Interpreting GPS observations of the megathrust earthquake cycle: insights from numerical models

Abstract

During a megathrust earthquake cycle, the plates accumulate strain in the interseismic stage due to locking of a portion of the megathrust that separates them. This strain is released during the earthquake and following rapid postseismic relaxation. As summarised in Chapter 1, this understanding was built through decades of seismological and geodetic observations and of advances in physics-based models. Models link properties and structures to observable quantities and are crucial to interpreting observed surface deformation in terms of the processes and materials in the inaccessible subsurface. However, different models can approximate the same observations roughly equally well. Furthermore, more sophisticated models do not necessarily reflect better the processes and properties of the subsurface; at the same time, simpler models that can explain some observations do not necessarily reflect the key processes occurring throughout one or multiple earthquake cycles. This thesis uses relatively simple three-dimensional models that still capture the key processes occurring during repeated earthquake cycles, without attempting to reproduce the structure or properties of specific subduction zones, to examine possible explanations of geodetic observations from global navigation satellite systems (GNSSs) at different stages of the cycle. Chapter 2 focuses on the interseismic, horizontal deformation of the overriding plate, which is in apparent contrast with observations of far-reaching coseismic displacement. We estimate the spatial patterns, with uncertainties, of GNSS velocities in South America, Southeast Asia, and northern Japan. Interseismic velocities with respect to the overriding plate generally decrease with distance from the trench with a steep gradient up to a “hurdle”, beyond which the gradient is distinctly lower and velocities are small. The hurdle is located 500-1000 km away from the trench for the trench-perpendicular velocity component, and either at the same distance or closer for the trench-parallel component. The trench-perpendicular hurdle generally follows major tectonic or geological boundaries and seismological contrasts. We formulate and test the hypothesis that both the interseismic hurdle and the coseismic response result from a mechanical contrast in the overriding plate. Our models show that overriding plates with a sufficient contrast respond to locked interseismic convergence similarly to observations. The compliance contrast is probably mainly responsible for the observed hurdle and in turn results from thermal, compositional and thickness contrasts. Chapter 3 is concerned with the increased landward velocities that were recorded onshore after 6 megathrust earthquakes in subduction zone regions adjacent to the ruptured portion. We investigate whether bending can be expected to reproduce this observed enhanced landward motion (ELM). We find that viscous relaxation, but not afterslip, produces ELM when a depth limit is imposed on afterslip. This ELM results primarily from in-plane elastic bending of the overriding plate due to trenchward viscous flow in the mantle wedge near the rupture. Modeled ELM is, however, incompatible with the observations, which are an order of magnitude greater and last longer. This conclusion does not significantly change when varying key model parameters. The observed ELM consequently appears to reflect faster slip deficit accumulation, implying a greater seismic hazard in lateral segments of the subduction zone. In Chapter 4, we study postseismic landward motion observed on the overriding plate in the vicinity of a major megathrust rupture. We argue that relocking of the megathrust, particularly at shallow depths, is needed for postseismic relaxation to produce landward motion on the tip of the overriding plate. We discuss how that this finding is consistent with previous simulations that implicitly relock the megathrust where afterslip is not included. We conclude that the Tohoku megathrust relocked within less than two months of the earthquake. This suggests that the shallow megathrust probably behaves as a true, unstably sliding asperity.