Vs30 is the time-averaged shear wave velocity of the upper 30 meters of soil or rock at a site, used as the primary parameter for site classification in modern building codes and ground motion prediction equations. It is expressed in m/s and correlates strongly with site amplification.
Vs30 is defined as the ratio of 30 meters to the total travel time of a shear wave through the upper 30 meters of the soil or rock profile. It is a weighted harmonic average of shear wave velocity with depth, and it serves as a single parameter that captures much of the site's dynamic stiffness characteristics. Formally, Vs30 = 30 / Ī£(dįµ¢ / Vsįµ¢), where dįµ¢ and Vsįµ¢ are the thickness and shear wave velocity of each layer within the top 30 meters.
Vs30 became the standard site parameter in the 1990s, when the NEHRP provisions and later ASCE 7 adopted site classification based on this value. The scheme assigns sites to classes A (hard rock, Vs30 > 1500 m/s), B (rock, 760ā1500 m/s), C (very dense soil and soft rock, 360ā760 m/s), D (stiff soil, 180ā360 m/s), and E (soft soil, < 180 m/s), with F reserved for special soils requiring site-specific analysis. Eurocode 8 uses similar classes (A through E) with slightly different boundaries. The Iranian Standard 2800 uses four site classes (1 through 4) with its own thresholds.
Vs30 is not without limitations. It collapses a complex velocity profile into a single number, losing information about layer thicknesses, impedance contrasts, and deep structure ā all of which can affect site response. For sites with strong velocity inversions, deep soft deposits (like Mexico City), or shallow bedrock overlain by stiff soil, Vs30 alone may not adequately characterize amplification. Recent practice increasingly supplements Vs30 with additional parameters such as the depth to bedrock (Z1.0, Z2.5) or the site's fundamental period (T0). Vs30 is typically measured via invasive methods (downhole, crosshole, suspension logging) or non-invasive methods (MASW, SASW, microtremor array, HVSR). Where direct measurements are unavailable, Vs30 is estimated from proxies such as topographic slope, geological maps, or SPT/CPT correlations.