Effect of various factors on spectral values
Soil Conditions

Plot of N-H spectra for different soils

For soft soils, ag remains the same or decreases relative to firm soil, but vg and dg increase, generally.

Layers of soft clay, such as the Young Bay Mud found in the San Francisco Bay area, can also act as a filter, and will amplify motion at the period close to the natural period of the soil deposit.

Layers of deep, stiff clay can also have a large effect on site response. For more information on site effects, see Geotechnical Earthquake Engineering by Kramer.