Megathrust Earthquakes: Study of Fault Slip and Stress Relaxation Using Satellite Gravity Observations

During earthquakes large scale mass displacements take place when slip on a fault deforms the earth’s crust. Besides, in the days to decades after the main shock ongoing deformation is usually observed that is related to relaxation of stresses caused by the earthquake. In this thesis I relate gravity changes as observed by the GRACE satellite mission to solid earth deformation caused by earthquakes. Special attention is given to modeling the contribution of ocean mass redistribution to gravity changes, and its relation to changes in bathymetry. I show that common modeling practices are usually not properly taking into account the effect of ocean water redistribution when computing seismic gravity changes. Using a combination of GRACE data and GPS observations I interpret ongoing gravity changes and crustal motions after the 2004 Sumatra-Andaman earthquake as dominantly caused by viscoelastic mantle flow. Contrasts in relaxation styles from both observation types are related to lateral variations in mantle rheology below the subduction zone. The combined analysis of GPS and GRACE data strongly suggest that the mantle at the continental side of the subduction zone is weaker than the oceanic side.Astrodynamics and Satellite MissionsAerospace Engineerin

Google in China

The Mekelprize 2007 was won by Taco Broerse, who wrote an essay titled Google in China. Is it morally justified for a Western company to go along with self-censorship, especially in a country like China, where freedom of expression is under pressure? The jury found this essay to provide a strong analysis. The central question is explored from different angles and well supported. In addition, this student shows himself to be a real engineer-to-be by not only giving an analysis, but also trying to come up with directions for solutions.Civil Engineering and Geoscience

Surface motion modeling for the southeastern Carpathians: Processing and analysis of GPS observation data from the Vrancea seismic region in Romania

The SUBDUCT (Surface Behavior and Dynamical Units of the Southeast Carpathians Tectonics) research program, in which the TU Delft participates, aims to model the surface kinematics of a 350 x 350 km wide area around the Vrancea seismic region in Romania by means of GPS observations. The goal of the project is to relate surface motions to the active crust-lithosphere dynamics of the southeast Carpathians. The main objective is to model the surface expressions of the subducted slab at 70 to 200 km depth, which is the common origin of earthquakes in the Vrancea region. Since 1995 GPS field campaigns have been performed by various research groups, and since 2002 in the framework of the subduct research program that increased the number of episodical observed sites to more than 50 and maintains 7 continuously observing permanent stations in the region. This master thesis focusses on the precise processing with the JPL GIPSY software of the GPS data from permanent and campaign stations in Romania from the years 2004 to 2006 and the combination of these newly processed solutions with previous obtained results from the period 1995 to 2003. From the total set of available solutions linear site velocities have been estimated and a complete velocity field for the entire observed region has been produced for the horizontal surface motions as well as the vertical motions. The GPS data showed to be of high quality, resulting in daily repeatabilities for campaign and permanent stations in the order of 1-2 mm for the north and east and 3-7 mm for the vertical position component. However due to the use of different antennas in subsequent observation considerable vertical position offsets were introduced for all campaign stations, which limit the reliability of the constructed vertical velocities. The most reliable estimated horizontal velocities are for the region in the order of 1-2 mm/yr with respect to the stable Eurasian plate, indicating only small relative motions in Romania. The 95% confidence limit for the sites with the longest observation history is around 0.5-2 mm/yr. The total figure of the combined horizontal site velocities shows for most areas stable and regional uniform motions. Even though the vertical site velocities are unreliable in quantitative terms, the complete vertical velocity field shows in a qualitative sense nonetheless uniform subduction and uplifting zones. Most notably is the uniformly uplifting zone north of the Trotus fauls and the subsidence in the Moesian Platform and Transilvanian Basin. After previous preliminary DEOS publications, this thesis presents a first reliable qualitative vertical velocity field of the region that can be used for geophysical interpretation.Aerospace Engineerin

Ocean contribution to seismic gravity changes: The sea level equation for seismic perturbations revisited

During megathrust earthquakes, great ruptures are accompanied by large scale mass redistribution inside the solid Earth and by ocean mass redistribution due to bathymetry changes. These large scale mass displacements can be detected using the monthly gravity maps of the GRACE satellite mission. In recent years it has become increasingly common to use the long wavelength changes in the Earth's gravity field observed by GRACE to infer seismic source properties for large megathrust earthquakes. An important advantage of space gravimetry is that it is independent from the availability of land for its measurements. This is relevant for observation of megathrust earthquakes, which occur mostly offshore, such as the Mw?9 2004 Sumatra–Andaman, 2010 Maule (Chile) and 2011 Tohoku-Oki (Japan) events. In Broerse et al., we examined the effect of the presence of an ocean above the rupture on long wavelength gravity changes and showed it to be of the first order. Here we revisit the implementation of an ocean layer through the sea level equation and compare the results with approximated methods that have been used in the literature. One of the simplifications usually lies in the assumption of a globally uniform ocean layer. We show that especially in the case of the 2010 Maule earthquake, due to the closeness of the South American continent, the uniform ocean assumption is not valid and causes errors up to 57?per?cent for modelled peak geoid height changes (expressed at a spherical harmonic truncation degree of 40). In addition, we show that when a large amount of slip occurs close to the trench, horizontal motions of the ocean floor play a mayor role in the ocean contribution to gravity changes. Using a slip model of the 2011 Tohoku-Oki earthquake that places the majority of slip close to the surface, the peak value in geoid height change increases by 50?per?cent due to horizontal ocean floor motion. Furthermore, we test the influence of the maximum spherical harmonic degree at which the sea level equation is performed for sea level changes occurring along coastlines, which shows to be important for relative sea level changes occurring along the shore. Finally, we demonstrate that ocean floor loading, self-gravitation of water and conservation of water mass are of second order importance for coseismic gravity changes. When GRACE observations are used to determine earthquake parameters such as seismic moment or source depth, the uniform ocean layer method introduces large biases, depending on the location of the rupture with respect to the continent. The same holds for interpreting shallow slip when horizontal motions are not properly accounted for in the ocean contribution. In both cases the depth at which slip occurs will be underestimated.Geoscience and Remote SensingCivil Engineering and Geoscience

An analog material for ductile non-linear lithosphere rheology : application to lithospheric tearing

Stress-dependent non-linear upper mantle rheology has a firm base in rock mechanical tests, where this non-linearity results from dislocation creep of minerals. Furthermore, many numerical geophysical models invoke the power-law viscosity reduction at increasing stress, as observed for dislocation creep in olivine aggregates, to explain localized deformation in the ductile parts of the lithosphere. In the last few decades there has been some attention to non-linear, power law, materials for application in analogue, laboratory models of geophysical problems. However, literature describing the rheology of analogue materials with the same non-linear dependency on stress as observed for lithospheric mantle materials at relevant stress levels, is still lacking. We have developed and rheologically tested materials based on combinations of silicone polymers and plasticine, with the aim of obtaining a material that can serve as a laboratory analog to the power-law rheology of olivine aggregates at lithospheric mantle conditions. From our steady-state creep tests we find that it is possible to obtain such a power-law material, with effective viscosities over relevant model stress ranges [5-4000 Pa] that allow for deformation at laboratory time scales. We apply the developed material to settings where localized deformation of the lithosphere can be expected: slab break-off and STEP tears. We study these processes using analogue models, where we apply the developed non-linear material to the lithospheric mantle domains, while we use Newtonian glucose to represent the low viscous asthenosphere. From these models we observe that lithospheric tearing occurs in a localized mode

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.Physical and Space Geodes

Reversal of the Direction of Horizontal Velocities Induced by GIA as a Function of Mantle Viscosity

In regions undergoing glacial isostatic adjustment present‐day horizontal surface motion is observed to point mostly, but not always, away from the former ice load. To interpret these observations, we investigate the direction of horizontal velocities using glacial isostatic adjustment models. The direction is controlled by the opposing actions of inward mantle flow and outward lithosphere motion. In contrast with the prevailing idea that glacial isostatic adjustment‐induced horizontal velocities point outward, we show that velocities can be either outward or inward. Immediately after deglaciation velocities point inward but change direction to outward after a time that is controlled by mantle viscosity. Present‐day horizontal velocities point outward for a uniform mantle viscosity below 1e20 Pa s and inward for above 1e22 Pa s, with a combination of outward and inward in between. Our results help to interpret GPS‐observed horizontal velocities in areas with varying mantle viscosity

An analog material for ductile non-linear lithosphere rheology : application to lithospheric tearing

Stress-dependent non-linear upper mantle rheology has a firm base in rock mechanical tests, where this non-linearity results from dislocation creep of minerals. Furthermore, many numerical geophysical models invoke the power-law viscosity reduction at increasing stress, as observed for dislocation creep in olivine aggregates, to explain localized deformation in the ductile parts of the lithosphere. In the last few decades there has been some attention to non-linear, power law, materials for application in analogue, laboratory models of geophysical problems. However, literature describing the rheology of analogue materials with the same non-linear dependency on stress as observed for lithospheric mantle materials at relevant stress levels, is still lacking. We have developed and rheologically tested materials based on combinations of silicone polymers and plasticine, with the aim of obtaining a material that can serve as a laboratory analog to the power-law rheology of olivine aggregates at lithospheric mantle conditions. From our steady-state creep tests we find that it is possible to obtain such a power-law material, with effective viscosities over relevant model stress ranges [5-4000 Pa] that allow for deformation at laboratory time scales. We apply the developed material to settings where localized deformation of the lithosphere can be expected: slab break-off and STEP tears. We study these processes using analogue models, where we apply the developed non-linear material to the lithospheric mantle domains, while we use Newtonian glucose to represent the low viscous asthenosphere. From these models we observe that lithospheric tearing occurs in a localized mode

Estimating decadal variability in sea level from tide gauge records: An application to the North Sea

One of the primary observational data sets of sea level is represented by the tide gauge record. We propose a new method to estimate variability on decadal time scales from tide gauge data by using a state space formulation, which couples the direct observations to a predefined state space model by using a Kalman filter. The model consists of a time-varying trend and seasonal cycle, and variability induced by several physical processes, such as wind, atmospheric pressure changes and teleconnection patterns. This model has two advantages over the classical least-squares method that uses regression to explain variations due to known processes: a seasonal cycle with time-varying phase and amplitude can be estimated, and the trend is allowed to vary over time. This time-varying trend consists of a secular trend and low-frequency variability that is not explained by any other term in the model. As a test case, we have used tide gauge data from stations around the North Sea over the period 1980–2013. We compare a model that only estimates a trend with two models that also remove intra-annual variability: one by means of time series of wind stress and sea level pressure, and one by using a two-dimensional hydrodynamic model. The last two models explain a large part of the variability, which significantly improves the accuracy of the estimated time-varying trend. The best results are obtained with the hydrodynamic model. We find a consistent low-frequency sea level signal in the North Sea, which can be linked to a steric signal over the northeastern part of the Atlantic