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  • 1
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    Ambient Noise Based Monitoring of Seismic Velocity Changes Associated with the 2014 Mw 6.0 South Napa Earthquake (2015)
    Wiley
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    Publication Date: 2015-08-11
    Description: We perform an ambient noise based monitoring to explore temporal variations of crustal seismic velocities before, during, and after the 24 August 2014 M w 6.0 South Napa earthquake. A velocity drop of about 0.08% is observed immediately after the South Napa earthquake. Spatial variability of the velocity reduction is most correlated with the pattern of the peak ground velocity of the South Napa mainshock, which suggests that fracture damage in rocks induced by the dynamic strain is likely responsible for the coseismic velocity change. About 50% of the velocity reduction is recovered at the first 50 days following the South Napa mainshock. This postseismic velocity recovery may suggest a healing process of damaged rocks.
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    Electronic ISSN: 1944-8007
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  • 2
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    Mantle-driven Uplift of Hangai Dome: New Seismic Constraints from Adjoint Tomography (2015)
    Wiley
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    Publication Date: 2015-08-11
    Description: The origin of Hangai Dome, an unusual large-scale, high-elevation low-relief landform in central Mongolia, remains enigmatic partly due to lack of constraints on its underlying seismic structure. Using adjoint tomography –a full waveform tomographic technique– and a large seismic waveform data set in East Asia, we discover beneath the dome a deep low shear wavespeed (low-V) conduit indicating a slightly warmer (54 K–127 K) upwelling from the transition zone. This upwelling is spatially linked to a broader uppermost mantle low-V region underlying the dome. Further observations of high compressional to shear wavespeed ratios and positive radial anisotropy in the low-V region suggest partial melting and horizontal melt transport. We propose that the mantle upwelling induced decompression melting in the uppermost mantle and that excess heat associated with melt transport modified the lithosphere that isostatically compensates the surface uplift at upper mantle depths (〉80 km).
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  • 3
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    Theoretical Basis for At-Many-Stations Hydraulic Geometry (AMHG) (2015)
    Wiley
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    Publication Date: 2015-08-13
    Description: At-many-stations hydraulic geometry (AMHG) is a recently discovered set of geomorphic relationships showing that the empirical parameters of at-a-station hydraulic geometry (AHG) are functionally related along a river. This empirical conclusion seemingly refutes previous decades of research defining AHG as spatially independent and site-specific. Furthermore, AMHG was the centerpiece of an unprecedented recent methodology that successfully estimated river discharge solely from satellite imagery. Despite these important implications, AMHG has remained an empirical phenomenon without theoretical explanation. Here, we provide the mathematical basis for AMHG, showing that it arises when independent AHG curves within a reach intersect near the same values of discharge and width, depth, or velocity. The strength of observed AMHG is determined by the degree of this convergence. Finally, we show that AMHG enables discharge estimation by defining a set of possible estimated discharges that often match true discharges, and propose its future interpretation as a fluvial index.
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  • 4
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    Increasing carbon dioxide concentration in the upper atmosphere observed by SABER (2015)
    Wiley
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    Publication Date: 2015-08-13
    Description: Carbon dioxide measurements made by the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument between 2002 and 2014 were analyzed to reveal the rate of increase of CO 2 in the mesosphere and lower thermosphere. The CO 2 data show a trend of ~5% per decade at ~80 km and below, in good agreement with the tropospheric trend observed at Mauna Loa. Above 80 km, the SABER CO 2 trend is larger than in the lower atmosphere, reaching ~12% per decade at 110 km. The large relative trend in the upper atmosphere is consistent with results from the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS). On the other hand, the CO 2 trend deduced from the Whole Atmosphere Community Climate Model (WACCM) remains close to 5% everywhere. The spatial coverage of the SABER instrument allows us to analyze the CO 2 trend as a function of latitude for the first time. The trend is larger in the northern hemisphere than in the southern hemisphere mesopause above 80 km. The agreement between SABER and ACE-FTS suggests that the rate of increase of CO 2 in the upper atmosphere over the past 13 years is considerably larger than can be explained by chemistry-climate models.
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  • 5
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    Projected changes in extreme temperature events based on the NARCCAP model suite (2015)
    Wiley
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    Publication Date: 2015-08-15
    Description: Once-per-year maximum temperature extremes in NARCCAP are projected to increase more (less) than mean daily maximum summer temperatures over much of the eastern (western) United States. In contrast, the models almost everywhere project greater warming of once-per-year minimum temperatures as compared to mean daily minimum winter temperatures. Under projected changes associated with extremes of the temperature distribution, Baltimore's maximum temperature that was met or exceeded once-per-year historically is projected to occur 17 times per season by mid-century, a 28% increase relative to projections based on summer mean daily maximum temperature change. Under the same approach, historical once-per-year cold events in Baltimore are projected to occur once-per-decade. The models are generally able to capture observed geopotential height anomalies associated with temperature extremes in two subregions. Projected changes in extreme temperature events cannot be explained by lower boundary conditions as reflected by soil moisture anomalies or snow water equivalent.
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  • 6
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    Effect of the total solar eclipse of 20 March 2015 on the ELF propagation over high-latitude paths (2015)
    Wiley
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    Publication Date: 2015-08-16
    Description: Propagation of the artificial electromagnetic waves with frequency of 82 Hz in the Earth-ionosphere waveguide was observed during the solar eclipse on both partially and totally obscured high-latitude paths with a length of 450–1200 km. Field excitation was monitored by the reference measurements in the near zone of the transmitter, which are free of the ionospheric influence. It is found that the amplitude of the field at the remote points varied depending on the solar illumination and solar elevation angle. We suppose that this effect was probably caused by the increase in the effective height of the ionospheric D-layer, just as it was previously observed in VLF. The obtained results demonstrate the response of the propagating extremely low frequency (ELF, 3–300 Hz) wave to the change in the ionospheric boundary. This effect has been for the first time observed in this frequency range during a total solar eclipse.
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  • 7
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    Slip-pulse rupture behavior on a 2 meter granite fault (2015)
    Wiley
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    Publication Date: 2015-08-16
    Description: We describe observations of dynamic rupture events that spontaneously arise on meter-scale laboratory earthquake experiments. While low-frequency slip of the granite sample occurs in a relatively uniform and crack-like manner, instruments capable of detecting high frequency motions show that some parts of the fault slip abruptly (velocity 〉100 mm∙s -1 , acceleration 〉20 km∙s -2 ) while the majority of the fault slips more slowly. Abruptly slipping regions propagate along the fault at nearly the shear wave speed. We propose that the dramatic reduction in frictional strength implied by this pulse-like rupture behavior has a common mechanism to the weakening reported in high velocity friction experiments performed on rotary machines. The slip pulses can also be identified as migrating sources of high frequency seismic waves. As observations from large earthquakes show similar propagating high frequency sources, the pulses described here may have relevance to the mechanics of larger earthquakes.
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  • 8
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    Contrasting human versus climatic impacts on erosion (2015)
    Wiley
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    Publication Date: 2015-08-04
    Description: Both human activity and climate change can influence erosion rates and initiate rapid landscape change. Understanding the relative impact of these factors is critical to managing the risks of extreme erosion related to flooding and landslide occurrence. Here we present a 2,100-year record of sediment mass accumulation and inferred erosion based on lacustrine sediment cores from Amherst Lake, Vermont, USA. Using deposition from August 2011 Tropical Storm Irene as a modern analogue, we identified distinct event deposits indicative of destructive erosion events. These deposits record a prolonged (multi-decadal) interval of enhanced erosion following the initial storm-induced landscape disturbance. The direct impact of human land cover alteration is minimal in comparison to the more recent, 20 th century increase in the occurrence of catastrophic erosion linked to overall wetter conditions that favor high erosion rates and more easily trigger landslides during periods of extreme precipitation.
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  • 9
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    A new solar signal: Average maximum sunspot magnetic fields independent of activity cycle (2015)
    Wiley
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    Publication Date: 2015-08-04
    Description: Over the past five years, 2010 – April 2015, we observed 4176 sunspot umbrae in the infrared (IR) to measure maximum magnetic field strengths from the Zeeman splitting of Fe 15648.5 Å. Herein we distinguish “field strengths” from “field flux”. Field strengths range from 1500 Gauss in pores to 3500+ in large spots. We made one observation per spot per observing day, ignoring spot size. We show that in the IR no activity cycle dependence on average maximum field strength (2070 ± 20 G) has been found. A similar analysis of 17,450 spots observed in space by the Helioseismic and Magnetic Imager reveal a similar cycle independence (2050 ± 0.18 G). Conclude: the average maximum umbral fields are constant with time and independent of the activity cycle within our time coverage.
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  • 10
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    Insights into high peak current in-cloud lightning events during thunderstorms (2015)
    Wiley
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    Publication Date: 2015-08-04
    Description: We investigated National Lightning Detection Network reports and lightning radio waveforms in a 44-day observation period to analyze the in-cloud (IC) events producing currents above 200 kA. The results show there are two distinct classes of IC lightning events with very high peak currents: the well-known narrow bipolar events (NBEs), and a previously unreported type that we call energetic in-cloud pulses (EIPs). Their temporal and spatial context shows that EIPs are generated from existing negative polarity leaders that are propagating usually upward but sometimes downward. The nearly identical characteristics of EIPs and some previously reported terrestrial gamma ray flashes (TGFs) indicates a likely connection between the two, which further suggests the possibility of downward-directed TGFs. These very high peak current IC events also suggest the association of EIPs with ionospheric perturbations and optical emissions known as elves.
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  • 11
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    Linking magma transport structures at Kīlauea volcano (2015)
    Wiley
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    Publication Date: 2015-08-04
    Description: Identifying magma pathways is important for understanding and interpreting volcanic signals. At Kīlauea volcano, seismicity illuminates subsurface plumbing, but the broad spectrum of seismic phenomena hampers event identification. Discrete, long-period events (LPs) dominate the shallow (5–10 km) plumbing, and deep (40+ km) tremor has been observed offshore. However, our inability to routinely identify these events limits their utility in tracking ascending magma. Using envelope cross-correlation, we systematically catalog non-earthquake seismicity between 2008–2014. We find the LPs and deep tremor are spatially distinct, separated by the 15–25 km deep, horizontal mantle fault zone (MFZ). Our search corroborates previous observations, but we find broader-band (0.5-20 Hz) tremor comprising collocated earthquakes and reinterpret the deep tremor as earthquake swarms in a volume surrounding and responding to magma intruding from the mantle plume beneath the MFZ. We propose the overlying MFZ promotes lateral magma transport, linking this deep intrusion with Kīlauea's shallow magma plumbing.
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  • 12
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    Revisiting the recent California drought as an extreme value (2015)
    Wiley
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    Publication Date: 2015-08-04
    Description: Spatially weighted averages of Palmer Drought Severity Index (PDSI) over central and southern California show that the 1-year 2014 drought was not as severe as previously reported, but it still is the most severe in the 1895–2014 instrumental record. Using the typical adjustment procedure that matches the mean and standard deviation of tree-ring PDSI values to those of instrumental data shows over ten droughts from 800–2006 that were more severe than the 1-year 2014 drought, with the 2014 drought having a return period of 140–180 years. Quantile mapping allows for a closer correspondence between instrumental and tree-ring PDSI probability distributions and produces return periods of 700–900 years for the 1-year 2014 drought. Associated cumulative 3- and 4-year droughts, however, are estimated to be much more severe. The 2012–2014 drought is nearly a 10,000-year event while the 2012–2015 drought has an almost incalculable return period and is completely without precedent.
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  • 13
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    A 15-year slow slip event on the Sunda megathrust offshore Sumatra (2015)
    Wiley
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    Publication Date: 2015-08-04
    Description: In the Banyak Islands of Sumatra, coral microatoll records reveal a 15-year-long reversal of interseismic vertical displacement from subsidence to uplift between 1966 and 1981. To explain these coral observations, we test four hypotheses, including regional sea-level changes and various tectonic mechanisms. Our results show that the coral observations likely reflect a 15-year-long slow slip event (SSE) on the Sunda megathrust. This long duration SSE exceeds the duration of previously reported SSEs and demonstrates the importance of multi-decade geodetic records in illuminating the full spectrum of megathrust slip behavior at subduction zones.
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  • 14
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    Tropical North Atlantic ocean–atmosphere interactions synchronize forest carbon losses from hurricanes and Amazon fires (2015)
    Wiley
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    Publication Date: 2015-08-05
    Description: We describe a climate mode synchronizing forest carbon losses from North and South America by analyzing time series of tropical North Atlantic sea surface temperatures (SSTs), landfall hurricanes and tropical storms, and Amazon fires during 1995–2013. Years with anomalously high tropical North Atlantic SSTs during March – June were often followed by a more active hurricane season and a larger number of satellite-detected fires in the southern Amazon during June – November. The relationship between North Atlantic tropical cyclones and southern Amazon fires ( r =0.61, p 〈0.003) was stronger than links between SSTs and either cyclones or fires alone, suggesting that fires and tropical cyclones were directly coupled to the same underlying atmospheric dynamics governing tropical moisture redistribution. These relationships help explain why seasonal outlook forecasts for hurricanes and Amazon fires both failed in 2013 and may enable the design of improved early warning systems for drought and fire in Amazon forests.
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  • 15
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    Systematic Decomposition of the Madden-Julian Oscillation into Balanced and Inertio-Gravity Components (2015)
    Wiley
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    Publication Date: 2015-08-05
    Description: We present a new method for the three-dimensional multivariate decomposition of the MJO into balanced and inertio-gravity (IG) components. The method analyzes global fields with no filtering involved and it provides a quantitative comparison between the contribution of the Rossby, Kelvin and other balanced and IG modes to the MJO circulation and its teleconnections. Results based on the ERA Interim reanalysis data and the multivariate MJO index show that the Rossby mode with the lowest meridional index is the largest contributor to the MJO circulation over the Pacific. A smaller role of the Kelvin mode is diagnosed over the Indian ocean and the maritime continent. The MJO teleconnections in the polar stratosphere appear associated with the leading balanced vertical modes. The presented method shows new ways of evaluating the MJO structure and its global impacts in weather and climate models.
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  • 16
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    Arctic sea ice freeboard from AltiKa and comparison with CryoSat-2 and Operation IceBridge (2015)
    Wiley
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    Publication Date: 2015-08-05
    Description: Satellite radar altimeters have improved our knowledge of Arctic sea ice thickness over the past decade. The main sources of uncertainty in sea ice thickness retrievals are associated with inadequate knowledge of the snow layer depth and the radar interaction with the snow pack. Here we adapt a method of deriving sea ice freeboard from CryoSat-2 to data from the AltiKa Ka-band radar altimeter over the 2013–14 Arctic sea ice growth season. AltiKa measures basin-averaged freeboards between 4.4 cm and 11.2 cm larger than CryoSat-2 in October and March respectively. Using airborne laser and radar measurements from spring 2013 and 2014 we estimate the effective scattering horizon for each sensor. Whilst CryoSat-2 echoes penetrate to the ice surfaceover first-year ice and penetrate the majority (82 ± 3%) of the snow layer over multi-year ice, AltiKa echoes are scattered from roughly the midpoint (46 ± 5%) of the snow layer over both ice types.
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  • 17
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    Partial melting of a Pb-Sn mushy layer due to heating from above, and implications for regional melting of Earth's directionally solidified inner core (2015)
    Wiley
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    Publication Date: 2015-08-05
    Description: Superimposed on the radial solidification of Earth's inner core may be hemispherical and/or regional patches of melting at the inner-outer core boundary. Little work has been carried out on partial melting of a dendritic mushy layer due to heating from above. Here we study directional solidification, annealing, and partial melting from above of Pb-rich Sn alloy ingots. We find that partial melting from above results in convection in the mushy layer, with dense, melted Pb sinking and re-solidifying at a lower height, yielding a different density profile than for those ingots that are just directionally solidified, irrespective of annealing. Partial melting from above causes a greater density deeper down and a corresponding steeper density decrease nearer the top. There is also a change in microstructure. These observations may be in accordance with inferences of east-west and perhaps smaller scale variations in seismic properties near the top of the inner core.
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  • 18
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    Flat Slab Deformation Caused by Interplate Suction Force (2015)
    Wiley
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    Publication Date: 2015-08-21
    Description: We image the structure at the southern end of the Peruvian flat subduction zone, using receiver function and surface wave methods. The Nazca slab subducts to ~100 km depth, then remains flat for ~300 km distance before it resumes the dipping subduction. The flat slab closely follows the topography of the continental Moho above, indicating a strong suction force between the slab and the over-riding plate. A high velocity mantle wedge exists above the initial half of the flat slab, and the velocity resumes to normal values before the slab steepens again, indicating the resumption of dehydration and ecologitization. Two prominent mid-crust structures are revealed in the 70 km thick crust under the Central Andes: molten rocks beneath the Western Cordillera, and the underthrusting Brazilian Shield beneath the Eastern Cordillera.
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  • 19
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    Evaporative-cooling amplification of the entrainment velocity in radiatively-driven stratocumulus (2015)
    Wiley
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    Publication Date: 2015-08-21
    Description: Evaporative cooling monotonically increases as the thermodynamical properties of the inversion allow for more evaporation in shear-free radiatively-driven stratocumulus. However, the entrainment velocity can deviate from the evaporative-cooling trend and even become insensitive to variations in the inversion properties. Here, the efficiency of evaporative cooling at amplifying the entrainment velocity is quantified by means of direct numerical simulations of a cloud-top mixing layer. We demonstrate that variations in the efficiency modulate the effect of evaporative cooling on entrainment, explaining the different trends. These variations are associated with the evaporation of droplets in cloud holes bellow the inversion point. The parametrization of the efficiency provides the evaporative amplification of the entrainment velocity as a function of a single parameter that characterizes the inversion. The resulting entrainment velocities match our experiments and previous measurements to within ±25%. The parametrization also predicts the transition to a broken cloud field consistently with observations.
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  • 20
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    TES Observations of the Interannual Variability of PAN over Northern Eurasia and the Relationship to Springtime Fires (2015)
    Wiley
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    Publication Date: 2015-08-23
    Description: Peroxyacetyl nitrate (PAN) plays an important role in atmospheric chemistry through its impact on remote oxidant and nitrogen budgets. PAN is formed rapidly in boreal fire plumes through the oxidation of short-lived volatile organic compounds in the presence of nitrogen oxide radicals. Here we present new satellite observations of PAN from the Tropospheric Emission Spectrometer (TES) over northern Eurasia for April 2006 – 2010. We observe large interannual variability in TES PAN observations, and we show that fires are one source of this variability using 1) Moderate Resolution Imaging Spectroradiometer (MODIS) Mean Fire Radiative Power (FRP) observations, and 2) Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) backward trajectories. We also show that cold springtime temperatures and enhanced vertical mixing in the lower free troposphere over northeastern Eurasia likely played a role in the detection of PAN from TES in April 2006 in this region.
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  • 21
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    Possible seasonality in large deep-focus earthquakes (2015)
    Wiley
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    Publication Date: 2015-08-23
    Description: Large deep-focus earthquakes (magnitude 〉 7.0, depth 〉 500km) have exhibited strong seasonality in their occurrence times since the beginning of global earthquake catalogs. Of 60 such events from 1900 to present, 42 have occurred in the middle half of each year. The seasonality appears strongest in the northwest Pacific subduction zones, and weakest in the Tonga region. Taken at face value, the surplus of summer events is statistically significant, but due to the ex post facto hypothesis testing, the absence of seasonality in smaller deep earthquakes, and the lack of a known physical triggering mechanism, we cannot rule out that the observed seasonality is just random chance. However, we can make a testable prediction of seasonality in future large deep-focus earthquakes, which, given likely earthquake occurrence rates, should be verified or falsified within a few decades. If confirmed, deep earthquake seasonality would challenge our current understanding of deep earthquakes.
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  • 22
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    Grounding line retreat of Totten Glacier, East Antarctica, 1996 to 2013 (2015)
    Wiley
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    Publication Date: 2015-08-23
    Description: Totten Glacier, East Antarctica, a glacier that holds a 3.9 m sea level change equivalent, has thinned and lost mass for decades. We map its grounding line positions in 1996 and 2013 using differential radar interferometry (InSAR) data, and develop precise, high-resolution topographies of its ice surface and ice draft using NASA Operation IceBridge data, InSAR data and a mass conservation method. We detect a 1 to 3 km retreat of the grounding line in 17 years. The retreat is asymmetrical along a two-lobe pattern, where ice is only grounded a few 10 m above sea level, or ice plain, which may unground further with only modest amounts of ice thinning. The pattern of retreat indicates ice thinning of 12 m in 17 years or 0.7±0.1 m/yr at the grounding line on average. Sustained thinning will cause further grounding line retreat, but may not be conducive to a marine instability.
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  • 23
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    Long-range transport across the Atlantic in summertime does not enhance the hygroscopicity of African mineral dust (2015)
    Wiley
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    Publication Date: 2015-08-23
    Description: We present the first direct evidence that the hygroscopic properties of super-micronic (〉1 µm) African dust particles did not change despite undergoing long-range transport across the Atlantic towards the Caribbean. Concurrent measurements of chemical composition show that most of mineral dust were chemically unprocessed and externally mixed. A minor portion of mineral dust was internally mixed with sulfate and chloride (~13–24% by number) or aggregated with sea-salt particles (~3–6%). Only dust particles aggregated with sea salt showed significant hygroscopic growth above 75% RH, resulting in a decrease in extinction mass efficiency by up to a factor 2.2. All other dust particles did not take up significant amounts of water when exposed to up to 94% RH. These results demonstrate that the direct radiative effect of African dust in this region remained independent of RH and an external mixing state could be considered for evaluating the climate effects of dust.
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  • 24
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    Overlap statistics of shallow boundary layer clouds: comparing ground-based observations with large-eddy simulations (2015)
    Wiley
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    Publication Date: 2015-08-23
    Description: High-resolution ground-based measurements are used to assess the realism of fine-scale numerical simulations of shallow cumulus cloud fields. The overlap statistics of cumuli as produced by large-eddy simulations (LES) are confronted with CloudNet datasets at the Jülich Observatory for Cloud Evolution (JOYCE). The CloudNet pixel is small enough to detect cumuliform cloud overlap. Cloud fraction masks are derived for five different cases, using gridded time-height datasets at various temporal and vertical resolutions. The overlap ratio (R), i.e. the ratio between cloud fraction by volume and by area, is studied as a function of the vertical resolution. Good agreement is found between R derived from observations and simulations. An inverse linear function is found to best describe the observed overlap behavior, confirming previous LES results. Simulated and observed decorrelation lengths are smaller (~300 m) than previously reported (〉 1km). A similar diurnal variation in the overlap efficiency is found in observations and simulations.
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    Observation of charged nanograins at comet 67P/Churyumov-Gerasimenko (2015)
    Wiley
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    Publication Date: 2015-08-23
    Description: Soon after the Rosetta Orbiter rendezvoused with Comet 67P/Churyumov-Gerasimenko at a solar distance of ~3.5 AU and began to fly in triangular-shaped trajectories around it, the Ion and Electron Sensor detected negative particles at energies from about one hundred eV/q to over 18 keV/q. The lower-energy particles came from roughly the direction of the comet; the higher-energy particles came from approximately the solar direction. These particles are interpreted as clusters of molecules, most likely water, which we refer to as nanograins because their inferred diameters are less than one hundred nm. Acceleration of the grains away from the comet is through gas drag by the expanding cometary atmosphere while acceleration back to the vicinity of the comet is caused partly by solar radiation pressure but mainly by the solar-wind electric field. These observations represent the first measurements of energetic charged submicron-sized dust or ice grains (nanograins) in a cometary environment.
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    Background seismicity rate at subduction zones linked to slab-bending-related hydration (2015)
    Wiley
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    Publication Date: 2015-08-23
    Description: Tectonic properties strongly control variations in seismicity among subduction zones. In particular, fluid distribution in subduction zones influences earthquake occurrence, and it varies among subduction zones due to variations in fluid sources such as hydrated oceanic plates. However, the relationship between variations in fluid distribution and variations in seismicity among subduction zones is unclear. Here, we divide Earth's subduction zones into 111 regions and estimate background seismicity rates using the epidemic type aftershock sequence (ETAS) model. We demonstrate that background seismicity rate correlates to the amount of bending of the incoming oceanic plate, which in turn is related to the hydration of oceanic plates via slab-bending-related faults. Regions with large bending may have high seismicity rates because a strongly hydrated oceanic plate causes high pore-fluid pressure and reduces the strength of the plate interface. We suggest that variations in fluid distribution can also cause variations in seismicity in subduction zones.
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    Understanding the contrasting North Atlantic Oscillation anomalies of the winters of 2010 and 2014 (2015)
    Wiley
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    Publication Date: 2015-08-23
    Description: The present study investigates the two contrasting winters of 2010 and 2014 during which the North Atlantic Oscillation (NAO) was mainly negative and positive respectively. In the North Pacific, contrasting anomalies were also present, with a straight zonal Pacific jet in 2010 and a strong poleward deviation of the Pacific jet in its exit region in 2014. Using reanalysis datasets and adopting a nonlinear initial-value approach with a quasi-geostrophic model, we show that the Pacific-North American anomalies are responsible for shaping synoptic wave trains propagating across North America. This in turn largely determines the nature of wave breaking and the synoptic eddy feedback onto the mean flow in the North Atlantic and finally the NAO phase. In such a proposed mechanism, synoptic wave activity forms the cornerstone of the dynamical relationship between the North Pacific and North Atlantic large-scale anomalies during the contrasting winters of 2010 and 2014.
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    Ion temperature anisotropy across a magnetotail reconnection jet (2015)
    Wiley
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    Publication Date: 2015-08-25
    Description: A significant fraction of the energy released by magnetotail reconnection appears to go into ion heating, but this heating is generally anisotropic. We examine ARTEMIS dual-spacecraft observations of a long-duration magnetotail exhaust generated by anti-parallel reconnection in conjunction with Particle-In-Cell simulations, showing spatial variations in the anisotropy across the outflow far (〉 100 d i ) downstream of the X-line. A consistent pattern is found in both the spacecraft data and the simulations: Whilst the total temperature across the exhaust is rather constant, near the boundaries T i ,|| dominates. The plasma is well-above the firehose threshold within patchy spatial regions at | B X | ∈ [0.1, 0.5] B 0 , suggesting that the drive for the instability is strong and the instability is too weak to relax the anisotropy. At the mid-plane (| B X |0.1  B 0 ), T i ,⊥  〉  T i ,|| and ions undergo Speiser-like motion despite the large distance from the X-line.
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    On the Confinement of Lunar Induced Magnetic Fields (2015)
    Wiley
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    Publication Date: 2015-08-25
    Description: We examine the confinement of induced magnetic fields on the lunar dayside and nightside, when the Moon is in the solar wind. We use a three-dimensional hybrid model of plasma and place a dipole magnetic field at the center of the Moon to mimic the induced magnetic field, which is the response of the lunar interior to the time-varying interplanetary magnetic field. Consistent with previous observations and theoretical predictions, we show that the induced magnetic fields on the dayside are confined within the lunar surface through a dayside current sheet. In contrast to previous work, we show that the induced magnetic fields are not confined in the lunar wake, and they leak out, sometimes even appearing as lunar limb compressions. Finally, we identify favorable places to observe induced magnetic fields by electromagnetic sounding techniques, which will help to better constrain the lunar electrical conductivity profile, and interior structure.
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    High effusion rates of the Cordón Caulle 2011-12 eruption (Southern Andes) and their relation with the quasi-harmonic tremor (2015)
    Wiley
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    Publication Date: 2015-08-24
    Description: High effusion rates of intermediate-to-high-silica lavas seem to be less uncommon than previously thought, in particular during their initial eruptive stages. In this study, we report satellite-based time-averaged discharge rates for the 2011-2012 effusive phase at Cordón Caulle, which are well correlated with the evolution of the quasi-harmonic tremor, the most significant seismic signal after the initial explosive stage. Such a correspondence could become a key method for detection of the onset of effusive phases, especially in remote and/or very cloudy areas, supplying an additional tool for effective warnings and near real-time hazards appraisal.
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    Evidence for the Episodic Erosion of the Medusae Fossae Formation Preserved within the Youngest Volcanic Province on Mars (2015)
    Wiley
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    Publication Date: 2015-08-25
    Description: We use orbital SHARAD sounder data to three-dimensionally visualize the subsurface structure of Elysium Planitia, the youngest volcanic province on Mars. Our results reveal an emplacement history consisting of multiple groups of overlapping lava flow units, originating from different sources. The uniquely complex “radar stratigraphy” of Elysium Planitia, relative to other volcanic regions, requires a distinct mechanism to generate the numerous reflectors observed in SHARAD data. Sedimentary deposits interbedded with successive batches of lava flows could account for the elaborate pattern of reflectors. We infer that widespread, rapidly emplaced material sourced from the enigmatic Medusae Fossae Formation (MFF) creates these sedimentary layers. This implies that episodes of atmospheric activity, perhaps linked with the obliquity of Mars, periodically erode and redeposit material from the MFF across a large region.
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    Observed eastward progression of the Fukushima 134Cs signal across the North Pacific (2015)
    Wiley
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    Publication Date: 2015-08-12
    Description: Radionuclide samples taken as part of hydrographic surveys at 30°N in the North Pacific reveal that the easternmost edge of Fukushima-derived 134 Cs observed at 174.3°W in 2012 had progressed eastward across the basin to 160.6°W by 2013. The 2013 30°N observations indicate surface 134 Cs concentrations of 3–5 Bq/m 3 between 160°E and 160°W, slightly lower concentrations west of 160°E and no detectable signal east of 160.6°W. Profile samples show 134 Cs penetration to 500 m west of 180° with shoaling penetration depth toward to the east. The near-uniform vertical distribution of 137 Cs between 152°W and 121.3°W in the top 500 m is indicative of trace amounts of radionuclides remaining from weapons testing. The physical processes responsible for the deep 134 Cs penetration in the western Pacific appear to be related to distinct water mass subduction pathways, however the timing and rapidity of deep penetration over the broad scales observed has yet to be clarified.
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    Lateral mixing across ice meltwater fronts of the Chukchi Sea shelf (2015)
    Wiley
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    Publication Date: 2015-08-15
    Description: Summer and fall hydrographic sections in the northeastern Chukchi Sea frequently capture 5 – 20 m thick intrapycnocline lenses or horizontal plumes of warm, moderately salty summer Bering Sea Water flowing northward from Bering Strait. These features occur within the shallow (~20 m depth) pycnocline separating cold, dilute, surface meltwater from near-freezing, salty, winter-formed waters beneath the pycnocline. An idealized numerical model suggests that the features arise from eddies and meanders generated by instability of the surface front separating meltwater from Bering Sea Water. Warm Bering Sea Water is transported across the front and into the pycnocline by the cross-frontal velocities associated with the instabilities. The accompanying lateral eddy heat fluxes may be important both in summer for promoting ice melt and in fall by delaying the onset of ice formation over portions of this shelf. Lateral heat flux magnitudes depend upon the stratification of the Bering Sea Water.
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    Statistics and accuracy of magnetic null identification in multi-spacecraft data (2015)
    Wiley
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    Publication Date: 2015-08-15
    Description: Complex magnetic topologies are ubiquitous in astrophysical plasmas. Analyzing magnetic nulls, regions of vanishing magnetic field, is one way to characterize 3D magnetic topologies. Magnetic nulls are believed to be important in 3D reconnection and turbulence. In the vicinity of a null, plasma particles become unmagnetized and can be accelerated to high energies by electric fields. We present the first statistical study of the occurrence of magnetic nulls and their types in the Earth's nightside magnetosphere. We are able to identify the nulls both in the tail and in the magnetopause current sheets. On average we find one null for every few current sheet crossings. We show that the type-identification of magnetic nulls may be sensitive to local fluctuations in the magnetic field. We develop and demonstrate a method to estimate the reliability of the magnetic null type-identification.
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    The 2003–2004 seismic swarm in the western Corinth rift: Evidence for a multiscale pore pressure diffusion process along a permeable fault system (2015)
    Wiley
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    Publication Date: 2015-08-15
    Description: Microseismic multiplets occurring in the western Corinth rift, Greece, during a large swarm are analyzed to retrieve their spatio-temporal characteristics. These multiplets activated small subfaults at depth (~7 km), up to 1 km long, at the root oftwo parallel active normal faults. The swarm migrates westward nearly horizontally over 10 km at an average velocity of 50 m/day with a diffusivity of 0.5 m 2 ∙s −1 . It successively activates the Aigion fault, a relay zone in its hanging wall, and the Fassouleika fault. Within each multiplet, hypocenters also migrate with diffusivities ranging from 0.001 to 0.4 m 2 ∙s −1 . The largest internal diffusivities appear at the core of the layer defined by the clusters. These results are interpreted as a hydroshear process caused by pore pressure migration within permeable corridors resulting from the intersection of the major faults with a brittle geological layer inherited from the Hellenic nappe stack.
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    On solar geoengineering and climate uncertainty (2015)
    Wiley
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    Publication Date: 2015-08-15
    Description: Uncertain climate system response has been raised as a concern regarding solar geoengineering. We explore the effects of geoengineering on one source of climate system uncertainty by evaluating the inter-model-spread across 12 climate models participating in the Geoengineering Model Intercomparison project (GeoMIP). The model-spread in simulations of climate change and the model-spread in the response to solar geoengineering are not additive but rather partially cancel. That is, the model-spread in regional temperature and precipitation changes is reduced with CO 2 and a solar reduction, in comparison to the case with increased CO 2 alone. Furthermore, differences between models in their efficacy (the relative global mean temperature effect of solar versus CO 2 radiative forcing) explain most of the regional differences between models in their response to an increased CO 2 concentration that is offset by a solar reduction. These conclusions are important for clarifying geoengineering risks regarding uncertainty.
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    On the escape of CH4 from Pluto's atmosphere (2015)
    Wiley
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    Publication Date: 2015-08-15
    Description: We adapted a multi-species escape model, developed for close-in extrasolar planets, to calculate the escape rates of CH 4 and N 2 from Pluto. In the absence of escape, CH 4 should overtake N 2 as the dominant species below the exobase. The CH 4 profile depends strongly on the escape rate, however, and the typical escape rates predicted for Pluto lead to a nearly constant mixing ratio of less than 1 % below the exobase. In this case the CH 4 escape rate is only 5–10 % of the N 2 escape rate. Observations of the CH 4 profile by the New Horizons/ALICE spectrograph can constrain the CH 4 escape rate and provide a unique test for escape models.
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    Imaging widespread seismicity at mid-lower crustal depths beneath Long Beach, CA, with a dense seismic array: Evidence for a depth-dependent earthquake size distribution (2015)
    Wiley
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    Publication Date: 2015-07-30
    Description: We use a dense seismic array composed of 5200 vertical geophones to monitor microseismicity in Long Beach, California. Poor signal-to-noise ratio due to anthropogenic activity is mitigated via downward-continuation of the recorded wave field. The downward-continued data are continuously back-projected to search for coherent arrivals from sources beneath the array, which reveals numerous, previously undetected events. The spatial distribution of seismicity is uncorrelated with the mapped fault traces, or with activity in the nearby oil-fields. Many events are located at depths larger than 20 km, well below the commonly accepted seismogenic depth for that area. The seismicity exhibits temporal clustering consistent with Omori's law, and its size distribution obeys the Gutenberg-Richter relation above 20 km, but falls off exponentially at larger depths. The dense array allows detection of earthquakes two magnitude units smaller than the permanent seismic network in the area. Because the event size distribution above 20 km depth obeys a power-law whose exponent is near one, this improvement yields a hundred-fold decrease in the time needed for effective characterization of seismicity in Long Beach.
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    What hindered the El Niño pattern in 2014? (2015)
    Wiley
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    Publication Date: 2015-07-30
    Description: At the beginning of 2014, an El Niño event was predicted to occur in the following winter. However, the El Niño that started to develop in 2014 was hindered in the boreal summer, and only the ocean reached a weak El Niño condition. This outcome was largely attributed to a suppressed ocean–atmosphere interaction caused by anomalous easterly winds in the eastern equatorial Pacific. These winds were related to negative sea surface temperature anomalies (SSTAs) in the southeastern subtropical Pacific (SESP). The negative phase of the Interdecadal Pacific Oscillation (IPO) laid the foundation for the persistence of cooler SSTAs and enhanced trade winds in the SESP after the year 2000. As the recent IPO downward trend continued, the SSTAs in SESP reached an extremely low value in the boreal summer of 2014 and imposed a serious obstacle to the evolution of a warming event.
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    Water exchange between the continental shelf and the cavity beneath Nioghalvfjerdsbræ (79 North Glacier) (2015)
    Wiley
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    Publication Date: 2015-08-04
    Description: The mass loss at Nioghalvfjerdsbræ is primarily due to rapid submarine melting. Ocean data obtained from beneath the Nioghalvfjerdsbræ ice tongue show that melting is driven by the presence of warm (1°C) Atlantic Intermediate Water (AIW). A sill prevents AIW from entering the cavity from Dijmphna Sund, requiring that it flow into the cavity via bathymetric channels to the south at a pinned ice front. Comparison of water properties from the cavity, Dijmphna Sund, and the continental shelf support this conclusion. Overturning circulation rates inferred from observed melt rates and cavity stratification suggest an exchange flow between the cavity and the continental shelf of 38 mSv, sufficient to flush cavity waters in under one year. These results place upper bounds on the timescales of external variability that can be transmitted to the glacier via the ice tongue cavity.
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    Atlantic opportunities for ENSO prediction (2015)
    Wiley
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    Publication Date: 2015-08-04
    Description: El Niño-Southern Oscillation (ENSO) is the dominant mode of inter-annual climate variability with worldwide impacts. The knowledge of ENSO drivers and the underlying mechanisms is crucial to improve ENSO prediction, which still remains a challenge. The recently discovered connection between an Atlantic Niño (Niña) and a Pacific Niña (Niño), through an air-sea coupled mechanism during the first and last decades of the 20 th century, highlights an opportunity for ENSO prediction. Here, a statistical cross-validated hindcast of ENSO along the 20 th century is presented, considering the Atlantic Sea Surface Temperatures as the unique predictor field, and a set of atmospheric and oceanic variables related to the Atlantic-Pacific connection as the predictand field. The observed ENSO phase is well reproduced and the skill is enhanced at the beginning and the end of the 20 th century. Understanding this multidecadal modulation of the Atlantic-Pacific connection could help to improve seasonal-to-decadal forecasts of ENSO and its associated impacts.
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    Temporal Variation in the Magnitude-Frequency Distribution during the Guy-Greenbrier Earthquake Sequence (2015)
    Wiley
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    Publication Date: 2015-08-04
    Description: The recent increase in earthquake activity in the central U.S. has led to concerns about the hazard posed by induced earthquakes. Understanding earthquake phenomena and monitoring in all settings can be improved by the detection of small events; however, the catalog of induced earthquakes is incomplete for small events due to the sparse instrumentation. This is particularly true in settings of low background seismicity, like the central U.S. We apply single-station template matching to detect small earthquakes during the Guy-Greenbrier sequence in central Arkansas, and find over 100 times more earthquakes than are registered in the ANSS catalog between July 2010 and October 2011. A complete catalog over nearly four units of magnitude enables us to analyze the magnitude-frequency distribution of induced earthquakes. We find that earthquakes deviated from the Gutenberg-Richter statistics during the operation of nearby injection wells, but returned to Gutenberg-Richter statistics after the wells were shut in.
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    Retrieving 65 years of volcano summit deformation from multi-temporal Structure-from-Motion: the case of Piton de la Fournaise (La Réunion Island) (2015)
    Wiley
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    Publication Date: 2015-08-04
    Description: The structure-from-Motion photogrammetry technique enables use of historical airborne photography to achieve high-resolution 3D terrain models. We apply this method on Piton de la Fournaise volcano (La Réunion), which allows a unique opportunity to retrieve high-resolution (1.5-0.11 m) Digital Elevation Models and precise deformation maps of the volcano since 1950. Our results provide evidence that the summit volume increased throughout the study period, at a stable rate of 2.2 Mm 3 /yr between 1950 and 2000, increasing by a factor of four (to 8.0Mm 3 /yr) prior to the major 2007 eruption which was accompanied by summit caldera collapse. At the same time, summit deformation was asymmetric, with 9.2±2.5 m of eastward seaward displacement, and 1.3±2.5 m to the west during 1950-2015. Our results reveal a temporal evolution in the volcano magma influx rate and deformation. Tracking these fluxes and the long-lived preferential eastern motion is crucial to mitigate risks associated to flank destabilization.
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    A Global assessment of accelerations in surface mass transport (2015)
    Wiley
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    Publication Date: 2015-08-04
    Description: Water mass transport in the Earth's dynamic surface layer of atmosphere, cryosphere, and hydrosphere driven by various global change processes has complex spatiotemporal patterns. Here we determine global patterns and regional mean values of accelerations in surface mass variations during the Gravity Recovery and Climate Experiment (GRACE) mission's data span from 2002.2 to 2015.0. GRACE gravity data are supplemented by surface deformation from 607 Global Navigation Satellite System (GNSS) stations, an ocean bottom pressure model, satellite laser ranging (SLR), and loose a priori knowledge on mass variation regimes incorporating high-resolution geographic boundaries. While Greenland and West Antarctica have strong negative accelerations, Alaska and the Arctic Ocean show significant positive accelerations. In addition, the accelerations are not constant in time with some regions showing considerable variability due to irregular interannual changes. No evidence of significant non-steric mean sea level acceleration has been found, but the uncertainty is quite large.
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    More tropical cyclones in a cooler climate? (2015)
    Wiley
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    Publication Date: 2015-08-04
    Description: Recent review papers reported that many high-resolution global climate models consistently projected a reduction of global tropical cyclone (TC) frequency in a future warmer climate, although the mechanism of the reduction is not yet fully understood. Here we present a result of 4K-cooler climate experiment. The global TC frequency significantly increases in the 4K-cooler climate compared to the present climate. This is consistent with a significant decrease in TC frequency in the 4K-warmer climate. For the mechanism of TC frequency reduction in a warmer climate, upward mass flux hypothesis and saturation deficit hypothesis have been proposed. The result of the 4K-cooler climate experiment is consistent with these two hypotheses. One very interesting point is that the experiment has clearly shown that TC genesis is possible at sea surface temperature (SST) well below 26°C which has been considered as the lowest SST limit for TC genesis.
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    An index to characterize the spatial distribution of land use within watersheds and implications for river network nutrient removal and export (2015)
    Wiley
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    Publication Date: 2015-08-05
    Description: The spatial distribution of land use and associated nutrient inputs may influence the efficacy of in-stream nutrient removal however, the effect of source location on N removal and watershed N export has not been quantified. We present the skewness index, a metric to quantify the spatial distribution of land use within watersheds. Using this index and a river network nitrogen removal model, we quantified the effect of varying the location of developed land use within two watersheds on nutrient removal and export. The quantity and location of developed land use as well as runoff affected nitrogen removal and export. Because river network nitrogen removal is bypassed when sources are skewed towards the watershed mouth, varying the location of land use alone can double aquatic nitrogen removal. Nutrient sources skewed towards the distant headwaters maximized in-stream removal which in turn can reduce watershed export.
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    Ambient resonance of Mesa Arch, Canyonlands National Park, Utah (2015)
    Wiley
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    Publication Date: 2015-08-06
    Description: We analyzed the resonance characteristics of a prominent natural arch in Canyonlands National Park, Mesa Arch, as measured from ambient seismic data. Evaluating spectral and polarization attributes, we distinguished the first four resonant frequencies of the arch: 2.9, 6.0, 6.9, and 8.5 Hz, as well as basic properties of the associated mode shapes. We then affirmed experimental data using 3D numerical modal analysis, providing estimates of material properties and clarifying vibrational mode shapes. Monitoring resonant frequencies over time, we searched for shifts associated with changing environmental conditions and long-term progressive damage. We measured ~3% direct daily variation in resonant frequency associated with changing rock temperature, thermal stress and stiffening of the rock matrix. Independent tilt data showed similar diurnal cycles associated with thermo-elastic stresses and deformation of the arch. We observed no permanent resonant frequency shifts related to irreversible damage of Mesa arch during our study period.
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    Observations of a large-scale gravity wave propagating over an extremely large horizontal distance in the thermosphere (2015)
    Wiley
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    Publication Date: 2015-08-08
    Description: In this paper we report the detection of a large-scale gravity wave propagating over an extremely large horizontal distance in the thermosphere on July 28, 2006. Specifically, after being launched at the northern auroral region on the dayside, this wave propagated equatorward with phase speeds on the order of ∼720 m/s, and finally almost traveled around the Earth once horizontally in the thermosphere prior to dissipation. The time taken to dissipate is about 15.5 hours. It is the farthest-traveling large-scale gravity wave currently tracked by satellite measurements, made possible by a sudden injection of energy in an unusually clean propagation environment. This experiment of opportunity serves as an important step in furthering our theoretical understanding of gravity wave propagation and dissipation in the thermosphere.
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    Differences in size spectra of electrified storms over land and ocean (2015)
    Wiley
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    Publication Date: 2015-08-08
    Description: Numerous studies demonstrate the marked difference in intensity of land and ocean convection, the most pronounced being the higher frequency of intense convection and lightning over land. This study contributes to the ongoing debate of the physical reasons for this difference, through the use of the Tropical Rainfall Measuring Mission (TRMM) Precipitation Feature (PF) database. We demonstrate that tropical oceanic PFs with lightning tend to be over 10 times larger and more mature than those over land. This relationship is most pronounced in archetypal land and ocean regimes (Congo and Central Pacific), and is also manifested throughout the entire (35 ∘ S to 35 ∘ N) TRMM domain, to varying extents. This relationship holds true when we select for PFs with 40 dBZ echo tops over at least 5.5 km in height, which continues to be an effective proxy for convective intensity and probability of lightning.
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    Feedback temperature dependence determines the risk of high warming (2015)
    Wiley
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    Publication Date: 2015-06-03
    Description: The long-term warming from an anthropogenic increase in atmospheric CO 2 is often assumed to be proportional to the forcing associated with that increase. This paper examines this linear approximation using a zero-dimensional energy balance model with a temperature-dependent feedback, with parameter values drawn from physical arguments and general circulation models. For a positive feedback temperature dependence, warming increases Earth's sensitivity, while greater sensitivity makes Earth warm more. These effects feed on each other, amplifying warming. As a result, for reasonable values of feedback temperature dependence and preindustrial feedback, Earth can jump to a warmer state under only one or two CO 2 doublings. The linear approximation breaks down in the long tail of high climate sensitivity commonly seen in observational studies. Understanding feedback temperature dependence is therefore essential for inferring the risk of high warming from modern observations. Studies that assume linearity likely underestimate the risk of high warming.
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    Time scales and ratios of climate forcing due to thermal versus carbon dioxide emissions from fossil fuels (2015)
    Wiley
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    Publication Date: 2015-06-03
    Description: The Earth warms both when fossil fuel carbon is oxidized to carbon dioxide and when greenhouse effect of carbon dioxide inhibits longwave radiation from escaping to space. Various important time scales and ratios comparing these two climate forcings have not previously been quantified. For example, the global and time-integrated radiative forcing from burning a fossil fuel exceeds the heat released upon combustion within 2 months. Over the long lifetime of CO 2 in the atmosphere, the cumulative CO 2 -radiative forcing exceeds the amount of energy released upon combustion by a factor 〉100,000. For a new power plant, the radiative forcing from the accumulation of released CO 2 exceeds the direct thermal emissions in less than half a year. Furthermore, we show that the energy released from the combustion of fossil fuels is now about 1.71% of the radiative forcing from CO 2 that has accumulated in the atmosphere as a consequence of historical fossil fuel combustion.
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    Differences in decadal trends of atmospheric mercury between the Arctic and northern mid-latitudes suggest a decline in Arctic Ocean mercury (2015)
    Wiley
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    Publication Date: 2015-06-03
    Description: Atmospheric mercury (Hg) in the Arctic shows much weaker or insignificant annual declines relative to northern mid-latitudes over the past decade (2000-2009), but with strong seasonality in trends. We use a global ocean-atmosphere model of Hg (GEOS-Chem) to simulate these observed trends and determine the driving environmental variables. The atmospheric decline at northern mid-latitudes can largely be explained by decreasing North Atlantic oceanic evasion. The mid-latitude atmospheric signal propagates to the Arctic but is there countered by rapid Arctic warming and declining sea ice, which suppresses deposition and promotes oceanic evasion over the Arctic Ocean. The resulting simulation implies a decline of Hg in the Arctic surface ocean that we estimate to be -0.67% yr -1 over the study period. Rapid Arctic warming and declining sea ice are projected for future decades and would drive a sustained decline in Arctic Ocean Hg, potentially alleviating the methylmercury exposure risk for northern populations.
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    Interpreting Fluid Pressure Anomalies in Shallow Intraplate Argillaceous Formations (2015)
    Wiley
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    Publication Date: 2015-06-03
    Description: Investigations have revealed several instances of apparently isolated highs or lows in pore fluid potential in shallow (〈 ~ 1 km depth) argillaceous formations in intraplate settings. Formations with the pressure anomalies are distinguished by (1) smaller ratios of hydraulic conductivity to formation thickness, and (2) smaller hydraulic (or pressure) diffusivities than those without anomalies. This is consistent with transient Darcian flow caused by strain at rates of ~ 10 -17 to 10 -16 s -1 , by significant perturbing events in the past 10 4 to 10 6 a, or by some combination of the two. Plausible causes include erosional downwasting, tectonic strain, and glaciation. In this conceptualization the anomalies provide constraints on formation-scale flow properties, flow history, and local geological forcing in the last 10 6 a, and in particular indicate zones of low permeability (10 -19 – 10 -22 m 2 ) that could be useful for isolation of nuclear waste.
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    Interannual variability of the Madden-Julian Oscillation and its impact on the North Atlantic Oscillation in the boreal winter (2015)
    Wiley
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    Publication Date: 2015-06-03
    Description: Interannual variability of the Madden-Julian Oscillation (MJO) and its influence on the extratropical teleconnection are analyzed. It is found that there is an interannual shift of the MJO activity between the Indian Ocean and the western-central Pacific. This is reflected by the year-to-year changes in the occurrence frequency of individual MJO phases. The leading mode of an empirical orthogonal function (EOF) analysis of occurrence frequency of MJO phases shows an out-of-phase relationship between MJO phase 7 and phases 3–5. During winters when the MJO convection occurs more frequently in the Indian Ocean (phases 3–5), a winter-mean positive North Atlantic Oscillation (NAO) tends to occur. These results indicate that the seasonal mean convection activity over the Indian Ocean related to the MJO is a possible driving forcing for the seasonal mean NAO variability in the boreal winter.
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    Barrier-induced supershear ruptures on a slip-weakening fault (2015)
    Wiley
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    Publication Date: 2015-06-04
    Description: Here we investigate the effects of a patch with elevated effective normal stress (barrier) on two dimensional in-plane supershear rupture propagation on a planar fault from numerical experiments. Our results confirm that the barrier may slow down or stop coseismic ruptures, but may also induce supershear ruptures. We demonstrate that the supershear rupture may emerge in a region that is delineated by two approximate linear boundaries. If the barrier size is below the lower boundary, ruptures can overcome the barrier and propagate at subshear speeds. If the barrier size is larger than the upper boundary, ruptures are always stopped by the barrier. Furthermore, we find that the barrier-induced supershear ruptures may eventually slow down into subshear speed, depending on the size and the location of the barrier. The duration of supershear ruptures increases as the barrier sizes grow from the lower to the upper boundary, which are proportional to the reduction in rupture speeds caused by the barrier. These results indicate that a barrier on the fault may not stop coseismic ruptures. Rather, the barrier may induce ruptures propagating at supershear speeds that play important roles in near-field ground shaking and damage.
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    Isolated cases of remote dynamic triggering in Canada detected using cataloged earthquakes combined with a matched-filter approach (2015)
    Wiley
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    Publication Date: 2015-06-04
    Description: Here we search for dynamically triggered earthquakes in Canada following global mainshocks between 2004 and 2014 with M S 〉 6, depth 〈 100 km, and estimated peak ground velocity 〉 0.2 cm/s. We use the Natural Resources Canada (NRCan) earthquake catalog to calculate β-statistical values in 1°×1° bins in 10-day windows before and after the mainshocks. The statistical analysis suggests triggering may occur near Vancouver Island, along the border of the Yukon and Northwest Territories, in western Alberta, western Ontario, and the Charlevoix seismic zone. We also search for triggering in Alberta where denser seismic station coverage renders regional earthquake catalogs with lower completeness thresholds. We find remote triggering in Alberta associated with three mainshocks using a matched-filter approach on continuous waveform data. The increased number of local earthquakes following the passage of mainshock surface waves suggests local faults may be in a critically stressed state.
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    Temporal stability and origin of chemoclines in the deep hypersaline anoxic Urania basin (2015)
    Wiley
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    Publication Date: 2015-06-04
    Description: Submarine brine lakes feature sharp and persistent concentration gradients between seawater and brine, though these should be smoothed out by free diffusion in open ocean settings. The anoxic Urania basin of the Eastern Mediterranean contains an ultra sulfidic, hypersaline brine of Messinian origin above a thick layer of suspended sediments. With a dual modeling approach we reconstruct its contemporary stratification by geochemical solute transport fundamentals, and show that thermal convection is required to maintain mixing in the brine and mud layer. The origin of the Urania basin stratification was dated to 1650 years before present, which may be linked to a major earthquake in the region. The persistence of the chemoclines may be key to the development of diverse and specialized microbial communities. Ongoing thermal convection in the fluid mud layer may have important, yet unresolved consequences for sedimentological and geochemical processes, also in similar environments.
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    Ocean Circulation and Biogeochemistry Moderate Inter-annual and Decadal Surface Water pH Changes in the Sargasso Sea (2015)
    Wiley
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    Publication Date: 2015-06-06
    Description: The oceans absorb anthropogenic CO 2 from the atmosphere lowering surface ocean pH, a concern for calcifying marine organisms. The impact of ocean acidification is challenging to predict as each species appears to respond differently, and because our knowledge of natural changes to ocean pH is limited in both time and space. Here we reconstruct 222 years of biennial seawater pH variability in the Sargasso Sea from a brain coral, Diploria labyrinthiformis . Using hydrographic data from the Bermuda Atlantic Time-Series Study (BATS) and the coral derived pH record, we are able to differentiate pH changes due to surface temperature versus those from ocean circulation and biogeochemical changes. We find that ocean pH does not simply reflect atmospheric CO 2 trends, but rather that circulation/biogeochemical changes account for 〉90% of pH variability in the Sargasso Sea and more variability in the last century than would be predicted from anthropogenic uptake of CO 2 alone.
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    What favors convective aggregation, and why? (2015)
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    Publication Date: 2015-06-06
    Description: The organization of convection is ubiquitous, but its physical understanding remains limited. One particular type of organization is the spatial self-aggregation of convection, taking the form of cloud clusters, or tropical cyclones in the presence of rotation. We show that several physical processes can give rise to self-aggregation, and highlight the key features responsible for it using idealized simulations. Longwave radiative feedbacks yield a “radiative aggregation" . In that case, sufficient spatial variability of radiative cooling rates yields a low-level circulation, which induces the up-gradient energy transport and radiative-convective instability. Not only do vertically-integrated radiative budgets matter, but the vertical profile of cooling is also crucial. Convective aggregation is facilitated when downdrafts below clouds are weak (“moisture-memory aggregation" ), and this is sufficient to trigger aggregation in the absence of longwave radiative feedbacks. These results shed some light on the sensitivity of self-aggregation to various parameters, including resolution or domain size.
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    Effects of initial conditions uncertainty on regional climate variability: An analysis using a low-resolution CESM ensemble (2015)
    Wiley
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    Publication Date: 2015-06-06
    Description: The uncertainties surrounding the initial conditions in Earth system models can considerably influence interpretations about climate trends and variability. Here we present results from a new climate change ensemble experiment using the Community Earth System Model (CESM), to analyze the effect of internal variability on regional climate variables that are relevant for decision-making. Each simulation is initialized from a unique and dynamically consistent model state sampled from a ~10,000 year fully-coupled equilibrium simulation, which captures the internal unforced variability of the coupled Earth system. We find that internal variability has a sizeable contribution to the modeled ranges of temperature and precipitation. The effects increase for more localized regions. The ensemble exhibits skill in simulating key regional climate processes relevant to decision makers, such as seasonal temperature variability and extremes. The presented ensemble framework and results can provide useful resources for uncertainty quantification, integrated assessment, and climate risk management.
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    Stratospheric sulfate aerosol injections cannot preserve the West Antarctic Ice Sheet (2015)
    Wiley
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    Publication Date: 2015-06-06
    Description: Injection of sulfate aerosols into the stratosphere has the potential to reduce the climate impacts of global warming, including sea level rise (SLR). However, changes in atmospheric and oceanic circulation that can significantly influence the rate ofbasal melting of Antarctic marine ice shelves and the associated SLR have not previously been considered. Here we use a fully-coupled global climate model to investigate whether rapidly increasing stratospheric sulfate aerosol concentrations after a period of global warming could preserve Antarctic ice sheets by cooling subsurface ocean temperatures. We contrast this climate engineering method with an alternative strategy in which all greenhouse gases (GHG) are returned to preindustrial levels. We find that the rapid addition of a stratospheric aerosol layer does not effectively counteract surface and upper level atmospheric circulation changes caused by increasing GHGs, resulting in continued upwelling of warm water in proximity of ice shelves, especially in the vicinity of the already unstable Pine Island Glacier in West Antarctica. By contrast, removal of GHGs restores the circulation, yielding relatively cooler subsurface ocean temperatures to better preserve West Antarctica.
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    Black Carbon Aerosols Induced Northern Hemisphere Tropical Expansion (2015)
    Wiley
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    Publication Date: 2015-06-06
    Description: Global Climate Models (GCMs) underestimate the observed trend in tropical expansion. Recent studies partly attribute it to black carbon aerosols (BC), which are poorly represented in GCMs. We conduct a suite of idealized experiments with the Community Atmosphere Model (CAM4) coupled to a slab ocean model forced with increasing BC concentrations covering a large swath of the estimated range of current BC radiative forcing while maintaining their spatial distribution. The Northern Hemisphere (NH) tropics expand polewards nearly linearly as BC radiative forcing increases (0.7 0 W −1 m 2 ), indicating that a realistic representation of BC could reduce GCM biases. We find support for the mechanism where BC induced midlatitude tropospheric heating shifts the maximum meridional tropospheric temperature gradient polewards resulting in tropical expansion. We also find that the NH poleward tropical edge is nearly linearly correlated with the location of the inter-tropical convergence zone (ITCZ), which shifts northwards in response to increasing BC.
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    Long-period ground motion simulation of a subduction earthquake using the offshore–onshore ambient seismic field (2015)
    Wiley
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    Publication Date: 2015-06-06
    Description: Large earthquakes that occur in subduction zones are likely to generate long-period ground motions that can cause severe damage even at great distances from the epicenter. We extracted surface-to-surface impulse response functions from the ambient seismic field recorded by offshore ocean bottom seismometers located atop the Nankai subduction zone and onshore stations, and showed that these offshore–onshore impulse response functions can be used to accurately simulate the long-period ground motions generated by an offshore moderate subduction earthquake. Moreover, we also found that the distributions of the earthquake and impulse response function pseudo-velocity response spectra have similar maximum amplifications in the same area close to the earthquake epicenter. This suggests that the ambient seismic field recorded by the increasing number of ocean bottom seismometers around the world can be used to assess seismic hazard related to offshore subduction earthquakes without prior knowledge of the velocity structure.
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    Stability and compressibility of a new iron-nitride β-Fe7N3 to core pressures (2015)
    Wiley
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    Publication Date: 2015-06-06
    Description: We have examined the phase relations in the iron-rich portion of the Fe-N binary system up to 150 GPa and 2720 K in a laser-heated diamond anvil cell (DAC). Synchrotron X-ray diffraction measurements revealed the formation of a new phase (β-Fe 7 N 3 ) above 41 GPa and ~1000 K. The β phase is structurally identical or very close to Fe 7 C 3 and the most iron-rich Fe-N intermediate compound at its stability field. We also measured the volume of β-Fe 7 N 3 to 132 GPa and found its elastic property very similar to that of Fe 7 C 3 . Recently, it has been proposed that the density and sound velocities, in particular shear wave velocity, in the Earth's inner core are reconciled with Fe 7 C 3 , but such properties could be explained by β-Fe 7 N 3 equally well. It is therefore possible that the inner core consists of Fe 7 (C,N) 3 , which is found as diamond inclusions and likely stable under inner core conditions.
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    Solar wind energy input to the magnetosheath and at the magnetopause (2015)
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    Publication Date: 2015-06-07
    Description: Using THEMIS observations, we show that the efficiency of the energy entry through the magnetopause as measured by the Poynting vector normal component depends on the combination of the solar wind speed and the southward component of the IMF: Most efficient energy transfer occurs when the IMF BZ is only moderately negative and the solar wind speed is high. This means that for the same level of solar wind driver parameters (electric field, epsilon, or other), different combinations of V and BZ will produce different driving at the magnetopause. The effect is strongest for low to moderate driving conditions, while the influence is smaller for the intense space weather events.
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    Southern Hemisphere extratropical circulation: Recent trends and natural variability (2015)
    Wiley
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    Publication Date: 2015-06-07
    Description: Changes in the Southern Annular Mode (SAM), Southern Hemi-sphere (SH) westerly jet location and magnitude are linked with changes in ocean circulation along with ocean heat and carbon uptake. Recent trends have been in observed these fields, but not much is known about the natural variability. Here we aim to quantify the natural variability of the SH extra tropical circulation by using Coupled Model Intercomparison Project Phase 5 (CMIP5) pre-industrial control model runs and compare with the observed trends in SAM, jet magnitude, and jet location. We show that trends in SAM are due partly to external forcing, but are not outside the natural variability as described by these models. Trends in jet location and magnitude, however, lie outside the unforced natural variability but can be explained by a combination of natural variability and the ensemble-mean forced trend. These results indicate that trends in these three diagnostics cannot be used inter-changeably.
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    Asymmetric Impact of Atlantic Multi-decadal Oscillation on El Niño and La Niña Characteristics (2015)
    Wiley
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    Publication Date: 2015-06-07
    Description: The long-lasting cold surface conditions of North Atlantic, i.e., the negative phase of Atlantic Multidecadal Oscillation (AMO), can intensify the El Niño-Southern Oscillation (ENSO) through the enhanced air-sea coupling under the increased central-to-eastern tropical Pacific mean sea surface temperature (SST). However, the impact of warmer mean SST is more efficient in the intensifying El Niño than La Niña, because of the nature of the exponential growth of atmospheric convection to SST change. Moreover, the farther eastward shift of the atmospheric convection during the negative AMO leads to the stronger El Niño due to the longer delayed negative feedback by oceanic waves. Therefore, the AMO mainly influences the El Nino intensity rather than La Nina intensity.
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    Timing and structure of Mega-SACZ events during Heinrich Stadial 1 (2015)
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    Publication Date: 2015-06-07
    Description: A substantial strengthening of the South American monsoon system (SAMS) during Heinrich Stadials (HS) points toward decreased cross-equatorial heat transport as the main driver of monsoonal hydroclimate variability at millennial time-scales. In order to better constrain the exact timing and internal structure of HS1 over tropical South America we assessed two precisely dated speleothem records from central-eastern and northeastern Brazil in combination with two marine records of terrestrial organic and inorganic matter input into the western equatorial Atlantic. During HS1 we recognize at least two events of widespread intensification of the SAMS across the entire region influenced by the South Atlantic Convergence Zone (SACZ) at 16.11-14.69 kyr BP and 18.1-16.66 kyr BP (labeled as HS1a and HS1c, respectively), separated by a dry excursion from 16.66-16.11 kyr BP (HS1b). In view of the spatial structure of precipitation anomalies, the widespread increase of monsoon precipitation over the SACZ domain was termed ‘Mega-SACZ’.
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    A two-way interaction between the Hainan plume and the Manila subduction zone (2015)
    Wiley
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    Publication Date: 2015-06-07
    Description: The interaction between mantle plumes and subducting slabs is well accepted, but the influence of slabs on plumes has more often been portrayed than the reverse. Here we present three-dimensional upper-mantle laboratory models in which a compositional plume rises underneath a subducting plate. Slab/plume buoyancy flux ratios ranged between 7 and 18. The models exhibit a two-way interaction. While the plume conduit increasingly tilts away from the trench as a result of slab rollback-induced toroidal mantle flow, the slab subduction rate decreases as a function of the amount of plume buoyancy opposing that of the slab, which gets subducted beneath the slab. We propose that our models apply to the Hainan-Manila system and explain the recently imaged tilt of the Hainan plume by the Manila slab-induced mantle return flow. The Hainan plume could lessen the Manila subduction rate from 8 million years into the future.
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    Spaceborne GNSS-Reectometry for ocean winds: First results from the UK TechDemoSat-1 mission (2015)
    Wiley
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    Publication Date: 2015-06-09
    Description: First results are presented for ocean surface wind speed retrieval from reflected GPS signals measured by the Low-Earth-Orbiting UK TechDemoSat-1 satellite (TDS-1). Launched in July 2014, TDS-1 provides the first new spaceborne Global Navigation Satellite System-Reflectometry (GNSS-R) data since the pioneering UK-Disaster Monitoring Mission experiment in 2003. Examples of onboard-processed delay Doppler Maps reveal excellent data quality for winds up to 27.9 m/s. Collocated ASCAT scatterometer winds are used to develop and evaluate a wind speed algorithm based on Signal-to-Noise ratio (SNR) and the Bistatic Radar Equation. For SNR greater than 3 dB, wind speed is retrieved without bias and a precision around 2.2 m/s between 3–18 m/s even withoutcalibration. Exploiting lower SNR signals however requires good knowledge of the antenna beam, platform attitude and instrument gain setting. This study demonstrates the capabilities of low-cost, low-mass, low-power GNSS-R receivers ahead of their launch on the NASA CYGNSS constellation in 2016.
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    How Does Increasing Horizontal Resolution in a Global Climate Model Improve the Simulation of Aerosol-Cloud Interactions? (2015)
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    Publication Date: 2015-06-09
    Description: The Community Atmosphere Model Version 5 is run at horizontal grid spacing of 2, 1, 0.5, and 0.25 degrees, with the meteorology nudged towards the Year Of Tropical Convection analysis, and cloud simulators and the collocated A-Train satellite observations are used to explore the resolution dependence of aerosol-cloud interactions. The higher-resolution model produces results that agree better with observations, showing an increase of susceptibility of cloud droplet size, indicating a stronger first aerosol indirect forcing (AIF), and a decrease of susceptibility of precipitation probability, suggesting a weaker second AIF. The resolution sensitivities of AIF are attributed to those of droplet nucleation and precipitation parameterizations. The annual average AIF in the northern hemisphere mid-latitudes (where most anthropogenic emissions occur) in the 0.25° model is reduced by about 1 W m -2 (-30%) compared to the 2° model, leading to a 0.26 W m -2 reduction (-15%) in the global annual average AIF.
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    Improved detection of pre-eruptive seismic velocity drops at the Piton de La Fournaise volcano (2015)
    Wiley
    Details
    Publication Date: 2015-07-30
    Description: The unexpected 2014 and 2015 Ontake (Japan) and Calbuco (Chile) eruptions proved that improving volcanic eruption prediction is still a great challenge. Decreases of seismic velocities of the Piton de la Fournaise volcano inferred from seismic noise correlations have been shown to precede eruptions. However, seismic velocities are strongly influenced by rainfall and subsequent pore pressure perturbations. Here we increase the detection of precursory seismic velocity changes to an eruption by removing the effects of pore pressure changes. During 2011–2013, the volcano exhibits a low eruptive activity during which we observe seismic velocity variations well correlated with rainfall episodes. We estimated the transfer function between fluid pressure and seismic velocity changes. We use these results to correct seismic velocity changes time series for pore pressure changes due to rainfall and found a pre-eruptive velocity drop (0.15%) associated with the 21 st of June 2014 eruption that was undetected before correction.
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    Generation of large post-injection induced seismic events by backflow from dead-end faults and fractures (2015)
    Wiley
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    Publication Date: 2015-08-04
    Description: Induced seismicity is common during hydraulic stimulation in fractured crystalline rock. Fluid injection pressurizes preexisting fractures, triggering slip and seismicity. Often, the largest induced events occur after the end of injection, which complicates efforts to manage seismic risk. In this study, a three-dimensional discrete fracture network simulator that couples fluid flow with earthquake simulation was used to investigate a novel hypothesis for why large post-injection seismic events occur. Fractures that form dead-end pathways differentially pressurize during injection. After injection is stopped, fluid backflows through the well from the dead-end fractures into larger fractures, inducing additional seismicity and potentially causing events larger than occurred during injection. Our simulations indicate that flowing fluid back to the surface immediately after injection could mitigate this effect and reduce post-injection seismicity.
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    Imprints of impulse-excited hydromagnetic waves on electrons in the Van Allen radiation belts (2015)
    Wiley
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    Publication Date: 2015-08-05
    Description: Ultralow frequency electromagnetic oscillations, interpreted as standing hydromagnetic waves in the magnetosphere, are a major energy source that accelerates electrons to relativistic energies in the Van Allen radiation belt. Electrons can rapidly gain energy from the waves when they resonate via a process called drift resonance, which is observationally characterized by energy-dependent phase differences between electron flux and electromagnetic oscillations. Such dependence has been recently observed and interpreted as spacecraft identifications of drift-resonance electron acceleration. Here we show that in the initial wave cycles, the observed phase relationship differs from that characteristic of well-developed drift-resonance. We further examine the differences and find that they are imprints of impulse-excited, coupled fast-Alfvén waves before they transform into more typical standing waves. Our identification of such imprints provides a new understanding of how energy couples in the inner magnetosphere, and a new diagnostic for the generation and growth of magnetospheric hydromagnetic pulsations.
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    Detection of recent regional sea surface temperature warming in the Caribbean and surrounding region (2015)
    Wiley
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    Publication Date: 2015-08-05
    Description: We show a sea surface temperature (SST) warming trend for the Caribbean and surrounding region over 1982 – 2012. Using an optimum interpolated SST product, a 30-year climatological analysis was generated to observe annual, monthly, and seasonal trends. Results show that SSTs are increasing annually for the region. For the two Caribbean rainy seasons, the Early Rainfall Season (ERS) and the Late Rainfall Season (LRS), estimated trends at 0.0161°C year −1 and 0.0209°C year −1 were observed, with high statistical significance. Sub-regional analysis revealed that warming is greatest in the Gulf of Mexico and North of South America during the ERS and LRS. Additionally, LRS averages for 1998–2012 reflect an increase in magnitude and intensity of the Atlantic Warm Pool (AWP) since the 1983–1997 period reflected in the AWP Area Index. Extreme increases/decreases in the time series show potential correlation with El Niño and the Southern Oscillation.
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    Role of the strengthened El Niño teleconnection in the May 2015 floods over the southern Great Plains (2015)
    Wiley
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    Publication Date: 2015-08-08
    Description: The climate anomalies leading to the May 2015 floods in Texas and Oklahoma were analyzed in the context of El Niño teleconnection in a warmer climate. El Niño tends to increase late-spring precipitation in the southern Great Plains and this effect has intensified since 1980. There was a detectable effect of anthropogenic global warming in the physical processes that caused the persistent precipitation in May of 2015: Warming in the tropical Pacific acted to strengthen the teleconnection towards North America, modification of zonal wave-5 circulation that deepened the anomalous trough to the west of Texas, and an enhanced Great Plains low-level southerlies increasing moisture supply from the Gulf of Mexico. Attribution analysis using the CMIP5 single-forcing experiments and the CESM Large Ensemble Project indicated a significant increase in the El Niño-induced precipitation anomalies over Texas and Oklahoma when increases in the anthropogenic greenhouse gases were taken into account.
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    Interplanetary shocks and the resulting geomagnetically induced currents at the equator (2015)
    Wiley
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    Publication Date: 2015-08-08
    Description: Geomagnetically induced currents (GICs) caused by interplanetary shocks represent a serious space weather threat to modern technological infrastructure. The arrival of interplanetary shocks drives magnetosphere and ionosphere currents systems, which then induce electric currents at ground level. The impact of these currents at high latitudes has been extensively researched, but the magnetic equator has been largely overlooked. In this paper, we investigate the potential effects of interplanetary shocks on the equatorial region and demonstrate that their magnetic signature is amplified by the equatorial electrojet. This local amplification substantially increases the region's susceptibility to GICs. Importantly, this result applies to both geomagnetic storms and quiet periods, and thus represents a paradigm shift in our understanding of adverse space weather impacts on technological infrastructure.
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    The low-degree shape of Mercury (2015)
    Wiley
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    Publication Date: 2015-08-08
    Description: The shape of Mercury, particularly when combined with its geoid, provides clues to the planet's internal structure, thermal evolution, and rotational history. Elevation measurements of the northern hemisphere acquired by the Mercury Laser Altimeter on the MESSENGER spacecraft, combined with 378 occultations of radio signals from the spacecraft in the planet's southern hemisphere, reveal the low-degree shape of Mercury. Mercury's mean radius is 2439.36±0.02 km, and there is a 0.14-km offset between the planet's centers of mass and figure. Mercury is oblate, with a polar radius 1.65 km less than the mean equatorial radius. The difference between the semimajor and semiminor equatorial axes is 1.25 km with the long axis oriented 15° west of Mercury's dynamically defined principal axis. Mercury's geoid is also oblate and elongated, but it deviates from a sphere by a factor of 10 less than Mercury's shape, implying compensation of elevation variations on a global scale.
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    The formation of graben morphology in the Dead Sea Fault, and its implications (2015)
    Wiley
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    Publication Date: 2015-08-08
    Description: The Dead Sea fault (DSF) is a 1000 km long continental transform. It forms a narrow elongated valley with uplifted shoulders showing an east–west asymmetry, which is not common in other continental transforms. This topography may have strongly affected the course of human history. Several papers addressed the geomorphology of the DSF, but there is still no consensus with respect to the dominant mechanism of its formation. Our thermo-mechanical modeling demonstrates that existence of a transform prior to the rifting predefined high strain softening on the faults in the strong upper crust and created a precursor weak zone localizing deformations in the subsequent transtensional period. Together with a slow rate of extension over the Arabian plate, they controlled a narrow asymmetric morphology of the fault. This rift pattern was enhanced by a fast deposition of evaporites from the Sedom Lagoon, which occupied the rift depression for a short time period.
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    Upper-ocean response to Hurricane Gonzalo (2014): salinity effects revealed by targeted and sustained underwater glider observations (2015)
    Wiley
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    Publication Date: 2015-08-12
    Description: During October 2014, Hurricane Gonzalo traveled within 85km from the location of an underwater glider situated north of Puerto Rico. Observations collected before, during, and after the passage of this hurricane were analyzed to improve our understanding of the upper-ocean response to hurricane winds. The main finding in this study is that salinity potentially played an important role on changes observed in the upper-ocean; a near-surface barrier-layer likely suppressed the hurricane-induced upper-ocean cooling, leading to smaller than expected temperature changes. Post-storm observations also revealed a partial recovery of the ocean to pre-storm conditions 11 days after the hurricane. Comparison with a coupled ocean-atmosphere hurricane model indicates that model-observations discrepancies are largely linked to salinity effects described. Results presented in this study emphasize the value of underwater glider observations for improving our knowledge of how the ocean responds to tropical cyclone winds and for tropical cyclone intensification studies and forecasts.
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    The fractured Moon: Production and saturation of porosity in the lunar highlands from impact cratering (2015)
    Wiley
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    Publication Date: 2015-08-12
    Description: We have analyzed the Bouguer anomaly (BA) of ~1200 complex craters in the lunar highlands from Gravity Recovery and Interior Laboratory (GRAIL) observations. The BA of these craters is generally negative, though positive BA values are observed, particularly for smaller craters. Crater BA values scale inversely with crater diameter, quantifying how larger impacts produce more extensive fracturing and dilatant bulking. The Bouguer anomaly of craters larger than km in diameter is independent of crater size, indicating that there is a limiting depth to impact-generated porosity, presumably from pore collapse associated with either overburden pressure or viscous flow. Impact-generated porosity of the bulk lunar crust is likely in a state of equilibrium for craters smaller than ~30 km in diameter, consistent with an ~8-km-thick lunar megaregolith, whereas the gravity signature of larger craters is still preserved and provides new insight into the cratering record of even the oldest lunar surfaces.
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    Solar Wind Conditions Leading to Efficient Radiation Belt Electron Acceleration: A Superposed Epoch Analysis (2015)
    Wiley
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    Publication Date: 2015-08-14
    Description: Determining preferential solar wind conditions leading to efficient radiation belt electron acceleration is crucial for predicting radiation belt electron dynamics. Using Van Allen Probes electron observations (〉1 MeV) from 2012 to 2015, we identify a number of efficient and inefficient acceleration events separately to perform a superposed epoch analysis of the corresponding solar wind parameters and geomagnetic indices. By directly comparing efficient and inefficient acceleration events, we clearly show that prolonged southward B z , high solar wind speed, and low dynamic pressure are critical for electron acceleration to 〉 1 MeV energies in the heart of the outer radiation belt. We also evaluate chorus wave evolution using the superposed epoch analysis for the identified efficient and inefficient acceleration events and find that chorus wave intensity is much stronger and lasts longer during efficient electron acceleration events, supporting the scenario that chorus waves play a key role in MeV electron acceleration.
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    Subduction Initiation at Relic Arcs (2015)
    Wiley
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    Publication Date: 2015-08-15
    Description: Although plate tectonics is well established, how a new subduction zone initiates remains controversial. Based on plate reconstruction and recent ocean drilling within the Izu-Bonin-Mariana, we advance a new geodynamic model of subduction initiation (SI). We argue that the close juxtaposition of the nascent plate boundary with relic oceanic arcs is a key factor localizing initiation of this new subduction zone. The combination of thermal and compositional density contrasts between the overriding relic arc and the adjacent old Pacific oceanic plate promoted spontaneous SI. We suggest that thermal rejuvenation of the overriding plate just before 50 Ma causes a reduction in overriding plate strength and an increase in the age contrast (hence buoyancy) between the two plates, leading to SI. The computational models map out a framework in which rejuvenated relic arcs are a favorable tectonic environment for promoting subduction initiation, while transform faults and passive margins are not.
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    Near-surface scholte-wave velocities at ekosk from short noise recordings by seismic noise gradiometry (2015)
    Wiley
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    Publication Date: 2015-08-15
    Description: We propose a new approach for imaging the subsurface using a stochastic wavefield of interface waves present in the ambient seismic field. Unlike seismic interferometry, our technique does not rely on cross-correlations to obtain the Green's function between two seismic receivers. Rather, it relies on the local measurements of phase-velocity obtained directly from the ratio between second-order temporal and spatial derivatives of the wavefield. We process 10 minutes of ambient seismic noise recording made using a large and dense array installed over Ekofisk. We image a subsidence-induced geomechanical imprint on the Scholte-wave phase velocities in the near surface. This resulting phase velocity pattern is verified by comparison to results from a seismic-noise cross-correlation tomography.
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    Earthquake characteristics before eruptions of Japan's Ontake volcano in 2007 and 2014 (2015)
    Wiley
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    Publication Date: 2015-08-15
    Description: We detect earthquakes associated with Japan's Ontake eruptions in 2007 and 2014 using the match-and-locate (M&L) method. 37 times more events (4949) than the Japan Meteorological Agency (JMA) catalog (133) and 30 times more events (1880) than the JMA catalog (62) are detected during the eruptions of 2007 and 2014, respectively. The detected earthquakes are further classified into three different types: long-period (LP) events, LP-associated events and volcano-tectonic (VT) earthquakes. Both LP and LP-associated events are observed in the two eruptions. We suggest that the observed intense seismicity before both eruptions indicates the rewaking of Ontake volcano and might be used to predict its impending eruption in the long term. The temporal persistence and spatial concentration of LP and LP-associated events may be used to predict the eventual eruption and eruption location, and the maximum magnitude of the LP events may be used as an indicator of eruption magnitude.
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    Paul-Mohr-Coulomb Failure Surface of Rock in the Brittle Regime (2015)
    Wiley
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    Publication Date: 2015-08-15
    Description: The Paul-Mohr-Coulomb failure criterion includes the intermediate principal stress σ II and friction angles at the limiting stress states of σ II = σ III and σ II = σ I , where σ I and σ III are major and minor principal stresses. Conventional triaxial compression ( σ II = σ III ), extension ( σ II = σ I ), and plane strain ( σ Ι ≠ σ ΙΙ ≠ σ III ) experiments were performed on dry rock. The failure data were plotted in principal stress space and material parameters were determined in the context of two internal friction angles and the theoretical uniform triaxial (all-around equal) tensile strength. Assuming isotropy, the triaxial compression and extension results were used to construct a six-sided pyramidal failure surface, and the extension friction angle was larger than the compression friction angle, a sufficient but not necessary condition of the intermediate stress effect. To capture the behaviour of the rock in multi-axial loading, the Paul-Mohr-Coulomb criterion was extended to form a twelve-sided pyramid with best-fit planes.
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    Coseismic displacements from SAR image offsets between different satellite sensors: Application to the 2001 Bhuj (India) earthquake (2015)
    Wiley
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    Publication Date: 2015-08-15
    Description: Synthetic Aperture Radar (SAR) image offset tracking is increasingly being used for measuring ground displacements, e.g., due to earthquakes and landslide movement. However, this technique has been applied only to images acquired by the same or identical satellites. Here we propose a novel approach for determining offsets between images acquired by different satellite sensors, extending the usability of existing SAR image archives. The offsets are measured between two multi-image reflectivity maps obtained from different SAR datasets, which provide significantly better results than with single pre- and post-event images. Application to the 2001 M w 7.6 Bhuj earthquake reveals, for the first time, its near-field deformation using multiple pre-earthquake ERS and post-earthquake Envisat images. The rupture model estimated from these cross-sensor offsets and tele-seismic waveforms shows a compact fault slip pattern with fairly short rise times (〈3 s) and a large stress drop (20 MPa), explaining the intense shaking observed in the earthquake.
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    The meteoroid stream of comet Encke at Mercury: implications for MESSENGER observations of the exosphere (2015)
    Wiley
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    Publication Date: 2015-08-15
    Description: We test the hypothesis that an annually repeatable Ca emission excess in Mercury's exosphere at a True Anomaly Angle (TAA) of 25°±5° [ Burger et al. , 2014; Killen and Hahn , 2015] is due to particles from comet 2P/Encke impacting the surface. By simulating the dynamical evolution of Encke particles under planetary perturbations and Poynting-Robertson drag, we find that mm-sized grains ejected 1-2x10 4 yr ago encounter Mercury at TAA =350°–30°. The timing of the excess emission is consistent with a major dust release episode ≲ 20 kyr ago, possibly due to Encke progenitor breakup [ Clube and Napier , 1984; Jenniskens , 2007]. The emission mechanism is likely the direct injection of impact-liberated Ca into sunlight rather than nightside surface adsorption for subsequent release at dawn. The timing of dust release from the comet depends on this mechanism; a 10 kyr age is implied by the direct-injection scenario.
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    Global and seasonal variations in three-dimensional gravity wave momentum flux from satellite limb sounding temperatures (2015)
    Wiley
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    Publication Date: 2015-08-11
    Description: Satellite limb sounding methods provide the best global temperature data available for simultaneous measurement of gravity wave horizontal and vertical structure needed to estimate momentum flux and constrain wave effects on general circulation. Gravity waves vary in the three spatial dimensions and time, so the ideal measurement observes all three dimensions at high resolution nearly simultaneously. High Resolution Dynamics Limb Sounder (HIRDLS) measurements, give near-simultaneous profiles in close proximity and at high vertical resolution, but these coincident profiles lie only along the plane of the measurement track. Here we combine HIRDLS and radio occultation datasets to obtain three-dimensional properties of gravity waves on a global scale as well as seasonal variations. The results show dramatic changes from previous estimates using either dataset alone. Changes include much larger momentum fluxes and latitudinal variations in propagation direction that support an enhanced role for gravity wave forcing of middle atmosphere circulation.
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    Line of Sight Displacement from ALOS-2 Interferometry: Mw 7.8 Gorkha Earthquake and Mw 7.3 Aftershock (2015)
    Wiley
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    Publication Date: 2015-08-11
    Description: Interferometric Synthetic Aperture Radar (InSAR) is a key tool for the analysis of displacement and stress changes caused by large crustal earthquakes, particularly in remote areas. A challenge for traditional InSAR has been its limited spatial and temporal coverage especially for very large events, whose dimensions exceed the typical swath width of 70 – 100 km. This problem is addressed by the ALOS-2 satellite, whose PALSAR-2 instrument operates in ScanSAR mode, enabling a repeat time of 2 weeks and a swath width of 350km. Here, we present InSAR line-of-sight displacement data from ALOS-2/PALSAR-2 observations covering the Mw 7.8 Gorkha, Nepal earthquake and its Mw 7.3 aftershock that were acquired within one week of each event. The data are made freely available and we encourage their use in models of the fault slip and associated stress changes. The Mw 7.3 aftershock extended the rupture area of the mainshock toward the east, but also left a 20 km gap where the fault has little or no co-seismic slip. We estimate this un-slipped fault patch has the potential to generate a Mw 6.9 event.
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    High emission rate of sulfuric acid from Bezymianny volcano, Kamchatka (2015)
    Wiley
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    Publication Date: 2015-08-14
    Description: High concentrations of primary sulfuric acid (H 2 SO 4 ) in fumarolic gases and high emission rate of sulfuric acid aerosol in the plume were measured at Bezymianny volcano, an active dome-growing andesitic volcano in central Kamchatka. Using direct sampling, filter pack sampling and DOAS measurements, we estimated an average emission of H 2 SO 4 at 243±75 t/d in addition to an average SO 2 emission of 212±65 t/d. The fumarolic gases of Bezymianny correspond to arc gases released by several magma bodies at different stages of degassing and contain 25–92% of entrained air. H 2 SO 4 accounts for 6–87 mol% of the total sulfur content, 45 mol% on average, and SO 2 is the rest. The high H 2 SO 4 in Bezymianny fumaroles can be explained by catalytic oxidation of SO 2 inside the volcanic dome. Because sulfate aerosol is impossible to measure remotely, the total sulfur content in a plume containing significant H 2 SO 4 may be seriously underestimated.
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    First Satellite Imaging of Auroral Pulsations by the Fast Auroral Imager on e-POP (2015)
    Wiley
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    Publication Date: 2015-08-14
    Description: We report the first satellite imaging of auroral pulsations by the Fast Auroral Imager (FAI) onboard the Enhanced Polar Outflow Probe (e-POP) satellite. The near-infrared camera of FAI is capable of providing up to two auroral images per second, ideal for investigation of pulsating auroras. The auroral pulsations were observed within the auroral bulge formed during a substorm interval on 2014 February 19. This first satellite view of these pulsations from FAI reveals that (1) several pulsating auroral channels (PACs) occur within the auroral bulge, (2) periods of the intensity pulsations span over one decade within the auroral bulge, and (3) there is no apparent trend of longer pulsation periods associated with higher latitudes for these PACs. Although PACs resemble in some respect stable pulsating auroras reported previously but they have several important differences in characteristics.
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    Tidally driven mixing and dissipation in the stratified boundary layer above steep submarine topography (2015)
    Wiley
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    Publication Date: 2015-08-15
    Description: A tidally driven, stratified boundary layer over supercritical topography is simulated numerically. The near-boundary flow is characterized by quasi-periodic, bore-like motions and episodic expulsion events where fluid is ejected into the stratified interior. The character of the bores is compared to the high-resolution ocean mooring data of van Haren [2006]. The diffusivity of the flow near the boundary is estimated by means of a synthetic dye tracer experiment. The average dissipation rate within the dye cloud is computed and combined with the diffusivity estimate to yield an overall mixing efficiency of 0.15. Both the estimated diffusivity and dissipation rates are in reasonable agreement with the microstructure observations of Kunze at al [2012] when scaled to the environmental conditions at the Monterey and Soquel Canyons and to the values estimated by van Haren and Gostiaux [2012] above the sloping bottom of the Great Meteor Seamount in the Canary Basin.
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    Slow electron holes in multi-component plasmas (2015)
    Wiley
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    Publication Date: 2015-08-15
    Description: Electrostatic solitary waves (ESWs), often interpreted as electron phase space holes, are commonly observed in plasmas and are manifestations of strongly nonlinear processes. Often slow ESWs are observed, suggesting generation by the Buneman instability. The instability criteria, however, are generally not satisfied. We show how slow electron holes can be generated by a modified Buneman instability in a plasma that includes a slow electron beam on top of a warm thermal electron background. This lowers the required current for marginal instability and allows for generation of slow electron holes for a wide range of beam parameters that covers expected plasma distributions in space, for example in magnetic reconnection regions. At higher beam speeds, the electron-electron beam instability becomes dominant instead, producing faster electron holes. The range of phase speeds for this model is consistent with a statistical set of observations at the magnetopause made by Cluster.
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    Slip model of the 2015 Mw 7.8 Gorkha (Nepal) earthquake from inversions of ALOS-2 and GPS data (2015)
    Wiley
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    Publication Date: 2015-08-15
    Description: We use surface deformation measurements including Interferometric Synthetic Aperture Radar (InSAR) data acquired by the ALOS-2 mission of the Japanese Aerospace Exploration Agency (JAXA) and Global Positioning System (GPS) data to invert for the fault geometry and coseismic slip distribution of the 2015 M w 7.8 Gorkha earthquake in Nepal. Assuming that the ruptured fault connects to the surface trace of the of Main Frontal Thrust fault (MFT) between 84.34°E and 86.19°E, the best-fitting model suggests a dip angle of 7°. The moment calculated from the slip model is 6.08x10 20 Nm, corresponding to the moment magnitude of 7.79. The rupture of the 2015 Gorkha earthquake was dominated by thrust motion that was primarily concentrated in a 150-km long zone 50 to 100 km northward from the surface trace of the Main Frontal Thrust (MFT), with maximum slip of ~ 5.8 m at a depth of ~8 km. Data thus indicate that the 2015 Gorkha earthquake ruptured a deep part of the seismogenic zone, in contrast to the 1934 Bihar-Nepal earthquake, which had ruptured a shallow part of the adjacent fault segment to the East.
    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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    Recent evidence for a strengthening CO2 sink in the Southern Ocean from carbonate system measurements in the Drake Passage (2002–2015) (2015)
    Wiley
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    Publication Date: 2015-09-11
    Description: We present a 13-year (2002–2015) semi-monthly time-series of the partial pressure of CO 2 in surface water (pCO 2surf ) and other carbonate system parameters from the Drake Passage. This record shows a clear increase in the magnitude of the sea-air pCO 2 gradient, indicating strengthening of the CO 2 sink in agreement with recent large-scale analyses of the world oceans. The rate of increase in pCO 2surf north of the Antarctic Polar Front (APF) is similar to the atmospheric pCO 2 (pCO 2atm ) trend, whereas the pCO 2surf increase south of the APF is slower than the pCO 2atm trend. The high-frequency surface observations indicate that an absence of a winter increase in total CO 2 (TCO 2 ) and cooling summer sea surface temperatures are largely responsible for increasing CO 2 uptake south of the APF. Muted winter trends in surface TCO 2 also provide temporary stability to the carbonate system that is already close to undersaturation with respect to aragonite.
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    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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  • 97
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    Liquid iron-hydrogen alloys at outer core conditions by first-principles calculations (2015)
    Wiley
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    Publication Date: 2015-09-11
    Description: We examined the density, bulk sound (compressional) velocity, and Grüneisen parameter of liquid pure Fe, Fe 100 H 28 (0.50 wt.% H), Fe 88 H 40 (0.81 wt.% H), and Fe 76 H 52 (1.22 wt.% H) at Earth's outer core pressure and temperature ( P-T ) conditions (~100 to 350 GPa, 4,000 to 7,000 K) based on first-principles molecular dynamics calculations. The results demonstrate that the thermodynamic Grüneisen parameter of liquid iron alloy decreases with increasing pressure, temperature, and hydrogen concentration, indicating a relatively small temperature gradient in the outer core when hydrogen is present. Along such temperature profile, both the density and compressional velocity of liquid iron containing ~1 wt.% hydrogen match seismological observations. It suggests that hydrogen could be a primary light element in the core, although the shear velocity of the inner core is not reconciled with solid Fe-H alloy and thus requires another impurity element.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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  • 98
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    Impact of different El Niño types on the El Niño/IOD relationship (2015)
    Wiley
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    Publication Date: 2015-09-11
    Description: Abstract Previous studies reported that positive phases of the Indian Ocean Dipole (IOD) tend to accompany El Niño during boreal autumn. Here we show that the El Niño/IOD relationship can be better understood when considering the two different El Niño flavors. Eastern-Pacific (EP) El Niño events exhibit a strong correlation with the IOD dependent on their magnitude. In contrast, the relationship between Central-Pacific (CP) El Niño events and the IOD depends mainly on the zonal location of the sea surface temperature anomalies rather than their magnitude. CP El Niño events lying further west than normal are not accompanied by significant anomalous easterlies over the eastern Indian Ocean along the Java/Sumatra coast, which is unfavorable for the local Bjerknes feedback and correspondingly for an IOD development. The El Niño/IOD relationship has experienced substantial changes due to the recent decadal El Niño regime shift, which has important implications for seasonal prediction.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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  • 99
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    Eurasian winter cooling in the warming hiatus of 1998–2012 (2015)
    Wiley
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    Publication Date: 2015-09-13
    Description: We investigate the relative magnitudes of the contributions of surface temperature trends from different latitude bands to the recent warming hiatus. We confirm from five different global datasets that the global-mean surface temperature trend in the period 1998–2012 is strongly influenced by a pronounced Eurasian winter cooling trend. To understand the drivers of this winter–cooling trend, we perform three twenty-member ensembles of simulations with different prescribed sea surface temperature and sea ice in the atmospheric model ECHAM6. Our experimental results suggest that the Arctic sea-ice loss does not drive systematic changes in the northern-hemisphere large-scale circulation in the past decades. The observed Eurasian winter cooling trend over 1998–2012 arises essentially from atmospheric internal variability and constitutes an extreme climate event. However, the observed reduction in Arctic sea ice enhances the variability of Eurasian winter climate and thus increases the probability of an extreme Eurasian winter cooling trend.
    Print ISSN: 0094-8276
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    Plasmapause location under quiet geomagnetic conditions (Kp≤ 1): THEMIS observations (2015)
    Wiley
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    Publication Date: 2015-09-13
    Description: We statistically examined the plasmapause location ( L pp ) under quiet geomagnetic conditions ( K p ≤ 1) using the electron density inferred from the THEMIS spacecraft potential for two-year period (2008 and 2009). 543 L pp samples were identified under steady quiet conditions with K p values ≤ 1 during 12 hours prior to the plasmapause crossing. From our large data set, we determined the medians and means of L pp in L and MLT. They are located near geosynchronous orbit and nearly circular. The L pp medians show a slight bulge located in post-dusk sector. Comparing with previous models, our median or mean L pp is extended ∼1-2 L from the earth than the model L pp along the local time from 0800 to 2400 MLT. That is, L pp locations in the previous models are underestimated during quiet geomagnetic conditions.
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