Abstract

Temporal changes of seismic velocities generated at the subsurface by the 2011 Mw 9 Tohoku earthquake in Japan are analyzed using interferometry and autocorrelation of waveforms recorded by surface and borehole sensors at the KiK‐net station IBRH16. Spectral ratios and interferometry of data recorded in moving time windows at the surface and a depth of 300 m exhibit clear time delays and reduction of the predominant frequency during the strong ground motion followed by partial recovery in the waveform coda. Converting the time‐delay evolution to velocity changes implies about 30% velocity reduction in the structure between the two sensors. Calculating temporal evolution with autocorrelation of data at the surface sensor in the moving time window indicates about 50% velocity reduction, reflecting changes of shallower material below the station. Computing autocorrelations of data recorded by the surface sensor with the Stockwell transform allows monitoring temporal changes with higher resolution that approaches the sampling rate of the waveforms. Using this technique, we estimate about 60% drop of seismic velocity below the surface sensor during strong ground motion followed again by partial recovery in the coda. These results provide fundamental information on in situ dynamic properties of soils and damaged shallow rocks that complement laboratory measurements, with important implications for constitutive equations of material degradation and healing and nonlinear site effects.

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