Seismic and aseismic fault slip before and during the 2011 off the Pacific coast of Tohoku Earthquake

The March 11, 2011, off the Pacific coast of Tohoku Earthquake is the first great subduction zone earthquake to be recorded by a dense, modern network of geodetic quality Global Positioning System (GPS) receivers, and hence presents an unprecedented opportunity to study the evolution of fault slip during the time periods immediately before, during, and after a truly great earthquake. We utilized sub-daily GPS data to constrain the slip distribution in the March 9 Mw 7.3 foreshock, the 50-hour time period between the foreshock, and the coseismic slip during the Mw 9.0 mainshock. We find that the foreshock ruptured downdip from its hypocenter, that there was considerable (Mw ~ 7.3) slip during the 50 hours between the foreshock and the mainshock, and that the peak mainshock slip was about 35 m. The mainshock’s epicenter may have been triggered by afterslip that followed the foreshock. Additionally, the mainshock slip distribution was centered further updip than the peak interseismic coupling estimated from GPS data in the ten years before the earthquake.