Characteristics and controlling factors of seismic site response at a crystalline bedrock site based on quality factor inversion using deep borehole data

This study evaluated epistemic uncertainty in shear-wave velocity (Vs) profiles and variability in shear-wave quality factor (Qs) profiles across broadband frequencies to investigate characteristics and controlling factors of seismic site response at a crystalline bedrock site. This study utilized deep borehole data down to the seismological bedrock at a depth of 2000 m. Multiple invasive measurements were conducted to estimate the Vs profiles. Three-component seismometers were installed at depths of 5, 160, 500, and 2000 m for Qs profiling. A logic tree with 128 analysis patterns was employed to capture the frequency-dependent percentiles of the amplification factor (AF) and the Qs parameter through spectral ratio Qs inversions. AFs from depths of 2000 m to 5 m showed the attenuation effect was dominant in the frequency range above 1 Hz. Sensitivity analysis revealed the dominant effects of the Qs profile below 2 Hz and above 15 Hz and the significant effects of the Vs profile at 2-15 Hz. The median AF curves were robust due to multiple invasive measurements and constraints from reliable seismic records. Local site responses captured from the 2000- m borehole station are expected to contribute to seismic hazard assessments as a reference of crystalline bedrock sites.