Modeling of the subsurface structure from the seismic bedrock to the ground surface for a broadband strong motion evaluation in Kumamoto plain

The 2016 Kumamoto earthquake happened in Masiki town. And that, both of the foreshock [Mjma6.5] on April 14 and mainshock [Mjma7.3] on April 16 measured 7 level on the Japanese intensity scale of 7. The earthquake caused heavy damage to buildings etc. near the faults. On distribution of building damage such as houses, it was found that damage was heavier roughly near the surface earthquake faults which caused the mainshock. On the other hand, although some areas suffered slighter damage despite just above the faults, other ones suffered severer damage despite away from the faults. It is said that these phenomena are likely to be caused by subsurface structure. Therefore, first, in mainly Kumamoto plain near the faults where damage was severe, we collected boring survey data and developed initial geological models. Secondly, we obtained records of microtremor observation and earthquake observation, and then improved layer thickness and S-wave velocity structure of initial geological models. Finally, we modeled subsurface structure from seismic bedrock to ground surface, and then compared our models with building damage distribution and investigated the relation. A certain correlation can be found between damage distribution and S-wave velocity structure one, and predominant period one for subsurface structure models. However, in order to obtain a consistent result with building damage about estimation of damage by means of strong ground-motion simulation with subsurface structure models, non-linear characteristics have to be taken into consideration.