Billings sits atop the Yellowstone River's alluvial terraces, where unconsolidated sands and silts deposited during the Holocene epoch dominate the subsurface profile. A shallow groundwater table—often encountered between 2 and 7 meters depth across the Heights and South Side—creates conditions where saturated granular soils are susceptible to cyclic mobility during a seismic event. The 1935–36 Helena earthquake sequence, though centered 160 miles west, produced Modified Mercalli Intensity IV shaking in Billings, a reminder that intermountain seismicity can propagate through Montana's crystalline basement rock. For design firms and site developers working under the current IBC and ASCE 7-22 provisions, a site-specific CPT test often supplements standard penetration testing to refine the fines content correction and obtain a continuous stratigraphic profile before calculating the factor of safety against liquefaction.
A single SPT blow count below 15 in a saturated sand lens beneath the Billings benchlands can trigger a mandatory liquefaction mitigation review under IBC 2024 Chapter 18.
