Frequency-dependent rupture process of the 2011 M(w) 9.0 Tohoku Earthquake: Comparison of short-period P wave backprojection images and broadband seismic rupture models

The frequency-dependent rupture process of the 11 March 2011 M(w)9.0 off the Pacific coast of Tohoku Earthquake is examined using backprojection (BP) imaging with teleseismic short-period (~1 s) P waves, and finite faulting models (FFMs) of the seismic moment and slip distributions inverted from broadband (>3 s) teleseismic P waves, Rayleigh waves and regional continuous GPS ground motions. Robust features of the BPs are initial down-dip propagation of the short-period energy source with a slow rupture speed (~1 km/s), followed by faster (2-3 km/s) rupture that progresses southwestward beneath the Honshu coastline. The FFMs indicate initial slow down-dip expansion of the rupture followed by concentrated long-period radiation up-dip of the hypocenter, then southwestward expansion of the rupture. We explore whether these differences correspond to real variations in energy release over the fault plane or represent uncertainties in the respective approaches. Tests of the BP results involve (1) comparisons with backprojection of synthetic P waves generated for the FFMs, and (2) comparisons of backprojection locations for aftershocks with corresponding NEIC and JMA locations. The data indicate that the down-dip environment radiates higher relative levels of short-period radiation than the up-dip regime for this great earthquake, consistent with large-scale segmentation of the frictional properties of the megathrust.