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  • 1
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    Ocean tide loading displacements in western Europe. Part 1: Validation of kinematic GPS estimates (2015)
    Wiley
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    Publication Date: 2015-08-15
    Description: GPS has been extensively used to estimate tidal ground displacements, but the accuracy of this has not been systematically verified. Using more than 20 sites distributed across western Europe, we show that post-processed kinematic precise point positioning GPS with appropriately-tuned process noise constraints is capable of recovering synthetic tidal displacements inserted into real data, with a typical accuracy of 0.2 mm depending on the time series noise. The kinematic method does not result in erroneous propagation of signals from one coordinate component to another, or to the simultaneously-estimated tropospheric delay parameters. It is robust to the likely effects of day-to-day equipment and reference frame changes, and to outages in the data. A minimum data span of 4 years with at least 70% availability is recommended. Finally, we show that the method of reducing apparent coordinate time series noise by constraining the tropospheric delay to values previously estimated in static batch GPS analysis, in fact results in the suppression of true tidal signals. Using our kinematic GPS analysis approach, periodic displacements can be reliably observed at the 0.2 mm level, which is suitable for the testing and refinement of ocean tide and solid Earth response models.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 2
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    Ocean tide loading displacements in western Europe. Part 2: GPS-observed anelastic dispersion in the asthenosphere (2015)
    Wiley
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    Publication Date: 2015-08-15
    Description: GPS-observed vertical ocean tide loading displacements show in Cornwall, southwest England and in Brittany, northwest France, discrepancies of 2–3 mm with predicted values based on isotropic PREM for the main tidal harmonic M 2 , yet in central Europe the agreement is better than 0.5 mm. By comparison of ocean tide models and validation with tide gauge observations, we demonstrate that the uncertainties in the former are too small to cause this disagreement. Furthermore, we find that different local models of the crust and different global elastic reference models derived from seismological observations can only reduce the observed discrepancies to 1–2 mm, which still exceeds the GPS observational uncertainty of 0.2-0.4 mm. It is customary to use the elastic properties of the Earth as given by seismic models. Previously, there has been insufficient evidence to determine how to modify these properties during the transformation from seismic to tidal frequencies to account for possible anelastic dispersion in the asthenosphere, and so this effect has been ignored. If we include this effect, then our discrepancies reduce further to 0.2-0.4 mm. This value is of the same order as the sum of the remaining errors due to uncertainties in the ocean tide models and in the GPS observations themselves. This research provides evidence in western Europe of a reduction of around 8-10% of the seismic shear modulus in the asthenosphere at tidal frequencies. In addition, we find that the asthenosphere absorption band frequencies can be represented by a constant quality factor Q .
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    Topics: Geosciences , Physics
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  • 3
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    The role of pyroxenite in basalt genesis: Melt-PX, a melting parameterization for mantle pyroxenites between 0.9 and 5 GPa (2016)
    Wiley
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    Publication Date: 2016-07-17
    Description: Geochemical and isotopic data suggest that the source regions of oceanic basalts may contain pyroxenite in addition to peridotite. In order to incorporate the wide range of compositions and melting behaviors of pyroxenites into mantle melting models, we have developed a new parameterization, Melt-PX, which predicts near-solidus temperatures and extents of melting as a function of temperature and pressure for mantle pyroxenites. We used 183 high-pressure experiments (25 compositions; 0.9–5 GPa; 1150–1675 °C) to constrain a model of melt fraction vs. temperature from 5% melting up to the disappearance of clinopyroxene for pyroxenites as a function of pressure, temperature, and bulk composition. When applied to the global set of experimental data, our model reproduces the experimental F -values with a standard error of estimate of 13% absolute; temperatures at which the pyroxenite is 5% molten are reproduced with a standard error of estimate of 30 °C over a temperature range of ~500 °C and a pressure range of ~4 GPa. In conjunction with parameterizations of peridotite melting, Melt-PX can be used to model the partial melting of multi-lithologic mantle sources—including the effects of varying the composition and the modal proportion of pyroxenite in such source regions. Examples of such applications include calculations of isentropic decompression melting of a mixed peridotite + pyroxenite mantle; these show that, although the potential temperature of the upwelling mantle plays an important role in defining the extent of magma production, the composition and mass fraction of the pyroxenite also exert strong controls.
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  • 4
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    Active hydrothermal discharge on the submarine Aeolian Arc (2011)
    Wiley
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    Publication Date: 2011-02-05
    Description: In November 2007 we conducted a water column and seafloor mapping study of the submarine volcanoes of the Aeolian Arc in the southern Tyrrhenian Sea aboard the R/V Urania. On 26 conductivity-temperature-depth casts and tows we measured temperature, conductivity, pressure, and light scattering and also collected discrete samples for helium isotopes, methane, and pH. The 3He/4He isotope ratio, an unambiguous indicator of hydrothermal input, showed a clear excess above background at 6 of the 10 submarine volcanoes surveyed. Marsili seamount had the highest anomaly, where the 3He/4He ratio reached a δ3He value of 23% at 610 m depth compared with background values of ∼5%. Smaller but distinct δ3He anomalies occurred over Palinuro, Enarete, Eolo, Sisifo, and Secca del Capo. Although hydrothermal emissions are known to occur offshore of some Aeolian subaerial volcanoes, and hydrothermal deposits have been sampled throughout the arc, our results are the first to confirm active discharge on Marsili, Enarete, Eolo, Sisifo, and Secca del Capo. Samples collected over Lametini, Filicudi North, Alicudi North, and Alcione had δ3He near the regional background values, suggesting either absence of, or very weak, hydrothermal activity on these seamounts. Hydrocasts between the volcanoes revealed a consistent δ3He maximum between 11% and 13% at 2000 m depth throughout the SE Tyrrhenian Sea. The volcanoes of the Aeolian arc and the Marsili back arc, all
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  • 5
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    Correction to “Active hydrothermal discharge on the submarine Aeolian Arc” (2011)
    Wiley
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    Publication Date: 2011-04-15
    Print ISSN: 0148-0227
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  • 6
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    Gravity changes and deformation at Kīlauea Volcano, Hawai‘i, associated with summit eruptive activity, 2009–2012 (2014)
    Wiley
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    Publication Date: 2014-08-21
    Description: Analysis of microgravity and surface displacement data collected at the summit of Kīlauea Volcano, Hawai‘i (USA), between December 2009 and November 2012, suggests a net mass accumulation at ~1.5 km depth beneath the northeast margin of Halema‘uma‘u Crater, within Kīlauea Caldera. Although residual gravity increases and decreases are accompanied by periods of uplift and subsidence of the surface, respectively, the volume change inferred from the modeling of interferometric synthetic aperture radar (InSAR) deformation data can account for only a small portion (as low as 8%) of the mass addition responsible for the gravity increase. We propose that, since the opening of a new eruptive vent at the summit of Kīlauea in 2008, magma rising to the surface of the lava lake outgasses, becomes denser, and sinks to deeper levels, replacing less-dense gas-rich magma stored in the Halema‘uma‘u magma reservoir. In fact, a relatively small density increase (〈200 kg m −3 ) of a portion of the reservoir can produce the positive residual gravity change measured during the period with the largest mass increase, between March 2011 and November 2012. Other mechanisms may also play a role in the gravity increase without producing significant uplift of the surface, including compressibility of magma, formation of olivine cumulates, and filling of void space by magma. The rate of gravity increase, higher than during previous decades, varies through time and seems to be directly correlated with the volcanic activity occurring at both the summit and the east rift zone of the volcano.
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  • 7
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    Thermal, Deformation, and Degassing Remote Sensing Time Series (CE 2000–2017) at the 47 most Active Volcanoes in Latin America: Implications for Volcanic Systems (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract Volcanoes are hazardous to local and global populations, but only a fraction are continuously monitored by ground‐based sensors. For example, in Latin America, more than 60% of Holocene volcanoes are unmonitored, meaning long‐term multiparameter data sets of volcanic activity are rare and sparse. We use satellite observations of degassing, thermal anomalies, and surface deformation spanning 17 years at 47 of the most active volcanoes in Latin America and compare these data sets to ground‐based observations archived by the Global Volcanism Program. This first comparison of multisatellite time series on a regional scale provides information regarding volcanic behavior during, noneruptive, pre‐eruptive, syneruptive, and posteruptive periods. For example, at Copahue volcano, deviations from background activity in all three types of satellite measurements were manifested months to years in advance of renewed eruptive activity in 2012. By quantifying the amount of degassing, thermal output, and deformation measured at each of these volcanoes, we test the classification of these volcanoes as open or closed volcanic systems. We find that ~28% of the volcanoes do not fall into either classification, and the rest show elements of both, demonstrating a dynamic range of behavior that can change over time. Finally, we recommend how volcano monitoring could be improved through better coordination of available satellite‐based capabilities and new instruments.
    Print ISSN: 2169-9313
    Electronic ISSN: 2169-9356
    Topics: Geosciences , Physics
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  • 8
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    Top-down inflation and deflation at the summit of Kīlauea Volcano, Hawai‘i observed with InSAR (2012)
    Wiley
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    Publication Date: 2012-12-15
    Description: We use interferometric synthetic aperture radar (InSAR) to study deformation of the summit caldera at Kīlauea Volcano during 2000–2008, which spanned both an east rift zone eruptive event in 2007 and the start of the ongoing summit eruption in 2008. The data set consists of small baseline subset (SBAS) time series generated from 270 acquisitions on three separate beam modes from the Radarsat-1 satellite. We identify 12 time periods with distinct patterns of displacement that we attribute until late 2003 to secular tectonic-driven deformation and from 2004 to 2008 to four different sources in the summit area. We model the shallow magmatic system as a spherical reservoir at 1.9 ± 0.1 km depth below the surface to the northeast of Halemaumau (source 1) and three vertically stacked sills at greater depths in the southern caldera area (source 2 at the southern edge of the caldera at 2.9 ± 0.2 km depth, source 3 to the south-southeast of the caldera at 3.4 ± 0.5 km depth, and source 4 south of the caldera at 3.6 ± 0.4 km depth). The sequence for filling of and withdrawal from these reservoirs reveals a top-down process, with sequences of both inflation and deflation initiating in the shallowest source. Inflation of source 3 is coincident with elevated seismic activity in the upper east rift zone in February 2006 and May 2007. Source 4 is elongated toward the southwest rift zone and also shows elevated seismicity that extends toward the southwest rift zone.
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  • 9
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    The role of viscous magma mush spreading in volcanic flank motion at Kīlauea Volcano, Hawai'i (2013)
    Wiley
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    Publication Date: 2013-04-25
    Description: [1]  Multiple mechanisms have been suggested to explain seaward motion of the south flank of Kīlauea Volcano, Hawai'i. The consistency of flank motion during both waxing and waning magmatic activity at Kīlauea suggests that a continuously acting force, like gravity body force, plays a substantial role. Using finite element models, we test whether gravity is the principal driver of long-term motion of Kīlauea'sflank. We compare our model results to geodetic data from GPS and InSAR during a time period with few magmatic and tectonic events (2000–2003), when deformation of Kīlauea was dominated by summit subsidence and seaward motion of the south flank. We find that gravity-only models can reproduce the horizontal surface velocities if we incorporate a regional decollement fault and a deep, low viscosity magma mush zone. To obtain quasi-steady state horizontal surface velocities that explain the long-term seaward motion of the flank, we find that an additional weak zone is needed, which is an extensional rift zone above the magma mush. The spreading rate in our model is mainly controlled by the magma mush viscosity, while its gravity plays a less significant role. We find that a viscosity of 2.5 x 10 17 – 2.5 x 10 19  Pa.s for the magma mush provides an acceptable fit to the observed horizontal surface deformation, and that to first order the low viscosity of the mush, and not a higher density, drives spreading of the deep rift zone. Using high magma mush viscosities, such as 2.5 x 10 19  Pa.s, the deformation rates remain more steady state over longer timescales. These models explain a significant amount of the observed subsidence at Kīlauea's summit. Some of the remaining subsidence is probably a result of magma withdrawal from subsurface reservoirs.
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  • 10
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    Remote sensing of ground deformation for monitoring groundwater management practices: application to the Santa Clara Valley during the 2012-2015 California drought (2017)
    Wiley
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    Publication Date: 2017-09-23
    Description: Groundwater management typically relies on water level data and spatially limited deformation measurements. While InSAR has been used to study hydrological deformation, its limited temporal sampling can lead to biases in rapidly changing systems. Here we use 2011-2017 COSMO-SkyMed data with revisit intervals as short as one day to study the response of the Santa Clara Valley (SCV) aquifer in California to the unprecedented 2012-2015 drought. Cross correlation and independent component analyses of deformation time series enable tracking water though the aquifer system. The aquifer properties are derived prior to and during the drought to assess the success of water-resource management practices. Subsidence due to groundwater withdrawal dominates during 2011-2017, limited to the confined aquifer and west of the Silver Creek Fault, similar to pre-drought summer periods. Minimum water levels and elevations were reached in mid-2014, but thanks to intensive groundwater management efforts the basin started to rebound in summer 2015, during the deepening drought. By 2017, water levels were back to their pre-drought levels, while elevations had not yet fully rebounded due to the delayed poroelastic response of aquitards and their large elastic compressibility. As water levels did not reach a new low stand, the drought led to only elastic and recoverable changes in the SCV. The SCV lost 0.09 km 3 during the drought while seasonal variations amount to 0.02 km 3 . Analysis of surface loads due to water mass changes in the aquifer system suggests that groundwater drawdowns could influence the stress on nearby faults.
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