Strain localization and the onset of dynamic weakening in calcite fault gouge

To determine the role of strain localization during dynamic weakening of calcite gouge at seismic slip rates, single-slide and slide–hold–slide experiments were conducted on 2–3-mm thick layers of calcite gouge at normal stresses up to 26 MPa and slip rates up to 1 m s−1. Microstructures were analyzed from short displacement (<35 cm) experiments stopped prior to and during the transition to dynamic weakening. In fresh calcite gouge layers, dynamic weakening occurs after a prolonged strengthening phase that becomes shorter with increasing normal stress and decreasing layer thickness. Strain is initially distributed across the full thickness of the gouge layer, but within a few millimeters displacement the strain becomes localized to a boundary-parallel, high-strain shear band c. 20 μm wide. During the strengthening phase, which lasts between 3 and 30 cm under the investigated conditions, the shear band broadens to become c. 100 μm wide at peak stress. The transition to dynamic weakening in calcite gouges is associated with the nucleation of micro-slip surfaces dispersed throughout the c. 100 μm wide shear band. Each slip surface is surrounded by aggregates of extremely fine grained and tightly packed calcite, interpreted to result from grain welding driven by local frictional heating in the shear band. By the end of dynamic weakening strain is localized to a single 2–3-μm wide principal slip surface, flanked by layers of recrystallized gouge. Calcite gouge layers re-sheared following a hold period weaken nearly instantaneously, much like solid cylinders of calcite marble deformed under the same experimental conditions. This is due to reactivation of the recrystallized and cohesive principal slip surface that formed during the first slide, reducing the effective gouge layer thickness to a few microns. Our results suggest that formation of a high-strain shear band is a critical precursor to dynamic weakening in calcite gouges. Microstructures are most compatible with dynamic weakening resulting from a thermally triggered mechanism such as flash heating that requires both a high degree of strain localization and a minimum slip velocity to activate. The delayed onset of dynamic weakening in fresh calcite gouge layers, particularly at low normal stresses, may inhibit large coseismic slip at shallow crustal levels in calcite-bearing fault zones

Rough Faults, Distributed Weakening, and Off-Fault Deformation

We report systematic spatial variations of fault rocks along non-planar strike-slip faults 11 cross-cutting the Lake Edison Granodiorite, Sierra Nevada, California (Sierran wavy fault) and 12 Lobbia outcrops of the Adamello Batholith in the Italian Alps (Lobbia wavy fault). In the case of 13 the Sierran fault, pseudotachylyte formed at contractional fault bends, where it is found as thin 14 (1-2 mm) fault-parallel veins. Epidote and chlorite developed in the same seismic context as the 15 pseudotachylyte and are especially abundant in extensional fault bends. We argue that the 16 presence of fluids, as illustrated by this example, does not necessarily preclude the development 17 of frictional melt. In the case of the Lobbia fault, pseudotachylyte thickness varies along the 18 length of the fault, but the pseudotachylyte veins thicken and pool in extensional bends. We 19 conduct a quantitative analysis of fault roughness, microcrack distribution, stress, and friction 20 along the Lobbia fault. 21 Numerical modeling results show that opening in extensional bends and localized thermal 22 weakening in contractional bends counteract resistance encountered by fault waviness, resulting 23 in an overall weaker fault than suggested by the corresponding static friction coefficient. The 24 models also predict static stress redistribution around bends in the faults which are consistent 25 with distributions of microcracks, indicating significant elastic and inelastic strain energy is 26 dissipated into the wall rocks due to non-planar fault geometry. Together these observations suggest that damage and energy dissipation occurs along the entire non-planar fault during slip, 28 rather than being confined to the region close to the dynamically propagating crack tip

Detection of high-velocity material from the wind-wind collision zone of Eta Carinae across the 2009.0 periastron passage

We report near-IR spectroscopic observations of the Eta Carinae massive binary system during 2008-2009 using VLT/CRIRES. We detect a strong, broad absorption wing in He I 10833 extending up to -1900 km/s across the 2009.0 spectroscopic event. Archival HST/STIS ultraviolet and optical data shows a similar high-velocity absorption (up to -2100 km/s) in the UV resonance lines of Si IV 1394, 1403 across the 2003.5 event. UV lines from low-ionization species, such as Si II 1527, 1533 and C II 1334, 1335, show absorption up to -1200 km/s, indicating that the absorption with v from -1200 to -2100 km/s originates in a region markedly faster and more ionized than the nominal wind of the primary star. Observations obtained at the OPD/LNA during the last 4 spectroscopic cycles (1989-2009) also display high-velocity absorption in He I 10833 during periastron. Based on the OPD/LNA dataset, we determine that material with v < -900 km/s is present in the phase range 0.976 < phi < 1.023 of the spectroscopic cycle, but absent in spectra taken at phi < 0.947 and phi > 1.049. Therefore, we constrain the duration of the high-velocity absorption to be 95 to 206 days (or 0.047 to 0.102 in phase). We suggest that the high-velocity absorption originates from shocked gas in the wind-wind collision zone, at distances of 15 to 45 AU in the line-of-sight to the primary star. Using 3-D hydrodynamical simulations of the wind-wind collision zone, we find that the dense high-velocity gas is in the line-of-sight to the primary star only if the binary system is oriented in the sky so that the companion is behind the primary star during periastron, corresponding to a longitude of periastron of omega ~ 240 to 270 degrees. We study a possible tilt of the orbital plane relative to the Homunculus equatorial plane and conclude that our data are broadly consistent with orbital inclinations in the range i=40 to 60 degrees.Comment: 18 pages, 15 figures, accepted for publication in A&A; high-resolution PDF version available also at http://www.mpifr.de/staff/jgroh/etacar.htm

Ultraviolet Imaging Observations of the cD Galaxy in Abell 1795: Further Evidence for Massive Star Formation in a Cooling Flow

We present images from the Ultraviolet Imaging Telescope of the Abell 1795 cluster of galaxies. We compare the cD galaxy morphology and photometry of these data with those from existing archival and published data. The addition of a far--UV color helps us to construct and test star formation model scenarios for the sources of UV emission. Models of star formation with rates in the range \sim5-20M_{\sun}yr$^{-1}$ indicate that the best fitting models are those with continuous star formation or a recent ($\sim4$ Myr old) burst superimposed on an old population. The presence of dust in the galaxy, dramatically revealed by HST images complicates the interpretation of UV data. However, we find that the broad--band UV/optical colors of this cD galaxy can be reasonably matched by models using a Galactic form for the extinction law with $E_{B-V}=0.14$. We also briefly discuss other objects in the large UIT field of view.Comment: To appear in the Astrophysical Journal. 14 AAS preprint style pages plus 7 figure

Mycobacterium avium paratuberculosis infection of calves – The impact of dam infection status

Johne’s disease, caused by Mycobacterium avium subsp. paratuberculosis (MAP), is a chronic condition of dairy cattle, and is endemic in the UK. Lack of understanding of the relative importance of different transmission routes reduces the impact of control scheme recommendations. The long incubation period for Johne’s disease makes evaluation of control schemes difficult, and so this long-term cohort study offers a rare and valuable insight into the disease epidemiology. A longitudinal study was carried out following a cohort of 440 UK dairy cows in 6 herds recruited in 2012-2013. Individuals entering the milking herd were routinely monitored for the presence of MAP using quarterly milk ELISA testing. Using a Cox proportional-hazards regression model the relationship between time until first detection of infection and dam MAP status was investigated. We then compared the magnitude of the effect of dam status with that of other risk factors in order to understand its relative importance. Dam status was found to be the only observed factor that was significantly associated with time to an individual testing MAP-positive (p = 0.012). When compared to negative dams, we found a marginally significant effect of having a positive dam at time of calving, that increased the hazard of an individual testing positive by a factor of 2.6 (95% confidence interval: 0.89–7.79, p = 0.081). Further positive associations were found with dams becoming positive after the birth of the subject; a dam seroconverting within 12 months post parturition being associated with a 3.6 fold increase in hazard (95% confidence interval: 1.32–9.77, p = 0.013), and dams seroconverting more than a year after calving increased the hazard by a factor of 2.8 (95% confidence interval: 1.39–5.76, p = 0.004). These results suggest that cows may be transmitting MAP to their offspring at an earlier stage than had previously been thought, and so raise important questions about how this transmission may be occurring. The results of the study may have important practical implications for the management on-farm of the offspring of MAP-positive animals, with the potential to vastly reduce the time required to eliminate this chronic disease

Catastrophic emplacement of giant landslides aided by thermal decomposition: Heart Mountain, Wyoming

The Heart Mountain landslide of northwest Wyoming is the largest known sub-aerial landslide on Earth. During its emplacement more than 2000 km3 of Paleozoic sedimentary and Eocene volcanic rocks slid >45 km on a basal detachment surface dipping 2°, leading to 100 yr of debate regarding the emplacement mechanisms. Recently, emplacement by catastrophic sliding has been favored, but experimental evidence in support of this is lacking. Here we show in friction experiments on carbonate rocks taken from the landslide that at slip velocities of several meters per second CO2 starts to degas due to thermal decomposition induced by flash heating after only a few hundred microns of slip. This is associated with the formation of vesicular degassing rims in dolomite clasts and a crystalline calcite cement that closely resemble microstructures in the basal slip zone of the natural landslide. Our experimental results are consistent with an emplacement mechanism whereby catastrophic slip was aided by carbonate decomposition and release of CO2, allowing the huge upper plate rock mass to slide over a ‘cushion’ of pressurized material

Dietary intake in post-menopausal women

Si has been suggested as an essential element, and may be important in optimal bone, skin and cardiovascular health. However, there are few estimates of dietary Si intakes in man, especially in a UK population. Following the development of a UK food composition database for Si, the aim of the present study was to investigate dietary intakes of Si amongst healthy women aged over 60 years and to identify important food sources of Si in their diet. Healthy, post-menopausal female subjects (&gt;60 years of age; n 209) were recruited from the general population around Dundee, Scotland as part of an unrelated randomised controlled intervention study where dietary intake was assessed using a self-administered, semi-quantitative food-frequency questionnaire at five time-points over a 2-year period. Food composition data on the Si content of UK foods was used to determine the Si content of food items on the food-frequency questionnaire. Mean Si intake was 18&middot;6 (sd 4&middot;6) mg and did not vary significantly across the 2 years of investigation. Cereals provided the greatest amount of Si in the diet (about 30%), followed by fruit, beverages (hot, cold and alcoholic beverages combined) and vegetables; together these foods provided over 75% about Si intake. Si intakes in the UK appear consistent with those reported previously for elderly women in Western populations, but lower than those reported for younger women or for men.<br /

Quantum and semiclassical study of magnetic anti-dots

We study the energy level structure of two-dimensional charged particles in inhomogeneous magnetic fields. In particular, for magnetic anti-dots the magnetic field is zero inside the dot and constant outside. Such a device can be fabricated with present-day technology. We present detailed semiclassical studies of such magnetic anti-dot systems and provide a comparison with exact quantum calculations. In the semiclassical approach we apply the Berry-Tabor formula for the density of states and the Borh-Sommerfeld quantization rules. In both cases we found good agreement with the exact spectrum in the weak magnetic field limit. The energy spectrum for a given missing flux quantum is classified in six possible classes of orbits and summarized in a so-called phase diagram. We also investigate the current flow patterns of different quantum states and show the clear correspondence with classical trajectories.Comment: 14 pages, 13 figure

Constraining the Absolute Orientation of Eta Carinae's Binary Orbit: A 3-D Dynamical Model for the Broad [Fe III] Emission

We present a three-dimensional (3-D) dynamical model for the broad [Fe III] emission observed in Eta Carinae using the Hubble Space Telescope/Space Telescope Imaging Spectrograph (HST/STIS). This model is based on full 3-D Smoothed Particle Hydrodynamics (SPH) simulations of Eta Car's binary colliding winds. Radiative transfer codes are used to generate synthetic spectro-images of [Fe III] emission line structures at various observed orbital phases and STIS slit position angles (PAs). Through a parameter study that varies the orbital inclination i, the PA {\theta} that the orbital plane projection of the line-of-sight makes with the apastron side of the semi-major axis, and the PA on the sky of the orbital axis, we are able, for the first time, to tightly constrain the absolute 3-D orientation of the binary orbit. To simultaneously reproduce the blue-shifted emission arcs observed at orbital phase 0.976, STIS slit PA = +38 degrees, and the temporal variations in emission seen at negative slit PAs, the binary needs to have an i \approx 130 to 145 degrees, {\theta} \approx -15 to +30 degrees, and an orbital axis projected on the sky at a PA \approx 302 to 327 degrees east of north. This represents a system with an orbital axis that is closely aligned with the inferred polar axis of the Homunculus nebula, in 3-D. The companion star, Eta B, thus orbits clockwise on the sky and is on the observer's side of the system at apastron. This orientation has important implications for theories for the formation of the Homunculus and helps lay the groundwork for orbital modeling to determine the stellar masses.Comment: 23 pages, 12 color figures, plus 2 online-only appendices (available in the /anc folder of the Source directory). Accepted for publication in MNRA

Electron scattering on circular symmetric magnetic profiles in a two-dimensional electron gas

The quasi-bound and scattered states in a 2DEG subjected to a circular symmetric steplike magnetic profile with zero average magnetic field are studied. We calculate the effect of a random distribution of such identical profiles on the transport properties of a 2DEG. We show that a nonzero Hall resistance can be obtained, although $=0$, and that in some cases it can even change sign as function of the Fermi energy or the magnetic field strength. The Hall and magnetoresistance show pronounced resonances apart from the Landau states of the inner core, corresponding to the so-called quasi-bound snake orbit states.Comment: 7 pages, 8 figure