A COMPARISON OF ISOSTATIC CORRECTION TECHNIQUES APPLIED TO WESTERN UNITED STATES TOPOGRAPHY

Crustal density estimates can be derived from isostatically corrected gravity anomalies. The accuracy of the anomaly data, as well as the use of geologic and geophysical constraints, is essential to the production of a reasonable density model, especially when small geographical areas are being considered.

To insure the greatest accuracy possible in the isostatic anomalies, we performed a sensitivity study of various isostatic compensation techniques. We used various techniques to calculate isostatic correction values for the western United States and compared the values to each other, with the goal of evaluating if the correction methods yield significantly different results (correction values).

The models used to calculate the isostatic corrections are: the Airy-Heiskanen, Pratt-Hayford, modified Pratt-Hayford, and the flexural isostatic compensation model. The corrections were calculated from the combined GTOPO30 gridded elevation set for the western U.S. and the Digital Bathymetric Database Variable Resolution (DBDB-V) for the eastern Pacific Ocean. All data were regridded to 1-minute resolution for greater processing speed.

Differences in correction values are seen among all model runs and vary with geographic location and elevation. When the correction values from two model runs are compared, differences of 15 mgal (positive or negative) between adjacent geographical areas are not uncommon. Any differences in correction values would translate directly into differences in anomaly values and so into density estimates. Simple calculations show that for a 15 mgal difference between adjacent areas, the resulting difference in the density estimation of a 4 km thick slab, 6 km under the surface is approximately 100 kg m^-3, roughly the difference between a granite and granodiorite.