Tremor and Inferred Slow Slip Associated With Afterslip of the 2011 Tohoku Earthquake

Characterizing shallow interplate slip is essential for modeling the potential generation of megathrust earthquakes and tsunamis. Large postseismic crustal deformation caused by the 2011 Tohoku-Oki earthquake suggests the occurrence of shallow afterslip, but it is unclear what kind of slip is dominant at the shallow plate interface. Here we report episodic tremor activity south of the primary rupture area based on ocean bottom seismometer observations. Five tremor episodes with recurrence intervals of ~60 days show migration behavior and shear mechanisms consistent with plate subduction, indicating the occurrences of the episodic slow slip events in the vicinity of the afterslip area. The association of slow slip events and afterslip sheds new light on the frictional nature and stress state of shallow megathrusts and may help predict the possible rupture extent of future earthquakes.