We use subdaily GPS time series of positions in the first 5 hours following the 2003 Tokachi‐oki earthquake ( M w = 8.0) located offshore of Hokkaido, Japan, to estimate frictional parameters for the afterslip zone on the subduction interface. The data show little motion immediately after the earthquake with sudden acceleration at about 1.2 hours after the main shock. This coincides with the largest aftershock ( M = 7.4), followed by gradual deceleration. We assume that early afterslip is the response of a fault patch to instantaneous stress perturbations caused by the main shock and the largest aftershock. Early afterslip is modeled with a spring‐slider system obeying a rate‐ and state‐dependent friction law. We develop and apply an inversion method to estimate friction parameters, D c , aσ , and ( a − b ) σ , where σ is effective normal stress. The estimated 95% confidence intervals of D c , aσ , and ( a − b ) σ are 2.6 × 10 −4 to 1.8 × 10 −3 m, 0.29 to 0.43 MPa, and 0.214 to 0.220 MPa, respectively. Estimated D c is 10 to 10 3 times larger than typical laboratory values. Estimated aσ and ( a − b ) σ values suggest that a and a − b are smaller than typical laboratory values and/or the pore pressure on the plate boundary is significantly elevated above the hydrostatic value. Our analyses show that the model can reproduce the observed GPS data and that the timing of the rapid acceleration of postseismic deformation is controlled by the frictional properties of the fault and stress change from the main shock, not by the timing of the largest aftershock.
Prior to the 2011 M9 Tohoku‐oki earthquake, subduction at the Japan Trench was characterized by M7‐8 earthquakes, sometimes rupturing the same source regions (seismic asperities), followed by extensive afterslip detected by GPS measurements. A physically‐based model consisting of velocity‐weakening asperities surrounded by aseismic creep on velocity‐strengthening regions (the ‘rate‐state asperity model’) became the prevailing conceptual model for earthquakes in this region. Theory and numerical simulation indicates that velocity‐weakening areas do not exhibit sustained afterslip, while velocity‐strengthening regions do not accumulate stress interseismically. Here we demonstrate that the rate‐state asperity model is contradicted by models of postseismic deformation following the Tohoku‐oki earthquake: afterslip in the first eight months either occurred on historical seismic asperities or stress accumulated in regions surrounding the asperities. Unsmoothed inversions of cumulative 8‐month postseismic GPS displacements that restrict afterslip to areas outside of historical ruptures cannot fit the data without afterslip exceeding the slip that fully relaxes the coseismic stress change. In contrast, similarly constrained inversions allowing slip within historical ruptures can satisfactorily fit the postseismic displacements. These results require a modification of the rate‐state asperity model and raise new questions about physical processes and properties of the subduction interface.
The earthquake nucleation process has been vigorously investigated based on geophysical observations, laboratory experiments, and theoretical studies; however, a general consensus has yet to be achieved. Here, we studied nucleation process for the 2014 Iquique, Chile Mw 8.2 megathrust earthquake located within the current North Chile seismic gap, by analyzing a long-term earthquake catalog constructed from a cross-correlation detector using continuous seismic data. Accelerations in seismicity, the amount of aseismic slip inferred from repeating earthquakes, and the background seismicity, accompanied by an increasing frequency of earthquake migrations, started around 270 days before the mainshock at locations up-dip of the largest coseismic slip patch. These signals indicate that repetitive sequences of fast and slow slip took place on the plate interface at a transition zone between fully locked and creeping portions. We interpret that these different sliding modes interacted with each other and promoted accelerated unlocking of the plate interface during the nucleation phase.
We present a unified theoretical framework and solution method for probabilistic, Bayesian inversions of crustal deformation data. The inversions involve multiple data sets with unknown relative weights, model parameters that are related linearly or non-linearly through theoretic models to observations, prior information on model parameters and regularization priors to stabilize underdetermined problems. To efficiently handle non-linear inversions in which some of the model parameters are linearly related to the observations, this method combines both analytical least-squares solutions and a Monte Carlo sampling technique. In this method, model parameters that are linearly and non-linearly related to observations, relative weights of multiple data sets and relative weights of prior information and regularization priors are determined in a unified Bayesian framework.
We have developed a new geodetic inversion method for space—time distribution of fault slip velocity with time-varying smoothing regularization in order to reconstruct accurate time histories of aseismic fault slip transients. We introduce a temporal smoothing regularization on slip and slip velocity through a Bayesian state space approach in which the strength of regularization (temporal smoothness of slip velocity) is controlled by a hyperparameter. The time-varying smoothing regularization is realized by treating the hyperparameter as a time-dependent stochastic variable and adopting a hierarchical Bayesian state space model, in which a prior distribution on the hyperparameter is introduced in addition to a conventional Bayesian state space model. We have tested this inversion method on two synthetic data sets generated by simulated aseismic slip transients. Results show that our method reproduces well both rapid changes of slip velocity and steady-state velocity without significant oversmoothing and undersmoothing, which has been hard to overcome by the conventional Bayesian approach with time-independent smoothing regularization. Application of this method to transient deformation in 2002 caused by a silent earthquake off the Boso peninsula, Japan, also shows similar advantages of this method over the conventional approach.
