Fault friction parameters inferred from the early stages of afterslip following the 2003 Tokachi-oki earthquake

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, a sigma, and (a - b)sigma, where sigma is effective normal stress. The estimated 95% confidence intervals of D-c, a sigma, and (a - b)sigma are 2.6 x 10(-4) to 1.8 x 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 a sigma and (a - b)sigma 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.