Faults vs. Fault Lines on a Map
In order to answer this question, we first need to explain some basics about faults. Faults are different from fault lines. A fault is a three-dimensional surface within the planet that might extend up to the surface or might be completely buried. In contrast, a fault line is where the fault cuts the Earth's surface… if indeed it does.
The most prominent faults in each state are usually shown on a state's geologic map as black lines. However, many faults are entirely beneath the surface and do not reach ground level. Therefore, these buried faults have no fault lines, and they are usually not shown on geologic maps. If a buried fault is known at all, information about it is usually published in technical articles in geological journals.
Why are most faults in the Central and Eastern U.S. buried?
The most significant historical earthquakes in the Central and Eastern U.S.--those that potentially did have a fault line at the surface--happened so long ago that any evidence of the fault line has since been obliterated by erosion and decomposition of the rocks and by sedimentary deposits that have covered the surface. The greater humidity in the eastern half of the country causes these processes to happen faster so that evidence of fault lines is erased much faster than in the West.
The New Madrid Seismic Zone
All of these factors contribute to fewer Quaternary faults mapped in the Central and Eastern United States (CEUS). Some of the best evidence of strong prehistoric earthquakes in the CEUS is from liquefaction features (sand boils and dikes) that are forced to the surface by ground shaking. Although liquefaction features can tell us when and where strong earthquakes have occurred, they don’t usually provide information about which fault specifically generated the earthquake. This is the case in many parts of the New Madrid Seismic Zone and other seismic zones in the CEUS.
We know there are faults in those parts of the country because there are earthquakes there, but we don’t know where the pre-existing fault lines were (if there were any), and there hasn’t been a recent earthquake large enough to create a new fault line. Despite these problems and shortcomings, the distribution of historical earthquakes and the geologic evidence of prehistoric earthquakes provide a reasonable guide to the seismic hazard in much of the CEUS. The good news is that much more data has been collected in recent years, particularly in and around the New Madrid Seismic Zone.