The magnitude 9 megathrust earthquake off Tohoku, Japan, in March 2011 had an unexpected size for a region which experienced only few events with magnitude larger than 8 in the past millennium. The event originated at crustal depths along a segment of the Japanese subduction zone, where large slip deficit and strong interplate coupling have been detected 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 are influenced by the presence of oceanic fracture zones.
To image the rupture process and the total slip in detail, we applied a novel amplitude backprojection technique to waveforms recorded by the dense Japanese accelerometer arrays. The waveforms were integrated twice to obtain ground displacement and were band-pass filtered between 0.05 and 0.4 Hz. In a series of synthetic tests, we verified that our method correctly retrieves relative locations of strongly radiating sources (asperities) and the temporal evolution.
Our results show that the earthquake began as a small-size bilateral rupture, slowly propagating mainly updip along the slab and triggering the breakage of a larger-size asperity at shallower depths nearby the trench. This resulted in a cumulative slip at the ocean bottom reaching values as large as 50 m and causing high-amplitude tsunami waves. For a long time the rupture remained confined in a 100-150 km wide slab segment, delimited by the westward prolongation of two oceanic fractures, before propagating further to the southwest, parallel to the trench.
The occurrence of large slip at shallow depths likely favored the rupture to propagate across contiguous slab segments and contributed to build up a giant size megathrust 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. Locations and rupture histories of the two main asperities suggest two seismic cycles with large events occurring every few decades at the deeper portion and megathrusts every thousand years at the shallow portion of the slab.