Nonlinear Site Response at a Soft Port Site: Vertical-Array Constraints for California's Liquefaction-Susceptible Waterfronts

Kenneth S. Hudson

Submitted August 30, 2026, SCEC Contribution #15648, 2026 SCEC Annual Meeting Poster #232

Nonlinear site response remains among the least constrained contributors to ground-motion uncertainty in California, and it matters most precisely where records are scarcest: soft, saturated, liquefaction-susceptible profiles such as the hydraulic fills beneath the Ports of Los Angeles and Long Beach. Vertical arrays capturing strong shaking at such sites are rare, so we exploit a well-instrumented analog — the Port and Airport Research Institute (PARI) array at Yatsushiro Port, Kumamoto, Japan, pairing a surface accelerometer with a downhole sensor at 34.43 m in a profile with Vs30 = 169 m/s, water table near 2–3 m, SPT N below 10 through most of the upper 30 m, and a velocity inversion at 24–29 m where N falls to 1–3.

The record inverts expected soft-site behavior. Linear 1D theory for the logged profile predicts resonance at 1.5 Hz with amplification near 30×; observed surface/downhole ratios instead peak at 0.46 Hz with amplification of 3, fall below unity from 1.5 to 15 Hz, and yield PGA ratios of 1.35 (N) and 0.83 (E). Response spectra show deamplification at 0.1–0.3 s and 2.4× amplification near 2.4 s.

Two independent, model-free measurements localize the transition. Sliding-window cross-correlation of the sensor pair gives apparent time-averaged Vs falling from 150 to 51 m/s — 28% of the 181 m/s borehole value — within 15 s, recovering only to ~100 m/s over the following two minutes. Stockwell transforms, ratioed between surface and downhole to remove the input motion, place the collapse of 4–15 Hz transmission at 3.6 s (N) and 3.8 s (E), roughly one second before peak acceleration, with the spectral centroid dropping from 6–7 Hz to 2.2 Hz.

Key Words
Nonlinear Site Response, Vertical Array, Liquefaction, Soft Soils, Ground Motion Amplification

Citation
Hudson, K. S. (2026, 08). Nonlinear Site Response at a Soft Port Site: Vertical-Array Constraints for California's Liquefaction-Susceptible Waterfronts. Poster Presentation at 2026 SCEC Annual Meeting.

Related Projects & Working Groups
Ground Motions (GM)