Where are the fault lines in the eastern United States (east of the Rocky Mountains)?

Faults are different from fault lines. A fault is a three-dimensional surface within the planet Earth. At the fault, rocks have broken. The rocks on one side of the fault have moved past the rocks on the other side. In contrast, a fault line is a line that stretches along the ground. The fault line is where the three-dimensional fault intersects the Earth's surface. Faults come in all sizes, from small ones whose short fault lines you can see in a single road cut, to huge faults whose long fault lines can be seen best in pictures taken from orbiting satellites. On continents, faults of all sizes are everywhere, and they formed at many different times during the Earth's long history.

The largest and most important faults in each state are usually shown on the state's geologic map. A geologic map shows the locations of rocks of different kinds and ages. Because the geologic map shows the rocks that are exposed at ground level, the map also shows fault lines. However, many faults are entirely buried 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 Eastern U.S. buried?

The answer is partly related to recognizing Quaternary faults (faults that have been active in the last 1.6 million years) and partly related to differences in geologic conditions east and west of the Rocky Mountains.

  • Many faults are present in the central and eastern U.S. (CEUS), but few of these faults have evidence of being active in Quaternary time. For example, if a fault is present only in pre-Quaternary rocks, then there may be no way to demonstrate Quaternary activity on the fault.
  • The types and ages of strata and deposits at the surface in the CEUS are commonly different from those in and west of the Rocky Mountains. In the last 15,000 years, large parts the CEUS were covered by massive continental glaciers that buried the landscape with glacial or wind-blown deposits. These young deposits can bury and conceal evidence of Quaternary fault movement that is older than these deposits.
  • The rate of movement on CEUS faults is generally significantly less than that on faults in the western U.S. Slower deformation rates mean that the evidence of Quaternary faulting will be subtler and is more likely to be missed, destroyed by erosion, or concealed by burial compared to areas having higher deformation rates.

All of these factors contribute to there being fewer Quaternary faults mapped east of the Rocky Mountains. 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. 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.

Earthquakes East of the U.S. Rocky Mountains

In general, east of the Rockies, known faults and fault lines are unreliable guides to the likelihood of earthquakes. In California, large earthquakes are generally associated with a particular fault because we have watched the fault break and offset the ground surface during the earthquake. In contrast, things are less straightforward east of the Rockies because it is rare for earthquakes to break the ground surface. In particular, east of the Rockies, most known faults and fault lines do not appear to have anything to do with modern earthquakes. We don't know why. An earthquake is as likely to occur on an unknown fault as on a known fault, if not more likely. The result of all this is that fault lines east of the Rockies are unreliable guides to where earthquakes are likely to occur.

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