Regional extent of the large coseismic slip zone of the 2011 Mw 9.0 Tohoku‐Oki earthquake delineated by on‐fault aftershocks

Since the 2011 Tohoku‐Oki earthquake, many fault source models have been constructed using a variety of data, such as teleseismic, strong ground motion, geodetic, and tsunami recordings. However, questions remain as to how far the associated large‐slip zone extended along the plate interface during the rupture. Here we delineate the outer edge of this large‐slip zone in detail, based on the sharp density contrast observed for interplate, repeating, and down‐dip compressional earthquakes induced by the Tohoku‐Oki mainshock. The seismicity rate of interplate earthquakes changed significantly after the mainshock, probably as a result of stress perturbations by the large‐slip, and here we use this information as qualitative constraints in constructing our model. The model that we present for the large‐slip zone of the Tohoku‐Oki earthquake incorporates the main features of previously proposed fault source models, and also the observed fine‐scale heterogeneities of fault slip associated with this event. It is important to highlight that the outer edge of this large‐slip zone shows a remarkably complex shape. In particular, it is narrow and elongate southward along the ~35 km iso‐depth contour of the plate boundary offshore of Fukushima and Ibaraki. This southward elongate slip zone corresponds to down‐dip regions that appear to have produced higher relative levels of short‐period seismic radiation.