Characteristics of ocean-floor time series of shallow crustal earthquakes recorded by DONET in Japan

Many offshore structures are constructed at locations that are earthquake prone; therefore, their associated seismic designs are required. However, the selection of the acceleration time series for seismic design is difficult because recorded ground motions at the ocean-bottom seafloor are limited. Recently, the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) installed the ocean-bottom seismology network (DONET) at the offshore of the Kii Peninsula in Japan. Twenty stations are recording three-component time series using accelerometers and seismometers. These time series are publicly available since 2012 at the JAMSTEC website (https://join-web.jamstec.go.jp/JSEIS/). This study selects representative time series of shallow crustal earthquakes and processes from DONET to develop a database of ocean-bottom ground motion. These time series are compared to the inland recordings from the Kyoshin network (K-NET). This study reviews the instrument responses of the accelerometers and seismograms of DONET. Fourier amplitude spectra (FAS) are also computed for different time windows, and the ocean floor and inland FAS are compared to understand the characteristics of ocean-floor recordings. Horizontal and vertical attenuations are also compared between ocean floor and inland and with the results of typical ground-motion prediction models. Finally, the horizontal-to-vertical spectral ratios (HVSRs) are computed for the ocean-floor stations to understand its local site effects.