165 research outputs found
The effect of grain texture on the occurrence of microstructural properties in subglacial till
No full textIn this paper, we examine whether till grain size affects the range and occurrence of micromorphological features associated with subglacial shear. Our till samples were collected from two glaciers in Iceland, and varied in texture from a coarse, sandy clast-rich till (Fjallsjökull) to a fine-grained silty-sandy till (Vestari-Hagafellsjökull). We found a wide range of deformational microstructures that included skelsepic plasmic fabric, intraclasts of pre-existing eroded bedrock (basalt) and weathered clay and ‘mini-shear zones’ between clasts. We classified our micromorphological data into three classes; rotational, intermediate and linear. In addition to these observations, we performed extensive microfabric analysis at different scales on all of our samples. We found that the coarse-grained till contained a greater number and variety of microstructures than the fine-grained till. In addition, the fine-grained till showed a distinct lack of rotational structures that we attribute to the lack of significantly sized clasts in the matrix. We argue that the varied texture of the coarse-grained till provides a greater degree of perturbation within the shearing layer and so more distinct microstructures form. In a more fine-grained till, shearing is more homogeneous since there are less perturbations in the matrix and this leads to a more singular kind of microstructure. Our observations suggest that subglacial shear occurs within a multi-layered patchwork of different grain sizes, competence and pore water pressures. It is these factors that are so crucial in determining the occurrence and type of microstructural evidence we see in subglacial tills
Report on An investigation of the dynamics driving the responses of ice caps to climatic forcing: a GPR survey of Vestari-Hagafellsjökull
No full textPerformance of fiber reinforcement in completely decomposed granite
No full textAdding discrete fibers to soils can improve their strength; however, fiber reinforcement remains scarce in practice. Previous studies on the performance of soils reinforced with discrete fibers consist mainly of laboratory studies with either clay or, most often, uniform sand as the host soil, so there is a lack of data on other types of soils such as weathered soils, which tend to be well graded. Unlike uniform soils, which are generally dilative, well-graded soils usually show a contractive behavior. This study examines the effect of adding fibers to a completely decomposed granite (CDG) typical of many residual soils, which has the characteristics to be sensitive to material and sample preparation and also to be compressive during shearing. It is found that adding discrete fibers to the CDG homogenizes it because the reinforced soil is not sensitive to the method of material or sample preparation. It is also found that, despite its compressive nature, fibers mobilize extra strength compared with the unreinforced soil, and this effect does not reduce at large confining stresses
A Nonextensive Statistical Physics Analysis of the 1995 Kobe, Japan Earthquake
No full textThis paper presents an analysis of the distribution of earthquake magnitudes for the period 1990–1998 in a broad area surrounding the epicenter of the 1995 Kobe earthquake. The frequency–magnitude distribution analysis is performed in a nonextensive statistical physics context. The nonextensive parameter q M , which is related to the frequency-magnitude distribution, reflects the existence of long-range correlations and is used as an index of the physical state of the studied area. Examination of the possible variations of q M values is performed during the period 1990–1998. A significant increase of q M occurs some months before the strong earthquake on April 9, 1994 indicating the start of a preparation phase prior to the Kobe earthquake. It should be noted that this increase coincides with the occurrence of six seismic events. Each of these events had a magnitude M = 4.1. The evolution of seismicity along with the increase of q M indicate the system’s transition away from equilibrium and its preparation for energy release. It seems that the variations of q M values reflect rather well the physical evolution towards the 1995 Kobe earthquake
Relationship between topography, rates of extension and mantle dynamics in the actively-extending Italian Apennines
No full textTo investigate the mechanism driving active extension in the central and southern Italian Apennines and the geography of seismic hazard, we compare spatial variations in upper crustal strain-rate measured across exposed fault scarps since 15 ± 3 ka with data on cumulative upper-crustal strain and topographic elevation, and free-air gravity, P-wave tomography and SKS splitting delay times that are a proxy for strain in the mantle. High extensional strain-rates across the Apennines since 15 ± 3 ka (0.4–3.1 mm/yr along 90 km transects) occur in two areas (Lazio-Abruzzo; SE Campania and Basilicata) where values for finite extensional strains that have developed since 2–3 Ma are highest (2–7 km cumulative throw), and where mean elevation in 5 × 90 km NE–SW boxes is > 600 m; the intervening area (NW Campania and Molise) with < 600 m mean elevation in 5 × 90 km boxes has extension-rates < 0.4 mm/yr and lower values for finite extensional strains (< 2 km cumulative throw). These two areas with high upper-crustal strain-rates overlie mantle that has relatively-long spatially-interpolated SKS delay times (1.2–1.8 s) indicating relatively-high mantle strains and free-air gravity values (140–160 mGals); the intervening area of lower extension-rate has shorter spatially-interpolated SKS delay times (0.8–1.2 s) and lower free-air gravity values (120 mGals). The two areas with high upper crustal strain-rates and strain, mean elevation, and mantle strain, coincide with the northern and southern edges of a slab window in the Tyrrhenian–Apennines subducting plate that has been inferred from published P-wave tomography. Together these correlations suggest that dynamic support of the topography by mantle flow through the slab window may control the present day upper crustal strain-rate field in the Apennines and the geography of seismic hazard in the region
Electric potential changes prior to shear fracture in dry and saturated rocks
No full textElectric potential changes before shear rupture were measured using Darley Dale sandstone (quartz-rich) and Icelandic basalt (quartz-free) on both dry specimens and in the presence of pore fluid. We find that electric potential changed markedly just prior to dynamic rupture in dry and saturated sandstones and saturated basalt but we did not detect precursory signals in dry basalt. The absence of signals in dry basalt provides strong evidence that the piezoelectric effect and electrokinetic effect are dominant sources for precursory signals. Moreover we find that the amplitude of the precursory signals due to electrokinetic effect in saturated sandstone were as large as the coseismic signals. We propose that this signal is caused by accelerating evolution of dilatancy as cracks grow in the rock before rupture, resulting in water flow into the dilatant region with an electric current produced concurrently
Temperature and pore pressure effects on the shear strength of granite in the brittle-plastic transition regime
No full textCurrently published lithospheric strength profiles lack constraints from experimental data for shear failure of typical crustal materials in the brittle-plastic transition regime in wet environments. Conventional triaxial shear fracture experiments were conducted to determine temperature and pore pressure effects on shear fracture strength of wet and dry Tsukuba granite. Experimental conditions were 70MPa < P-C < 480MPa, 10MPa < P-p < 300MPa, 25 A degreesC < T < 480 degreesC, at a constant strain rate of 10(-5)s(-1). An empirical relation is proposed which can predict the shear strength of Tsukuba granite, within the range of experimental conditions. Mechanical pore pressure effects are incorporated in the effective stress law. Chemical effects are enhanced at temperatures above 300 degreesC. Below 300 degreesC wet and dry granite strengths are temperature insensitive and wholly within the brittle regime. Above 400 degreesC, semi-brittle effects and ductility are observed
Horizontal strain-rates and throw-rates across breached relay zones, central Italy: implications for the preservation of throw deficits at points of normal fault linkage
No full textIn order to investigate the relationship between the throws and 3D orientation of breaching faults crossing relay zones, kinematic data, throw-rates and total throws have been measured for an active normal fault in the Italian Apennines that displays a relay zone at its centre. The c. 0.8 km long breaching fault dips at 67 ± 5° and strikes obliquely to c. 2–3 km long faults outside the relay zone which dip at 61 ± 5°. Total throws of pre-rift limestone define a throw profile with a double maximum (370 ± 50 m; 360 ± 50 m) separated by an area of lower throw (100 ± 50 m) where the breaching fault is growing. Throw-rates implied by offsets across bedrock scarps of Late Pleistocene–Holocene landforms (15 ± 3 ka) are higher across the breaching fault (0.67 ± 0.13 mm/yr) than for locations of throw maxima on the neighbouring faults (0.38 ± 0.07 mm/yr; 0.55 ± 0.11 mm/yr). The deficit in total throw will be removed in 0.68–1.0 Myr if these deformation rates continue. To investigate why the highest throw-rates occur in the location with lowest total throw, Kostrov horizontal strain-rate tensors were calculated in 1 × 2 km boxes. We show that the oblique strike and relatively high dip of the breaching fault mean that it must have a relatively high throw-rate in order for it to have a horizontal strain-rate concomitant with its position at the centre of the overall fault. We show that whether throw minima at locations of fault linkage are preserved during progressive fault slip depends on the 3D orientation of the breaching fault. We use the above to discuss the longevity of throw deficits and multiple throw maxima along faults in relation to seismic hazard and landscape evolution
Sammonds, P. B.
No full textSee entry in St. Clair County volume 1, page 20: https://digital.archives.alabama.gov/digital/collection/voter/id/289
Volcanic Hazard from Iceland: analysis and implications of the Eyjafjallajokull eruption
No full text- …
