ASCE 7-22 Chapter 20 requires a measured VS30 for any Stockton project seeking a site-specific seismic design category. The default assumption of Site Class D often overestimates the hazard on the deep, soft alluvial deposits of the eastern San Joaquin Valley. Stockton sits at roughly 13 feet above sea level, with Quaternary basin fill extending hundreds of feet down before reaching competent bedrock. This means a generic code approach can miss the amplification effects that saturated sands and silts produce during a long-duration event on the San Andreas or Hayward faults. Our team runs active-source MASW surveys with 24-channel seismographs and 4.5 Hz geophones, pairing the dispersion analysis with passive microtremor arrays when depth targets exceed 30 meters. The result is a VS30 value that reflects actual subsurface stiffness, not a conservative placeholder. For sites near the Deep Water Ship Channel or the Calaveras River, we often combine the seismic refraction profile to map the water table interface, which strongly influences the shallow velocity structure. The downhole seismic method provides a complementary check when borings are already part of the exploration program.
A measured VS30 of 175 m/s in south Stockton can double the seismic base shear compared to the Site Class D default of 260 m/s — the code penalty is real.
