Transient Brittle Creep mechanism explains early postseismic phase of
the 2011 Tohoku-Oki megathrust earthquake: observations by high-rates
GPS solutions
Abstract
The early stage of the postseismic phase is characterized by a large
deformation rate. Its analysis is thus key to decipher the role played
by different mechanisms (afterslip and viscoelasticity) at various time
scales. Here, we process GPS data to obtain 30-seconds kinematic
position time series recording the surface deformation following the Mw
9.0 Tohoku-Oki megathrust earthquake (2011), and combine them with
static solutions over 9 years. We analyze the temporal evolution of the
time series and use these observations to image the postseismic slip. We
find that the first month of deformation following Tohoku-Oki can be
explained by an afterslip mechanism, that exhibits an “Omori-like”
decay, with a p-value around 0.75 almost everywhere with the
exception of a small region around Ibaraki prefecture where
p~1 is observed. This p<1
indicates that the postseismic displacements do not increase
logarithmically with time as predicted by rate-and-state rheology.
Instead, we argue that early afterslip is associated to a transient
brittle creep mechanism. We use numerical simulations to show that an
exponent of p<1 can be explained by a combination of
thermal activation of local slips and elastic interactions. Over longer
time scales, an additional mechanism is required to explain the observed
deformation signal, and the transient brittle creep mechanism is
combined with viscoelastic relaxation modeled by a Newtonian flow. The
spatial analysis reveals two distinct afterslip regions, a major one on
the North, associated with a p-value around 0.75, and a smaller
one close to the Ibaraki aftershock, associated to
p~1.