Envelope Inversion Analysis for High-Frequency Seismic Energy Radiation from the 2011 Mw 9.0 Off the Pacific Coast of Tohoku Earthquake

Abstract

Abstract I study S ‐wave energy radiation in higher frequencies of 1–8 Hz from the 2011 M w 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 5.8×10 16 J for 1–2 Hz, 4.5×10 16 J for 2–4 Hz, 1.5×10 16 J for 4–8 Hz, and totaling 1.2×10 17 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 frequency‐dependent rupture may accord with structural changes along the dip direction. Online material: Snapshots of high‐frequency energy radiation from the fault plane and results of estimating scattering parameters by the multiple lapse time window analysis in 1–2 and 2–4 Hz.