Formation and growth of normal faults at the divergent plate boundary in Iceland     

Agust Gudmundsson 《地学学报》1992,4(4):464-471

The divergent plate boundary in Iceland is characterized by 40–80 km long and 5–10 km wide swarms of tension fractures (∼10² m long) and normal faults (∼10³ m long). The upper part of the crust is mainly composed of lava flows, with abundant columnar joints that are mostly perpendicular to the lava contacts. The lava flows are horizontal at the surface of the rift zone but become tilted at the rate of 1° for every 150 m depth in the crust. At the surface of the rift zone the joints are vertical and parallel to the vertical principal stress. Because of tilting of the lava pile, the columnar joints become oblique to this stress, hence becoming potential shear fractures, and form echelon sets at greater depths in the crust. Theoretical considerations suggest that normal faults start to nucleate on sets of en echelon columnar joints and/or large-scale tension fractures at crustal depths of 0.5–1.5 km. The width (depth) must be the smallest (controlling) dimension of many faults. Nevertheless, there is a positive linear relation (r = 0.91) between maximum throw and length of the Holocene faults. If the faults grow as self-similar structures, the throw-length relationship can be explained by a similar relation between fault length and width.  相似文献   

On the structure and formation of fracture zones     

Agust Gudmundsson 《地学学报》1993,5(3):215-224

Studies of the oceanic fracture zones, as well as field observations of the on-land parts of a fracture zone in Iceland, show that there are numerous tension fractures, normal faults, small-scale grabens and dykes within, and trending subparallel with, the fracture zones. These structures indicate that, in addition to the shear displacement, there is considerable extension associated with the development of fracture zones and that many of them may be regarded as complex graben structures. The ridges surrounding the continents of Africa and Antarctica are examples of mid-ocean ridges that are moving away from the continental margins where they originated and therefore expanding. Here it is suggested that as the perimeter of an expanding ridge increases, the tensile stresses associated with the ridge extension may contribute to the formation of fracture zones.  相似文献   

Stratigraphy and paleomagnetism of a 2.9-km composite lava section in Eyjafjördur,Northern Iceland: a reconnaissance study     

Leo?KristjanssonEmail author  Agust?Gudmundsson  Bj?rn?S.?Hardarson 《International Journal of Earth Sciences》2004,93(4):582-595

Eight mountainside profiles in lava flows south of the fjord Eyjafjördur, Northern Iceland, were sampled for paleomagnetic studies. The sampling was concurrent with pilot stratigraphic mapping of the lava sequences in these and several supplementary profiles. The eight profiles are correlated with minor overlaps so that they form a composite section of 2.9 km thickness, estimated to cover the age range between about 9 and 5 million years ago (Hardarson et al. 1999). Paleomagnetic measurements made on 319 lavas generally yield primary remanence directions of high stability and within-unit consistency. Evidence for at least 17 reversals of the geomagnetic field is seen, as well as numerous field excursions. Frequent clustering of directions in successive lavas indicates that the volcanism was episodic.  相似文献   

The effects of layering and local stresses in composite volcanoes on dyke emplacement and volcanic hazards     

Agust Gudmundsson 《Comptes Rendus Geoscience》2005,337(13):1216-1222

Since most volcanic eruptions are fed by dykes, any assessment of volcanic hazards in an area must include an evaluation of the probability of injected dykes either reaching the surface or becoming arrested. Composite volcanoes are normally composed of alternating stiff (high Young's modulus) and soft (low Young's modulus) layers. Numerical models indicate that during unrest periods with magma-chamber inflation, the local stresses in composite volcanoes commonly prevent dyke-fed eruptions: while the stresses in the stiff layers may favour dyke propagation and seismogenic faulting, the local stresses in the soft layer remain seismically quiet and favour dyke arrest. Geodetic and field studies also indicate that most dykes never reach the surface, and that only a small fraction of the magma volume injected from a chamber erupts at the surface. I propose that for a dyke-fed eruption to occur, all the layers along the potential pathway of the dyke must have local stresses that favour magma-driven extension-fracture propagation. Thus, the stress field along the pathway must be homogenised. To cite this article: A. Gudmundsson, C. R. Geoscience 337 (2005).  相似文献   

Propagation pathways and fluid transport of hydrofractures in jointed and layered rocks in geothermal fields     

Agust Gudmundsson  Ingrid Fjeldskaar  Sonja L. Brenner 《Journal of Volcanology and Geothermal Research》2002,116(3-4)

Palaeofluid-transporting systems, observed as networks of mineral-filled veins in deeply eroded parts of extinct geothermal fields, indicate that hydrofractures commonly supply fluids to geothermal fields. Here we examine well-exposed vein networks that occur at crustal depths of around 1.5 km below the initial surface of the Tertiary lava pile in North Iceland. The veins are located in the damage zone of a major fault zone that dissects basaltic lava flows, the most common host rocks of geothermal fields in Iceland. The lava flows contain numerous weaknesses, particularly columnar (cooling) joints and contacts. For hydrofractures to supply fluids to geothermal fields, the fractures must be able to propagate, and transport fluids, to the surface. We explore hydrofracture pathway formation using boundary-element models of hydrofractures with fluid overpressure varying linearly from 10 MPa at the fracture centre to 0 MPa at the fracture tip (or the fluid front). The hydrofractures propagate through a vertically jointed and horizontally layered pile of lava flows with a general rock-matrix Young’s modulus of 1×10¹⁰ Pa and a Poisson’s ratio of 0.25. The joints and contacts between layers are modelled as internal springs, each with a stiffness (‘strength’) of 6 MPa/m. The location and sizes of discontinuities, as well as the location of the hydrofracture tip, vary between the models. The results indicate that tensile stresses generated at the tip of an overpressured hydrofracture can open up horizontal and vertical discontinuities out to a considerable distance from the tip, and that these discontinuities eventually link up to form the hydrofracture pathway. Analytical models indicate that for a hot spring of a given yield associated with a fault, the dimensions of the fluid-transporting part of the fault are likely to be similar for a typical normal fault and a strike-slip fault. Also, a hot spring of yield 180 l/s (the maximum in the low-temperature fields of Iceland) can be supplied through a hydrofracture of aperture 3 mm and trace length 1.2 m. These dimensions are very similar to those of typical veins in the studied networks. Buoyancy, rather than excess pressure in the fluid source, appears to be the primary driving force of hydrofractures in the geothermal fields of Iceland.  相似文献   

Bottom-up and top-down control of seagrass overgrazing by the sea urchin Tripneustes gratilla     

Agustín Moreira-Saporiti  Dieuwke Hoeijmakers  Hauke Reuter  Flower E. Msuya  Katrin Gese  Mirta Teichberg 《Marine Ecology》2023,44(1):e12734

The lack of top-down control on Tripneustes gratilla, a sea urchin commonly known to graze on seagrass, and the bottom-up control of its feeding preference, led to the overgrazing of seagrass meadows of the species Thalassodendron ciliatum in Changuu Island (Zanzibar Archipelago). The impact of overgrazing on seagrasses was assessed by mapping the presence of grazed versus non-grazed seagrass patches in the study site, while the top-down control on T. gratilla was assessed by measuring the abundance of its fish predators. The feeding preference and distribution of T. gratilla were characterized by calculating the electivity indexes for each seagrass species and measuring sea urchin density, respectively. Approximately half of the patches of T. ciliatum were overgrazed, while predatory fishes of T. gratilla were absent from the site. The Vanderploeg and Scavia's Relativized Electivity Index indicated that T. gratilla had a feeding preference for T. ciliatum, which was also supported by higher urchin densities within T. ciliatum dominated patches. Bottom-up control of grazing activity was observed by quantifying and analyzing morphological, nutritional, and the chemical defense traits of the seagrass in relation to feeding preference and urchin density. Feeding was positively correlated to the seagrass tissue C:P ratio (ρ = 0.9), whereas urchin density showed no correlations. The bottom-up control of the feeding preference and agglomeration of T. gratilla in T. ciliatum meadows, together with the lack of evidence of substantial top-down control and the long recovery time of T. ciliatum led to the overgrazing of this species at this site. Overgrazing, therefore, was shown to be the result of multiple factors ranging from the traits of the seagrass and feeding preference of T. gratilla, to the abundance of predators in this area.  相似文献   

Volcanoes as elastic inclusions: Their effects on the propagation of dykes,volcanic fissures,and volcanic zones in Iceland     

Ruth E.B. Andrew  Agust Gudmundsson 《Journal of Volcanology and Geothermal Research》2008

Mechanically, many volcanoes may be regarded as elastic inclusions, either softer (with a lower Young's modulus) or stiffer (with a higher Young's modulus) than the host-rock matrix. For example, many central volcanoes (stratovolcanoes, composite volcanoes) are composed of rocks that are softer than the crustal segments that host them. This is particularly clear in Iceland where central volcanoes are mostly made of soft rocks such as rhyolite, pyroclastics, hyaloclastites, and sediments whereas the host rock is primarily stiff basaltic lava flows. Most active central volcanoes also contain fluid magma chambers, and many have collapse calderas. Fluid magma chambers are best modelled as cavities (in three dimensions) or holes (in two dimensions), entire calderas as holes, and the ring faults themselves, which commonly include soft materials such as breccias, as soft inclusions. Many hyaloclastite (basaltic breccias) mountains partly buried in the basaltic lava pile also function as soft inclusions. Modelling volcanoes as soft inclusions or holes, we present three main numerical results. The first, using the hole model, shows the mechanical interaction between all the active central volcanoes in Iceland and, in particular, those forming the two main clusters at the north and south end of the East Volcanic Zone (EVZ). The strong indication of mechanical interaction through shared dykes and faults in the northern cluster of the EVZ is supported by observations. The second model, using a soft inclusion, shows that the Torfajökull central volcano, which contains the largest active caldera in Iceland, suppresses the spreading-generated tensile stress in its surroundings. We propose that this partly explains why the proper rift zone northeast of Torfajökull has not managed to propagate through the volcano. Apparently, Torfajökull tends to slow down the rate of southwest propagation of the rift-zone part of the EVZ. The third model, again using a soft inclusion, indicates how the lateral propagation of a segment of the 1783 Laki fissure became arrested in the slopes of the hyaloclastite mountain Laki.  相似文献   

Seismic hazard in Buenos Aires,Argentina: A preliminary study on the effects of long‐distance earthquakes on tall buildings          下载免费PDF全文

Raúl D. Bertero  Sebastián Vaquero  Juan M. Mussat  Agustín Bertero 《地震工程与结构动力学》2018,47(11):2333-2339

The 2015 Illapel earthquake produced self‐evacuation of tall buildings in the city of Buenos Aires, Argentina, located 1280 km away from the epicenter. The ground motions in Buenos Aires due to the main event (M_w 8.3) and its aftershocks were registered by a new seismometer. The data collected allowed to estimate the maximum story drift ratios and top floor accelerations for tall buildings in Buenos Aires. The similarities between the response spectra and the Fourier amplitude spectra for the mainshock and its aftershocks show the influence that the dynamic properties of the 300‐m soil deposit have on the large acceleration amplification produced in these groups of buildings.  相似文献   

Non-stationary future return levels for extreme rainfall over Extremadura (southwestern Iberian Peninsula)     

Francisco Javier Acero  Sylvie Parey  Thi Thu Huong Hoang  Didier Dacunha-Castelle  José Agustín García  María Cruz Gallego 《水文科学杂志》2017,62(9):1394-1411

Based on a previous study for temperature, a new method for the calculation of non-stationary return levels for extreme rainfall is described and applied to Extremadura, a region of southwestern Spain, using the peaks-over-threshold approach. Both all-days and rainy-days-only datasets were considered and the 20-year return levels expected in 2020 were estimated taking different trends into account: first, for all days, considering a time-dependent threshold and the trend in the scale parameter of the generalized Pareto distribution; and second, for rainy days only, considering how the mean, variance, and number of rainy days evolve. Generally, the changes in mean, variance and number of rainy days can explain the observed trends in extremes, and their extrapolation gives more robust estimations. The results point to a decrease of future return levels in 2020 for spring and winter, but an increase for autumn.  相似文献   

Volcanic stratigraphy of a high-altitude Mammuthus columbi (Tlacotenco,Sierra Chichinautzin), Central México     

Marie-Noelle Guilbaud  Lilia Arana-Salinas  Claus Siebe  Luis Alberto Barba-Pingarrón  Agustín Ortiz 《Bulletin of Volcanology》2015,77(3):1-16

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