Groundwater extraction decreases water pressure in aquifer systems, causing reversible or irreversible deformation of the water-bearing layers that manifests as recoverable or permanent displacements of the land surface, respectively. Detecting and forecasting when and where an aquifer system transitions from a reversible, poroelastic regime, to an irreversible, inelastic regime remains a crucial challenge given the complex, heterogeneous nature of aquifer systems. Here we leverage high-resolution measurements of ground deformation and groundwater levels from 2016 to 2022 to characterize both regimes at the regional scale and show that a critical transition occurred in large areas of the Sacramento Valley during California's 2020-2022 extreme drought. Our analysis reveals that, while deformation remained primarily poroelastic during the 2016-2020 interdrought period, land subsidence in areas of intense groundwater extraction accelerated abruptly in 2021, with subsidence rates exceeding the inferred poroelastic rates by several decimeters per year. Such rapid and extensive land subsidence indicates severe inelastic compaction and loss of storage capacity of the underlying aquifer system, which pose a serious threat to California's water resources and infrastructure. A comparison of present-day deformation with historical groundwater levels reveals that this abrupt transition was not predictable based on the available groundwater records alone.

This study presents a framework for quantifying large-scale interseismic deformation of a tectonic plate overlying a viscoelastic asthenosphere. We explain and compare four common methods for modelling upper-plate surface deformation induced by time-dependent slip on the subduction interface. One method is a widely used, purely elastic model. The other three are viscoelastic models, in which slip on the subduction interface is either steady over time, steady over time but with the impact of the last earthquake taken into account, or represented as periodic earthquake cycles. For the three viscoelastic models, two asthenospheric rheologies are investigated. The first is a Maxwell rheology with a single viscosity, inferred from early postseismic deformation (3 x 10^18 Pa.s). This model reasonably reproduces short-term deformation. The second is a Burgers rheology with a comparable short-term viscosity, but with a higher long-term effective viscosity (3 x 10^19 Pa.s). This higher long-term viscosity is constrained by decadal-to-centennial observations. Therefore, this model alone accounts for deformation across all timescales and spatial scales. These observations include a trenchward deformation persisting 50 years after the 1960 M_w9.5 Valdivia earthquake, and deformation from northern Chile, where deformation reflects coupling along segments that have not ruptured for decades. All models are evaluated using a finite-element mesh spanning an 8000 km-wide region around the Chilean subduction margin. For the same coupling, near the coast, all models yield similar deformation, with differences smaller than 20%. However, discrepancies increase markedly around 200 km inland. In the continental interior, the elastic model consistently underestimates interseismic velocities. Only the Burgers rheology, combined with a realistic cyclic slip history, reproduces the observed deformation over the entire continent and across annual to centennial timescales. We constrain the acceptable parameter range of this simplified Burgers rheology. This requires a Kelvin-Voigt shear modulus 4 to 15 times smaller than the elastic shear modulus. These results demonstrate that the effective viscosity of the asthenosphere increases markedly from annual to decadal or longer-term timescales. They also show that millennium-scale strain accumulation must be accounted for when interpreting interseismic deformation. They call into question estimates of coupling depth and sliver motion based on purely elastic models.

We conduct a comprehensive comparison of ice mass balance (IMB) estimates for Greenland derived from satellite observations of ice surface elevation changes (SEC), gravity and global navigation satellite system (GNSS) observations. Our analysis integrates data from the ICESat and CryoSat-2 satellite altimetry missions, augmented by optical stereo-imagery for peripheral glaciers, and GRACE satellite gravimetry mission, spanning the 2003-2008 and 2011-2015 periods. We also consider three firn densification models (FDM) and five glacial isostatic adjustment (GIA) models for correcting the data sets for these effects when necessary. Our results reveal significant differences among FDM corrections applied to SEC observations, with particularly large variations in IMB estimates reaching up to 90 Gt yr-1. To address this, we develop an innovative method for estimating equivalent firn corrections to the ice elevation observations, based on a least-squares fit of filtered ice SEC observations to GRACE mass-change estimates. This approach is both simple and independent from climate models assumptions and shows minimal sensitivity to GIA model differences. Using this method, we estimate IMBs for Greenland at -217.6 $\pm$ 15.7 Gt yr-1 for 2003-2008 and -253.2 $\pm$ 18.8 Gt yr-1 for 2011-2015. Importantly, these values indicate an acceleration of the thinning rate, not consistently captured by the IMB estimates inferred from the ice SEC observations corrected by FDMs. Finally, we compute elastic ground deformation induced by ice mass change during 2011-2015, using the four proposed mass-variation distributions and compare the predicted vertical velocities with GNSS observations in Greenland, accounting for all GIA models. While all models are consistent with most of the GNSS-derived uplift rates, they cannot fully explain the observed vertical velocities, especially in the South-East Greenland, which confirms the need to refine our understanding of GIA contributions in this region.

Megaearthquakes (Mw > 8) cause continental-scale, long-lasting surface deformation, mainly due to viscoelastic relaxation of the asthenosphere. To investigate the links between this deformation and the slip history along subduction interfaces—including earthquakes, postseismic slip, and interseismic coupling—large 3D spherical finite-element meshes are required. This technical report introduces the various steps to build Chile_Mesh_v1.0, a customizable mesh for the Chilean subduction zone, designed as a robust platform for testing various viscoelastic rheologies. It spans ~8500 km in longitude, ~7300 km in latitude, encompassing the entire South American plate, and from the surface to 2900 km depth. Special care was taken to reproduce the complex slab geometry, especially in flat-slab regions such as the Pampean and Peruvian segments, following the Slab2 model. We show that accurately modeling both coseismic and postseismic deformation over large scales requires realistic meshed domains, extending down to the Core-Mantle boundary and thousands of kilometers from the trench. In some cases, depth-reduced meshes can be used to model viscoelastic postseismic deformation, but they fail to simultaneously capture coseismic deformation accurately. We hope this open-access mesh proves valuable for researchers studying subduction dynamics in Chile and supports the development of similar models for other regions.

GPS positioning offers millimetric precision in measuring deformation of the lithosphere during the seismic cycle. In particular, during the post-seismic phase, long-lasting and large-scale deformation are measured. They result from the viscoelastic relaxation in the asthenosphere. Consequently, the post-seismic phase is currently modeled using viscoelastic rheologies (e.g., Maxwell or Burgers viscous models). On the other hand, the inter-seismic phase is mainly modeled using purely elastic models. In particular, coupling models, widely used to quantify the accumulation of deformation on the subduction fault, are therefore used to evaluate earthquake hazard. However, such elastic models fail to explain mid-field deformation without the use of an external hypothesis (e.g., a third plate called sliver). The study of post-seismic deformation has provided important insights into the rheological properties of the asthenosphere during the post-seismic phase. For example, viscous creep has been found Newtonian since the cumulative post-seismic displacements normalized by the co-seismic offset, as a function of distance to the trench, superimpose very well for earthquakes of different magnitudes [Boulze et al. 2022]. By incorporating these different results and using the backslip theory [Savage 1983], we model the inter-seismic phase using viscoelastic models. We explore the impact on coupling distribution along the Chilean subduction zone, in particular discussing differences with the elastic model in terms of depth and lateral extension. We also examine the impact of viscoelastic models in a region of Chile (Taltal region, 25.2°S) where elastic models currently fail to reproduce deformation in the near-field [Klein et al. 2018]. Finally, we show that a 2-Burgers viscous model is necessary to reproduce deformation in Argentina in 2010, before the Maule earthquake.

Ever since the Maule earthquake (Mw8.8, 2010), a quick vertical uplift is measured thanks to GNSS in the Andes, facing the rupture zone (~250 km to the trench). Models built for the Maule earthquake [Klein et al. 2016] have highlighted that a low-viscosity channel is required to explain the post-seismic uplift. This channel is located along the slab between 50 km and 130 km depth and has a viscosity of a few 1017 Pa.s - lower than in the asthenosphere, 1018 Pa.s. After the Illapel earthquake (Mw8.3, 2015), simple observations on GNSS time-series show that no uplift occurred in the Andes at an equivalent distance to the trench than in the case of the Maule earthquake. The subduction in the Illapel region is characterized by a flat-slab (called the Pampean flat-slab) in contrast with the normal-dipping subduction in the region of Maule.Here, we investigate what is the impact of the Pampean flat-slab on the post-seismic deformations of the Illapel earthquake. In particular, we try to understand whether the presence of the flat-slab inhibits the effect of the low-viscosity channel. For that purpose we compare GNSS vertical displacements with predictions in both regions of Maule and Illapel from a 3D spherical finite-element model that accounts for the slab geometry of the Chilean subduction zone.

In continental interiors, tectonically-driven deformation rates are low, often to the point where they are unde-tectable with modern geodesy. However, a range of non-tectonic surface processes, particularly relating to hy-drological, cryospheric, and sedimentological mass changes, can produce strain-rates which on geologically-short timescales are substantially greater than those produced by tectonics. Here, we illustrate the problem that such transient strain rates may pose in low-strain environments by considering the impact that the growth and decay of the Fennoscandian and Laurentian ice sheets over the Holocene had on Europe and North America respec-tively. Induced deformation extended far beyond the periphery of the ice sheets, with the potential to impact on seismicity rates thousands of kilometres south of the maximum ice extent. We consider how the modelled non -tectonic deformation would have interacted with several known active fault systems, including the European Cenozoic Rift System and the New Madrid fault system. In low strain continental interiors, seismic hazard assessment - crucial for the long-term planning of critical infrastructure, including nuclear waste disposal - is often dependent on sparse information from observational and historical seismicity, and from paleoseismological studies of surface fault systems. We recommend that for a more complete seismic hazard assessment, the impact of non-tectonic transients should be considered - both in the context of the role such transients may have played in recent seismicity, and the role they may play in seismicity to come. Whilst such consideration has previously been given to the direct impact on glacial loading in areas directly glaciated, we show that it should also be considered much more broadly.

<p>Thanks to space geodesy we know with a millimetric precision how the lithosphere deforms at each stage of the seismic cycle. In particular, during the post-seismic phase, it can deform over thousands of kilometers and for decades. These deformations are partly due to viscoelastic relaxation of the asthenosphere.</p> <p>In a previous work, we have shown that at the temporal and spatial scale of the seismic cycle, the viscoelastic relaxation can be modeled by a linear creep law [Boulze et al. 2022]. Therefore, because of the linearity of the creep law, the superposition principle applies and the present day deformation is simply the sum of the post-seismic deformations induced by past earthquakes. Based on this result, the objective of our work is to determine what slip history is needed on the Chilean subduction interface to reproduce the current deformation of South America, which is well measured by GNSS.</p> <p>To investigate this challenging problem, we first develop a 3D spherical finite-element model of the Chilean subduction zone. This model covers the entire South American continent and incorporates a slab with a geometry described by Slab2.0 model [Hayes et al. 2018]. Then, we compare different ways to model the seismic cycle using the backslip theory [Savage 1983]. Finally, by comparing GPS time-series with our seismic cycle model prediction, we discuss many ingredients of the model: e.g. the viscosity of the asthenosphere (Maxwell, Burgers), the impact of a flat slab and low viscosity zones, the magnitude and extent of historical earthquakes.</p>

Monitoring the evolution of freshwater water resources worldwide is challenging yet essential to prevent severe water scarcity. In that regard, the Gravity Recovery and Climate Experiment and Follow-On satellite missions (GRACE/-FO) have successfully proven to complement in-situ hydrological observations by giving access to variations in continental water storage through measurements of the Earth’s gravity field since 2002.However, the measurement noise and errors inherent to the GRACE/-FO missions design and data processing cause estimation bias and spatial leakage that limit applications to large-scale hydrological basins and complicate geophysical interpretation. Moreover, missing observations during and between the missions, and in particular the 11-month gap between missions make if difficult to monitor long-term mass variations. To overcome these issues, we build a procedure, based on a spectral analysis by Multichannel – Singular Spectrum Analysis (M-SSA), that uses spatio-temporal correlations of the GRACE/-FO time series to fill data gaps and reduce a significant portion of the distinctive noise pattern while maintaining the best possible spatial resolution. This processing reveals hydrological signals that are less well or not resolved by other processing strategies.We seek validation of the GRACE/-FO M-SSA solution by comparison with independent estimates of regional hydrological mass balance. We first develop a new forward modeling approach to derive regional hydrological balance that accounts for potential bias caused by necessary filtering of the GRACE/-FO data. In particular, we account for signal leakage of a known source while estimating regional mass balance.The Caspian Sea, due to its relative isolation, large size and significant mass variations over the past decades, offers an interesting example to validate mass balance derived from GRACE/-FO measurements. We compare space-gravity derived hydrological mass balance with independent altimetry data, accounting for thermosteric effects using in-situ data and ground deformation caused by mass redistribution, and show consistent results on the long term variations. However, we also discuss potential causes of a significant discrepancy in seasonal amplitude.

Gravity Recovery And Climate Experiment and Follow On (GRACE/‐FO) global monthly measurements of Earth's gravity field have led to significant advances in quantifying mass transfer. However, a significant temporal gap between missions hinders evaluating long‐term mass variations. Moreover, instrumental and processing errors translate into large non‐physical North‐South stripes polluting geophysical signals. We use Multichannel Singular Spectrum Analysis (M‐SSA) to overcome both issues by exploiting spatio‐temporal information of Level‐2 GRACE/‐FO solutions, filtered using the DDK7 decorrelation and a new complementary filter, built based on the residual noise between fully processed data and a parametric fit to observations. Using an iterative M‐SSA on Equivalent Water Height (EWH) time series processed by Center of Space Research, GeoForschungsZentrum, Institute of Geodesy at Graz University of Technology, and Jet Propulsion Laboratory, we replace missing data and outliers to obtain a combined evenly sampled solution. Then, we apply M‐SSA to retrieve common signals between each EWH time series and its same‐latitude neighbors to further reduce residual spatially uncorrelated noise. Comparing GRACE/‐FO M‐SSA solution with Satellite Laser Ranging and Swarm low‐degree Earth's gravity field and hydrological model demonstrates its ability to satisfyingly fill missing observations. Our solution achieves a noise level comparable to mass concentration (mascon) solutions over oceans (3.0 mm EWH), without requiring a priori information nor regularization. While short‐wavelength signals are challenging to capture using highly filtered spherical harmonics or mascons solutions, we show that our technique efficiently recovers localized mass variations using well‐documented mass transfers associated with reservoir impoundments.

The continuous redistribution of water involved in the hydrologic cycle leads to deformation of the solid Earth. On a global scale, this deformation is well explained by the loading imposed by hydrological mass variations and can be quantified to first order with space‐based gravimetric and geodetic measurements. At the regional scale, however, aquifer systems also undergo poroelastic deformation in response to groundwater fluctuations. Disentangling these related but distinct 3D deformation fields from geodetic time series is essential to accurately invert for changes in continental water mass, to understand the mechanical response of aquifers to internal pressure changes as well as to correct time series for these known effects. Here, we demonstrate a methodology to accomplish this task by considering the example of the well‐instrumented Ozark Plateaus Aquifer System (OPAS) in the central United States. We begin by characterizing the most important sources of groundwater level variations in the spatially heterogeneous piezometer dataset using an Independent Component Analysis. Then, to estimate the associated poroelastic displacements, we project geodetic time series corrected for hydrological loading effects onto the dominant groundwater temporal functions. We interpret the extracted displacements in light of analytical solutions and a 2D model relating groundwater level variations to surface displacements. In particular, the relatively low estimates of elastic moduli inferred from the poroelastic displacements and groundwater fluctuations may be indicative of aquifer layers with a high fracture density. Our findings suggest that OPAS undergoes significant poroelastic deformation, including highly heterogeneous horizontal poroelastic displacements.

SUMMARY Over the last decade, three major subduction earthquakes, Maule Mw 8.8 (2010), Illapel Mw 8.3 (2015) and Iquique Mw 8.1 (2014), occurred in Chile and generated significant post-seismic deformations. These large scale and long lasting deformations can be quantified with modern GNSS precise positioning and highlight viscoelastic processes in the asthenosphere. Here, we calculate the ratios of cumulative post-seismic displacements after 5 yr over the coseismic offsets. We find that at any distance from the trench, ratios are similar for the three earthquakes despite their different magnitudes which imply induced stresses that are more than one order of magnitude apart. This observation suggests that the post-seismic deformation is related to the same effective viscosity for the three earthquakes, indicating Newtonian rheology, rather than power-law rheology in the asthenosphere.

Earth and Space Science Open Archive This work has been accepted for publication in Journal of Geophysical Research - Solid Earth. Version of RecordESSOAr is a venue for early communication or feedback before peer review. Data may be preliminary. Learn more about preprints. preprintOpen AccessYou are viewing the latest version by default [v2]Understanding the geodetic signature of large aquifer systems: Example of the Ozark Plateaus in Central United StatesAuthorsStacyLarochelleiDKristelChanardiDLuceFleitoutJerome NicolasFortiniDAdrianoGualandiiDLaurentLonguevergneiDPaulRebischungiDSophieVioletteJean-PhilippeAvouacSee all authors Stacy LarochelleiDCorresponding Author• Submitting AuthorCalifornia Institute of TechnologyiDhttps://orcid.org/0000-0001-6161-5605view email addressThe email was not providedcopy email addressKristel ChanardiDIPGPiDhttps://orcid.org/0000-0001-9934-9621view email addressThe email was not providedcopy email addressLuce FleitoutENS-CNRSview email addressThe email was not providedcopy email addressJerome Nicolas FortiniDEcole Normale SupérieureiDhttps://orcid.org/0000-0002-6341-3318view email addressThe email was not providedcopy email addressAdriano GualandiiDIstituto Nazionale di Geofisica e VulcanologiaiDhttps://orcid.org/0000-0002-3100-8932view email addressThe email was not providedcopy email addressLaurent LonguevergneiDCNRS - Université Rennes 1iDhttps://orcid.org/0000-0003-3169-743Xview email addressThe email was not providedcopy email addressPaul RebischungiDInstitut National de l'Information Géographique et Forestière (IGN)iDhttps://orcid.org/0000-0002-2462-1886view email addressThe email was not providedcopy email addressSophie VioletteUniversité Pierre et Marie CURIE-Sorbonne Universités & CNRSview email addressThe email was not providedcopy email addressJean-Philippe AvouacCalifornia Institute of Technologyview email addressThe email was not providedcopy email address

Spatial geodesy through GNSS allows to measure with a millimetric precision the displacement of the lithosphere during the seismic cycle. The post-seismic part of this cycle can last for decades, traduced by long-lasting surface deformations. One of the main physical processes involved in the postseismic deformation is the viscoelastic relaxation in the asthenosphere. However, a long term debate persists about the involved rheology of the asthenosphere: Is the viscosity highly variable from one region to the next and is effective viscosity Newtonian (linear) or non-Newtonian (non linear)? To investigate these questions, we compare the horizontal post-seismic deformations induced by three Chilean megathrust earthquakes: Maule Mw8.8 (2010), Illapel Mw8.3 (2015) and Iquique Mw8.1 (2014). For each earthquake, we select permanent GPS stations along profiles perpendicular to the trench, extending as far as 1400 km. We calculate the ratio of the cumulative post-seismic (post) over 5 years and the coseismic (co) displacements for each station. Remarkably, at a given distance to the trench, the post/co ratios from the three earthquakes differ only slightly. What can be the interpretation of this observation in terms of rheology of the asthenosphere? First we can analyse the response of the asthenosphere in the case of homothetic earthquakes of different magnitude. The post/co ratio obeys simple analytical relationships: For a Newtonian rheology, it is simply a function of the (time/viscosity) ratio. For a non-Newtonian viscosity with a stress exponent n=3, the timescale becomes inversely proportional to M**2, where M is the moment of the earthquake. We show that these relationships are only slightly modified when the earthquakes are no longer homothetic and that the post/co ratio is a good proxy to quantify the strain-rate and stress ratio in the underlying asthenosphere. As a conclusion, the post-seismic deformation following the three Chilean earthquakes reveals very similar viscosity. In particular, a Newtonian, rather than a non-Newtonian, effective viscosity is required to explain the post-seismic deformation process.

The Gravity Recovery And Climate Experiment (GRACE; April-2002-June 2017) and current GRACE-Follow On (GRACE-FO; June 2018-present) missions have provided monthly global measurements of the space and time varying Earth’s gravity field, monitoring changes in the ice-sheets and glaciers, hydrological water storage, sea level and within solid Earth. Yet, temporal gaps, including the long 11 months gap between missions, prevent the interpretation of long-term mass variations. Moreover, despite the data processing strategy adopted, GRACE and GRACE-FO solutions show high level of distinctive unphysical noise. Consequently, we use the Multi-Channel Singular Spectrum Analysis (MSSA) and exploit both spatial and temporal information contained in multiple solutions of GRACE and GRACE-FO to fill the observational gaps and develop a data-driven spatio-temporal filter to enhance the data signal-to-noise ratio. First, we use the well-established decorrelation DDK7 filter to remove a large part of the distinctive noise in a North-South striping patterns. Because we detect persisting noise at high orders, we develop a complementary filter based on the residual noise between fully-processed data and parametric fit to the observations. We then fill observational gaps using an iterative M-SSA approach and series of equivalent water height from four Level-2 solutions (CSR, GFZ, JPL, TU-GRAZ). The method is validated on a synthetic test, where we remove and reconstruct one year of the time series. By using multiple solutions in the process, we form a combined solution based on their common modes of variability. Finally, we take full advantage of the M-SSA to reduce residual spatially uncorrelated noise, namely stripes, by conserving common signals between times series of each point on the globe and its neighbors. Comparison of the GRACE-GRACE-FO M-SSA solution with independent observations of the low-degree Earth’s gravity field via Satellite Laser Ranging validates the method’s potential to recover missing observations. Furthermore, comparisons with other solutions show a significant noise reduction compared to spherical harmonic solutions, and the ability to retrieve short-wavelengths geophysical signals masked by Mascons-type processing strategy.

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Onset of small-scale instabilities at the base of the lithosphere : scaling laws and role of pre-existing structures C. Dumoulin, Doin M.P., D. Arcay, L. Fleitout

The evolution of the Greenland Ice Sheet (GIS) is an important indicator of climate change and driver of sea level rise. However, providing accurate GIS ice mass balance remains a challenge today. Here, we propose to combine a unique set of geodetic measurements to improve our knowledge of the GIS spatial and temporal evolution. We attempt at reconciling satellite observations of ice volume with regional GNSS velocities estimates and time variable space gravity measurements over the 2003-2009 and 2011-2015 periods. The GIS mass variations are inferred from satellite altimetry for large ice sheets (IceSat and CryoSat-2; Sorensen et al.,2018, Simonsen et al.,2017) and digital elevation models (DEMs) generated from multiple satellite archives for peripheral glaciers (Hugonnet et al.,2020), associated with IMAU-FDM firn model (Ligtenberg et al., 2011). The spatial and temporal variations of the gravity field are given by the GRACE mission for which we use a solution where smaller wavelength signals are preserved (Prevost et al., 2019). To resolve short wavelengths load variations affecting the displacement of nearby GNSS stations, we use Green’s functions for vertical crustal displacements assuming purely elastic Earth properties (Martens et al., 2019). We first assume that the deformation is entirely due to recent ice melting and show that vertical elastic displacements predicted by our refined ice loading model, while in good agreement with observations in some regions, cannot explain observations overall. In particular, observations and model disagree in the Southeastern and the Northern parts of Greenland. We then explore potential viscoelastic deformation associated with short-term rheology of the asthenosphere induced by recent ice melting that could explain the observed GNSS displacements. We define a history of ice loading from 1900 to 2009 using both in situ and satellite altimetric measurements, compute today’s associated viscoelastic deformation for various mantle rheologies and discuss the potential contribution of ice melting since the little ice age to current observations. Remaining differences between observations and viscoelastic models may reflect a viscoelastic deformation induced by glacial isostatic adjustment. We discuss implications in terms of regional rheological constraints, and impact on estimates of present-day GIS ice mass budget. Hugonnet, R. (2020). A globally complete, spatially, and temporally resolved estimate of glacier mass change: 2000 to 2019. In EGU 2020. Ligtenberg, S. R. M., et al (2011). An improved semi-empirical model for the densification of Antarctic firn. The Cryosphere, 5, 809-819. Martens, H. R.,et al (2019). LoadDef: A Python‐based toolkit to model elastic deformation caused by surface mass loading on spherically symmetric bodies. Earth and Space Science, 6(2), 311-323. Prevost, P., et al (2019). Data-adaptive spatio-temporal filtering of GRACE data. Geophysical Journal International, 219(3), 2034-2055. Simonsen, S. B., & Sørensen, L. S. (2017). Implications of changing scattering properties on Greenland ice sheet volume change from Cryosat-2 altimetry. Remote Sensing of Environment, 190, 207-216. Sørensen, L. S., et al (2018). 25 years of elevation changes of the Greenland Ice Sheet from ERS, Envisat, and CryoSat-2 radar altimetry. Earth and Planetary Science Letters, 495, 234-241.