Modeling Surface Mass Load Displacements in the Western US

The surface of the Earth is under constant stress from a variety of mass loads. Surface mass loads, such as oceans, atmosphere, glaciers, seasonal snowpack, and ground water reservoirs, exert forces on the surface of the Earth, causing elastic crustal deformation. Surface mass loads migrate across the Earth’s surface on a range of time scales from daily to several thousand years. Horizontal and vertical displacement responses of the Earth can be recorded using Global Positioning System (GPS) receivers. Modeling and removing surface-mass loading signals, which are present in all GPS time series, can reduce the variance in these time series. My research project focuses on using the python-based software program LoadDef to accurately compute displacement responses of the Earth’s surface to surface mass loads. The modeled mass load responses are compared to the observed GPS displacement responses measured by the Plate Boundary Observatory (PBO), and then removed to determine the relative contributions of each loading source at each station in the PBO network throughout the Western US. These contributions are mapped and colored based on value contribution. Currently, we have already shown that atmospheric mass loading (ATML) contributes a large portion to GPS time series in the western US. Contributions vary spatially with distance from the ocean, with over 25% RMS reduction for stations 1000 km inland from the coast versus about 12% contribution within 100 km of the coast. We are collaborating with NASA’s Jet Propulsion Laboratory to better constrain snow and water storage in the western US from GPS using our daily estimates of ATML. ATML models can be used to correct GPS time series for atmospheric loading effects. GPS data is also important in understanding plate motions at subduction zones. Subduction zones are capable of causing some of the most destructive earthquakes on Earth. By improving the ability to characterize loading deformation in GPS time series, we can improve the ability to monitor tectonic deformation

Modeling Surface Mass Load Displacements Along The Cascadia Subduction Zone

The Earth’s surface is under constant strain from different mass loads. Surface mass loads, such as the oceans, atmosphere, and continental water reservoirs, exert forces on the elastic solid Earth, inducing crustal deformation. These loads move over Earth\u27s surface on time scales varying from less than a day to many thousand years. Since the Earth is elastic and not perfectly rigid, the pressure from these loads deforms the shape of Earth’s surface. Horizontal and vertical displacement responses due to a load can be recorded using Global Positioning System (GPS) receivers. Modeling and removing surface-mass loading signals, which are present in all GPS time series, can reduce the variance in the time series. Surface deformation is of particular interest along subduction zones. A subduction zone is an area of tectonic plate collision where the more dense plate subducts, or moves underneath, the less dense plate. The Cascadia Subduction Zone extends from Vancouver Island down to Northern California. This research project focuses on using the python-based software program LOADDEF to accurately compute displacement responses of the Earth to oceanic, atmospheric, and hydrologic loads. These modeled responses are then compared to the observed displacement responses measured by the Plate Boundary Observatory along the Cascadia Subduction Zone

Exploring Effects of GPS Processing on Atmospheric Responses of Earth Deformation

Surface mass loads, such as oceans, atmosphere, glaciers, snowpack, and freshwater exert forces on Earth’s surface, causing the crust to change shape, also known as crustal deformation. The response of Earth’s crust to these mass loads is often easily predicted and removed from crustal measurements, except atmospheric response, which is much more variable from weather conditions. The Global Positioning System (GPS), which transmits radio signals between satellites and ground receivers, can monitor millimeter changes to the crust’s shape. This data is recorded by over 1200 GPS stations from the Plate Boundary Observatory network all across the western United States. However, methods for processing raw GPS data differ among analysis centers, producing different positions. One difference in GPS processing methods is the treatment of satellite signals delayed in the troposphere, the region of the atmosphere with the most mass. This research is significant to helping develop a set of standard methods for GPS processing and therefore making GPS more accurate. We investigated inconsistencies in three-dimensional (east, north, up) positions from five datasets created by the following three processing centers: NASA’s Jet Propulsion Laboratory, the Nevada Geodetic Laboratory, and UNAVCO consortium. We used the software program LoadDef to model the response of Earth’s surface to changes in atmospheric pressure at each station, which were then compared with observed GPS positions. We find a trend that GPS datasets produced with highly simplified assumptions about tropospheric delays have smaller reductions in root-mean-square scatter after correcting for atmospheric loading. Up to 50 % of the scatter in the residual GPS time series normally considered noise can be explained by crustal deformation responses from atmospheric mass loading. We conclude that differences in GPS time-series solutions can be partially explained by the temporal treatment of tropospheric signal delays during initial processing of raw GPS data

Vacancy ordering effects on the conductivity of yttria- and scandia-doped zirconia

Polarizable interaction potentials, parametrized using ab initio electronic structure calculations, have been used in molecular dynamics simulations to study the conduction mechanism in Y2 O3 - and Sc2 O3 -doped zirconias. The influence of vacancy-vacancy and vacancy-cation interactions on the conductivity of these materials has been characterised. While the latter can be avoided by using dopant cations with radii which match those of Zr4+ (as is the case of Sc3+), the former is an intrinsic characteristic of the fluorite lattice which cannot be avoided and which is shown to be responsible for the occurrence of a maximum in the conductivity at dopant concentrations between 8 and 13 %. The weakness of the Sc-vacancy interactions in Sc2 O3 -doped zirconia suggests that this material is likely to present the highest conductivity achievable in zirconias.Comment: 17 pages, 6 figur

Exploring Effects of GPS Processing on Atmospheric and Hydrologic Pressure-induced Crustal Responses

The Earth’s crust is in continuous motion from changes in fluid pressures associated with the redistribution of mass at the surface. These forces, known as surface mass loading, make up a significant amount of signal within GPS time series. This thesis is broken up into two projects exploring atmospheric and hydrologic pressure-induced crustal responses. The first project focuses on effects of GPS processing on corrections of atmospheric loading. We use data from over 1100 GPS stations within the Western US to investigate crustal displacements from atmospheric surface pressure variations. We find that modeling and removing atmospheric mass loading reduces root mean square (RMS) scatter of residual GPS time series by 16 % on average and up to 50 % for inland stations. We observe a trend of larger RMS reduction with increasing distance from the ocean, due to the inverted barometer effect. We then compare five sets of processed GPS data from three different processing centers (JPL, NGL, UNAVCO) and attempt to isolate possible causes for variations in the GPS displacements. The GPS products with the largest reductions in RMS scatter were generated using the more accurate, high resolution troposphere delays, with the UNAVCO data product providing the best retention of atmospheric mass loading (ATML) in the time series. The retention of ATML in the time series is affected by the temporal resolution of the tropospheric model used in initial processing of raw GPS signal. Mismodeling troposphere delays can lead to an inaccurate distance estimate between satellite and receiver, thereby limiting retention of atmospheric surface pressure-induced crustal displacements in the time series. As such, we recommend using high resolution tropospheric delays when possible. The second project focuses on isolating and quantifying hydrologic loading signal sources within GPS stations near the Columbia River along the Washington-Oregon border. We attempt to correlate seasonal river discharge with horizontal motions present within the GPS time series using particle motions ellipses. We also attempt correlation between sub-seasonal signals of displacement with changes in river discharge measured by USGS river gauges

Cation composition effects on oxide conductivity in the Zr_2Y_2O_7-Y_3NbO_7 system

Realistic, first-principles-based interatomic potentials have been used in molecular dynamics simulations to study the effect of cation composition on the ionic conductivity in the Zr2Y2O7-Y3NbO7 system and to link the dynamical properties to the degree of lattice disorder. Across the composition range, this system retains a disordered fluorite crystal structure and the vacancy concentration is constant. The observed trends of decreasing conductivity and increasing disorder with increasing Nb5+ content were reproduced in simulations with the cations randomly assigned to positions on the cation sublattice. The trends were traced to the influences of the cation charges and relative sizes and their effect on vacancy ordering by carrying out additional calculations in which, for example, the charges of the cations were equalised. The simulations did not, however, reproduce all the observed properties, particularly for Y3NbO7. Its conductivity was significantly overestimated and prominent diffuse scattering features observed in small area electron diffraction studies were not always reproduced. Consideration of these deficiencies led to a preliminary attempt to characterise the consequence of partially ordering the cations on their lattice, which significantly affects the propensity for vacancy ordering. The extent and consequences of cation ordering seem to be much less pronounced on the Zr2Y2O7 side of the composition range.Comment: 22 pages, 8 figures, submitted to Journal of Physics: Condensed Matte

Massive molecular outflows at high spatial resolution

We present high-spatial resolution Plateau de Bure Interferometer CO(2-1) and SiO(2-1) observations of one intermediate-mass and one high-mass star-forming region. The intermediate-mass region IRAS20293+3952 exhibits four molecular outflows, one being as collimated as the highly collimated jet-like outflows observed in low-mass star formation sources. Furthermore, comparing the data with additional infrared H2 and cm observations we see indications that the nearby ultracompact HII region triggers a shock wave interacting with the outflow. The high-mass region IRAS19217+1651 exhibits a bipolar outflow as well and the region is dominated by the central driving source. Adding two more sources from the literature, we compare position-velocity diagrams of the intermediate- to high-mass sources with previous studies in the low-mass regime. We find similar kinematic signatures, some sources can be explained by jet-driven outflows whereas other are better constrained by wind-driven models. The data also allow to estimate accretion rates varying from a few times 10^{-5}Msun/yr for the intermediate-mass sources to a few times 10^{-4}Msun/yr for the high-mass source, consistent with models explaining star formation of all masses via accretion processes.Comment: 14 pages text, 4 tables, 8 figures, accepted for Ap

Observations on the Formation of Massive Stars by Accretion

Observations of the H66a recombination line from the ionized gas in the cluster of newly formed massive stars, G10.6-0.4, show that most of the continuum emission derives from the dense gas in an ionized accretion flow that forms an ionized disk or torus around a group of stars in the center of the cluster. The inward motion observed in the accretion flow suggests that despite the equivalent luminosity and ionizing radiation of several O stars, neither radiation pressure nor thermal pressure has reversed the accretion flow. The observations indicate why the radiation pressure of the stars and the thermal pressure of the HII region are not effective in reversing the accretion flow. The observed rate of the accretion flow, 0.001 solar masses/yr, is sufficient to form massive stars within the time scale imposed by their short main sequence lifetimes. A simple model of disk accretion relates quenched HII regions, trapped hypercompact HII regions, and photo-evaporating disks in an evolutionary sequence

High-pressure behaviour of GeO2: a simulation study

In this work we study the high pressure behaviour of liquid and glassy GeO2 by means of molecular dynamics simulations. The interaction potential, which includes dipole polarization effects, was parameterized from first-principles calculations. Our simulations reproduce the most recent experimental data to a high degree of precision. The proportion of the various GeOn polyhedra is determined as a function of the pressure: a smooth transition from tetrahedral to octahedral network is observed. Finally, the study of high-pressure, liquid germania confirms that this material presents an anomalous behaviour of the diffusivity as observed in analog systems such as silica and water. The importance of penta-coordinated germanium ions for such behaviour is stressed.Comment: 16 pages, 4 figures, accepted as a Fast Track Communication on Journal of Physics: Condensed Matte