AGU Fall Meeting, San Francisco, USA, 5 - 9 December, 2011
Nils Maercklin, Gaetano Festa, Simona Colombelli, Aldo Zollo
The Mw 9.0 megathrust earthquake off northeastern Honshu, Japan, in March 2011 had an unexpected size for a region which experienced only few events with magnitude larger than 8.0 in the past millennium. The event originated at crustal depths along a segment of the Pacific slab of the Japanese subduction zone. Large slip deficit and strong interplate coupling have been previously detected there by inland deformation measurements. The pattern of seismicity occurrence and the mechanical coupling between the different sectors of the Japan slab suggest that its morphology and segmentation may be strongly influenced by the presence of landward oceanic fracture zones.
The aim of this study is to image the locations of strongly radiating sources and the rupture development during the faulting process. We used strong-motion records from the dense Japanese accelerometer arrays, integrated twice to obtain ground displacement, and filtered in different frequency bands between 0.04 Hz and 2.0 Hz. We applied a move-out and stacking technique to back-project the S-wave displacement amplitudes onto the subducting plate boundary, including the proper correction for geometrical spreading and source radiation pattern. Thus, the resulting images are consistently mapped into the slip distribution during the rupture development. Image resolution and sensitivity to processing parameters is assessed by synthetic tests. Our results show that the great Tohoku earthquake started as a smaller size rupture, slowly propagating upward along the slab segment and triggering the break of a larger size asperity at shallower depths near the trench. In that region also the largest slip has been observed in various studies. For a large amount of its duration, the rupture remained confined in a 100-150 km wide slab stripe, delimited by two Northwest-Southeast trending oceanic fractures. After about a minute, the rupture propagated at relatively high speed toward Southwest, parallel to the trench.
The occurrence of large slip amplitudes at shallow depths likely favored the rupture to propagate across contiguous slab segments and contributed to build up a giant size earthquake. The lateral variations in the slab surface geometry may act as geometrical and/or mechanical barriers finally controlling the earthquake rupture nucleation, evolution and arrest.