Subduction zone; Earthquake cycle; Deformation; GPS; Finite Element Method modeling;
550 Geowissenschaften 558 Geologie Südamerikas
Modern GPS measurements have provided essential constraints on the kinematics of the continental lithosphere at an unprecedented spatial and temporal resolution and have, in turn, revolutionized our view of crustal deforming processes spanning the earthquake cycle in the subduction zone. These measurements have been particularly useful in constraining viscous deformation of the asthenosphere. The accumulation of geodetic time series in many subduction zones has led to many significant refinements on the concept of subduction earthquake cycle. In this thesis, I present a broad spectrum of interrelated topics about the underlying deformation mechanisms during the subduction zone earthquake cycles. I integrate Finite Element Method (FEM) modeling and geodetic constraints from GPS observations to geomechanically explore the tectonophysical processes at different stages of the earthquake cycle with case studies mainly confined to the Chilean plate boundary margin.
For the interseismic period, I investigate the control of viscoelasticity of the asthenosphere on interseismic deformation and its effects on the apparent locking degree determination. Most previous models explain the interseismic deformation with purely elastic solution and neglect the potential viscoelastic effects, hence the associated interpretations are potentially misleading. To highlight the pitfalls of interpreting the geodetic data with purely elastic models for both the forward and inverse problems, I develop a novel FEM-based viscoelastic inversion method and apply it to the Peru-North Chile subduction zone. My results confirm that elastic models are prone to overestimating the interseismic locking depth and indicate that the signals interpreted as back-arc shortening in the elastic model can be alternatively explained by viscoelastic deformation, which, in turn, dramatically refines the interseismic locking pattern in both dip and strike directions. Hence it is necessary to thoroughly reevaluate existing locking models that are based on purely elastic models, some of which attribute viscoelastic deformation to different sources such as microplate
For the coseismic period, I investigate the influence that megathrust earthquake slip has on the activation of splay faults, taking into account the effects of gravity and variations in the frictional strength properties of splay faults. My results indicate that the static triggering process is controlled by a critical depth of megathrust slip distribution. Megathrust slip concentrated at depths shallower than the critical depth will favor normal displacement, while slip concentrated at depths deeper than the critical depth is likely to result in reverse motion. This work thus provides a useful tool for predicting the activation of secondary faults and may have direct implications for tsunami hazard research.
For the earthquake cycle, especially the postseismic period, I investigate how the effective viscosity varies in asthenosphere. We use a set of 3-D FEM models and continuous GPS observations to constrain the effective viscosities of the asthenosphere and investigate the spatio-temporal variability of the effective viscosity. Our results reveal a sudden decrease in effective viscosities in near field following the earthquake and the slow recovery of these effective viscosities during the postseismic phase. While in far field, there is no sudden effect, rather a gradual viscosity decrease. The variations of the viscosity in these bodies may reflect a dependence of the viscosity on the stress state of the materials, which is suddenly elevated by coseismic-introduced stress perturbation. Therefore, we suggest this geophysical process may explain the first order change in wavelength of surface deformation away from the trench before and after a great earthquake. While the viscosity variation of the asthenosphere is significant enough to be measured by geodetic instruments, significant challenges remain for refining the model of viscoelastic deformation in the subduction earthquake cycle.
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