Estimation of weak and strong ground motions based on diffuse field concept for earthquake for steps 2 and 3 of blind prediction exercise

For the accurate prediction of earthquake ground motions, the investigation of three elements in its process, namely the source, path, and site effects, is crucial, and their accuracy and reliability should be evaluated. The Committee of the 6th Effect of Surface Geology on Seismic Motion Symposium conducted a blind prediction exercise to investigate the accuracy and reliability of estimating the subsurface velocity structure related to the site effect and for predicting weak and strong ground motions. We participated in a blind prediction exercise and estimated the weak and strong ground motions that occurred during the 2016 Kumamoto earthquake sequence. First, the velocity structures at a target site KUMA and our reference site EEB were identified based on the diffuse field concept for earthquake (DFCe). The identified velocity structures at KUMA and EEB reproduced the observed horizontal-to-vertical spectral ratios of earthquakes well and were considered to be able to reproduce the horizontal and vertical site amplification factors based on DFCe. Second, the incident waves at the seismic bedrock of EEB were estimated. For weak and strong ground motions, the spectral amplitudes of the three components of the incident waves were estimated from the vertical motions observed on the surface at EEB assuming the generation of a diffuse field for a single earthquake record and the linearity of the vertical amplification factor, even during strong shaking. Finally, the ground motions on the surface at KUMA were estimated using the incident waves estimated at the seismic bedrock, the inverted velocity structure, and the dynamic deformation characteristics distributed by the organizer of the blind prediction exercise for shallow soil sediments. The spectral amplitudes of the estimated strong ground motions were smaller at around 1 Hz and larger at around 10 Hz than the observed ones. However, the estimated overall spectral levels of all components corresponded to the observed ones. Although several parameters require further research, such as the damping factor of soil sediments, we have shown that the proposed method can estimate the overall seismic ground motions at a target site in the blind. Graphical Abstract