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How to Calculate Average Vs and Vs30m

The average shear-wave velocity (Vs) for the upper 30-m depth is denoted as Vs30m (or Vs-30, Vs-30m, etc.).  This is calculated from a Vs measurement presented in various

Because of the gravitational influence, the property of ground materials is usually presented by, so-called, a "
layered-earth-model" in which the earth's properties change only
vertically and are represented by a collection of distinctive layers.  Each layer is then considered a homogeneous material with the same seismic properties in S and P waves
(Vs and Vp), and density.  In addition, because of the more rapid property change at shallower depths, the thickness of each layer in a layered-earth-model tends to be smaller
at the top, and increases with depth.

Vs measurements are also presented by a layered format as graphically illustrated below and also in
some field examples of Vs30m.  Calculation of the average Vs for a
certain depth range (for example, top 30 m) can be accomplished in two different ways:  (1) based on relative thickness-contribution of each layer (Method 1 in the illustration
below), and (2) based on the definition of velocity ─ total thickness (∑di) divided by total travel time (∑ti) that is calculated by summation of thickness (di) divided by velocity (Vsi)
of each layer (Method 2 in the illustration).  Both methods can yield significantly different results for the same Vs profile as illustrated below by using a simple 2-layer Vs profile.  
Vs30m as defined in
International Building Code (IBC 2000 and later editions) uses the second method (2):

Vs30m = ∑di / ∑ti  = 30 / ∑(di/Vsi)