Modeling the foreshock sequence prior to the 2011, MW9.0 Tohoku, Japan, earthquake

The 2011 M(W)9.0 Tohoku earthquake, Japan, was preceded by a 2 day-long foreshock sequence, initiated by a M(W)7.3 earthquake. We analyze this foreshock sequence, with the aim of detecting possible aseismic deformation transients that could have driven its evolution. Continuous broad-band recordings at F-net stations are processed to identify as exhaustive a set of m(JMA) > 1.2 earthquakes as possible. We moreover directly quantify with these recordings the changes in detection level associated with changes in seismic or environmental noise. This earthquake data set is then modeled, to show that the whole sequence can be readily explained without the need to invoke aseismic transients. The observation of a 3-hour long low-frequency noise increase, concurrent with an apparent migration of seismicity toward the epicenter of the impending M(W)9.0 mega-thrust earthquake, however suggests that some premonitory slip could have played a role in loading the asperity which failure initiated the M(W)9.0 shock. We thus propose that this aseismic slip, if it really existed, had only a minor role in triggering and southward displacing the foreshock sequence, as compared to earthquake interaction mechanisms that allow earthquakes to trigger one another.