Spectra (left column) and 0.115–0.145-Hz-Butterworth-filtered waveforms (right column) of station J30A (top row) and J31A (bottom row) records in March 4–5 (blue) and March 6–7 (red). Spectra were calculated using Welch's method with 50 per cent overlapping 256-s data segments. The corner frequencies of the filter are indicated by the dashed lines. The small local earthquake on March 6 was removed, indicated by the gap.