Ground Shaking and Seismic Source Spectra for Large Earthquakes around the Megathrust Fault Offshore of Northeastern Honshu, Japan

Large earthquake ruptures on or near the plate boundary megathrust fault offshore of northeastern Honshu, Japan, produce variable levels of regional highfrequency ground shaking. Analyses of 0.1–10 Hz strong ground motion recordings from K-NETand KiK-net stations and 0.3–3.0 Hz short-period recordings from Hi-net stations establish that the shaking variations result from a combination of differences in seismic source spectra and path attenuation. Eleven earthquakes with Mw 6.0–7.6 are analyzed, including interplate events at different positions on the megathrust within the rupture zone of the 11 March 2011 Tohoku (Mw 9.0) event and nearby intraplate events within the underthrusting Pacific slab. The relative ground shaking for frequencies of 5–10 Hz is strongest for the 7 April 2011 (Mw 7.2) intraslab event near the coast, followed by intraplate events beneath the outer-trench slope. Decreasing levels of high-frequency shaking are produced by interplate megathrust events moving from the down-dip edge of the seismogenic zone to the up-dip region near the trench. Differential attenuation measurements from averaged spectral ratios of strong-motion recordings indicate that average path attenuation is lower for events deeper on the megathrust or within the slab below the coast. Empirical Green’s function analysis isolates the source spectra for the passband 0.3–3.0 Hz, indicating higher corner frequencies for intraplate events and deep megathrust events than for shallow megathrust events. Similar differences in average source spectra are found for teleseismic Pwaves. Depth-varying source radiation and path attenuation thus account for the high-frequency shaking for the 2011 Tohoku mainshock originating from the down-dip portion of the megathrust. Online Material: Figures of spectral ratios, regression of averaged spectral ratios, and stacked spectral ratios.