Spatial distribution of ground motion during the 2018 Northern Osaka earthquake (Mw5.6) based on a microtremor survey in Ibaraki city

Abstract

The 2018 Northern Osaka earthquake (Mw5.6) caused residential damages in the Ibaraki and Takatsuki cities in Osaka. However, the distribution of damage was not uniform and there were locally damaged areas. To clarify the cause of this damage distribution, seismic records are pulled back to the engineering basement by using the velocity models evaluated from the microtremor surveys, and the ground motion distribution in Ibaraki city is calculated. The velocity models are evaluated by performing microtremor array observations in the vicinity of the stations where the seismic records are available. Phase velocity dispersion curves are estimated from the microtremor array observation records, and an initial velocity model is established to explain the phase velocity as well as the HVSR and nearby borehole data. A genetic algorithm is, then, used to optimize the model to fit the phase velocity more precisely. Using this model, the input motions on the engineering basement are estimated from the records at each station. The peak velocities at the engineering basement in the locally damaged areas are larger than those in the other areas. Since the effect of site amplification near the surface is excluded, it may represent source-induced differences such as forward directivity. A S-wave velocity model derived from borehole data is used to map the distribution of peak ground velocities in Ibaraki city. The results show a good agreement with the distribution of the residential damages. The amplification factors from the engineering basement to the ground surface are also greater in the locally damaged areas than in the other areas. This implies that the cause of the damage in the local areas was due to the combined effects of the forward directivity and the site amplifications.