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 fault cuts 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 are everywhere, of all sizes, 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 US buried?

The answer is partly related to recognizing Quaternary faults and partly related to differences in geologic conditions in the areas east of and west of the Rocky Mountains.

First, 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.

Second, the types and ages of strata and deposits at the surface in the CEUS are commonly different from those from the Rocky Mountains west.  In late Quaternary time, large parts the CEUS were covered by massive continental glaciers, which buried the landscape with glacial till (coarse glacial debris) and outwash deposits (glacial debris transported by water), or by wind-blown deposits (silty loess or eolian sand).  Many of these deposits are geologically very young, typically <15,000 years old.  These young deposits can bury and conceal evidence of Quaternary fault movement that is older than these deposits.

Third, generally the rate of movement on CEUS faults is significantly less than that on faults in the western U.S.  Slower rates of deformation mean that the evidence of Quaternary faulting will be subtler and is therefore, more likely to be missed, destroyed by erosion, or concealed by burial compared to areas having higher deformation rates.

All of these factors contribute 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), which form as poorly consolidated, water-saturated sediment is 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, individual known faults and fault lines are unreliable guides to the likelihood of earthquakes.  In California, a large earthquake can generally be associated with a particular fault because we have watched the fault break and offset the ground surface during the earthquake.  In contrast, east of the Rockies things are less straightforward, 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.

Accordingly, the best guide to earthquake hazard east of the Rockies is probably the earthquakes themselves.  This doesn't mean that future earthquakes will occur exactly where past ones did, although that can happen.  It means that future earthquakes are most likely to occur in the same general regions that had past earthquakes.  Some future earthquakes are likely to occur far from past ones, in areas that have had few or no past earthquakes.  However, these surprises are not too common.  Most earthquakes tend to occur in the same general regions that are already known to have earthquakes.  Even if we could pin an individual earthquake on an individual fault, that would still be only part of the answer we want.  There is, in general, no reliable way to know where or when the NEXT damaging earthquake will occur, and that is the earthquake that is of the greatest interest to society.


Sources of Information

  • State Geological Surveys or State Geologists - Most state geological surveys sell copies of the geologic maps of their states. Many state surveys also have booklets on the earthquakes of their state, as well as lots of other information on their state's faults, rocks, minerals, general and environmental geology, natural resources, and natural and environmental hazards.
  • Seismicity of the Earth Maps and Information by State/Territory
  • U.S. Seismic Hazard Maps - Every few years the USGS produces a national map of seismic hazards. These maps are probably the best single reference for earthquake hazard in a particular state, but it is technical in nature because it is aimed at engineers who design buildings and other structures. Of course, no one can predict the future, and earthquake prediction remains beyond our present knowledge, so these maps express the hazard as probabilities.


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