Abstract

We investigate the high‐frequency content of the initial 50 s of waveforms from the 2011 Off the Pacific Coast of Tohoku earthquake (Mw 9.0, 11 March 2011). The strong‐motion spectra for the Mw 9.0 mainshock are richer in high‐frequency (>10 Hz) content than those of the Mw 7.3 foreshock compared to low frequencies. The mainshock spectra are flat up to at least 20 Hz, which is at variance with the general source spectra models such as the ω‐square and fmax models. The spectral ratios of the Mw 9.0–7.3 events show that after 20 s from the P‐wave arrival time, the high‐frequency strong motion of the mainshock is distributed in the southern Tohoku district. Analyses of past moderate magnitude earthquakes (Mw 6.0–7.3) in the area around the mainshock hypocenter indicate that the radiation from the deeper regions is the main cause of the high‐frequency strong motion.

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