Seismic waves travel faster through hard rock than through softer rock and sediments. As the waves pass from harder to softer rocks, the waves slow down and their amplitude increases. Thus shaking tends to be stronger at sites with softer surface layers, where seismic waves move more slowly. Ground motion above an unconsolidated landfill or soft soils can be more than 10 times stronger than at neighboring sites on rock for small ground motions.
The effect of the underlying soil on the local amplification is called the site effect.