EXPERIMENTAL INVESTIGATION OF MONUMENT STABILITY IN THE NEW MADRID SEISMIC ZONE

MATTIOLI, G.S., JASMA, J., and DAVIS, C.Y., Department of Geosciences, University of Arkansas, Fayetteville, AR 72701, and DAVIS, J., and SMALLEY, R., Center for Earthquake Research and Information, University of Memphis, Memphis, TN 38152, mattioli@uark.edu.

Smalley et al. (2005) recently proposed that strain rates based on GPS observations from the GAMA network are well above background and may be as high as plate boundary zones. Calais et al. (2005) have argued that the GAMA data are still too uncertain and provide only an upper bound on strain accumulation in the NMSZ. Strain rate may yet be zero. Errors from monument instability and other effects may mask any small tectonic signal. Spatial averaging can reduce the size of both white and random-walk noise, but it does not mitigate their relative effects on strain accumulation models. Long-term correlations thus can have a large effect on estimating deformation rates.

Because several years may be required to obtain accurate site velocity estimates from GPS data time series in areas of small strain such as the NMSZ, a variety of errors with different timescales may corrupt the data, and the errors may change with time. Although averaging can minimize white noise, these methods are less useful for time-correlated effects. Velocity estimates that incorporate only white noise underestimate uncertainty. Regionally-correlated noise is reduced by subtracting common mode signals from GPS time series. Monument behavior is not well constrained for different types from various geologic settings. Monument motion is likely significant in the NMSZ, which is dominated by unconsolidated sediments of the Lower Mississippi Valley. We have installed two new cGPS sites at existing GAMA sites to analyze monument stability and noise. Our new sites use SCIGN drilled, deep-braced monuments, precision levels, and radomes, but in every other way are identical to GAMA sites ~10 m away. Our experimental purpose is twofold: 1) to assess quantitatively monument motion in different geological substrates; and 2) to evaluate the suitability of different monument types in low strain environments.