Pre- and postseismic slow slip surrounding the 2011 Tohoku-oki earthquake rupture

Slow (aseismic) slip that accommodates part of the long-term plate motion on subduction megathrusts is thought to be strongly related to the occurrence of large earthquakes on the same fault zone. However, the temporal evolution and spatial distribution of the aseismic slip before major earthquakes and of accelerated postseismic afterslip are largely unconstrained. We estimate cumulative offsets of small repeating earthquakes that are interpreted to reflect the in situ aseismic slip history on the subduction zone offshore northeastern Japan. These data reveal contrasting aseismic slip patterns between the coseismic rupture area of the Mw 9.0 Tohoku-oki earthquake and surrounding portions of the subduction thrust. The rupture area is characterised by low and variable slip rates before 2008, and the slip stopped almost completely after the earthquake. The region surrounding the rupture area exhibited higher aseismic fault slip rates before the earthquake and clear postseismic slip of up to 1.6 m within 9 months following the main shock. The frictional fault properties and complete relief of ambient stress in the central rupture zone of the main shock probably control the observed distribution. The postseismic slip shows a more abrupt increase in the region closer to the source, suggesting outwards propagation of afterslip. Small but distinct increases in the slip rate in the ~3 yr before the earthquake near the area of large coseismic slip suggests preseismic unfastening of the locked area in the last stage of the earthquake cycle.