Paul Tregoning

Motion and rigidity of the Pacific Plate and implications for plate boundary deformation

[1] Using up to 11 years of data from a global network of Global Positioning System (GPS) stations, including 12 stations well distributed across the Pacific Plate, we derive present-day Euler vectors for the Pacific Plate more precisely than has previously been possible from space geodetic data. After rejecting on statistical grounds the velocity of one station on each of the Pacific and North American plates, we find that the quality of fit of the horizontal velocities of 11 Pacific Plate (PA) stations to the best fitting PA Euler vector is similar to the fit of 11 Australian Plate (AU) velocities to the AU Euler vector and � 20% better than the fit of nine North American Plate (NA) velocities to the NA Euler vector. The velocities of stations on the Pacific and Australian Plates each fit a rigid plate model with an RMS residual of 0.4 mm/yr, while the North American velocities fit a rigid plate model with an RMS velocity of 0.6 mm/yr. Our best fitting PA/AU relative Euler vector is located � 170 km southeast of the NUVEL-1A pole but is not significantly different at the 95% confidence level. It is also close (<70 km in position and <3% in rate) to a pole derived from transform faults identified from satellite altimetry, suggesting that the vector has not changed significantly over the past 3 Myr. Our relative Euler vector is also consistent with all known geological and geodetic evidence concerning the AU/PA plate boundary through New Zealand. The GPS sites offshore of southern California are presently moving 4–5 ± 1 mm/yr relative to predicted Pacific velocity, with their residual velocities in approximately the opposite direction to PA/NA relative motion. Likewise, the easternmost sites in South Island, New Zealand, are moving � 3 ± 1 mm/yr relative to predicted Pacific velocity, with the residuals in approximately the opposite direction to PA/AU relative motion. These velocity residuals are in the same sense as predicted by elastic strain accumulation on known plate boundary faults but are of a significantly higher magnitude in both southern California and New Zealand, implying that the plate boundary zones in both regions are wider than previously believed. INDEX TERMS: 1206 Geodesy and Gravity: Crustal movements—interplate (8155); 1208 Geodesy and Gravity: Crustal movements—intraplate (8110); 1243 Geodesy and Gravity: Space geodetic surveys; 8158 Tectonophysics: Evolution of the Earth: Plate motions—present and recent (3040); 8159 Tectonophysics: Evolution of the Earth: Rheology—crust and lithosphere; KEYWORDS: Pacific plate motion, plate rigidity, plate boundary deformation

Accuracy of absolute precipitable water vapor estimates from GPS observations

We present GPS, radiosonde and microwave radiometer (MWR) estimates of precipitable water vapor (PW) at Cape Grim, Tasmania, during November and December 1995. The rms differences between GPS and radiosonde, MWR and radiosonde and GPS and MWR estimates of PW were 1.5 mm, 1.3 mm and 1.4 mm, respectively, whilst the biases between the three systems were ∼0.2 mm. However, there are occasions when the amount of PW was underestimated by GPS whilst at other times was over-estimated by MWR. The average overlap error of the GPS estimates of PW between adjacent daily solutions is related to the orbit overlap error and we removed a 2 mm bias introduced using International GPS Service orbits by estimating more accurate global orbits. The discrepancies of up to 3–4 mm between the MWR and GPS systems are not caused by rain, waveguide losses, varying waveguide temperature, detector non-linearity or inaccurate estimates of the mean radiating temperature of the atmosphere. However, small differences between mapping functions at low elevations can produce biases comparable with the bias between the two systems. Consequently, we suspect that the biases arise because the mapping functions do not represent the localized atmospheric conditions at Cape Grim. The most accurate GPS estimates are achieved when the GPS analysis contains station separations of more than 2000 km, an elevation cutoff angle of 12° is used and the CFA2.2 wet mapping function is used to map the wet delay at any angle to the delay in the zenith.

Estimation of current plate motions in Papua New Guinea from Global Positioning System observations

Plate tectonic motions have been estimated in Papua New Guinea from a 20 station network of Global Positioning System sites that has been observed over five campaigns from 1990 to 1996. The present velocities of the sites are consistent with geological models in which the South Bismarck, Woodlark, and Solomon Sea Plates form the principal tectonic elements between the Pacific and Australian Plates in this region. Active spreading is observed on the Woodlark Basin Spreading Centre but at a rate that is about half the rate determined from magnetic reversals. The other major motions observed are subduction on the New Britain Trench, seafloor spreading across the Bismarck Sea Seismic Lineation, convergence across the Ramu-Markham Fault and left-lateral strike slip across the Papuan Peninsula. These motions are consistent with a 8.2° Myr -1 clockwise rotation of the South Bismarck Plate about a pole in the Huon Gulf and a rotation of the Woodlark Plate away from the Australian Plate. Second order deformation may also be occurring; in particular, Manus Island and northern New Ireland may be moving northward relative to the Pacific Plate at ∼5-8 mm yr -1 (significant at the 95% but not at the 99% confidence level) which may suggest the existence of a North Bismarck Plate.

Absolute Calibration in Bass Strait, Australia: TOPEX, Jason-1 and OSTM/Jason-2

Updated absolute bias estimates are presented from the Bass Strait calibration site (Australia) for the TOPEX/Poseidon (T/P), Jason-1 and the Ocean Surface Topography Mission (OSTM/Jason-2) altimeter missions. Results from the TOPEX side A and side B data show biases insignificantly different from zero when assessed against our error budget (−15 ± 20 mm, and −6 ± 18 mm, respectively). Jason-1 shows a considerably higher absolute bias of +93 ± 15 mm, indicating that the observed sea surface is higher (or the range shorter), than truth. For OSTM/Jason-2, the absolute bias is further increased to +172 ± 18 mm (determined from T/GDR data, cycles 001–079). Enhancements made to the Jason-1 and OSTM/Jason-2 microwave radiometer derived products for correcting path delays induced by the wet troposphere are shown to benefit the bias estimate at the Bass Strait site through the reduction of land contamination. We note small shifts to bias estimates when using the enhanced products, changing the biases by +11 and +3 mm for Jason-1 and OSTM/Jason-2, respectively. The significant, and as yet poorly understood, absolute biases observed for both Jason series altimeters reinforces the continued need for further investigation of the measurement systems and ongoing monitoring via in situ calibration sites.

Impact of solid Earth tide models on GPS coordinate and tropospheric time series

Unmodelled sub-daily periodic signals can propagate into time series of daily geodetic coordinates and tropospheric estimates at various different frequencies. Geophysical interpretations of geodetic products, particularly at seasonal timescales, can therefore be affected by poorly modelled signals in the geodetic analysis. In this study, we use two solid Earth tide models (IERS2003 and IERS1992) and analyses of global GPS data to demonstrate how this process occurs. Aliased annual and semi-annual signals are evident in the vertical component of the GPS time series, with the amplitudes increasing as a function of latitude up to approximately 2.0 and 0.4 mm, respectively. Tropospheric zenith delay estimates show differences at the 2mm level, with a dominant diurnal frequency. These results have significant implications in regard to the geophysical interpretation of GPS time series computed using the outdated IERS1992 model and, more generally, for any mis- or unmodelled periodic signals that affect geodetic sites.

Glacial isostatic adjustment and nonstationary signals observed by GRACE

[1] Changes in hydrologic surface loads, glacier mass balance, and glacial isostatic adjustment (GIA) have been observed using data from the Gravity Recovery and Climate Experiment (GRACE) mission. In some cases, the estimates have been made by calculating a combination of the linear rate of change of the time series and periodic seasonal variations of GRACE estimates, yet the geophysical phenomena are often not stationary in nature or are dominated by other nonstationary signals. We investigate the variation in linear rate estimates that arise when selecting different time intervals of GRACE solutions and show that more accurate estimates of stationary signals such as GIA can be obtained after the removal of model-based hydrologic effects. We focus on North America, where numerical hydrological models exist, and East Antarctica, where such models are not readily available. The root mean square of vertical velocities in North America are reduced by ~20% in a comparison of GRACE- and GPS-derived uplift rates when the GRACE products are corrected for hydrological effects using the GLDAS model. The correlation between the rate estimates of the two techniques increases from 0.58 to 0.73. While acknowledging that the GLDAS model does not model all aspects of the hydrological cycle, it is sufficiently accurate to demonstrate the importance of accounting for hydrological effects before estimating linear trends from GRACE signals. We also show from a comparison of predicted GIA models and observed GPS uplift rates that the positive anomaly seen in Enderby Land, East Antarctica, is not a stationary signal related to GIA.

Motion of the South Bismarck Plate, Papua New Guinea

The absolute motion of the South Bismarck Plate was first estimated by Tregoning et al. [1998] from three site velocities estimated from Global Positioning System (GPS) observations. We report an improved estimate of the Euler vector for this plate using site velocities derived from new GPS data which include the velocity of a site located ∼25 km from the pole of rotation. The GPS velocities of Madang, Witu, Jacquinot Bay and Finschafen can be modelled to within ∼3 mm/yr using a single pole of rotation located at 6.75°S, 147.98°E with a clockwise rotation rate of 8.11°/My. The known tectonic features and available geophysical data surrounding the South Bismarck Plate can also be explained by a rotation of the South Bismarck Plate about this pole.

Is the Australian Plate deforming? A space geodetic perspective

Measurements at discrete points spanning the Australian Plate have been made for over a decade using the Global Positioning System (GPS) space geodetic technique. These measurements show that, to within the resolution of the technique (~2 mm/y at 95% confidence level), there are no significant changes in the dimensions of the Australian Plate across the Australian continent. That is, no changes in baseline lengths are evident between any sites located in Australia when taking into account the measurement and modelling errors. However, during the past two decades, several significant earthquakes have occurred within the Australian Plate indicating that at times stress failure levels are reached, resulting in failure within the crust. Therefore, the rate at which stress accumulates must be slower than is visible in the geodetic measurements. With the exception of two sites affected by earthquake co-seismic displacement and equipment failure, all time series of sites in the interior of the Australian Plate are linear and site velocities are not significantly different from the predicted motion of the ‘rigid’ Australian Plate. However, the northern margin of the plate in Papua New Guinea is undergoing regional deformation. This is probably a result of the interaction with neighbouring plates and the proximity of the GPS site to nearby plate-boundary zones.

Present-day crustal motion in Papua New Guinea

Papua New Guinea is one of the most active tectonic regions in the world. It comprises several microplates and deforming zones trapped in the collision of the Australian and Pacific Plates. GPS observations have been used to estimate plate velocities across a network of sites spanning most of the country. We present new velocites in the northwestern region of New Guinea, and look in detail at the strain accumulation region between the South Bismarck and Pacific Plates in the New Ireland/New Britain region.

Improved water balance component estimates through joint assimilation of GRACE water storage and SMOS soil moisture retrievals

The accuracy of global water balance estimates is limited by the lack of observations at large scale and the uncertainties of model simulations. Global retrievals of terrestrial water storage (TWS) change and soil moisture (SM) from satellites provide an opportunity to improve model estimates through data assimilation. However, combining these two data sets is challenging due to the disparity in temporal and spatial resolution at both vertical and horizontal scale. For the first time, TWS observations from the Gravity Recovery and Climate Experiment (GRACE) and near-surface SM observations from the Soil Moisture and Ocean Salinity (SMOS) were jointly assimilated into a water balance model using the Ensemble Kalman Smoother from January 2010 to December 2013 for the Australian continent. The performance of joint assimilation was assessed against open-loop model simulations and the assimilation of either GRACE TWS anomalies or SMOS SM alone. The SMOS-only assimilation improved SM estimates but reduced the accuracy of groundwater and TWS estimates. The GRACE-only assimilation improved groundwater estimates but did not always produce accurate estimates of SM. The joint assimilation typically led to more accurate water storage profile estimates with improved surface SM, root-zone SM, and groundwater estimates against in situ observations. The assimilation successfully downscaled GRACE-derived integrated water storage horizontally and vertically into individual water stores at the same spatial scale as the model and SMOS, and partitioned monthly averaged TWS into daily estimates. These results demonstrate that satellite TWS and SM measurements can be jointly assimilated to produce improved water balance component estimates.