Post-seismic slip of the 2011 Tohoku-Oki earthquake from GPS observations: implications for depth-dependent properties of subduction megathrusts

Here we inverted the GPS data to infer the coseismic slip of the Tohoku-Oki earthquake
and the time-dependent afterslip distribution in the 4 months following the main shock. The
Tohoku-Oki earthquake showed an unexpected magnitude and a characteristic depth-dependent
differentiation of seismic energy radiation. In this context the estimation and comparison of
the distribution of the fault portions that slip coseismically and post-seismically contribute
to a better understanding of the variation of frictional characteristics of the plate interface.
The inferred coseismic slip extends in a relatively compact region located updip from the
hypocentre and reaches its highest value (about 60 m) near the trench. Afterslip occurs mostly
outside the coseismic rupture and is distributed in two main modal centres. It reaches its largest
values in an area located downdip of the coseismic slip and extends to a depth of 80 km. In the
depth range between 30 and 50 km afterslip overlaps the portion of the fault that experienced
historical moderate earthquakes, high-frequency seismic radiation and thrust-type aftershocks.
The behaviour of this area can be explained by a rheologically heterogeneous region made of
a ductile fault matrix interspersed with compact brittle asperities. On the contrary, the region
beneath 50–60 km depth is probably characterized by a fully velocity strengthening behaviour.
Southern afterslip, located off-Chiba Prefecture, is probably related to the Mw 7.9 Ibaraki-Oki
aftershock. The northward extension of the afterslip stops at a latitude of about 40◦ N, just
south of the off-Aomori region. This may be related to three large events occurred in this
area during the last century and the consequent strong coupling or complete depletion of the
accumulated strain that characterize this region.