From Daily Deformation to Millennial Mechanics: Insights from Subduction Zone Earthquake Cycle Models

Subduction zones have hosted all the largest five earthquakes in the last one hundred years, including the 2011 Mw 9.1 Tohoku-oki earthquake on 11 March 2011, one of the largest natural disasters in history. While the general mechanism of these thrust-style earthquakes is well described by stress accumulation due to the locking between the incoming and the overriding tectonic plates, many questions remain as to the size and longevity of the asperities which host the coseismic rupture, the rheological models best describing the rock in and around the fault zone, and the effects of stress shadows and interactions between different asperities on the same plate interface. These questions are addressed by using large earthquakes as natural experiments, which we can observe using geodetic, seismic, and other techniques. However, ambiguities in the modeling results point to the inherent problem of non-uniqueness when interpreting surface observations of single events to infer complex processes at depth. This dissertation presents a new framework to study subduction zones and their rheological properties by extending both the time period modeled and the observations considered to all phases of the seismic cycle, on a fault interface that experiences earthquakes at multiple points in space and time. The motivating concept is that the recovery of rheological parameters could be greatly improved when considering that the constitutive laws for fault material must be able to reproduce all phases of the earthquake cycle, since it is the same physical material. Here, we (1) develop a timeseries analysis software that enables the efficient processing of large geodetic networks with long timeseries, allowing us to extract the relevant subduction-zone related signal in the observations, (2) formulate a forward model that, based on ancillary historical and seismic datasets as well as a candidate rheological model, simulates surface motion over multiple earthquake cycles, and (3) use a probabilistic inverse method to estimate the best-fitting rheological parameters given the postprocessed surface deformation timeseries and model uncertainties. We validate the timeseries analysis software on the transient volcanic deformation of Long Valley Caldera, California, USA, before extracting the megathrust component of the surface observations on Northern Honshu Island, Japan. We then estimate the rheological properties of the Northern Japanese subduction zone using our inversion method, simultaneously producing time-varying estimates of kinematic coupling, slip deficit, and surface deformation. Our model predictions match the pre- and postseismic displacement timeseries of the 2011 Tohoku-oki earthquake well. On the steadily creeping part of the plate interface, we infer rate-dependent frictional parameters generally increasing with depth, but with second-order along-strike variation. Finally, we discuss the potential impact of our cycle-spanning, probabilistic inversion method on the field of subduction zone studies, and present possible avenues for further improvements to our framework.