Where are the Fault Lines in the United States East of the Rocky Mountains?


This message is intended for people who inquire where the fault lines are in some particular state east of the Rocky Mountains. Most such inquiries are actually stimulated by interest in earthquakes and earthquake hazards instead of geologic faults. Accordingly, what follows is slanted toward earthquakes. I've tried to make the message as non-technical as I can, so that it will be useful to the largest number of people. If you already have some background in geology, you might want to skim the early sections and concentrate on the last section titled Sources of Information.

The next section (Faults East of the Rocky Mountains) tells you where to find maps that show the locations of fault lines east of the Rocky Mountains. However, as I'll explain later (Faults and Earthquakes East of the Rocky Mountains), fault lines are not a particularly reliable guide to earthquakes or earthquake hazards east of the Rockies. The earthquakes themselves are the best guide. The last section (Sources of Information) describes various books, maps, and World Wide Web pages that show earthquake locations and estimates of earthquake hazards. Most of these books and maps should be available in large libraries, particularly libraries of universities that have geology departments. I'll also tell you where to buy copies yourself.

Faults East of the U.S. 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. Any particular block of the Earth has room inside for more small faults than big ones, so most faults are small. 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.

Just as the U.S. government has a U.S. Geological Survey to serve the needs of the entire country, each state has a state geological survey or a state geologist to serve the citizens of the state. Most state geological surveys sell copies of the geologic maps of their states. Prices range from a few dollars to a few tens of dollars, depending on the size of the state and the complexity of its geology. The scales of the state geologic maps vary, and so do the sizes of the maps. Most are brightly colored sheets of paper several feet long and several feet wide. To contact any state geological survey or state geologist, see Sources of Information below.

Faults are typically shown on geologic maps as black lines. Be warned that, east of the Rockies, faults are usually not visible to a person standing on the ground. Partly that's because faulting breaks rocks and tends to grind them into small bits that decompose faster than the nearby, unfaulted rocks. The result is that most fault lines are obscured by thick soils. That's especially the case in the more humid eastern states, because moisture speeds up the decomposition. In the northern states, fault lines and the adjoining rocks are also likely to be covered by sand, clay, or gravel that were left by melting glaciers. Thus, in most places east of the Rockies, the presence of a fault is inferred by a geologist because rocks exposed here and there on both sides of the fault would usually not be found so close together. The geologist infers that the once-distant rocks have been brought close together by vertical or horizontal movement on a fault.

Earthquakes East of the U.S. Rocky Mountains

Earthquakes occur when the two sides of a fault slip suddenly past each other. East of the Rockies, most earthquakes probably occur by new slip on existing faults. The earthquake is generated when the two sides of the slipping part of the fault grind against each other and send out shock waves or vibrations. The vibrations travel through the Earth and, if they are strong enough, they reach ground level, and can shake and damage any man-made structures there. In general, the bigger the patch of the fault that slips, and the more it slips, the bigger the earthquake. Big faults can include big slipping patches and make big earthquakes. As explained earlier under Faults East of the Rocky Mountains, there are more small faults than big ones, and there are more small earthquakes than big ones.

The size of an earthquake is typically measured in two ways: magnitude and intensity.

Most earthquakes east of the Rockies are too small to be felt or to cause damage, and most earthquakes large enough to be felt are still too small to cause damage. Damaging earthquakes east of the Rockies are rare. Of those that do damage buildings or other man-made structures, most cause only slight, localized damage with few injuries. However, the U.S. east of the Rockies is such an enormous region that, every few years, even one of the rare damaging earthquakes can occur somewhere.

Faults and 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. We do know that most earthquake locations cannot be measured very accurately east of the Rockies. Earthquakes typically occur several miles deep within the Earth. Their locations, including their depths, are usually uncertain by a mile or more. Although the larger faults extend from their fault lines downward deep into the Earth, their locations at earthquake depths are usually wholly unknown. The uncertain underground locations of earthquakes and faults make it terrifically hard to determine whether a particular earthquake occurred on a particular known fault. We also know that there are many faults hidden underground that are large enough to generate damaging earthquakes, but which are also too small to extend from earthquake depths all the way up to ground level where we have the best chance of seeing the faults. These hidden faults are likely to be at least as numerous as the faults we know about. Accordingly, 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

Last revised April 30, 2003