David LaVallee

Crustal displacements due to continental water loading

The effects of long-wavelength (> 100 km), seasonal variability in continental water storage on vertical crustal motions are assessed. The modeled vertical displace- ments (ARM) have root-mean-square (RMS) values for 1994- 1998 as large as 8 mm, with ranges up to 30 mm, and are predominantly annual in character. Regional strains are on the order of 20 nanostrain for tilt and 5 nanostrain for hori- zontal deformation. We compare ArM with observed Global Positioning System (GPS) heights (Aro) (which include ad- justments to remove estimated effects of atmospheric pres- sure and annual tidal and non-tidal ocean loading) for 147 globally distributed sites. When the Aro time series are ad- justed by ArM, their variances are reduced, on average, by an amount equal to the variance of the ArM. Of the Aro time series exhibiting a strong annual signal, more than half

Widespread low rates of Antarctic glacial isostatic adjustment revealed by GPS observations

Bedrock uplift in Antarctica is dominated by a combination of glacial isostatic adjustment (GIA) and elastic response to contemporary mass change. Here, we present spatially extensive GPS observations of Antarctic bedrock uplift, using 52% more stations than previous studies, giving enhanced coverage, and with improved precision. We observe rapid elastic uplift in the northern Antarctic Peninsula. After considering elastic rebound, the GPS data suggests that modeled or empirical GIA uplift signals are often over?estimated, par t icularly the magnitudes of the signal maxima. Our observation that GIA uplift is misrepresented by modeling (weighted root?meansquares of observation?model differences: 4.9–5.0 mm/yr) suggests that, apart from a few regions where large ice mass loss is occurring, the spatial pattern of secular ice mass change derived from Gravity Recovery and Climate Experiment (GRACE) data and GIA models may be unreliable, and that several recent secular Antarctic ice mass loss estimates are systematically biased, mainly too high.

Signal and noise in Gravity Recovery and Climate Experiment (GRACE) observed surface mass variations

The Gravity Recovery and Climate Experiment (GRACE) product used for this study consists of 43 monthly potential coefficient sets released by the GRACE science team which are used to generate surface mass thickness grids expressed as equivalent water heights (EQWHs). We optimized both the smoothing radius and the level of approximation by empirical orthogonal functions (EOFs) and found that 6.25° and three modes are able to describe more than 73.5% of the variance. The EQWHs obtained by the EOF method describe all known variations in the continental hydrology, present?day ice sheet melting, and global isostatic adjustment. To assess the quality of the estimated grids, we constructed degree error spectra of EQWHs. We conclude that a significant part of the errors in GRACE can be explained by a scaling factor of 0.85 relative to degree error estimates provided by the GGM02C gravity model but that the present?day errors in the GRACE data are a factor 2 to 5 larger than forecasted by tide model differences and atmospheric pressure differences. Comparison to a network of 59 International GNSS Service (IGS) stations confined the filter parameter settings to three EOF modes and 5° or 6.25° smoothing radius. Residuals that remain after the EOF method do exhibit S2 aliasing errors and a semiannual continental hydrology signal contained in the Global Land Data Assimilation Systems (GLDAS) model. Further analysis of the residual EOF signal revealed alternating track correlation patterns that are partially explained by the GRACE covariance matrix and the handling of nuisance parameters in the GRACE data processing.

On the determination of a global strain rate model

The objective of this paper is to outline the fundamental concepts underlying the estimation of a global strain rate model. We use a variant of the method first introduced by Haines and Holt (1993) to estimate the strain rate tensor field within all of the Earth’s deforming regions. Currently the observables used are ~1650 geodetic velocities, seismic moment tensors from the Harvard CMT catalog, and Quaternary fault slip rate data. A model strain rate field and velocity field are obtained in a least-squares fit to both the geodetic velocities and the observed strain rates inferred from fault slip rates. Seismic moment tensors are used to provide a priori constraints on the style and direction (not magnitude) of the model strain rate field for regions where no fault slip rate data are available. The model will soon be expanded to include spreading rates, ocean transform azimuths, and more fault slip rate data. We present a first estimate of the second invariant of the global model strain rate field. We also present Euler poles obtained by fitting geodetic vectors located on defined rigid plates. We find that 17% of the total model moment rate is accommodated in zones of (diffuse) continental deformation.

Sea‐level fingerprint of continental water and ice mass change from GRACE

The Gravity Recovery and Climate Experiment satellites (GRACE) provide, for the first time, a method to directly measure mass exchange between the land and oceans over time. The dominant components of this exchange are due to continental ice loss/gain and land hydrology. Here, we determine the secular trend in these two components during the GRACE measurement era: 2003–2009. For each component, we model the distinct regional signatures or fingerprints of relative sea?level (RSL) change, obtaining maxima at low latitudes between ±40° N/S, but with particularly strong regional patterns. We estimate that the total ice and water mass loss from the continents is causing global mean sea?level to rise by 1.0 ± 0.4 mm/yr. Isolating the ice and hydrological signals, we find that the former is the sole net contributor to the global mean, while the latter dominates regional RSL changes in many coastal areas.

Higher?order ionospheric effects on the GPS reference frame and velocities

We describe how GPS time series are influenced by higher?order ionospheric effects over the last solar cycle (1995–2008) and examine implications for geophysical studies. Using 14 years of globally reprocessed solutions, we demonstrate the effect on the reference frame. Including second? and third?order ionospheric terms causes up to 10 mm difference in the smoothed transformation to the International Terrestrial Reference Frame (ITRF) 2005, with the Z translation term dominant. Scale is also slightly affected, with a change of up to ?0.05 ppb. After transformation to ITRF2005, residual effects on vertical site velocities are as high as 0.34 mm yr?1. We assess the effect of the magnetic field model on the second?order term and find a time?varying difference of 0–2 mm in the Z translation. We also assess the effect of omitting the third?order term. We find that while the second?order term is responsible for almost all the Z translation effects, it is the combination of the second? and third?order terms that causes the effect on scale. Comparison of our GPS reprocessing with ITRF2005 suggests that GPS origin rates may vary with time period. For example, we find Z translation rates of ?0.82 ± 0.17 mm yr?1 for 1995–2008 and 0.17 ± 0.24 mm yr?1 for 1995–2005. If GPS were to contribute to origin rate definition for future ITRFs, higher?order ionospheric corrections would need to be applied due to their effect on translation parameters during solar maximum.

Degree‐2 harmonics of the Earth's mass load estimated from GPS and Earth rotation data

A fluid, mobile atmosphere and oceans surrounds the solid Earth and upon its land surface lays a continually changing distribution of ice, snow, and ground water. The changing distribution of mass associated with the motion of these surficial fluids changes the Earths rotation by changing its inertia tensor and changes the Earths shape by changing the load on the solid Earth. It has recently been demonstrated that large-scale changes of the Earths shape, and hence of the mass load causing the Earths shape to change, can be measured using the global network of GPS receivers. Here, the degree-2 mass load coefficients determined from GPS data are compared with those obtained from Earth orientation observations from which the effects of tides, winds, and currents have been removed. Good agreement is found between these two estimates of the degree-2 mass load, particularly at seasonal frequencies.

Seasonal variations in sea level induced by continental water mass: First results from GRACE

Variations in the Earths water cycle are commonly quantified by their effect on global mean sea?level. However, the interaction between passive adjustment of the ocean to changes in gravitational attraction due to mass redistribution, the related deformation of the solid Earth and disturbances in the Earths rotation vector will yield a distribution that is more complicated than a uniform rise or fall of the oceans surface. In this study, we present the first estimates of seasonal changes in passive sea?level (which we define as the height difference between the sea surface at rest and ocean floor, excluding steric and dynamical effects) based on direct observations of surface mass redistribution, made by the Gravity Recovery and Climate Experiment (GRACE) between 2003 and 2010. We show that this “selfgravitation?effect” causes seasonal variations of the sea?level of up to 1 cm – comparable to the amplitude of the long?period tides – and that inclusion in numerical ocean models results in a better agreement between observed and modelled ocean bottom pressure variations, particularly in coastal zones.

J2: An evaluation of new estimates from GPS, GRACE, and load models compared to SLR

Changes in J2, resulting from past and present changes in Earths climate, are traditionally observed by Satellite Laser ranging (SLR). Assuming an elastic Earth, it is possible to infer changes in J2 from changes in Earths shape observed by GPS. We compare estimates of non-secular J2 changes from GPS, SLR, GRACE, and a load model. The GPS and SLR annual signals agree but are different (16%) to the load model. Subtraction of the load model removes the annual variation from GPS, SLR, and GRACE, and the semi-annual variation in GPS. The GPS and SLR long-term signals are highly correlated, but GPS is better correlated with the loading model. Subtraction of the load model removes the 1998 anomaly from the GPS J2 series but not completely from the SLR J2 series, suggesting that the SLR anomaly may not be entirely due to mass re-distribution as has been presumed.

Project helps constrain continental dynamics and seismic hazards

The Global Strain Rate Map project II-8, initiated in 1998 by the International Lithosphere Program (ILP), provides constraints for understanding continental dynamics and for quantifying seismic hazards in general. To date, the Global Strain Rate Map (GSRM) model is a numerical velocity gradient tensor fi eld solution (i.e., spatial variations of horizontal strain rate tensor components and rotation rates) for the entire Earth surface