M. R. Craymer

Present-day tilting of the Great Lakes region based on water level gauges

By using monthly mean water levels at 55 sites around the Great Lakes, a regional model of vertical crustal motion was computed for the region. In comparison with previous similar studies over the Great Lakes, 15 additional gauge sites, data from all seasons instead of the 4 summer months, and 8 additional years of data were used. All monthly water levels available between 1860 and 2000, as published by the U.S. National Ocean Survey and the Canadian Hydrographic Service, were used. For each lake basin, the vertical velocities of the gauge sites relative to each other were simultaneously computed, using the least-squares adjustment technique. Our algorithm solves for and removes a monthly bias common to all sites, as well as site-specific biases. It also properly weighs the input water levels, resulting in a realistic estimation of the uncertainties in tilting parameters. The relative velocities obtained for each lake were then combined to obtain relative velocities over the entire Great Lakes region. Finally, the gradient of the relative rates for the regional model was found to agree best with the ICE-3G global isostatic model of Tushingham and Peltier, whereas the ICE-4G gradients were too small around the Great Lakes.

Realization and Unification of NAD83 in Canada and the U.S. via the ITRF

A more unified realization of the North American Datum of 1983 (NAD83) was recently implemented by Canada and the U.S. to provide more accurate and convenient access to this reference system. In both countries, NAD83 is the adopted datum for spatial positioning. It was initially realized in the mid-1980s by adjusting primarily classical geodetic observations that connected a network of monumented control points spanning North America. Although the new NAD83 realization is compatible with the original one within the accuracies of those classical observations, the wide use of high precision GPS surveys since then have required a more rigorous definition of NAD83 in terms of network scale, ellipsoidal heights and crustal motion. In an effort to provide a more precise realization of a 3D NAD83 common to both countries, and to comply with LAG resolutions recommending that reference systems be tied to the ITRS, Canada and the U.S. have collaborated in the determination of a common transformation between the ITRF and the fundamental 3Ddatum of NAD83. This conformal similarity transformation is based upon ITRF96, the latest realization of ITRF. Through the use of continuously operating GPS stations (CACS in Canada and CORS in the US) forming part of the ITRF, this transformation also provides more accurate and convenient access to the NAD83 reference frame. It also allows for the conversion of IGS precise GPS ephemerides from ITRF to NAD83, thereby enabling high accuracy GPS surveys to be performed entirely within NAD83.

Modeling 3-D crustal velocities in the United States and Canada

A numerical model for three-dimensional (3-D) crustal velocities has been derived for most of the United States and Canada, primarily from repeated geodetic data. This model provides a foundation for a prototype of the TRANS4D software. TRANS4D is being developed to enable geospatial professionals and others to transform 3-D positional coordinates across time. The derived model reveals several macroscopic features of the 3-D velocity field, including the pervasive presence of the glacial isostatic adjustment associated with the past melting of the ice fields that formed more than 19,000 years ago during the Last Glacial Maximum. In this study, the present-day 3-D velocity field associated with this melting (as estimated via the recently published ICE-6G_C (VM5a) model) was subtracted from this studys total 3-D velocity field to identify features of the residual velocity field. In particular, this study introduces the NA_ICE-6G reference frame in which residual horizontal velocities have magnitudes that are less than 2 mm/yr everywhere east of longitude 104°W and south of latitude 60°N, except in southern Texas. Residual horizontal velocities of greater magnitude are found west and/or north of these two boundaries, and they are due mostly to interactions among tectonic plates with localized pockets due to other geophysical phenomena. Large residual vertical velocities, some with values exceeding 30 mm/yr, are found in southeastern Alaska. The uplift occurring here is due to present-day melting of glaciers and ice fields formed during the Little Ice Age glacial advance that occurred between 1550 A.D. and 1850 A.D.

Estimation of rod scale errors in geodetic leveling

Comparisons among repeated geodetic levelings have often been used for detecting and estimating residual rod scale errors in leveled heights. Individual rod-pair scale errors are estimated by a two-step procedure using a model based on either differences in heights, differences in section height differences, or differences in section tilts. It is shown that the estimated rod-pair scale errors derived from each model are identical only when the data are correctly weighted, and the mathematical correlations are accounted for in the model based on heights. Analyses based on simple regressions of changes in height versus height can easily lead to incorrect conclusions. We also show that the statistically estimated scale errors are not a simple function of height, height difference, or tilt. The models are valid only when terrain slope is constant over adjacent pairs of setups (i.e., smoothly varying terrain). In order to discriminate between rod scale errors and vertical displacements due to crustal motion, the individual rod-pairs should be used in more than one leveling, preferably in areas of contrasting tectonic activity. From an analysis of 37 separately calibrated rod-pairs used in 55 levelings in southern California, we found eight statistically significant coefficients that could be reasonably attributed to rod scale errors, only one of which was larger than the expected random error in the applied calibration-based scale correction. However, significant differences with other independent checks indicate that caution should be exercised before accepting these results as evidence of scale error. Further refinements of the technique are clearly needed if the results are to be routinely applied in practice.

Detection of Vertical Temporal Behaviour of IGS Stations in Canada Using Least Squares Spectral Analysis

Unambiguous, consistent and homogeneous GPS station coordinates are the fundamental requirement in the appropriate determination of geodetic velocities that are often used for the derivation of geodetic and geophysical models for a variety of applications [Segall and Davis, Ann Rev Earth Planet Sci 23:201–336, 1997]. Because of this, there have been significant efforts to improve the modeling and parameterization of global GPS solutions in order to get stable and homogeneous positions and velocities. This paper presents a study aiming at detecting least-squares spectral peaks present at the best available (at the time) IGS weekly vertical component time series of five permanent stations in Canada. These peaks are the result of short and long term effects of mismodelled and unmodelled geophysical phenomena on the height. The LSSA approach is used. Results show strong periodic constituents in the LSSA spectrum below or at the 1 year window but most notably constituents with periods longer than a year.

Structural Inheritance Control on Intraplate Present‐Day Deformation: GPS Strain Rate Variations in the Saint Lawrence Valley, Eastern Canada

Structural inheritance is one of the key factors commonly proposed to control the localization of strain and seismicity in continental intraplate regions, primarily on the basis of a first‐order spatial correlation between seismicity and inherited tectonic structures. In this paper, we present new GPS (Global Positioning System) velocity and strain rate analyses that provide strong constraints on the magnitude and style of present‐day strain localization associated with the inherited tectonic structures of the Saint Lawrence Valley, eastern Canada. We analyze 143 continuous and campaign GPS stations to calculate velocity and strain rate patterns, with specific emphases on the combination of continuous and campaign velocity uncertainties, and on the definition of robustness categories for the strain rate estimations. Within the structural inheritance area, strain rates are on average 2–11 times higher than surrounding regions and display strong lateral variations of the style of deformation. These GPS velocity and strain rate fields primarily reflect ongoing glacial isostatic adjustment (GIA). Their comparison with GIA model predictions allows us to quantify the impact of the structural inheritance and the associated lithosphere rheology weakening. Outside of the major tectonic inheritance area, GPS and GIA model strain rates agree to first order, both in style and magnitude. In contrast, the Saint Lawrence Valley displays strong strain amplification with GPS strain rates 6–28 times higher than model‐predicted GIA strain rates. Our results provide the first quantitative constraints on the impact of lithospheric‐scale structural inheritance on strain localization in intraplate domains.

Aquistore project: ground deformation retrieved by InSAR during May 2012 – May 2013

Objectives of the Aquistore CO2 storage project are to design, adapt, and test non-seismic monitoring methods that have not been systematically utilized to date for monitoring CO2 storage, and to integrate the data from these various monitoring tools to obtain quantitative estimates of the change in subsurface fluid distributions, pressure changes and associated surface deformation. Since spring of 2012 RADARSAT-2 data from five beams (ascending and descending Spotlight, Wide UltraFine and Fine Quad-Pol) were regularly (with the individual frequency of 24 days) collected and used for calculation of ground deformation time series over the Aquistore CO2 storage site located in SE Saskatchewan. The initial InSAR analysis revealed slow ground deformation not related to CO2 injection but caused by various natural and anthropogenic processes - snow melt, surface moisture variation, ground and surface water level changes and post-mining activities. In this work we provide updated results based on over one hundred RADARSAT-2 images acquired during May 2012 - May 2013.