SNARF Working Group - Report of the First SNARF Workshop
Underlying Models: What is Useful, and what is Possible?
Intraplate deformation occurs across North America on a variety of timescales. Removing accurate predictions generated from geophysical models of the deformational processes will allow us to obtain a more reliable realization of SNARF. On daily to annual timescales, atmospheric and hydrological variations cause changes in vertical station positions on the order of several millimeters. Vertical motions from atmospheric variations can be removed by using pressure data from models developed by the U.S. National Centers for Environmental Prediction (NCEP) and European Center for Medium-Range Weather Forecasts (ECMWF) combined with software based on the elastic response of an elastic Earth model. Similar, position changes cause by water loading can be removed with using hydrological data sets (e.g., the Land Dynamics model), which will benefit from additional constraints obtained from the Gravity Recovery and Climate Experiment (GRACE) satellite mission.
A significant contribution to the secular signal across North America is the continued recovery from loading effects caused by the Late Pleistocene ice sheets (glacial isostatic adjustment or GIA). The crustal motions caused by this process are quite large (horizontal motions on the order of 1 mm/yr across the plate and vertical motions of over 1 cm/yr near the loading centers). Unfortunately, both the Late Pleistocene ice history and the viscosity model of the solid Earth remain uncertain. The range of model predictions caused by reasonable variations in these inputs can be as large as the signals. Thus, the use of predictions from a single model at this initial stage is unreasonable. However, we propose an iterative process to determine the best model for use with SNARF. Initially, we will generate a range of model predictions using both ICE-I (a history based on geological data alone) and a suite of more recent models constrained, in part, from geophysical data sets (e.g., ICE-3G) as well as viscosity models that include a range of lithospheric thicknesses and upper and lower mantle viscosities. These model results will be examined to determine if there are regions were the predictions are either consistent or small. If such regions exist, they could form a basis for site selection criteria. Once these sites are used to determine an initial realization of SNARF, motions from a larger set of North American stations could then be considered to further constrain the GIA models.
As with all model data, it is important to remember the reference frame in which the predictions are calculated. Most modeling uses loading theory developed in Farrell (1972), which calculates results in a center of mass of the solid Earth frame. Results from all models should be represented in a reference frame that is consistent with the observational data.
Last modified: 2019-12-24 02:12:54 America/Denver