Precursors to PP

The results of a theoretical study of the interpretation of precursors to PP (Δ ～ 100°) in terms of scattering by small-scale random irregularities in the crust and uppermost mantle are presented. These results are compared with observational data from two nuclear explosions recorded at the Warramunga array and three earthquakes recorded at the NORSAR array. This comparison reveals that the scattering interpretation offers a plausible and adequate explanation of the detailed observational data, namely: onset times, duration, slowness, azimuth and amplitude variations. The scattering interpretation of PP precursors, along with a similar interpretation of precursors to PKPPKP, removes the need for postulating several sharp-reflecting discontinuities in the uppermost few hundred kilometers of the crust and upper mantle.     

Ukinrek Maars, Alaska, I. April 1977 eruption sequence, petrology and tectonic setting

During ten days of phreatomagmatic activity in early April 1977, two maars formed 13 km behind the Aleutian arc near Peulik volcano on the Alaska Peninsula. They have been named “Ukinrek Maars”, meaning “two holes in the ground” in Yupik Eskimo. The western maar formed at the northwestern end of a low ridge within the first three days and is up to 170 m in diameter and 35 m in depth. The eastern maar formed during the next seven days 600 m east of West Maar at a lower elevation in a shallow saddle on the same ridge and is more circular, up to 300 m in diameter and 70 m in depth. The maars formed in terrain that was heavily glaciated in Pleistocene times. The groundwater contained in the underlying till and silicic volcanics from nearby Peulik volcano controlled the dominantly phreatomagmatic course of the eruption.During the eruptions, steam and ash clouds reached maximum heights of about 6 km and a thin blanket of fine ash was deposited north and east of the vents up to a distance of at least 160 km. Magma started to pool on the floor of East Maar after four days of intense phreatomagmatic activity.The new melt is a weakly undersaturated alkali olivine basalt (Ne = 1.2%) showing some transitional character toward high-alumina basalts. The chemistry, an anomaly in the tholeitic basalt-andesite-dominated Aleutian arc, suggests that the new melt is primitive, generated at a depth of 80 km or greater by a low degree of partial melting of garnet peridotite mantle with little subsequent fractionization during transport.The Pacific plate subduction zone lies at a depth of 150 km beneath the maars. Their position appears to be tectonically controlled by a major regional fault, the Bruin Bay fault, and its intersection with cross-arc structural features. We favor a model for the emplacement of the Ukinrek Maars that does not link the Ukinrek conduit to the plumbing system of nearby Peulik volcano. The Ukinrek eruptions probably represent a genetically distinct magma pulse originating at asthenospheric depths beneath the continental lithosphere.     

Q and its effect on the observation of upper mantle travel-time branches

There is broad agreement among various seismological studies that the upper mantle has two regions where high positive velocity gradients or transition zones exist. The presence of these zones implies that two major triplications should exist in the travel-time curve for distances less than 30°. Approximately 200 earthquakes from the New Guinea, New Britain, and Solomon Island regions recorded at the Warramunga Array were analyzed using adaptive processing methods in an attempt to identify the positions of the later arrival branches. From measurements made along the first 20 sec of the arrivals, a retrogade travel-time branch associated with the “650-km” discontinuity was clearly identified as extending from 21° to 26°, and some evidence was found near 16° for the lower portion of the triplication associated with the “400-km” discontinuity. A careful search revealed however that the upper portions of the replicated travel-time branches were missing. There were no observed values ofdt/dΔ in the 12–13 sec/deg range for Δ greater than 20°. In this study it was found that if anelastic effects (Q) were not taken into consideration or ifQ were kept constant, the models derived from observed travel-time data all predicted large amplitude arrivals where non existed. The difficulty with the first triplication was resolved by the introduction of a lowQ region at depths of 85–315 km. This region may be associated with “the low-velocity region” but it is not necessary to decrease the P velocity to explain the observations.The difficulty with the second triplication was resolved by the introduction of a layer at a depth of 575–657 km which has no velocity gradient and a value ofQ significantly less than that for the material just below the “650-km” discontinuity. This layer may well represent the return path for an upper mantle convection cell.     

Size,shape and spatial arrangement of mega‐scale glacial lineations from a large and diverse dataset

Mega‐scale glacial lineations (MSGLs) are a characteristic landform on ice stream beds. Solving the puzzle of their formation is key to understanding how ice interacts with its bed and how this, in turn, influences the dynamics of ice streams. However, a comprehensive and detailed characterization of this landform's size, shape and spatial arrangement, which might serve to test and refine formational theories, is largely lacking. This paper presents a detailed morphometric analysis and comparison of 4043 MSGLs from eight palaeo‐ice stream settings: three offshore (Norway and Antarctica), four onshore (Canada), and one from under a modern ice stream in West Antarctica. The length of MSGLs is lower than previously suggested (mode 1000–2000 m; median 2892 m), and they initiate and terminate at various locations on an ice stream bed. Their spatial arrangement reveals a pattern that is characterized by an exceptional parallel conformity (80% of all mapped MSGLs have an azimuth within 5° from the mean values), and a fairly constant lateral spacing (mode 200–300 m; median 330 m), which we interpret as an indication that MSGLs are a spatially self‐organized phenomenon. Results show that size, shape and spatial arrangement of MSGLs are consistent both within and also generally between different ice stream beds. We suggest this results from a common mechanism of formation, which is largely insensitive to local factors. Although the elongation of MSGLs (mode 6–8; median 12.2) is typically higher than features described as drumlins, these values and those of their width (mode 100–200 m; median 268 m) overlap, which suggests the two landforms are part of a morphological continuum and may share a similar origin. We compare their morphometry with explicit predictions made by the groove‐ploughing and rilling instability theories of MSGL formation. Although the latter was most compatible, neither is fully supported by observations. © 2014 The Authors. Earth Surface Processes and Landforms Published by John Wiley & Sons Ltd.     

The Hubble constant from galaxy lenses: impacts of triaxiality and model degeneracies

The Hubble constant can be constrained using the time delays between multiple images of gravitationally lensed sources. In some notable cases, typical lensing analyses assuming isothermal galaxy density profiles produce low values for the Hubble constant, inconsistent with the result of the HST Key Project  (72 ± 8 km s⁻¹ Mpc⁻¹)  . Possible systematics in the values of the Hubble constant derived from galaxy lensing systems can result from a number of factors, for example, neglect of environmental effects, assumption of isothermality, or contamination by line-of-sight structures. One additional potentially important factor is the triaxial structure of the lensing galaxy halo; most lens models account for halo shape simply by perturbing the projected spherical lensing potential, an approximation that is often necessary but that is inadequate at the levels of triaxiality predicted in the cold dark matter paradigm. To quantify the potential error introduced by this assumption in estimates of the Hubble parameter, we strongly lens a distant galaxy through a sample of triaxial softened isothermal haloes and use an Markov Chain Monte Carlo method to constrain the lensing halo profile and the Hubble parameter from the resulting multiple image systems. We explore the major degeneracies between the Hubble parameter and several parameters of the lensing model, finding that without a way to accurately break these degeneracies accurate estimates of the Hubble parameter are not possible. Crucially, we find that triaxiality does not significantly bias estimates of the Hubble constant, and offer an analytic explanation for this behaviour in the case of isothermal profiles. Neglected triaxial halo shape cannot contribute to the low Hubble constant values derived in a number of galaxy lens systems.     

Assessment of polyaromatic hydrocarbon degradation by potentially pathogenic environmental Vibrio parahaemolyticus isolates from coastal Louisiana, USA

A presumed Vibrio parahaemolyticus isolate from Chesapeake Bay, Maryland, USA was previously reported to grow on phenanthrene, a polyaromatic hydrocarbon (PAH) found in crude oil. Following the Deepwater Horizon oil spill in the Gulf of Mexico, concerns were raised that PAH-degrading V. parahaemolyticus could increase in abundance, leading to elevated risks of disease derived from shellfish consumption. To assess this possibility, we examined responses to naphthalene and phenanthrene of 17 coastal Louisiana environmental V. parahaemolyticus isolates representing five distinct genotypes. Isolates were obtained immediately after the spill began and after oil had reached the Louisiana coast. None of the isolates grew on or oxidized either substrate and a naphthalene degradation product, 1-naphthol, substantially inhibited growth of some isolates. The use of PAH by V. parahaemolyticus is unusual, and an increase in human health risks due to stimulation of V. parahaemolyticus growth by oil-derived PAH under in situ conditions appears unlikely.     

Inference of Global Reservoir Connectivity from Static Pressure Data with Fast Coarse-Scale Simulation Models

Mathematical Geosciences - Characterization of field-scale reservoir connectivity is critical for production optimization and field development planning. The information content of the data...