Abstract
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.
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