Releveling Evidence for Crustal Deformation in the United States

Abstract We have reduced and analyzed over 90% of existing U.S. National Geodetic Survey releveling observations, in order to evaluate the implications of these measurements for contemporary crustal movements. The data are displayed both in profile form (movement versus distance along route) and, where adequate coverage is available, as maps of apparent elevation change. Our analysis involves: 1. (1) evaluating the reliability of the leveling measurements using modified geodetic checks as well as through comparison with independent measurements of crustal movement (e.g., tide and lake level measurements, seismological evidence, tilt and strain observations); 2. (2) interpreting what appear to be real movements in light of other geological and geophysical information on crustal dynamics. Empirical analysis of the U.S. releveling data base indicates that apparent tilts of less than 3 microradians are pervasive and must be regarded as typical signal levels and/or as typical levels of systematic error. Systematic leveling errors are suggested in some cases by close correlations between apparent elevation change and topography, disagreements between leveling and tide gauge measurements, and internal inconsistencies in leveling circuits. Apparent movements which correlate with topography are observed in many areas of the U.S. Such apparent movements often reach 35 mm per 100 m of relief. Disagreements between leveling and tide gauge measurements along the east and west coasts and internal inconsistencies in leveling circuits suggest errors reaching 1 mm/km which can accumulate monotonically for distances exceeding 1000 km. Regional maps of apparent elevation change for the Eastern U.S. show serious inconsistencies with similar maps for adjacent portions of Canada suggesting that such maps may be more indicative of systematic leveling errors than real crustal movements. The largest signals (apparent tilts greater than 4.5 microradians) are for the most part associated with: 1. (1) coseismic and postseismic movements of major ( M >6) dip-slip earthquakes, 2. (2) regions of contemporary magmatic activity, 3. (3) subsidence due to fluid withdrawal, 4. (4) topography-correlated apparent movements. Coseismic movements reported for eleven U.S. earthquakes are roughly consistent with simple models of elastic rebound. At least some postseismic movements appear consistent with after-slip on the fault or an extension of the fault that ruptured during the earthquake, although other explanations have been proposed (e.g., viscoelastic relaxation of the lithosphere-asthenosphere). Deformations possibly associated with contemporary magmatic processes are not restricted to volcanically active areas (e.g. Hawaii) having been observed in the Rio Grande rift and in Yellowstone National Park as well. Subsidence due to fluid withdrawal, well known in areas of intense pumping, is considerably more widespread than previously reported occuring throughout much of the Atlantic and Gulf coastal plains, along the Mississippi Valley and within many smaller sedimentary basins which have been exploited for groundwater. The association of many of the largest signals with real surface movements further demonstrates the ability of historic leveling to detect relatively subtle deformation.