Deformation in the New Madrid Seismic Zone from Continuous GPS Geodesy

A key question for the cause of the New Madrid earthquakes is "what is driving the earthquakes?" In order to address this question, several ongoing GPS based geodetic projects are being performed in the New Madrid seismic zone. Initial results from campaign style GPS networks in the New Madrid area reported deformation rates suggestive of plate boundary zones, but refined estimates of the deformation rate determined after additional GPS campaigns, are much lower and may not be statistically distinguishable from zero. We report here on the preliminary results from the GPS array for Mid-America (GAMA), an 11 station, continuous GPS network that began operation four years ago and has been in full operation for the last two years. To establish a reference frame for North America, we processed over four years of continuous GPS data from a set of stations in cratonic North America. These sites have a mean residual rms motion with respect to a "stable" North America of slightly less than 1 mm/yr, which is in agreement with previous GPS determinations of the stability of North America. This defines our lower limit for detection of deformation with respect to North America. The mean residual rms motion of the GAMA sites, with respect to the reference frame, is about 1.5 to 2 mm/yr, which is near the limit of detectablility for 2 years of data. The results from the continuous GPS network contributes to a better identification of zones with possible crustal strain accumulation as well as improving our ability to constrain geodynamic models that have been proposed for the New Madrid seismic zone.