Viscoelastic Earth Model Research Articles

A Hybrid Normal Mode‐Collocation Method for Finding the Response of Laterally Homogeneous Compressible Maxwell Viscoelastic Earth Models

A Hybrid Normal Mode‐Collocation Method for Finding the Response of Laterally Homogeneous Compressible Maxwell Viscoelastic Earth Models

Non‐Linear Vertical Land Motion of Coastal Chile and the Antarctic Peninsula Inferred From Combining Satellite Altimetry, Tide Gauge and GPS Data

AbstractWe developed an enhanced Kalman‐based approach to quantify abrupt changes and significant non‐linearity in vertical land motion (VLM) along the coast of Chile and the Antarctic Peninsula using a combination of multi‐mission satellite altimetry (ALT), tide gauge (TG), and GPS data starting from the early 1990s. The data reveal the spatial variability of co‐seismic and post‐seismic subsidence at TGs along the Chilean subduction zone in response to the Mw8.8 Maule 2010, Mw8.1 Iquique 2014, and Mw8.3 Illapel 2015 earthquakes that are not retrievable from the interpolation of sparse GPS observations across space and time. In the Antarctic Peninsula, where continuous GPS data do not commence until ∼1998, the approach provides new insight into the ∼2002 change in VLM at the TGs of +5.3 ± 2.2 mm/yr (Palmer) and +3.5 ± 2.8 mm/yr (Vernadsky) due to the onset of ice‐mass loss following the Larsen‐B Ice Shelf breakup. We used these data to constrain viscoelastic Earth model parameters for the northern Antarctic Peninsula, obtaining a preferred lithosphere thickness of 115 km and upper mantle viscosity of 0.9 × 1018 Pa s. Our estimates of regionally‐correlated ALT systematic errors are small, typically between ∼±0.5–2.5 mm/yr over single‐mission time scales. These are consistent with competing orbit differences and the relative errors apparent in ALT crossovers. This study demonstrates that, with careful tuning, the ALT‐TG technique can provide improved temporal and spatial sampling of VLM, yielding new constraints on geodynamic models and assisting sea‐level change studies in otherwise data sparse regions and periods.

Timescales of glacial isostatic adjustment in Greenland: is transient rheology required?

SUMMARY The possibility of a transient rheological response to ice age loading, first discussed in the literature of the 1980s, has received renewed attention. Transient behaviour across centennial to millennial timescales has been invoked to reconcile apparently contradictory inferences of steady-state (Maxwell) viscosity based on two distinct data sets from Greenland: Holocene sea-level curves and Global Navigation Satellite System (GNSS) derived modern crustal uplift data. To revisit this issue, we first compute depth-dependent Fréchet kernels using 1-D Maxwell viscoelastic Earth models and demonstrate that the mantle resolving power of the two Greenland data sets is highly distinct, reflecting the differing spatial scale of the associated surface loading: the sea-level records are sensitive to viscosity structure across the entire upper mantle while uplift rates associated with post-1000 CE fluctuations of the Greenland Ice Sheet have a dominant sensitivity to shallow asthenosphere viscosity. Guided by these results, we present forward models which demonstrate that a moderate low viscosity zone beneath the lithosphere in Maxwell Earth models provides a simple route to simultaneously reconciling both data sets by significantly increasing predictions of present-day uplift rates in Greenland whilst having negligible impact on predictions of Holocene relative sea-level curves from the region. Our analysis does not rule out the possibility of transient deformation, but it suggests that it is not required to simultaneously explain these two data sets. A definitive demonstration of transient behaviour requires that one account for the resolving power of the data sets in modelling the glacial isostatic adjustment process.

On torsional wave in void type porous layers between viscoelastic and piezoelectric media with parabolic irregularity

The viscoelastic earth model is increasingly utilized by seismologists for studying surface seismic wave propagation through Earth’s porous crustal regions such as granular materials, including rock, soils, and synthetic porous bodies. The present study characterizes torsional wave propagation in an irregular void-type porous layer sandwiched between an initially stressed inhomogeneous viscoelastic layer and a piezoelectric substratum. Displacement fields for the layer and half spaces are obtained separately using a variable separable technique, whereas volume fraction and electric potential field are derived for layer and substratum, respectively using the matrix calculus approach. Dispersion relations have been derived for both electrically open and short circuit conditions and agree with standard results. Particular cases have been deduced to validate the finding of the study with literature results. Effects of change in relevant parameters such as initial stress, heterogeneities, attenuation coefficient, void parameters, piezoelectricity and dissipation factor have been analyzed on the phase velocity of the torsional wave through various plots and summarized in a tabular form for ready reference to researchers. Analysis of the present geophysical problem may be useful in characterizing torsional velocity in porous silicate rocks which are abundantly found in the transition zone between oceanic and continental crust called Conrad discontinuity. Highlights Torsional propagation in a void-type porous layer crammed between an initially stressed inhomogeneous viscoelastic layer and piezoelectric half-space has been investigated. Variable separable technique, the matrix calculus approach is used for computing the displacement fields of respective media. A comparative study has been carried out for both electrically open and short conditions. The impact of pertinent parameters including initial stress, heterogeneities, attenuation coefficient, void parameters, piezoelectricity, and dissipation factor on the phase velocity of the torsional wave has been examined.

Periodic surface loading using GRACE data for a layered viscoelastic earth model based on a direct relationship between geoid and displacement

SUMMARY Loading theory is fundamental in explaining deformation resulting from surface mass changes. Although various theoretical methods, including the classical elastic loading theory by W. E. Farrell and I. M. Longman, have been proposed, a viscoelasticity-based theory may be required to address long timescale loading problems, such as annual and interannual deformation, as well as longer timescale loading effects. In this study, we use a semi-analytical approach to simulate the continuous periodic loading deformation of a viscoelastic, spherical, layered earth model with linear rheology profiles. We provide a series of formulas in the spectral domain for the spatiotemporal displacement, which establish connections between mass, geoid and displacement solely through the utilization of complex Love numbers and Stokes coefficients, thereby circumventing the need for viscoelastic Green's function. Using our newly proposed method, we investigate the viscoelastic loading deformation caused by annual cyclic mass loading, considering both steady-state creep and additional transient creep with a wide range of viscosities. The results indicate that when utilizing steady-state viscosity values constrained by GIA data, the viscoelastic effect is not evident in the annual cyclic load deformation. However, incorporating the Burgers model with transient creep mainly constrained by post-seismic deformation influences the amplitude and phase of the annual cyclic loading, highlighting the role of rheology. Furthermore, we observe that the horizontal displacement in periodic load deformation exhibits a higher sensitivity to the viscosity of the model compared to the geoid and vertical displacement, regardless of the rheological model used.

Comparative analysis on seismicity and stress triggering of strong earthquakes sequence in Central Tibet

Central Tibet constitutes part of the central part of the Qinghai–Tibetan Plateau, which is one of the highest seismically active areas in China. This paper discusses the regularity of seismic activity in this area. Based on a stratified viscoelastic earth model, we calculated the Coulomb stress changes imparted from four strong earthquakes (M ≥ 6.3) along the Bengco–southeastern piedmont of the Nyainqentanglha mountain fault zone in this region. The result shows that the study area may be entering a new active period from 2020. There was a trigger between the strong earthquakes (M ≥ 6.3) on the Bengco fault zone. The post-seismic viscous relaxation effect of a strong earthquake had a significant impact on subsequent earthquakes (M ≥ 6.3). In the next 100 years, the Coulomb stress loading is more than 1.0 MPa in the northwest section of the Bengco fault and the central part of Nimu segment of the southeastern piedmont of the Nyainqentanglha mountain fault. Thence, strong earthquakes may occur along these fault segments. The maximum magnitude of the earthquake could be M6.7 in the next 100 years.

Total isostatic response to the complete unloading of the Greenland and Antarctic Ice Sheets

The land surface beneath the Greenland and Antarctic Ice Sheets is isostatically suppressed by the mass of the overlying ice. Accurate computation of the land elevation in the absence of ice is important when considering, for example, regional geodynamics, geomorphology, and ice sheet behaviour. Here, we use contemporary compilations of ice thickness and lithospheric effective elastic thickness to calculate the fully re-equilibrated isostatic response of the solid Earth to the complete removal of the Greenland and Antarctic Ice Sheets. We use an elastic plate flexure model to compute the isostatic response to the unloading of the modern ice sheet loads, and a self-gravitating viscoelastic Earth model to make an adjustment for the remaining isostatic disequilibrium driven by ice mass loss since the Last Glacial Maximum. Feedbacks arising from water loading in areas situated below sea level after ice sheet removal are also taken into account. In addition, we quantify the uncertainties in the total isostatic response associated with a range of elastic and viscoelastic Earth properties. We find that the maximum change in bed elevation following full re-equilibration occurs over the centre of the landmasses and is +783 m in Greenland and +936 m in Antarctica. By contrast, areas around the ice margins experience up to 123 m of lowering due to a combination of sea level rise, peripheral bulge collapse, and water loading. The computed isostatic response fields are openly accessible and have a number of applications for studying regional geodynamics, landscape evolution, cryosphere dynamics, and relative sea level change.

Glacially Induced Stress Across the Arctic From the Eemian Interglacial to the Present—Implications for Faulting and Methane Seepage

AbstractStrong compressive and shear stresses generated by glacial loading and unloading have a direct impact on near‐surface geological processes. Glacial stresses are constantly evolving, creating stress perturbations in the lithosphere that extend significant distances away from the ice. In the Arctic, periodic methane seepage and faulting have been recurrently associated with glacial cycles. However, the evolution of the Arctic glacial stress field and its impact on the upper lithosphere have not been investigated. Here, we compute the evolution in space and time of the glacial stresses induced in the Arctic lithosphere by the North American, Eurasian and Greenland ice sheets during the latest glaciation. We use glacial isostatic adjustment (GIA) methodology to investigate the response of spherical, viscoelastic Earth models with varying lithospheric thickness to the ice loads. We find that the GIA‐induced maximum horizontal stress (σH) is compressive in regions characterized by thick ice cover, with magnitudes of 20–25 MPa in Fennoscandia and 35–40 MPa in Greenland at the last glacial maximum. Simultaneously, a tensile regime with σH magnitude down to −16 MPa dominates across the forebulges with a mean of −4 MPa in the Fram Strait. At present time, σH in the Fram Strait remains tensile with an East‐West orientation. The evolution of GIA‐induced stresses from the last glaciation to present could destabilize faults along tensile forebulges, for example, the west‐coast of Svalbard. A more tensile stress regime as during the Last Glacial Maximum would have more impact on pre‐existing faults that favor gas seepage from gas reservoirs.

Ice age effects on the satellite-derived [formula omitted] datum: Mapping the sensitivity to 3D variations in mantle viscosity

Studies of glacial isostatic adjustment (GIA) based on spherically symmetric viscoelastic Earth models have argued that the rate of change of the degree 2 zonal harmonic of the Earth's geopotential, or J˙2, provides an important constraint on mean viscosity in the deep mantle (Mitrovica and Peltier, 1993; Nakada et al., 2015; Lau et al., 2016). To refine this constraint, we compute Fréchet kernels using an adjoint methodology that reveal the sensitivity of the datum to 3D variations in mantle viscosity. We demonstrate that the mantle sensitivity of the datum is largely limited to the region below the ancient Laurentide ice sheet that covered Canada and significant portions of the northeastern United States at Last Glacial Maximum (LGM). In the bottom half of the lower mantle, this region of maximum sensitivity lies outside the location of Large Low Shear Velocity Provinces (LLSVPs) imaged from seismic tomographic studies. Thus, if the low shear velocity of these provinces originates from thermal effects, previous inferences of viscosity based upon the J˙2 datum are likely higher than the actual mean viscosity of the lower mantle.

The impact of 3-D Earth structure on far-field sea level following interglacial West Antarctic Ice Sheet collapse

Prior to inferring ice sheet stability from past interglacial sea-level records, these records must first be corrected for the contaminating effects of glacial isostatic adjustment (GIA). Typical GIA corrections, however, neglect variability in the signal that may be introduced by Earth's 3-D rheological structure. We predict sea-level changes due to a collapse of the West Antarctic Ice Sheet (WAIS) over an idealized 6 kyr-duration interglacial using four viscoelastic Earth models. Two of these are 3-D viscosity models inferred from seismic tomography. The third is a 1-D (i.e., depth varying) viscosity model that is equivalent to the spherically averaged “background” viscosity profile adopted in both 3-D Earth models. The fourth is a 1-D model that has a higher upper mantle viscosity but still falls within the class of models inferred from independent global GIA studies. We find that the discrepancy between 3-D and 1-D Earth model calculations of sea level in the far field of the melt zone is of order 0.3 m or less, with the 1-D Earth models producing higher sea level than the 3-D simulations. This value is 10% of the global mean sea-level (GMSL) rise associated with modeled ice sheet collapse by the end of the model interglacial (∼3 m) and a similar fraction of far-field sea-level changes. However, the value is a significantly larger fraction (∼60%) of the geographically variable (i.e., non-GMSL) component of the far-field sea-level signal due to GIA associated with modeled WAIS collapse (±0.5 m). Neglecting lateral variations in Earth structure in modeling the response to excess melting of WAIS during the interglacial compounds any error introduced by neglecting such structure in predictions of interglacial sea-level change driven by the preceding glacial cycle.

Theoretical viscoelastic deformations due to an expansion source in a spherical earth model

SUMMARYSources of expansion, such as nuclear explosions and magma chambers, can cause various surface effects, such as displacement and gravity field change on the Earth. The deformation simulation of an expansion source can be traced back to certain classic works nearly 70 yr ago, which were primarily based on a half-space earth model. However, the deformation of expansion sources in a spherical earth model has not been presented in the context of the semi-analytical viscoelastic Love numbers framework. In this study, we present the deformation Green's function for an expansion source in a spherical, non-rotating, viscoelastic, isotropic layered earth model using a semi-analytical method. We further obtain the analytical Green's functions for an elastic model in the near field. In addition, the influence of the stratification, curvature, and viscosity on the deformation is analysed using a layered spherical earth model. The suitability and effectiveness of the formulas are verified by numerical experiments and compared with previous results. The method presented in this paper is useful for explaining the viscoelastic deformations caused by the deep expansion source, for example, large magma chambers and upwelling of a mantle plume, in the layered viscoelastic earth model.

Optimized approximate inverse Laplace transform for geo-deformation computation in viscoelastic Earth model

SUMMARY Integral transformations, especially the inverse Laplace transform, are powerful techniques for resolving a wide range of geophysical and geodynamic simulation problems in viscoelastic materials. The exact location or distribution range of poles of the image function in a complex plane is usually necessary for applying numerical algorithms such as contour integration. Unfortunately, there are innumerable poles (such as those of post-seismic deformations) in a realistic Earth model with continuous stratification, finite compressibility and self-gravitation. Here, an optimized method to effectively calculate the inverse Laplace transform is presented. First, the integral kernel is approximated as a rational function with two parameters (a and m). Thereafter, the residue theorem is analytically applied to the approximated integrand. Finally, a series formula of the inverse Laplace transform sampling of image functions along a contour line parallel to the image axis is obtained. The proposed approximate scheme of the inverse Laplace transform is discussed by some common geophysical signals and the optimized selection of two parameters (a = 6 and m = 4) is conducted after a detailed analysis. The proposed method is anticipated as being able to help performing certain theoretical studies related to geodynamic problems with viscoelastic deformations.

Coseismic and post-seismic gravity disturbance induced by seismic sources using a 2.5-D spectral element method

SUMMARYI present a prescription for computing free-air coseismic and post-seismic gravity changes induced by seismic sources in a viscoelastic earth model. I assume a spherical earth geometry and a 2.5-D calculation, that is, 3-D motions that satisfy the equations of quasi-static equilibrium on a 2-D viscoelastic structure. The prescription permits application to regional gravity computations where a 2-D structure adequately represents the structural heterogeneity. I use a hybrid approach where deformation is computed on a discretized domain and the resulting density perturbations are expanded with spherical harmonics to produce the free-air gravity field. Starting with a solution to the equations of quasi-static displacements in the Laplace transform domain for a given dislocation source, I solve Poisson’s equation using Lagrangian interpolation on spectral element nodes to compute the required deformation quantities that contribute to free-air gravity. A numerical inverse Laplace transform then yields time domain results. This methodology is tested with analytic solutions on a spherically stratified viscoelastic structure, then applied to evaluate the effect of a descending slab of relatively high viscosity on post-seismic gravity in a megathrust faulting setting.

Viscous Effects in the Solid Earth Response to Modern Antarctic Ice Mass Flux: Implications for Geodetic Studies of WAIS Stability in a Warming World

AbstractThe West Antarctic Ice Sheet (WAIS) overlies a thin, variable-thickness lithosphere and a shallow upper-mantle region of laterally varying and, in some regions, very low (~1018 Pa s) viscosity. We explore the extent to which viscous effects may affect predictions of present-day geoid and crustal deformation rates resulting from Antarctic ice mass flux over the last quarter century and project these calculations into the next half century, using viscoelastic Earth models of varying complexity. Peak deformation rates at the end of a 25-yr simulation predicted with an elastic model underestimate analogous predictions that are based on a 3D viscoelastic Earth model (with minimum viscosity below West Antarctica of 1018 Pa s) by ~15 and ~3 mm yr−1 in the vertical and horizontal directions, respectively, at sites overlying low-viscosity mantle and close to high rates of ice mass flux. The discrepancy in uplift rate can be reduced by adopting 1D Earth models tuned to the regional average viscosity profile beneath West Antarctica. In the case of horizontal crustal rates, adopting 1D regional viscosity models is no more accurate in recovering predictions that are based on 3D viscosity models than calculations that assume a purely elastic Earth. The magnitude and relative contribution of viscous relaxation to crustal deformation rates will likely increase significantly in the next several decades, and the adoption of 3D viscoelastic Earth models in analyses of geodetic datasets [e.g., Global Navigation Satellite System (GNSS); Gravity Recovery and Climate Experiment (GRACE)] will be required to accurately estimate the magnitude of Antarctic modern ice mass flux in the progressively warming world.

Joint estimate of the co-seismic 2011 Tohoku earthquake fault slip and post-seismic viscoelastic relaxation by GRACE data inversion

SUMMARY Satellite-derived gravity data offer a novel perspective for understanding the physics of megathrust earthquakes at subduction zones. Nonetheless, their temporal resolution and observational errors make it difficult to discern the different phases of the seismic cycle, as the elastostatic deformation (co-seismic) and the stress relaxation by viscous flow (post-seismic). To overcome these difficulties and to take advantage of the physical constraints on the temporal evolution and on the spatial pattern of the earthquake-induced gravity disturbances, we have jointly estimated the fault slip of the 2011 Tohoku earthquake and the rheological stratification by means of a Bayesian inversion of GRACE data time series and within the framework of spherically symmetric self-gravitating compressible viscoelastic Earth models. This approach, in addition to improve the exploitation of satellite-derived gravity data, allows us (i) to constrain the fault slip taking advantage of information from both the co- and post-seismic signatures and (ii) to investigate the trade-off between the fault slip and the shallow rheological stratification. In this respect, it can be used to improve the modelling of crustal displacements from GPS data, even if their higher accuracy and temporal resolution allow to discriminate well the co-seismic signature from the others.

Green’s Functions for Post-seismic Strain Changes in a Realistic Earth Model and Their Application to the Tohoku-Oki Mw 9.0 Earthquake

Based on a spherically symmetric, self-gravitating viscoelastic Earth model, we derive a complete set of Green’s functions for the post-seismic surface strain changes for four independent dislocation sources: strike-slip, dip-slip, and horizontal and vertical tensile point sources. The post-seismic surface strain changes caused by an arbitrary earthquake can be obtained by a combination of the above Green’s functions. The post-seismic surface strain changes in the near field agree well with the results calculated by the method in a half-space Earth model (Wang et al. in Comupt Geosci 32:527–541, 2006), which verifies our Green’s functions. With an increase in the epicentral distance, the effect of the curvature on both the co- and post-seismic strain changes clearly increases, revealing the importance of our spherical theory for far-field calculations. Next, we use our Green’s functions to simulate the post-seismic surface strain changes that were caused by the viscoelastic relaxation of the mantle over the 6-year period after the Tohoku-Oki Mw 9.0 earthquake. Based on continuous Global Positioning System (GPS) observations around Honshu Island of Japan, Northeastern China, South Korea and the Russian Far East, we also deduce the post-seismic strain changes caused by the Tohoku-Oki Mw 9.0 earthquake. Overall, the distributions of the calculated and GPS-derived strain changes agree well each other. Finally, we compare the relative error between the observed and simulated strain changes over the 3.0–4.5-year period after the earthquake in both the near and far field. We find that the relative errors decrease as the epicentral distance increases, which validates our Green’s functions for research in the far field.

Geodetically Inferred Locking State of the Cascadia Megathrust Based on a Viscoelastic Earth Model

AbstractIn a viscoelastic Earth, stresses slowly built up due to fault locking are relaxed concurrently during the entire interseismic period. This interseismic stress relaxation causes crustal deformation much farther away from the locked fault than can be explained using elastic models that neglect the relaxation. Here we develop a viscoelastic geodetic inversion model to address this problem at Cascadia. We invert ~500 horizontal velocity vectors based on continuous and campaign geodetic measurements over the past two decades. Ambiguities arising from long‐term rotation of upper‐plate crustal blocks are addressed by test‐correcting the geodetic velocities with two different block‐motion models. Fault back slip (i.e., slip deficit) Green's functions are derived using a Maxwell viscoelastic finite element model with realistic subduction zone structure and megathrust geometry. The preferred model features a narrow and shallow megathrust locked zone, consistent with earlier thermorheological reasoning. For an elastic model to fit the data to the same fidelity, megathrust locking has to extend to much greater depths. However, even with the viscoelastic model, the land‐based geodetic data still cannot resolve whether there is some creep (incomplete locking) in the shallowest part of the megathrust far offshore. Neither can the land data fully resolve along‐strike variations of the locking state. These ambiguities can be resolved only when adequate seafloor geodetic data are obtained.

Spatial variation in Late Ordovician glacioeustatic sea-level change

Mass extinction of Late Ordovician marine fauna closely coincided with southern hemisphere glaciation. The sequence stratigraphic architecture of shallow marine deposits informs estimates of glacioeustatic sea-level change at sites both proximal and distal to the reconstructed Ordovician ice sheet(s) and contemporaneous changes in ice volume. A recent correlation framework for the stratigraphic architectures of one near and one far field Late Ordovician margin concluded that the Late Ordovician glaciation encompassed multiple long-term cycles of ice volume growth and retreat with superimposed higher frequency cycles. Here we posit that—similar to Cenozoic glacial cycles—glacial isostatic adjustment can preclude synchronous and similar magnitude (or directional) changes in Late Ordovician sea level between ice proximal and ice distal locations and, hence, distort a globally correlative sequence stratigraphy. We explored whether long-duration (i.e., million year) Late Ordovician glacial cycles should produce a globally coherent, eustatic record of sea-level change between ice proximal and ice distal margins using a gravitationally self-consistent theory that accounts for the deformational, gravitational and rotational perturbations to sea level on a viscoelastic Earth model. We adopted a Late Ordovician paleogeography and a synthetic continental ice-sheet distribution and volume informed by the areal extent of glaciogenic deposits and geochemical records, respectively. We demonstrate that modeled million year Late Ordovician glacial cycles produce sea-level histories on near and far field margins that differ from eustasy, and from one another, due primarily to elastic flexure and associated gravitational effects. While predicted far-field sea-level histories faithfully preserve the temporal structure of modeled glacioeustasy, their amplitude may differ from eustasy by as much as 30–40%. The impact of glacial isostatic adjustment is largest at the margins of glaciated continents, and these effects can be of the same order of magnitude as the eustatic, and even induce a local sea-level rise during an episode of ice growth and eustatic sea-level fall, and vice versa. In this regard, stratal surfaces of maximum regression and flooding expressed at near-field margins need not reflect global (‘eustatic’) trends in ice sheet growth and decay, respectively, and thus may not provide chronostratigraphic horizons for correlation with far-field sequence stratigraphic architectures.

Constraints on Transient Viscoelastic Rheology of the Asthenosphere From Seasonal Deformation

AbstractWe discuss the constraints on short‐term asthenospheric viscosity provided by seasonal deformation of the Earth. We use data from 195 globally distributed continuous Global Navigation Satellite System stations. Surface loading is derived from the Gravity Recovery and Climate Experiment and used as an input to predict geodetic displacements. We compute Green's functions for surface displacements for a purely elastic spherical reference Earth model and for viscoelastic Earth models. We show that a range of transient viscoelastic rheologies derived to explain the early phase of postseismic deformation may induce a detectable effect on the phase and amplitude of horizontal displacements induced by seasonal loading at long wavelengths (1,300–4,000 km). By comparing predicted and observed seasonal horizontal motion, we conclude that transient asthenospheric viscosity cannot be lower than 5 × 1017 Pa.s, suggesting that low values of transient asthenospheric viscosities reported in some postseismic studies cannot hold for the seasonal deformation global average.

Contributions of a Strengthened Early Holocene Monsoon and Sediment Loading to Present‐Day Subsidence of the Ganges‐Brahmaputra Delta

AbstractThe contribution of subsidence to relative sea level rise in the Ganges‐Brahmaputra delta (GBD) is largely unknown and may considerably enhance exposure of the Bengal Basin populations to sea level rise and storm surges. This paper focuses on estimating the present‐day subsidence induced by Holocene sediment in the Bengal Basin and by oceanic loading due to eustatic sea level rise over the past 18 kyr. Using a viscoelastic Earth model and sediment deposition history based on in situ measurements, results suggest that massive sediment influx initiated in the early Holocene under a strengthened South Asian monsoon may have contributed significantly to the present‐day subsidence of the GBD. We estimate that the Holocene loading generates up to 1.6 mm/yr of the present‐day subsidence along the GBD coast, depending on the rheological model of the Earth. This rate is close to the twentieth century global mean sea level rise (1.1–1.7 mm/yr). Thus, past climate change, by way of enhanced sedimentation, is impacting vulnerability of the GBD populations.