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  • 1
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    Imaging supershear rupture for the 2014 Mw 6.9 Northern Aegean earthquake by backprojection of strong motion waveforms (2014)
    Wiley
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    Publication Date: 2014-12-31
    Description: The seismic source spatio-temporal evolution of the M w 6.9 event on 24 May 2014 in Northern Aegean is imaged by backprojection of strong motion envelopes. The results indicate that the event ruptured on two different fault segments. In the first one, rupture propagated from the hypocenter westwards for ~ 20 km. In the second delayed segment to the east, rupture propagated eastwards for ~ 65 km with a supershear velocity (~5.5 km/s). At the end of this rupture the largest stacking amplitudes are imaged, associated with possible stopping phases from the abrupt cessation of a fast slip. Low aftershock seismicity on the supershear segment implies a simple and linear fault geometry there. This is the third large event in the Northern Aegean Trough - Northern Anatolian Fault zones that has ruptured with supershear speed. This characteristic should be taken into account in studying past events and estimating seismic hazard in this area.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
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    Electrostatic solitary waves with distinct speeds associated with asymmetric reconnection (2014)
    Wiley
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    Publication Date: 2014-12-31
    Description: Electrostatic solitary waves (ESWs) are observed at the magnetopause with distinct time scales. These ESWs are associated with asymmetric reconnection of the cold dense magnetosheath plasma with the hot tenuous magnetospheric plasma. The distinct time scales are shown to be due to ESWs moving at distinct speeds and having distinct length scales. The length scales are of order 5–50 Debye lengths and the speeds range from ~50 kms −1 to ~1000 kms −1 . The ESWs are observed near the reconnection separatrices. The observation of ESWs with distinct speeds suggests that multiple instibilities are occurring. The implications for reconnection at the magnetopause are discussed.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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  • 3
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    Observed multi-variable signals of late 20th and early 21st century volcanic activity (2014)
    Wiley
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    Publication Date: 2014-12-31
    Description: The relatively muted warming of the surface and lower troposphere since 1998 has attracted considerable attention. One contributory factor to this “warming hiatus" is an increase involcanically-induced cooling over the early 21st century. Here, we identify the signals of late 20th and early 21st century volcanic activity in multiple observed climate variables. Volcanic signals are statistically discernible in spatial averagesof tropical and near-global SST, tropospheric temperature, net clear-sky short-wave radiation, and atmospheric water vapor. Signals of late 20th and early 21st century volcanic eruptions are also detectable in near-global averages of rainfall. In tropical-average rainfall, however, only a Pinatubo-caused drying signal is identifiable. Successful volcanic signal detection is critically dependent on removal of variability induced by the El Niño/Southern Oscillation (ENSO).
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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  • 4
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    Volcanic ash layer depth: Processes and mechanisms (2014)
    Wiley
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    Publication Date: 2014-12-31
    Description: The long duration of the 2010 Eyjafjallajökull eruption provided a unique opportunity to measure a widely dispersed volcanic ash cloud. Layers of volcanic ash were observed by the European Aerosol Research LIdar NETwork (EARLINET) with a mean depth of 1.2 km and standard deviation of 0.9 km. In this paper we evaluate the ability of the UK Met Office's Numerical Atmospheric-dispersion Modelling Environment (NAME) to simulate the observed ash layers and examine the processes controlling their depth. NAME simulates distal ash layer depths exceptionally well with a mean depth of 1.2 km and standard deviation of 0.7 km. The dominant process determining the depth of ash layers over Europe is the balance between the vertical wind shear (which acts to reduce the depth of the ash layers) and vertical turbulent mixing (which acts to deepen the layers). Interestingly, differential sedimentation of ash particles and the volcano vertical emission profile play relatively minor roles.
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  • 5
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    Ocean glider observations of iceberg-enhanced biological production in the northwestern Weddell Sea (2014)
    Wiley
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    Publication Date: 2014-12-30
    Description: Icebergs affect local biological production around Antarctica. We used an ocean glider to observe the effects of a large iceberg that was advected by the Antarctic Slope Current along the continental slope in the northwestern Weddell Sea in early 2012. The high resolution glider data reveal a pronounced effect of the iceberg on ocean properties, with oxygen concentrations of (13 ± 4) μ mol kg −1 higher than levels in surrounding waters, which are most likely due to positive net community production. This response was confined to three areas of water in the direct vicinity of the iceberg track, each no larger than 2 km 2 . Our findings suggest that icebergs have an impact on Antarctic production presumably through local micronutrient injections, on a scale smaller than typical satellite observations of biological production in the Southern Ocean.
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  • 6
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    Controls on the geometry of potholes in bedrock channels (2014)
    Wiley
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    Publication Date: 2014-12-30
    Description: Potholes (circular depressions carved into bedrock) are the dominant roughness elements in many bedrock channels. Here we show, using data from previous studies and new data from the Smith River, Oregon, that pothole depths increase in proportion to both the mean pothole radius (such that the most common pothole depth-to-radius ratio is 2) and the diameter of the largest clasts episodically stored in potholes. We present a theory for these observations based on computational fluid dynamics (CFD) and sediment transport modeling of vortices in cylindrical cavities of different shapes and sizes. We show that the shear stress at the bottom of a pothole (which controls the rate of pothole growth) is maximized for potholes with a depth-to-radius ratio of approximately 1 and decreases nonlinearly with increasing depth-to-radius ratio such that potholes with depth-to-radius ratios larger than 3 are uncommon. Our model provides a mechanistic explanation for pothole shapes and sizes.
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  • 7
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    Net primary production in the Gulf Stream sustained by quasi-geostrophic vertical exchanges (2014)
    Wiley
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    Publication Date: 2014-12-30
    Description: We analyze 12 years of mesoscale vertical motion derived from an observation-based product in the top 1000 m of the North West Atlantic Ocean. Vertical velocities [O(10 m day −1 )] associated with Gulf Stream instabilities consist of alternating cells of upwelling and downwelling. Here we show that the magnitude of the vertical motions decay exponentially southwards with an e-folding length scale is informative on the dynamics of the system. We further investigate the impact of the vertical supply of nutrients on phytoplankton growth with a conceptual model incorporating the mean effect of nutrient distribution, quasi-geostrophic dynamics and Ekman suction/pumping. Results confirm that the mean effect of mesoscale vertical velocity variability alone can sustain observed levels of net primary production in the immediate vicinity of the Gulf Stream, while other mechanisms, including horizontal advection and submesoscale dynamics, need to be considered when moving towards the subtropical gyre.
    Print ISSN: 0094-8276
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    Topics: Geosciences , Physics
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  • 8
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    Effects of Arctic haze on surface cloud radiative forcing (2014)
    Wiley
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    Publication Date: 2014-12-30
    Description: From 4 years of observations from Barrow, Alaska, it is shown that the cloud radiative impact on the surface is a net warming effect between October and May and a net cooling in summer. During episodes of high surface haze aerosol concentrations and cloudy skies, both the net warming and net cooling are amplified, ranging from +12.2 Wm -2 in February to -11.8 Wm -2 in August. In liquid clouds, approximately 50%-70% of this change is caused by changes in cloud particle effective radius, with the remainder being caused by unknown atmospheric feedbacks that increase cloud water path. While yearly-averaged, the warming and cooling effects nearly cancel, the timing of the forcing may be a relevant control of the amplitude and timing of sea-ice melt.
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  • 9
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    Observation of deep water microseisms in the North Atlantic Ocean using tide modulations (2014)
    Wiley
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    Publication Date: 2014-12-30
    Description: Ocean activity produces continuous and ubiquitous seismic energy mostly in the 2 – 20 s period band, known as microseismic noise. Between 2 and 10 s period, secondary microseisms (SM) are generated by swell reflections close to the shores and/or by opposing swells in the deep ocean. However, unique conditions are required in order for surface waves generated by deep-ocean microseisms to be observed on land. By comparing short-duration power spectral densities at both Atlantic shoreline and inland seismic stations, we show that ocean tides strongly modulate the seismic energy in a wide period band except between 2.5 and 5 s. This tidal proxy reveals the existence of an ex situ short-period contribution of the SM peak. Comparison with swell spectra at surrounding buoys suggests that the largest part of this extra energy comes from deep-ocean-generated microseisms. The energy modulation might be also used in numerical models of microseismic generation to constrain coastal reflection coefficients.
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    Topics: Geosciences , Physics
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  • 10
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    Role of Mantle Flow in Nubia-Somalia Plate Divergence (2014)
    Wiley
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    Publication Date: 2014-12-29
    Description: Present-day continental extension along the East African Rift System (EARS) has often been attributed to diverging sub-lithospheric mantle-flow associated with the African Superplume. This implies a degree of viscous coupling between mantle and lithosphere that remains poorly constrained. Recent advances in estimating present-day opening rates along the EARS from geodesy offer an opportunity to address this issue with geodynamic modeling of the mantle-lithosphere system. Here, we use numerical models of the global mantle-plates coupled system to test the role of present-day mantle-flow in Nubia-Somalia plate divergence across the EARS. The scenario yielding the best-fit to geodetic observations is one where torques associated with gradients of gravitational potential energy stored in the African highlands are resisted by weak continental faults and mantle basal drag. These results suggest thet shear tractions from diverging mantle flow play a minor role in present-day Nubia-Somalia divergence.
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    Topics: Geosciences , Physics
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