Site Effects

Seismic waves are influenced by local geological conditions. These variations are known as site effects, and they can strongly influence the nature and severity of shaking at a given site. Site effects include the following factors:

The elastic properties of Earth materials range from stiff (i.e., difficult to deform, such as granite) to compliant (easy to deform, such as loosely packed sand). Seismic waves travel faster through stiffer rocks than through more compliant rocks and sediments, and rocks are more compliant near the Earth’s surface. As the waves pass from deep (stiff) rocks to shallow (more compliant) rocks they slow down and get larger in amplitude as the energy accumulates. Softer, more compliant soils amplify ground motion. 

The geometry of the compliant rock or sediments can further distort ground motion at soft rock sites. Seismic waves entering sediment-filled valleys with thick layers of compliant materials, such as the Duwamish River Valley, can trap seismic energy such that it reverberates like sound in an echo chamber. This can lead to both higher amplitudes and longer durations of shaking. Because such effects are geometric in nature, they depend on the characteristics of the incoming wave, and its direction of approach. Site effects of this type can be very difficult to predict.

This factor is similar to the previous one, but works at a broader scale. The deeper geologic structure impacts lower frequency seismic waves, and can have more widespread effects. It can also influence the shaking experienced in large structures that tend to be more sensitive to low-frequency motions. For example, high rise buildings are more likely to experience severe damage by low-frequency waves, as low-frequency waves will match the natural resonance of a tall building, causing it to sway more.

Deep sedimentary basins can have a large effect on ground motion above them. Earthquake waves traveling at high velocity through the stiff, crystalline rock of the crust refract and slow dramatically when entering a sedimentary basin. This increases the amplitude of the earthquake waves, and the sharp density contrast of the soft basin rocks with surrounding material can cause waves to reflect, trapping energy in the basin for a period of time. This extends the duration of shaking. Due to their size, these deep geologic structures can influence shaking over a wide area. 

Ground failure can be as spectacular as a large landslide or much more subtle, such as when sub-surface liquefied soils lead to differential settlement of a building. In either case, the results of both types of failure can lead to large losses.

For more information about ground failure during an earthquake, see: Liquefaction and Landslides.

The topographic features present at the Earth's surface also influence shaking intensity by focusing seismic wave energy in various ways. Studies of the distribution of the intensity of shaking experienced in an earthquake have concluded that hilltop sites often shake at an intensity level higher than nearby sites with flatter topography.

The following video provides some additional context on how site conditions can impact the strength of seismic waves during an earthquake.