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.[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

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.