Ground motion noises observed by Seafloor observation network for earthquakes and tsunamis along the Japan Trench (S-net)

S-net project started in 2011 and construction of S-net has finished in 2017. The S-net is a large-scale seafloor network of cable-linked seismic and tsunami observatories. S-net has 150 stations and covers the area about 1000km x 300 km from off-Hokkaido to off-Kanto around Japan Trench and Kuril Trench in Japan. In this presentation, we report the ground motion noise level of the S-net stations and discuss the performance of S-net.

Two or more sets of tsunami meters and seismometers are installed in one station for redundancy. Two sets of three component servo accelerometers, a set of three component quartz type accelerometers (frequency outputs), a set of three component velocity seismometers will be installed, and two sets of quartz type depth sensors (frequency outputs) are installed as tsunami meters. Theses observation devices are housed in a cylindrical pressure vessel made of beryllium copper alloy with a diameter of 34 cm and a length of 226 cm. We do not use any gimbal structures, so the direction of the three axes of the seismograph is in the direction according to the installation condition. Therefore, analysis was carried out by assuming that the installation direction at the time of laying is the longitudinal direction of the observation unit and converting to the up-down component, the north-south component, the east-west component from the acceleration value ratio of the accelerometer. We estimated the seismic noise level by the probability density function (PDF) of the power spectral density (PSD) by the method of McNamara and Boaz (2005).

We buried the observation units and cables beneath seafloor in the area shallower than 1,500m water depth. On the other hand, we installed them on the seafloor in the area deeper than 1,500m water depth. In order to estimate the effect of burying, we calculated PDF for each water depth and compared. Regarding the horizontal component, the noise level at the buried station was lower by about 10 dB for the 3 to 9 Hz band than for the non-buried station. In general, it is considered that the noise level of the ocean floor becomes higher as the water depth gets shallower due to the influence of the bottom current. Therefore, we consider that there is a great effect of noise reduction by burying the observation unit.