Offshore ground motion characteristics on the horizontal PGA, spectral acceleration, frequency content and significant duration from the 2021 Mw 7.1 and 2022 Mw 7.4 offshore earthquakes near the Japan Trench area

The two large offshore earthquakes that occurred in Japan's eastern Fukushima region on February 13, 2021, Mw 7.1 and March 17, 2022, Mw 7.4 are the two largest offshore earthquakes recorded by the large-scale permanent ocean-bottom seismograph network (S-net). The two earthquakes are both subduction slab earthquakes and caused great damage to onshore structures. The offshore ground motions from the S-net within a fault distance of 300 km were selected to investigate the characteristics of the strong offshore motions. The onshore ground motions are also selected from the K-net to compare the differences in offshore and onshore ground motions. These onshore and offshore datasets were used to calculate the representative ground motion parameters: the peak ground acceleration (PGA), spectral acceleration, frequency content and significant duration of the earthquakes. Here we present the comparison of the offshore and onshore ground motions in amplitude, duration and frequency content, as well as the applicability of the representative onshore ground motion model (GMM) to the offshore region. Additionally, we provide the spatial contours of the parameters used in this study and the residual analysis to examine the effects of the water depth, sedimentary layer and station deployment method on the offshore ground motions. We found that the PGA and spectral accelerations of unburied seafloor stations are larger than those of buried stations, and the attenuation rate observed from the spatial contours between the offshore and onshore regions is different, which shows the necessity of proposing a new prediction model for the PGA and spectral accelerations in offshore regions. The significant duration of offshore ground motions is consistent with the onshore prediction model of significant duration in the subduction zone but is on average greater than those of onshore ground motions. For the frequency content, there is no prediction model for subduction zones at present. The frequency-content parameters of offshore ground motions are relatively consistent with the frequency-content model for greater depths. On the whole, the complex environment at the ocean bottom has the most significant influence on the spectral accelerations over long periods and has some influence on the duration and frequency content, but the overall difference is smaller than the PGA and spectral accelerations.