Peak ground acceleration underestimates narrow-band seismic amplification on mountain slopes

Topographic amplification is widely invoked to explain strong shaking and slope failure in mountainous terrain, yet direct field observations remain limited. Here we analyze 24 regional earthquakes recorded by a four-station strong-motion array spanning a ridge crest, a valley bottom, and two contrasting slope facets on a single ridge in central Japan. Peak ground acceleration (PGA) and frequency-dependent measures reveal markedly different representations of local seismic response. PGA ratios vary among events and are positively associated with epicentral distance; however, earthquake magnitude also increases with distance, so source-size and propagation-distance effects cannot be separated in this catalog. In contrast, the NS slope exhibits a persistent narrow-band station-to-valley Fourier spectral-ratio peak at 3.5–4.0 Hz that remains stable across events and processing choices. Absolute spectra confirm enhanced energy at the slope in this band, while also showing that a relative spectral depression at the valley reference contributes to the large ratio magnitude. Most importantly, the geometric-mean 5%-damped response-spectral ratio reaches 6.67 at 0.277 s (3.61 Hz), compared with a corresponding full-record geometric-mean horizontal PGA ratio of 1.13. These observations show that PGA can substantially under-represent frequency-specific elastic response on mountain slopes, while the responsible mechanism cannot be uniquely attributed to topography or near-surface structure alone.