The effects of an earthquake will be dependent upon a number of variables, including the strength and depth of the quake, soil characteristics, the distance from the epicenter, and type of building construction.
In St. Charles County, we have seen over the years that a smaller earthquake, such as a 4.3 can be felt by those who live close to the rivers, in the floodplain, and not felt by anyone else. This is due to the geological makeup of the ground nearest the river; more sand and more shallow water tables.
When earthquake vibrations pass through soil, which has a high liquid water content, the soil loses the properties of a solid and takes on those of a semi-liquid, like quicksand or pudding. This process is called liquefaction. Building foundations can lose the support of the soil, and they may shift, settle, or sustain more damage than buildings in areas not subject to liquefaction. Further west, the soil composition contains more clay and bedrock.
Better Built Buildings
Over the years, as more is learned about earthquakes and their effects, building codes have become stricter and the use of seismic engineering more prevalent. Structures built in compliance with stricter codes will have a better chance of riding out an earthquake; however, this does not mean that it will not sustain any damage. Certainly the older the structure, the more likely it could see significant damage. But this area has not experienced a major earthquake since 1811-1812.