Horizontal ground-motion model for subduction slab earthquakes using offshore ground motions in the Japan Trench area

The S-net, a large-scale network of permanent ocean-bottom seismographs in the Japan Trench, consisting of 150 stations, has recorded a large number of offshore ground motions that can be used to establish the empirical attenuation relationship of offshore ground motions in the subduction zone. Due to the different attenuation characteristics among different earthquake types in subduction zones, the earthquakes are classified into four tectonic types of subduction slab, interface, shallow crustal, and upper-mantle earthquakes according to the classification scheme of Zhao et al. (2015). Predictive models and uncertainties in offshore ground motions were investigated for different earthquake types. This study establishes a horizontal offshore subduction slab earthquake ground-motion model (GMM) and compares the offshore and onshore slab earthquake GMMs. As the site conditions at ocean-bottom stations are different from those at land stations, the effects of water depth and sediment thickness are taken into account in the offshore slab earthquake GMM. Due to differences in the burial methods of ocean-bottom stations, stations were divided into buried and unburied to investigate the effects of the burial methods. Therefore, regression analysis was used to propose an offshore slab earthquake GMM considering the magnitude, focal depth, distance, water depth, sediment thickness, and burial method. By separating the within-event residuals, the single-station standard deviation is presented. Compared to the onshore GMM, the predicted spectra of the offshore GMM are significantly larger at long periods. For the attenuation rate, the offshore attenuation rate is lower than that of the onshore attenuation rate for short periods, but is basically consistent with the onshore attenuation rate for long periods. The proposed GMM can be used to predict the offshore ground motions for slab earthquakes with rupture distances less than 300 km, focal depths less than 110 km, and moment magnitude between 4 and 7.4.