite and aragonite overgrowths collected from the modern water table in coastal caves on Mallorca (Cova de Cala Varques A and Cova des Pas de Vallgornera) were determined using high‐precision MC‐ICPMS techniques. U‐Th ages indicate that phreatic carbonate deposition occurred between ca 2·8 and at least 0·6 ka BP and are in accord with an archeologically estimated age of 3·7–3·0 ka BP for a drowned prehistoric construction at a depth of 1 m below current sea level in a cave from the same area. Speleothem δ13C and δ18O and chemical composition of cave pools provide supportive evidence that POS reflect mixing between seawater and brackish water table. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
998.
P.A. Przybylski E. Głowniak J.G. Ogg P. Ziółkowski M. Sidorczuk J. Gutowski M. Lewandowski 《Earth and Planetary Science Letters》2010,289(3-4):417-432
A nearly continuous magnetostratigraphic polarity pattern was compiled from several ammonite-zoned carbonate successions of southern Poland and from a composite magnetostratigraphy from the Iberian Range of Spain. The array of sections spans the middle two-thirds of the Oxfordian within the Sub-Mediterranean Province (Cordatum through Bifurcatus ammonite zones). The average paleopole calculated from eight of these Polish sections is at 78.5°N, 184.9°E (δp = 2.6°, δm = 3.5°). The Sub-Mediterranean polarity pattern is consistent with an independent polarity pattern derived from the Boreal-realm sections of the British Isles, and improves the inter-correlation between these faunal realms. Cycle stratigraphy published for these ammonite subzones from southern France enabled temporal scaling of the polarity pattern, thereby facilitating correlation to marine magnetic anomalies M28 through M33 as modeled from deep-tow magnetometer surveys in the Western Pacific. The bases of the Middle and Upper Oxfordian substages as defined in the Sub-Mediterranean zonation in Poland correspond approximately to chrons M33 and M29 of that Pacific M-sequence model. 相似文献
999.
R.G. Ezquer M.A. Cabrera J. López M. Albornoz E. Garay 《Journal of Atmospheric and Solar》2010,72(2-3):206
1000.
Stéphane Couturier Jean‐François Mas Erna López‐Granados Jorge Benítez Valdemar Coria‐Tapia Álvaro Vega‐Guzmán 《Singapore journal of tropical geography》2010,31(2):163-179
The accuracy of the Mexican National Forest Inventory (NFI) map is derived in four distinct ecogeographical areas, using an assessment design tailored for the project. A main achievement of the design was to integrate the high diversity of classes encompassed at the most detailed subcommunity level of the classification scheme within a cost‐controlled statistically sound assessment. A hybrid double sampling strategy was applied to the 2.5 million‐ha study area. A total of 5955 reference sites were verified against their NFI map label. The availability of detailed quasi‐synchronous reference data for the 2000 Landsat‐derived NFI and the high diversity of mapped classes allowed a careful thematic analysis on the selected regions, relevant for national extrapolation. Global accuracy estimates of 64–78 per cent were registered among the four ecogeographical areas (two with mainly temperate climate and the other two with mainly tropical climate), with the lower accuracy levels found in areas more densely covered with forests. According to the estimates, the NFI map tends to underestimate the presence of temperate forest (especially oak) and overestimate the presence of tropical forest in the areas investigated. The analysis of confusions reveals difficulties in unambiguously interpreting or labelling forests with secondary vegetation, herbaceous and/or shrub‐like vegetation as well as distinguishing between aquatic vegetation types. The design proved useful from the perspective of accuracy assessments of regional maps in biodiverse regions. 相似文献