Fig. 5

The probability density function of ambient seismic noise spectra from three-component records of the anchored-buoy OBSs. The original data from the OBSs have been converted to the records for up–down, north–south and east–west directions using information on the attitude of the packages. Data from 14:31 on July 5, 2019, to 12:14 on July 13, 2019, were used for the estimation. We estimated spectra every 1 min with a time window of 32.768 s, and probability was estimated using 11,411 spectra. Although the seismic sensors of the anchored-buoy OBS have a natural frequency of 15 Hz, the sensor response was not compensated for the estimation. The High Noise Model and Low Noise Model of Peterson (1993) are shown in frequencies lower than 10 Hz. Black, gray, and white lines indicate averages of seismic noise spectra for the same period from the records of the land stations, E.NZMY, E.KNSY, and E.YAMY, respectively. A 15-Hz high-pass filter was applied for data from the land stations to compare those from the anchored-buoy OBSs. The ambient noise levels of the anchored-buoy OBSs are sufficiently small to pick up arrivals and comparable to the land stations, in spite of shallow water depths. It is believed that the flat shape of the recorder is effective for reducing ambient noises