Tilt from releveling: An overview of the U.S. data base

Abstract Because of existing uncertainties as to the influence of errors and non-tectonic movements on releveling estimates of vertical crustal movement, we have taken an empirical approach to investigate some characteristics of the U.S. releveling data base as a whole. Our primary purpose is to establish typical magnitudes of apparent tilt which can serve as a basis for defining anomalous apparent tilts (the term “apparent” is used since differences between levelings can result from errors as well as surface movement). Our analysis is sensitive to apparent tilts with characteristic lengths (i.e. lengths over which tilt is coherent) of 10 to about 70 km, a range which is relevant to many tectonic processes. The apparent tilts developed over a given characteristic length tend to decline as the characteristic length increases. That is the “apparent tilt spectrum” has most “energy” at small spatial wavelengths. For the entire data base the typical magnitudes of apparent tilt for characteristic lengths between 20 and 30 km and between 60 and 70 km are 2.0 and 0.7 μrad respectively. These values are approximately three times larger than the magnitude of apparent tilt expected from random errors. The typical magnitude of apparent tilt averaged over all lengths in the western U.S. (∼- 2.6 μrad) is about 1 μrad higher than that in the east (∼- 1.6 μrad). The largest observed apparent tilts for the entire data base occur along approximately flat routes. For the most part, these large apparent tilts result from real surface movement and are directly associated with sediment compaction due to water withdrawal in flat lying sedimentary areas. While a number of tectonically active areas are characterized by high average apparent tilts, the average magnitude of apparent tilt in releveling data defining the southern California uplift, a feature of much current debate, is not significantly different from that for the entire western U.S. This analysis emphasizes that real surface movements (tectonic and nontectonic) are a significant factor contributing to the high magnitude of apparent tilt relative to random error estimates.