Probabilistic estimation of postseismic relaxation parameters and processes following the 2011 Tohoku earthquake

Geodetic observations of 3-component surface motions following a megathrust earthquake are key to a better understanding of features and processes controlling the dynamics at subduction margins. The relative contributions of dominant drivers during the postseismic phase, such as viscoelastic relaxation, afterslip, and relocking, remain difficult to estimate individually and are often derived at the end of an observation period, without showing the temporal evolution of the processes. Data assimilation combines physical models with observations, and can be a way to constrain these contributions by estimation of model parameters, the associated uncertainties, and identifying parameter tradeoffs. We use Bayesian inference in the form of an ensemble smoother to estimate geodynamic parameters during the postseismic phase of the megathrust earthquake cycle. The ensemble smoother uses a Monte Carlo approach to represent the probability density distribution (pdf) of model states with a finite number of realizations. Prior estimates of the imperfect physical model are combined with the likelihood of noisy observations to estimate the posterior pdf of model parameters. We first discuss a synthetic data experiment where observations are sampled from a 3D earthquake cycle model and where we added variable levels of noise. We assimilate 3-component surface displacements into a 2D finite element viscoelastic earthquake cycle model. We incorporate an a priori heterogeneous temperature field to estimate the asthenospheric viscosity distribution through power-law parameters (e.g., stress power). Preliminary results show that model parameters, such as the extent of the cold nose, maximum depth of afterslip, and power-law parameters can be recovered remarkably well by assimilating synthetic on- and offshore surface observations. We show preliminary results of data assimilation of postseismic surface displacements following the 2011 Tohoku earthquake.