Strong localized asperity of the 2011 Tohoku-Oki earthquake, Japan

Our moment tensor inversion of the 2011 Tohoku-Oki earthquake, using regional broadband strong-motion waveforms, indicates that the earthquake can be approximated by a point source. The amplitude spectra of the observed displacement seismograms were fitted by the omega-square model, which resulted in the corner frequency of around 0.016 Hz. This implies a large slip over a circular fault having a radius of 70 km, with a rupture duration of about 40 s. The moment-rate function estimated from the inversion shows a large impulse of similar duration. We interpret this impulse to correspond to the rupture estimated from the corner frequency. From the seismic moment released during the impulse, we have estimated the average slip and stress drop over the fault to be 50 m and 40 MPa, respectively. This stress drop corresponds to an effective normal stress larger than 200 MPa, indicating that a strong localized asperity (mega asperity) was ruptured during the earthquake. Previous simulation studies suggested the importance of a large effective normal stress at a shallow plate interface, which was explained by a pore pressure distribution along the plate. We have explored the possibility of a subducted seamount to be the origin of the mega asperity.