The effects of an earthquake will be dependant upon where you live. 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. The foundations of heavy buildings suddenly lose the support of the soil, and they may topple, or settle deeper into the Earth.
Further west is more solid with clay and bedrock.
Within the past 15 years building codes have become more strict. Those buildings built within this period 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 as we all know this area has not experienced a major earthquake since 1811-1812. Felt indoors by few, especially on upper floors, or by sensitive or nervous persons.