Fig. 11

Average of power spectral density (PSD) before and after sidereal filtering made by stacking. a North component. The black line indicates PSD before sidereal filtering while colored lines indicate PSD after sidereal filtering in which a sidereal filter is made by stacking one (green), two (pink), or five (light blue) blocks preceding a block of interest. Vertical thick line indicates a period of 200 s. b, c Same as a but for east and up components. d–f Same as a–c but PSDs at periods from 50 to 1000 s are magnified for clarity