Why are there earthquakes but so few Quaternary faults east of the Rocky Mountains?
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
