Aftershocks of the 2011 M9.0 Tohoku-Oki earthquake

The giant 2011 Tohoku-Oki earthquake ruptured the Pacific plate – North american plate interface, over about 400 km, and was characterized by up to 50 m of slip near the trench. This very large slip, occurring over a relatively compact zone, means that the stress drop was large (of the order of 10 MPa).
The earthquake then initiated a very active aftershock sequence, but that lacked thrust earthquakes along the main slip interface ; this implies that the stress drop was effectively so large that no remanent deviatoric stress was left within the rupture zone to trigger subsequent earthquake.

Nearly 10 years after the mainshock, it is timely to (re)analyze this sequence, in the light of extensive data of incomparable quality, both geodetic and seismic. The goal of this work would be to study and model how the aftershock sequence evolved during 10 years, depending on focal mechanism and location (close or further away from the main slip area ; with respect to depth and inter-seismic coupling), and how it relates to afterslip (as measured by GPS). Specific attention will be paid to the observed seismic quiescence in the main rupture area related to the postulated near complete (deviatoric) stress drop. The dataset will consist in the NIED F-Net moment tensor catalog, complemented by the JMA earthquake catalog (that does not give focal mechanisms but documents the seismicity down to low magnitudes). This work will be done in collaboration with an on-going PhD on quantifying and modelling the afterslip of the 2011 mainshock.

Key questions in earthquake physics that will be investigated during this work are :

• How long does it take for a seismicity quiescence to start ? How long does it last ? How are these time scales controlled (by stress change ? Its spatial variability ?) ?
• Can we make prediction about the future evolution of the aftershock sequence ?
• Can we distinguish specific evolution for specific mechanisms ? (eg : thrust aftershocks on or near the slip interface ; normal aftershocks in the upper plate).
• Can the contribution of strong aftershocks into re-activating the overall aftershock sequence be evaluated ?
• Do afterslip and aftershock rates evolve similarly ? Can these two quantities be compared also for strong aftershocks ?