Slab tears and intermediate‐depth seismicity

Author Posting. © American Geophysical Union, 2013. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 40 (2013): 4244-4248, doi:10.1002/grl.50830.Active tectonic regions where plate boundaries transition from subduction to strike slip can take several forms, such as triple junctions, acute, and obtuse corners. Well‐documented slab tears that are associated with high rates of intermediate‐depth seismicity are considered here: Gibraltar arc, the southern and northern ends of the Lesser Antilles arc, and the northern end of Tonga trench. Seismicity at each of these locations occurs, at times, in the form of swarms or clusters, and various authors have proposed that each marks an active locus of tear propagation. The swarms and clusters start at the top of the slab below the asthenospheric wedge and extend 30–60 km vertically downward within the slab. We propose that these swarms and clusters are generated by fluid‐related embrittlement of mantle rocks. Focal mechanisms of these swarms generally fit the shear motion that is thought to be associated with the tearing process

Seismic evidence for a slab tear at the Puerto Rico Trench

Author Posting. © American Geophysical Union, 2013. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Solid Earth 105 (2013): 2915-2923, doi:10.1002/jgrb.50227.The fore‐arc region of the northeast Caribbean plate north of Puerto Rico and the Virgin Islands has been the site of numerous seismic swarms since at least 1976. A 6 month deployment of five ocean bottom seismographs recorded two such tightly clustered swarms, along with additional events. Joint analyses of the ocean bottom seismographs and land‐based seismic data reveal that the swarms are located at depths of 50–150 km. Focal mechanism solutions, found by jointly fitting P wave first‐motion polarities and S/P amplitude ratios, indicate that the broadly distributed events outside the swarm generally have strike‐ and dip‐slip mechanisms at depths of 50–100 km, while events at depths of 100–150 km have oblique mechanisms. A stress inversion reveals two distinct stress regimes: The slab segment east of 65°W longitude is dominated by trench‐normal tensile stresses at shallower depths (50–100 km) and by trench‐parallel tensile stresses at deeper depths (100–150 km), whereas the slab segment west of 65°W longitude has tensile stresses that are consistently trench normal throughout the depth range at which events were observed (50–100 km). The simple stress pattern in the western segment implies relatively straightforward subduction of an unimpeded slab, while the stress pattern observed in the eastern segment, shallow trench‐normal tension and deeper trench‐normal compression, is consistent with flexure of the slab due to rollback. These results support the hypothesis that the subducting North American plate is tearing at or near these swarms. The 35 year record of seismic swarms at this location and the recent increase in seismicity suggest that the tear is still propagating

Manufacturing Development of the NCSX Modular Coil Windings

The modular coils on the National Compact Stellarator Experiment (NCSX) present a number of significant engineering challenges due to their complex shapes, requirements for high dimensional accuracy and the high current density required in the modular coils due to space constraints. In order to address these challenges, an R&D program was established to develop the conductor, insulation scheme, manufacturing techniques, and procedures. A prototype winding named Twisted Racetrack Coil (TRC) was of particular importance in dealing with these challenges. The TRC included a complex shaped winding form, conductor, insulation scheme, leads and termination, cooling system and coil clamps typical of the modular coil design. Even though the TRC is smaller in size than a modular coil, its similar complex geometry provided invaluable information in developing the final design, metrology techniques and development of manufacturing procedures. In addition a discussion of the development of the copper rope conductor including "Keystoning" concerns; the epoxy impregnation system (VPI) plus the tooling and equipment required to manufacture the modular coils will be presented

Vacuum-ultraviolet photoabsorption imaging system for laser plasma plume diagnostics

We describe a recently designed and constructed system based on a 1 m normal incidence vacuum monochromator with corrected (toroidal) optics that produces a wavelength tuneable and collimated vacuum-ultraviolet (VUV) (λ=30–100 nm) beam. The VUV continuum source is a laser-generated gold plasma. The primary function of the system is the measurement of time resolved “images” or spatial distributions of photoabsorption/photoionization in expanding laser plasma plumes. This is achieved by passing the beam through the sample of interest (in our case a second synchronised plasma) and recording the “footprint” of the attenuated beam on a charge coupled device. Using this VUV photoabsorption imaging or “shadowgraphy” technique we track and extract column density distributions in expanding plasma plumes. We can also measure the plume front velocity. We have characterized the system, particularly in relation to spectral and spatial resolution and the experimental results meet very well the expectations from ray tracing done at the design phase. We present first photoabsorption images and column density distributions of laser produced Ca plumes from the system

The Sources of Inflammatory Mediators in the Lung after Silica Exposure

The expression of 10 genes implicated in regulation of the inflammatory processes in the lung was studied after exposure of alveolar macrophages (AMs) to silica in vitro or in vivo. Exposure of AMs to silica in vitro up-regulated the messenger RNA (mRNA) levels of three genes [interleukin-6 (IL-6), monocyte chemoattractant protein-1 (MCP-1), and macrophage inflammatory protein-2 (MIP-2)] without a concomitant increase in the protein levels. AMs isolated after intratracheal instillation of silica up-regulated mRNA levels of four additional genes [granulocyte/macrophage-colony stimulating factor (GM-CSF), IL-1β, IL-10, and inducible nitric oxide synthase]. IL-6, MCP-1, and MIP-2 protein levels were elevated in bronchoalveolar lavage fluid. Fibroblasts under basal culture conditions express much higher levels of IL-6 and GM-CSF compared with AMs. Coculture of AMs and alveolar type II cells, or coculture of AMs and lung fibroblasts, in contact cultures or Transwell chambers, revealed no synergistic effect. Therefore, such interaction does not explain the effects seen in vivo. Identification of the intercellular communication in vivo is still unresolved. However, fibroblasts appear to be an important source of inflammatory mediators in the lung

Reconstructing promoter activity from Lux bioluminescent reporters

The bacterial Lux system is used as a gene expression reporter. It is fast, sensitive and non-destructive, enabling high frequency measurements. Originally developed for bacterial cells, it has also been adapted for eukaryotic cells, and can be used for whole cell biosensors, or in real time with live animals without the need for euthanasia. However, correct interpretation of bioluminescent data is limited: the bioluminescence is different from gene expression because of nonlinear molecular and enzyme dynamics of the Lux system. We have developed a computational approach that, for the first time, allows users of Lux assays to infer gene transcription levels from the light output. This approach is based upon a new mathematical model for Lux activity, that includes the actions of LuxAB, LuxEC and Fre, with improved mechanisms for all reactions, as well as synthesis and turn-over of Lux proteins. The model is calibrated with new experimental data for the LuxAB and Fre reactions from Photorhabdus luminescens --- the source of modern Lux reporters --- while literature data has been used for LuxEC. Importantly, the data show clear evidence for previously unreported product inhibition for the LuxAB reaction. Model simulations show that predicted bioluminescent profiles can be very different from changes in gene expression, with transient peaks of light output, very similar to light output seen in some experimental data sets. By incorporating the calibrated model into a Bayesian inference scheme, we can reverse engineer promoter activity from the bioluminescence. We show examples where a decrease in bioluminescence would be better interpreted as a switching off of the promoter, or where an increase in bioluminescence would be better interpreted as a longer period of gene expression. This approach could benefit all users of Lux technology

A RAC/CDC-42–Independent GIT/PIX/PAK Signaling Pathway Mediates Cell Migration in C. elegans

P21 activated kinase (PAK), PAK interacting exchange factor (PIX), and G protein coupled receptor kinase interactor (GIT) compose a highly conserved signaling module controlling cell migrations, immune system signaling, and the formation of the mammalian nervous system. Traditionally, this signaling module is thought to facilitate the function of RAC and CDC-42 GTPases by allowing for the recruitment of a GTPase effector (PAK), a GTPase activator (PIX), and a scaffolding protein (GIT) as a regulated signaling unit to specific subcellular locations. Instead, we report here that this signaling module functions independently of RAC/CDC-42 GTPases in vivo to control the cell shape and migration of the distal tip cells (DTCs) during morphogenesis of the Caenorhabditis elegans gonad. In addition, this RAC/CDC-42–independent PAK pathway functions in parallel to a classical GTPase/PAK pathway to control the guidance aspect of DTC migration. Among the C. elegans PAKs, only PAK-1 functions in the GIT/PIX/PAK pathway independently of RAC/CDC42 GTPases, while both PAK-1 and MAX-2 are redundantly utilized in the GTPase/PAK pathway. Both RAC/CDC42–dependent and –independent PAK pathways function with the integrin receptors, suggesting that signaling through integrins can control the morphology, movement, and guidance of DTC through discrete pathways. Collectively, our results define a new signaling capacity for the GIT/PIX/PAK module that is likely to be conserved in vertebrates and demonstrate that PAK family members, which are redundantly utilized as GTPase effectors, can act non-redundantly in pathways independent of these GTPases