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With the improvement of the world's largest seafloor observation network (S-net) and the increase in the quantity and quality of records, the ergodic assumptions can be further relaxed in the modeling of offshore ground motion models (GMMs). This allows accounting for systematic and repeatable source, site, and path effects to further understand the characteristics of offshore ground motion in the Japan Trench region. We developed an offshore ergodic backbone GMM for subduction earthquakes and classified the sites into four categories using horizontal–vertical response spectral ratio to investigate site amplification. The offshore ergodic GMM is applicable for subduction earthquake scenarios with moment magnitudes ranging from 4.0 to 7.4 and rupture distances ranging from 10 to 300 km. Comparing offshore ergodic GMMs with onshore GMMs for subduction earthquakes, we found that offshore GMMs were significantly different from onshore GMMs, especially in the long-period and unburied states. Then a new offshore non-ergodic GMM was developed based on the offshore ergodic GMM. The systematic and repeatable source and site effects were captured by the spatially varying coefficients represented by Gaussian processes, while the systematic and repeatable path effects were captured by cell-specific anelastic attenuation proposed by Dawood and Rodriguez-Marek (2013), calculated with the Cohen-Sutherland computer graphics algorithm. The non-ergodic GMM revealed systematic and repeatable source, site, and path effects that were not captured by the ergodic GMM. Moreover, the non-ergodic GMM showed reduced aleatory variability and epistemic uncertainty on ground motion estimation compared to ergodic GMM. The reduction of aleatory variability and epistemic uncertainty had a significant impact on probabilistic seismic hazard analysis. Quantifications of these results are contributed to conduct reasonable seismic design and seismic risk assessment for marine engineering in offshore regions vulnerable to strong subduction earthquakes. [ABSTRACT FROM AUTHOR]

Probabilistic seismic hazard analysis (PSHA) is carried out during the seismic design of structures, and a seismic load corresponding to a certain return period is calculated. In PSHA, there are cases where a renewal process is adopted for some large specific earthquakes. When the probability of the occurrence of an earthquake increases within a certain period, the seismic load based on PSHA also increases. Here, we study the temporal change of the seismic hazard and the influence of the epistemic uncertainty on the occurrence probability of a large earthquake around Japan. When calculating the occurrence probability of earthquakes around the Nankai Trough and the Sagami Trough, the occurrence history is not clearly known, hence, a large epistemic uncertainty arises about the parameters of the Brownian Passage Time (BPT) distribution, such as the average recurrence interval, and the aperiodicity parameter alpha. In order to consider such uncertainty, we employ the Monte Carlo method with a large number of input parameters to grasp the temporal change of the occurrence probability. In addition, the calculated occurrence probability is incorporated into the seismic hazard analysis. Here, we conduct a risk assessment for sample sites and discuss the notion that the influence of the epistemic uncertainty on seismic loads varies greatly from one region to another. [ABSTRACT FROM AUTHOR]

Ground motion prediction equations (GMPEs) have a major impact on seismic hazard estimates, because they control the predicted amplitudes of ground shaking. The prediction of ground-motion amplitudes due to mega-thrust earthquakes in subduction zones has been hampered by a paucity of empirical ground-motion data for the very large magnitudes (moment magnitude ( M) $$>$$ 7) of most interest to hazard analysis. Recent data from Tohoku M9.0 2011 earthquake are important in this regard, as this is the largest well-recorded subduction event, and the only such event with sufficient data to enable a clear separation of the overall source, path and site effects. In this study, we use strong-ground-motion records from the M9 Tohoku event to derive an event-specific GMPE. We then extend this M9 GMPE to represent the shaking from other M $$>$$ 7 interface events in Japan by adjusting the source term. We focus on events in Japan to reduce ambiguity that results when combining data in different regions having different source, path and site effect attributes. Source levels (adjustment factors) for other Japanese events are determined as the average residuals of ground-motions with respect to the Tohoku GMPE, keeping all other coefficients fixed. The mean residuals (source terms) scale most steeply with magnitude at the lower frequencies; this is in accord with expectations based on overall source-scaling concepts. Interpolating source terms over the magnitude range of 7.0-9.0, we produce a GMPE for large interface events of M7- M9, for NEHRP B/C boundary site conditions (time-averaged shear-wave velocity of 760 m/s over the top 30 m) in both fore-arc and back-arc regions of Japan. We show how these equations may be adjusted to account for the deeper soil profiles (for the same value of $$\hbox {V}_\mathrm{S30})$$ in western North America. The proposed GMPE predicts lower motions at very long periods, higher motions at short periods, and similar motions at intermediate periods, relative to the simulation-based GMPE model of Atkinson and Macias () for the Cascadia subduction zone. [ABSTRACT FROM AUTHOR]