Identifying geographical patterns of transient deformation in the geological sea level record

In this study, we examine the effect of transient mantle creep on the prediction of glacial isostatic adjustment (GIA) signals. Specifically, we compare predictions of relative sea level (RSL) change from GIA from a set of Earth models in which transient creep parameters are varied in a simple Burgers model to a reference case with a Maxwell viscoelastic rheology. The model predictions are evaluated in two ways: first, relative to each other to quantify the effect of parameter variation, and second, for their ability to reproduce well-constrained sea level records from selected locations. Both the resolution and geographic location of the RSL observations determine whether the data can distinguish between model cases. Model predictions are most sensitive to the inclusion of transient mantle deformation in regions that are near-field and peripheral relative to former ice sheets. This sensitivity appears particularly true along the North American west coast in the region of the former Cordilleran Ice Sheet, which experienced rapid sea-level fall following deglaciation between 14 and 12 kyr BP. Relative to the Maxwell case, Burgers models better reproduce this rapid phase of regional postglacial sea-level fall. As well, computed goodness-of-fit values in this region show a clear preference for models where transient deformation is present in the whole or lower mantle, and for models where the rigidity of the Kelvin element is weakened relative to the rigidity of the Maxwell element. In contrast, model predictions of relative sea-level change in the far-field show weak sensitivity to the inclusion of transient deformation

A tsunami generated by a strike-slip event.: constraints from GPS and SAR data on the 2018 Palu earthquake

A devastating tsunami struck Palu Bay in the wake of the 28 September 2018 Mw = 7.5 Palu earthquake (Sulawesi, Indonesia). With a predominantly strike-slip mechanism, the question remains whether this unexpected tsunami was generated by the earthquake itself, or rather by earthquake-induced landslides. In this study we examine the tsunami potential of the co-seismic deformation. To this end, we present a novel geodetic dataset of GPS and multiple SAR-derived displacement fields to estimate a 3D co-seismic surface deformation field. The data reveal a number of fault bends, conforming to our interpretation of the tectonic setting as a transtensional basin. Using a Bayesian framework, we provide robust finite fault solutions of the co-seismic slip distribution, incorporating several scenarios of tectonically feasible fault orientations below the bay. These finite fault scenarios involve large co-seismic uplift ( > 2 m) below the bay due to thrusting on a restraining fault bend that connects the offshore continuation of two parallel onshore fault segments. With the co-seismic displacement estimates as input we simulate a number of tsunami cases. For most locations for which video-derived tsunami waveforms are available our models provide a qualitative fit to leading wave arrival times and polarity. The modeled tsunamis explain most of the observed runup. We conclude that co-seismic deformation was the main driver behind the tsunami that followed the Palu earthquake. Our unique geodetic dataset constrains vertical motions of the sea floor, and sheds new light on the tsunamigenesis of strike-slip faults in transtensional basins

A Tsunami Generated by a Strike-Slip Event: Constraints From GPS and SAR Data on the 2018 Palu Earthquake

A devastating tsunami struck Palu Bay in the wake of the 28 September 2018 Mw = 7.5 Palu earthquake (Sulawesi, Indonesia). With a predominantly strike-slip mechanism, the question remains whether this unexpected tsunami was generated by the earthquake itself, or rather by earthquake-induced landslides. In this study we examine the tsunami potential of the co-seismic deformation. To this end, we present a novel geodetic data set of Global Positioning System and multiple Synthetic Aperture Radar-derived displacement fields to estimate a 3D co-seismic surface deformation field. The data reveal a number of fault bends, conforming to our interpretation of the tectonic setting as a transtensional basin. Using a Bayesian framework, we provide robust finite fault solutions of the co-seismic slip distribution, incorporating several scenarios of tectonically feasible fault orientations below the bay. These finite fault scenarios involve large co-seismic uplift (>2 m) below the bay due to thrusting on a restraining fault bend that connects the offshore continuation of two parallel onshore fault segments. With the co-seismic displacement estimates as input we simulate a number of tsunami cases. For most locations for which video-derived tsunami waveforms are available our models provide a qualitative fit to leading wave arrival times and polarity. The modeled tsunamis explain most of the observed runup. We conclude that co-seismic deformation was the main driver behind the tsunami that followed the Palu earthquake. Our unique geodetic data set constrains vertical motions of the sea floor, and sheds new light on the tsunamigenesis of strike-slip faults in transtensional basins

Presence and significance of backstops in the overriding plate during the megathrust earthquake cycle

Seismological and geodetic observations indicate that similar physical processes are active at different subduction margins and provide information about the deformation at the different stages of the earthquake cycle. We analyze geodetic observations along sections of the South American subduction zone during the inter-seismic stage. Results show that overriding plates shorten from the trench to a “backstop”, where horizontal inter-seismic velocities become close to zero. In most, but not all regions, the backstop location from trench-perpendicular GPS velocities agrees with that from trench-parallel velocities. The distance of the backstop from the trench varies along the western South America margin. Backstop locations shows some correlation with gradients in the effective elastic thickness of the overriding plate. An apparently conflicting observation is that co-seismic and early post-seismic GPS-displacements during the 2010 Maule earthquake extended well beyond the backstop into eastern South America. Similarly conflicting observations were made in the overriding plate of the 2004 Sumatra earthquake and the 2011 Tohoku earthquake. We use cyclic 3D numerical models with dynamically driven co-seismic and afterslip to test the hypothesis that lateral contrasts in the thickness and/or elasticity of the overriding plate explain the observations. The model setup allows us to explore the sensitivity of geodetically observable surface motion to the mechanical structure of the subduction system during all parts of the earthquake cycle. We conclude that the observations can be explained by a lateral contrast. Such contrast restricts inter-seismic horizontal velocities in the region between the trench and the backstop, controlling their gradient, while allowing deformation due to coseismic slip and afterslip to reach well into the far field. One particularly interesting finding from our models is that stress accumulation in the overriding plate is controlled by the distance to the backstop

Classifying large strains from digital imagery: application to analogue models of lithosphere deformation

We are interested in reconstructing the time evolution of 2D plane deformation of analogue models of tectonic processes. Under relevant forcings, these models develop internal deformation, such as faults, and broader zones of deformation. We use Particle Image Velocimetry (PIV) to derive incremental displacements from top-view images that we use in subsequent steps to calculate the shape changes that come with large deformation. Because PIV describes displacement in a spatial reference, and material moves through the area in view, displacements at any given time refer to fixed locations in space, and not to specific material points. By reconstructing the path of material, we can follow small regions of material while they translate, rotate and change shape. To aid the qualitative interpretation of this deformation, we have developed a novel method that can qualitatively describe shape changes coming from extensional, shortening and horizontal shearing (strike-slip) deformation or combinations of these. This method is based on a logarithmic measure of stretch and results agree well with the visual interpretation of structures that we observe in our models. Thus, we provide tools with which the evolution of 2D tectonic deformation can be interpreted in a physically meaningful manner, but our method may be useful outside the realm of tectonics. Our software to compute deformation is freely available and can be used to post-process incremental displacements from PIV or similar autocorrelation methods

Mapping and classifying large deformation from digital imagery: application to analogue models of lithosphere deformation

Particle image velocimetry (PIV), a method based on image cross-correlation, is widely used for obtaining velocity fields from time-series of images of deforming objects. Rather than instantaneous velocities, we are interested in reconstructing cumulative deformation, and use PIV-derived incremental displacements for this purpose. Our focus is on analogue models of tectonic processes, which can accumulate large deformation. Importantly, PIV provides incremental displacements during analogue model evolution in a spatial reference (Eulerian) frame, without the need for explicit markers in a model. We integrate the displacements in a material reference (Lagrangian) frame, such that displacements can be integrated to track the spatial accumulative deformation field as a function of time. To describe cumulative, finite deformation, various strain tensors have been developed, and we discuss what strain measure best describes large shape changes, as standard infinitesimal strain tensors no longer apply for large deformation. PIV or comparable techniques have become a common method to determine strain in analogue models. However, the qualitative interpretation of observed strain has remained problematic for complex settings. Hence, PIV-derived displacements have not been fully exploited before, as methods to qualitatively characterize cumulative, large strain have been lacking. Notably, in tectonic settings, different types of deformation—extension, shortening, strike-slip—can be superimposed. We demonstrate that when shape changes are described in terms of Hencky strains, a logarithmic strain measure, finite deformation can be qualitatively described based on the relative magnitude of the two principal Hencky strains. Thereby, our method introduces a physically meaningful classification of large 2-D strains. We show that our strain type classification method allows for accurate mapping of tectonic structures in analogue models of lithospheric deformation, and complements visual inspection of fault geometries. Our method can easily discern complex strike-slip shear zones, thrust faults and extensional structures and its evolution in time. Our newly developed software to compute deformation is freely available and can be used to post-process incremental displacements from PIV or similar autocorrelation methods

Data supplement to: Mapping and classifying large deformation from digital imagery: application to analogue models of lithosphere deformation

The dataset consists of the original top-view and cross-section digital photographs of the model presented in Broerse et al. (2020) (Mappingand classifying large deformation from digital imagery: application to analogue models of lithosphere deformation.). The top-view images have been taken at a regular interval of 45 seconds during the entire experiment. Cross-section images have been taken after the experiment was finished. The purpose of the top-view images is to track incremental displacement fields of the model surface in time, estimated by Particle Image Velocimetry (PIV) based on the digital images. The cross-section images serve to inspect the final internal deformation along vertical planes. The images are located in two folders; top-views contains the full range of top-view images as used in the PIV analysis, starting at the original undeformed state until the final deformed state at the end of the experiment. A sub-folder not used in PIV contains images from before the start and after the stop of the experiment, and that are not used in the accompanying manuscript. The folder cross-sections contains an overview image of the locations of all cross-sections taken (IMG 7243) and images of vertical model cross-sections. In total 13 cross-sections have been made, where the cross-section numbering starts from the north, and continues southward (see image IMG 7243). The cross-sections a-a’, b-b’ and c-c’ that are used in Broerse et al. (2020) correspond to numbers 2, 6 and 13, respectively. The cross- section images have been taken with a southward view, while in Broerse et al. (2020) we have mirrored the images to present a northward view on the cross-sections. All photographs are in .jpg format and their names are original generic names created by the camera software at the moment of acquisition. Contact person is Taco Broerse - [email protected]

Postseismic GRACE and GPS observations indicate a rheology contrast above and below the Sumatra slab

More than 7 years of observations of postseismic relaxation after the 2004 Sumatra-Andaman earthquake provide an improving view on the deformation in the wide vicinity of the 2004 rupture. We include both Gravity Recovery and Climate Experiment (GRACE) gravity field data that show a large postseismic signal over the rupture area and GPS observations in the back arc region. With increasing time GPS and GRACE show contrasting relaxation styles that were not easily discernible on shorter time series. We investigate whether mantle creep can simultaneously explain the far-field surface displacements and the long-wavelength gravity changes. We interpret contrasts in the temporal behavior of the GPS-GRACE observations in terms of lateral variations in rheological properties of the asthenosphere below and above the slab. Based on 1-D viscoelastic models, our results support an (almost) order of magnitude contrast between oceanic lithosphere viscosity and continental viscosity, which likely means that the low viscosities frequently found from postseismic deformation after subduction earthquakes are valid only for the mantle wedge. Next to mantle creep, we also consider afterslip as an alternative mechanism for postseismic deformation. We investigate how the combination of GRACE and GPS data can better discriminate between different mechanisms of postseismic relaxation: distributed deformation (mantle creep) versus localized deformation (afterslip). We conclude that the GRACE-observed gravity changes rule out afterslip as the dominant mechanism explaining long-wavelength deformation even over the first year after the event