.[1] We examine absolute gravity (AG) and vertical Global Positioning System (GPS) time series between 1995 and 2010 at eight collocated sites in mid-continent North America. The comparison of AG and GPS rates aligned to ITRF2005 yields a gravity/uplift ratio of À0.17 AE 0.01 mGal mm À1(1 mGal = 10 nm s À2) and an intercept of À0.1 AE 0.5 mm yr À1. In contrast, aligning the GPS velocities to ITRF2000 results in a gravity/uplift intercept of À1.3 AE 0.5 mm yr À1. The near-zero gravity/uplift offset for the ITRF2005 (or ITRF2008) results shows a good alignment of the GPS vertical velocities to Earth's center of mass, and confirms that GPS velocities in this reference frame can be compared to predictions of geodynamic processes such as glacial isostatic adjustment (GIA) or sea-level rise. The observed gravity/uplift ratio is consistent with GIA model predictions. The ratio remains constant in regions of fast and slow uplift, indicating that GIA is the primary driving process and that additional processes such as local hydrology have a limited impact on a decadal time-scale. Combining AG and GPS measurements can provide significant constraints for geodetic, geodynamic, and hydrological studies. Citation: Mazzotti, S., A. Lambert, J. Henton, T. S. James, and N. Courtier (2011), Absolute gravity calibration of GPS velocities and glacial isostatic adjustment in

Subsidence is a common cause of amplifi ed relative sea-level rise, fl ooding, and erosion in coastal environments. In particular, subsidence due to sediment consolidation can play a signifi cant role in relative sea-level rise in large deltas. We use a combination of InSAR (interferometric synthetic aperture radar), leveling, and global positioning system data to map absolute vertical land motion in the Fraser River delta, western Canada. We show that primary consolidation of shallow Holocene sediments is the main cause for the slow subsidence (−1 to −2 mm/a) affecting the delta lowlands. In addition, parts of the delta undergo increased anthropogenic subsidence. Rapid subsidence rates (−3 to −8 mm/a) are associated with recent artifi cial loads and exhibit a fi rst-order exponential decrease with a time constant of ~20 years, consistent with the theory of consolidation. Assuming two sea-level rise scenarios of 30 or 100 cm by the end of the twenty-fi rst century, natural subsidence will augment relative sea-level rise in the Fraser Holocene lowlands by ~50% or ~15%. Anthropogenic subsidence will augment relative sea-level rise by ~130% or ~40%, potentially raising it to as much as 1-2 m. In deltaic, lacustrine, and alluvial environments, anthropogenic sediment consolidation can result in signifi cant amplifi cation and strong spatial variations of relative sea-level rise that need to be considered in local planning.

Abstract. Repeated absolute gravity measurements have been made over a period of several years at six sites along a 3000 kmlong, mid-continental, North American profile from the coast of Hudson Bay southward to Iowa. With the exception of the southern-most site, the observed rates of change of gravity are significantly higher than rates predicted by current models, such as ICE-3G and a laterally homogeneous, standard Earth. The observed gravity change rates suggest significant modifications, such as a 2 to 3-fold increase in lower mantle viscosity or a 50% increase in Laurentide ice sheet thickness west of Lake Superior. Results

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We analyze hourly data from five tremor episodes in the northern Cascadia subduction zone over the period 2003–2005 provided by the Tremor Activity Monitoring System (TAMS). All five tremor episodes correspond to slow slip events observed by GPS. Fourier decomposition is used to separate the hourly tremor counts for each episode into “long‐period” (0 < f < 0.8 cpd), “tidal” (0.8 < f < 2.2 cpd), and “short‐period” (f > 2.2 cpd) components. The tidal component of the observations is compared with theoretical stress variations at depths of 20, 30, and 40 km, with 40 km being the depth of the interpreted subduction thrust interface. The stress variations are predicted by a 2‐D ocean tide loading model combined with estimates of stress variations from Earth tides. We find that the shear stress in the thrust direction and the compressive normal stress on shallow dipping surfaces correlates with the data significantly better than the confining stress over the range of depths investigated. The relative amplitudes of tidal shear stress and compressive normal stress result in positive Coulomb stress favoring slip. Peak tremor activity occurs at times of maximum tidal shear stress in the thrust direction, which would assist slow slip and would suggest that tidal tremor and slip are colocated. The response of the tremor to tidal shear stress is roughly proportional to the mean activity level, controlled by tectonic conditions of stress and pore pressure. A significant, nontidal, daily variation in tremor activity of unknown origin is identified.

[1] We combine data from nine GPS, absolute gravity, and tide gauge stations to estimate the relation between sea-level rise, vertical motion, and solid Earth processes in the Pacific Northwest. GPS vertical velocities (in ITRF2000) and absolute gravity rates are well correlated, with a gradient of 0.2 ± 0.1 mGal mm À1 , but show a significant offset of 0.53 ± 0.30 mGal yr À1 (2.2 ± 1.3 mm yr À1 ) (95% confidence). Tide gauge and GPS data indicate a northeast Pacific regional sea-level rise of 1.7 ± 0.5 mm yr À1 , aligned to ITRF2000, or an unlikely regional sea-level fall of À0.5 ± 0.5 mm yr À1 , aligned to absolute gravity. Although we cannot rule out a bias in the GPS reference-frame alignment, our results suggest a possible absolute gravity bias by a long-period mass increase from an unknown near-surface or deep-seated source. The impact of such a mass increase on gravity, vertical motion, and sea level remains to be defined.

International audienceGravity Recovery and Climate Experiment (GRACE) satellite-derived total water storage can be obscured by glacial isostatic adjustment. In order to solve this problem for the Nelson River drainage basin in Canada, a gravity rate map from 110 months (June 2002 to October 2011) of GRACE gravity data was corrected for glacial isostatic adjustment using an independent gravity rate map derived from updated GPS vertical velocities. The GPS-based map was converted to equivalent gravity rate using a transfer function developed from GPS and absolute-g data at colocated sites. The corrected GRACE gravity rate map revealed a major positive anomaly within the drainage basin, which was independently shown by hydrological data to be due to changes in water storage. The anomaly represents a cumulative increase at its center of about 340 mm of water, reflecting a progression from extreme drought to extremely wet conditions

Crustal motion predicted by the ICE‐3G glacial rebound model exhibits a pattern of tangential (horizontal) divergence away from the centres of uplift, which in North America and Europe are located around Hudson Bay and the Gulf of Bothnia. Tangential velocities reach peak magnitudes of 1–2 mm/yr, and must be included when predicting Very‐Long‐Baseline‐Interferometry (VLBI) baseline‐length change rates due to post‐glacial rebound. Out of 18 observed VLBI baselines examined 3 are situated such that their predicted length rates are around their 2σ uncertainties or greater. It is encouraging that 2 of these baselines exhibit predicted length rates within 2σ of the observed rates.