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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.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
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    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).
    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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  • 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.
    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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  • 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.
    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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  • 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.
    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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  • 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.
    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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  • 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.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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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.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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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.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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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.
    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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