Site Classifications

The International Building Code (IBC) has developed seismic site classifications based on the following criteria.

Seismic Site Classifications from the IBC.

Site classifications E-A are established based on shear wave velocity ranges. The average shear-wave velocity for the top 30 meters of the subsurface is the value of importance for IBS classificaiton. How a site reacts geologically while transmitting compressional, shear, or surface waves is of importance for understanding earthquake hazards.

Ambient noise tomography methods, a seismic subsection of multi-channel analysis of surface waves (MASW), is one of the easiest methods to establish a site classification for a location. Typically, for a passive surface wave survey, the distance between geophones is also your penetration depth. A well-designed shear-wave velocity (AVS) survey will have a variety and consistently spaced number of geophone spacings, from 1 to 50+ meters. Depending on the characteristics of the site, this will provide data for the top 30-80m. In some instances where low frequencies wavelengths resonate well, data up to 250 meters in depth can be achieved with a 50m array.

To acquire data deeper than 30 meters, geophones must be spaced further apart. Again survey size and penetration depth is most often 1:1.

Site Class A Vs > 1500 m/s

The NVSP has yet to encounter a site class with an AVS30 > 1500 m/s within the Napa Valley. Only hard rock like granite, gneiss, shale have these shear wave velocity speeds. It is suspected that some sites in the Vaca Mountains and Berryessa region might be confirmed as site class A through passive-seismic methods.

Site Class B 760 < Vs < 1500 m/s

N60 phase velocity dispersion curve and coherencies

Rock at the surface of a site class B is common, though often a shallow layer of alluvium is on top. The rock making up a site class C could be wide ranging and potentially weathered and/or fractured. Many rock types have shear wave velocities in this range, so local reconnaissance like outcroppings and vegetation will provide clues.

A standard house will react to a earthquake on a site class B much better vs how it will fare situated on a site class D.

The effect depends on the magnitude of the earthquake, but a site class B being much stiffer means a building will experience a much small fraction of the energy released from an earthquake that a structure on a site class D would.

Site Class C 360 < Vs < 760 m/s

N27 phase velocity dispersion curve and coherencies

A site class C will fare well with an earthquake, depending on its magnitude and proximity to the epicenter. The ground is relatively stiff, at the interchange between soft rock and stiff soil. You can see here that the phase velocity drops rapidly after 10 Hz, and settles at a low 200 m/s.

An AVS30 is the average shear wave velocity of the top 30 meters, but passive seismic methods are able to penetrate much deeper with proper field methods and good survey geometry.

If conducting a site survey for a site assessment, it is smart to collect data to depths 50-100 m. As site regulations become stricter and more comprehensive, a deeper initial survey will future-proof the dataset compared to a strict AVS30 survey.

Site Class D 180 < Vs < 360 m/s

N39 phase velocity dispersion curve and coherencies

The bottom of a valley or small basin will gather alluvium, and if developed enough, this soil will take on some structural integrity. Compared to a site class C-A, a site class D is quite weak at smoothly propagating shear waves. Instead, the looser medium will shake and liquefy to a greater extend with shear and surface waves.

Since a site class D isn’t situated too far away from more stable bedrock, the phase velocity will typically start to notably increase with decreasing frequency (aka increased depth), as data is observed for deeper geologic layers.

This bedrock layer is typically beyond the data range of an AVS30, and as such a deeper passive-seismic investigation is recommend in order to more fully understand the geologic characteristics of the subsurface.

Site Class E Vs < 180 m/s

Site Class E phase velocity dispersion curve and coherencies

A site class E represents very weak soil. There is little structural integrity with a site class E, and a location with geological qualities like this will fare very poorly with an earthquake. Additionally, structural components build out of these materials, like a levee, are liable to fail during a severe weather event or catastrophe like a hurricane, tsunami, or tornado.

This data was collected in New Orleans using 6 Atom-1C Seismographs. Atoms are nodal, so three separate arrays were setup and the dispersion curves from each were combined to create the velocity model to the right.

Site classifications are important, but there is important geologic information that lies beyond the top 30 meters of the subsurface.

SeisImager AVS30+ Database

See AVS30 values from around the world, powered by

Seismic Site Classification Webinar

To learn more about IBC geologic site classifications, watch the below video from our friends at Olsen Engineering.