This article presents a large-scale surface gravity network in Colombia, developed during a 3-year collaboration between Ohio State University (OSU) and Colombia’s national mapping agency, the Instituto Geográfico Agustín Codazzi. The network spans approximately one-third of the country’s surface area and consists of 498 stations, including 22 absolute gravity (AG) constraints. We applied the OSU field protocol and adjustment technique, extending it to address the challenge of traffic-induced noise at benchmarks located near major roads. Our methodology employed measurements with an unusually high degree of redundancy, with 87% of the gravity lines surveyed using a minimum of four relative gravimeters. Additionally, we utilized both automatic (Scintrex CG-6) and manual (LaCoste and Romberg Model G) instruments to leverage their complementary strengths in different environments. The resulting gravity values were determined with a typical uncertainty (1 sigma) of ± 0.03 mGal. The leave-one-out cross-validation tests using the AG stations demonstrated the robustness of our solution, with all residuals statistically indistinguishable from zero. Our findings show that the combination of enhanced observational redundancy and the use of multiple gravimeter types effectively mitigates measurement challenges under suboptimal conditions, with only a modest increase in field time.

The Central and South-Central Andes form a “two-sided” mountain belt bounded by distinct zones of convergence in the western forearc and eastern foreland flanks. Previous geodetic studies of interseismic deformation in the Bolivian Subandes and the Argentine Precordillera found that the forearc to foreland velocity field decayed too slowly to be explained purely by elastic shortening driven by locking of the Nazca megathrust. The velocity field is more precisely explained if elastic deformation is augmented by eastward displacement of the entire Andes. Here, we extend the earlier interpretation of interseismic motion and argue that foreland décollements can participate in the co- and postseismic phases of the earthquake deformation cycle associated with the Nazca megathrust. These findings have direct implications in estimating recurrence interval, slip rate, and probabilistic seismic hazard analysis on both sides of the orogen.

Abstract. For nearly three decades, geodetic GNSS measurements in Antarctica have provided direct observations of bedrock displacement, which is linked to various geodynamic processes, including plate motion, post-seismic deformation and glacial isostatic adjustment (GIA). Previous geodynamic studies in Antarctica, especially those pertaining to GIA, have been constrained by the limited availability of GNSS data. This is due to the fact that GNSS data are collected by a wide range of institutions and network operators, with the raw observational data either not publicly available or scattered across various repositories. Further, the metadata necessary for rigorous data processing has often not been available or reliable. Consequently, the potential of GNSS observations for geodynamic studies in Antarctica has not been fully exploited yet. Here, we present consistently processed coordinate time series for GNSS sites in Antarctica and the sub-Antarctic region for the time span from 1995 to 2021. The data set is composed of 286 continuous and episodic sites, with 258 sites having a time span longer than three years. The coordinate time series were obtained from a combination of four independent processing solutions using different GNSS software and products, allowing the identification of inconsistencies in individual solutions. From these, we infer a reliable and robust combined solution. A key issue was the thorough reassessment of station metadata to minimise artefacts and biases in the coordinate time series. The resulting data set provides coordinate time series with unprecedented spatio-temporal coverage, promising significant advancements in future geodynamic studies.

Like many geophysical observations, relative gravity (RG) measurements are affected by random errors, systematic errors, and occasional blunders. When RG measurements are used to build large gravity networks in remote areas under adverse environmental or logistical conditions (such as extreme temperatures, heavy precipitation, rugged terrain, difficult or dangerous roads, and high altitudes), it is more likely for significant errors to occur and accumulate. Therefore, obtaining accurate gravity estimates at regional gravity networks largely depends on defensive data collection protocols and robust adjustment techniques. In this work, we present a measurement field protocol based on highly redundant observation patterns, and a two-step least squares adjustment scheme implemented as a MATLAB package. This software helps us identify blunders, mitigates the impact of random errors, and downweights or removes outlier observations. The methodology also guarantees that adjusted gravity values have well-constrained standard error estimates. We illustrate the capabilities of our approach through the case study of the Bolivian gravity network, where we determined the acceleration due to gravity at 2548 stations that spread over difficult and sometimes extreme environments, with a typical level of uncertainty of 0.10–0.15 mGal.

ANET-POLENET (Antarctic Network of the Polar Earth Observing Network) bedrock GNSS sites in the Ross Sea region of Antarctica surround an LGM load center in the Siple region of the Ross Embayment and record crustal motion due to GIA. Rather than a radial pattern of horizontal motion away from the former load, we instead observe three primary patterns of deformation; 1) motions are reversed towards the load in the southern region of the Transantarctic Mountains (TAM), 2) motions are radially away from the load in the Marie Byrd Land (MBL) region, and 3) an overall gradient in motion is present, with magnitudes progressively increasing from East to West Antarctica. We investigate the effects of alternative Earth model and ice loading scenarios, with the goal of understanding these distinct patterns of horizontal bedrock motion and their drivers. Using GIA models with a range of 1D Earth models, alternative ice loading scenarios for the Wilkes Subglacial Basin (LGM time scale) and the Siple Coast (centennial and millennial time scales) are explored. We find that no 1D model, regardless of the Earth model and ice loading scenario used, reproduces all three distinct patterns of observed motion at the same time. For select ice loading scenarios we also examine the influence of more complex rheology by invoking a boundary in Earth properties beneath the Transantarctic Mountains. This approach accounts for the strong lateral gradient in Earth properties across the continent by effectively separating East and West Antarctica into two different Earth model profiles. Some of our GIA models utilizing 3D Earth structure reproduce predicted motions that match all three observed patterns of deformation, and we find that a multiple order magnitude of change in upper mantle viscosity between East and West Antarctica is required to fit the observations.

For many practical applications, ranging from cadastre and engineering to scientific, GNSS locations must refer to a specific epoch in a known reference frame to establish a consistent spatial relationship between georeferenced features measured at different times. When an earthquake occurs, an effectively instantaneous coseismic offset in position is observed. This offset varies as a function of distance and direction from the earthquake’s rupture zone and depends on its type and magnitude. When GNSS is used to measure the position of a point after an earthquake, the result includes the coseismic displacement suffered by that point and this displacement must be removed to provide coordinates in the conventional epoch. When post-event GNSS observations are far from continuous GNSS monitoring stations, their coseismic displacements are unknown and must be estimated using surrounding continuous GNSS stations. Interpolation of coseismic displacements, however, is difficult unless a sufficiently dense continuous GNSS network exists, especially in the near-field. We present a methodology for estimating coseismic displacements in areas with low-density continuous GNSS coverage by using geophysical models in a hybrid (dynamic-kinematic) mode. We do this using elastic deformation of a spherical earth to constrain the overall coseismic displacement field without imposing the usual geodynamic constraints on fault slip distribution. Application of this methodology to the 2010 Maule and 2015 Illapel, Chile, earthquakes provides coseismic estimates on survey GNSS stations with rms (95

<p>Twelve continuous GNSS systems are deployed on bedrock across the Amundsen Embayment region, spanning the Pine Island, Thwaites and Pope-Smith-Kohler (PSK) glacial drainage network of the West Antarctic Ice Sheet.  Continuous daily position time series for these sites range from 4 to 12 years, yielding reliable crustal motion velocity solutions at these fast-moving bedrock sites. Remarkably, multiple stations record sustained uplift of 40-50 mm/yr.  Maximum uplift defined by the current distribution of sites is centered on the Pope-Smith-Kohler glaciers, where rapid thinning and grounding line retreat is well documented. Horizontal bedrock displacements, which are particularly sensitive to the location of changing surface mass loads, show a clear radial pattern with motion outward away from upstream portions of the Pope/Smith glaciers. Several modeling studies suggest there is a viscous deformation response to this decadal mass loss. Our modeling, however, shows that elastic deformation response explains nearly the entire measured signal at the PSK region sites. We will present new modeling results and discuss implications for ongoing cryosphere-solid Earth interactions.</p>

The Patagonia Icefields (PIF) are the largest non-polar ice mass in the southern hemisphere. The icefields cover an area of approximately 16,500 km 2 and are divided into the northern and southern icefields, which are ~ 4000 km 2 and ~ 12,500 km 2 , respectively. While both icefields have been losing mass rapidly, their responsiveness to various climate drivers, such as the El Niño-Southern Oscillation, is not well understood. Using the elastic response of the earth to loading changes and continuous GPS data we separated and estimated ice mass changes observed during the strong El Niño that started in 2015 from the complex hydrological interactions occurring around the PIF. During this single event, our mass balance estimates show that the northern icefield lost ~ 28 Gt of mass while the southern icefield lost ~ 12 Gt. This is the largest ice loss event in the PIF observed to date using geodetic data.

Deformation associated with plate convergence at subduction zones is accommodated by a complex system involving fault slip and viscoelastic flow. These processes have proven difficult to disentangle. The 2010 M w 8.8 Maule earthquake occurred close to the Chilean coast within a dense network of continuously recording Global Positioning System stations, which provide a comprehensive history of surface strain. We use these data to assemble a detailed picture of a structurally controlled megathrust fault frictional patchwork and the three-dimensional rheological and time-dependent viscosity structure of the lower crust and upper mantle, all of which control the relative importance of afterslip and viscoelastic relaxation during postseismic deformation. These results enhance our understanding of subduction dynamics including the interplay of localized and distributed deformation during the subduction zone earthquake cycle.

From early 2003 to mid-2013, the total mass of ice in Greenland declined at a progressively increasing rate. In mid-2013, an abrupt reversal occurred, and very little net ice loss occurred in the next 12-18 months. Gravity Recovery and Climate Experiment (GRACE) and global positioning system (GPS) observations reveal that the spatial patterns of the sustained acceleration and the abrupt deceleration in mass loss are similar. The strongest accelerations tracked the phase of the North Atlantic Oscillation (NAO). The negative phase of the NAO enhances summertime warming and insolation while reducing snowfall, especially in west Greenland, driving surface mass balance (SMB) more negative, as illustrated using the regional climate model MAR. The spatial pattern of accelerating mass changes reflects the geography of NAO-driven shifts in atmospheric forcing and the ice sheet's sensitivity to that forcing. We infer that southwest Greenland will become a major future contributor to sea level rise.

The very large and rare Mw ~7.9 Earthquake of 3 May 2006 in the Kingdom of Tonga aroused great interest among both Tongan scientists and their colleagues in Australia, New Zealand, and the United States. To investigate the earthquake we formed a collaborative research group of scientists from Australia, New Zealand, Tonga, and the United States. We brought in seven seismographs from Australia and the US to supplement the three-station network already in Tonga and eight GPS receivers primarily for the islands west of the earthquake epicenter. In addition, we made coastal observations to determine the regional pattern of subsidence associated with the earthquake. The GPS instruments can measure horizontal and vertical motion quite precisely, but only after the earthquake from the time of deployment onward, except for some sites on Tongatapu, Vava’u, and Lifuka that had been occupied by GPS receivers in the past. This report describes our efforts.

Chile is one of the world's most seismically active regions and is therefore extensively studied by the earthquake sciences. The great length of the country hosts a variety of measurement systems allowing for the characterization of earthquake processes over a wide range of timescales and in different phases of the seismic cycle. Starting in the early 1990s, several research groups began to deploy continuously operating geodetic networks in Chile, forming the core of the modern network of Global Navigation Satellite Systems (GNSS) receivers used to monitor geodynamics from the southern tip of the Americas to the central Andes. Today, the Centro Sismologico Nacional (CSN) of the Universidad de Chile maintains and improves this network, increasing its coverage and spatial density while greatly reducing solution latency. We present the status of the GNSS network, its data streams, and the real-time analysis system used to support real-time modeling of earthquakes. The system takes 2 s, on average, to collect raw data, estimate positions, and stream results. Such low latency is essential to enabling early warning of earthquakes and tsunamis in Chile.

The marine portion of the West Antarctic Ice Sheet (WAIS) in the Amundsen Sea Embayment (ASE) accounts for one-fourth of the cryospheric contribution to global sea-level rise and is vulnerable to catastrophic collapse. The bedrock response to ice mass loss, glacial isostatic adjustment (GIA), was thought to occur on a time scale of 10,000 years. We used new GPS measurements, which show a rapid (41 millimeters per year) uplift of the ASE, to estimate the viscosity of the mantle underneath. We found a much lower viscosity (4 × 1018 pascal-second) than global average, and this shortens the GIA response time scale from tens to hundreds of years. Our finding requires an upward revision of ice mass loss from gravity data of 10% and increases the potential stability of the WAIS against catastrophic collapse.