Envelope inversion analysis for high-frequency seismic energy radiation from the 2011 M_w 9.0 Off the Pacific Coast of Tohoku earthquake

I study S-wave energy radiation in higher frequencies of 1-8 Hz from the 2011 Mw 9.0 Off the Pacific Coast of Tohoku, Japan, earthquake (hereafter called the Tohoku-Oki earthquake) based on a seismogram envelope inversion method. I first evaluate two medium parameters of scattering attenuation and intrinsic absorption for S waves using spatial and temporal distributions of seismic energy from aftershock records, which are necessary for calculating Green's functions. Next, applying the envelope inversion method to 27 near-field strong-motion records from the mainshock, I estimate the S-wave energy radiation from 40 subfaults on the fault of 500 km × 250 km. Each subfault is allowed to rupture for eight consecutive source time windows. Rupture velocity and the duration of energy radiation for each time window are determined by a grid search to be 2:5 km=s and 8.0 s, respectively. Energy radiated from the entire fault is 5:8 × 10¹⁶ J for 1-2Hz, 4:5 × 10¹⁶ J for 2-4Hz, 1:5 × 10¹⁶ J for 4-8 Hz, and totaling 1:2 × 1017 J in the 1-8 Hz range. Strong energy emanated twice from greater depths of the off-Miyagi region, corresponding to two peaks seen at the northern stations. And energy radiation at greater depths off the Fukushima-Ibaraki border is compatible with a single peak found at the southern stations. Strong high-frequency radiations at greater depths strikingly contrast with large slips at shallower depths estimated in lower frequencies. This frequencydependent rupture may accord with structural changes along the dip direction.