QUANTIFYING SEISMIC HAZARD IN THE SOUTHERN ROCKY MOUNTAINS THROUGH GPS MEASUREMENTS OF CRUSTAL DEFORMATION

The Colorado Front Range and the Rio Grande Rift have experienced moderate seismic activity in recent times, including a magnitude ~6.5 earthquake in north central Colorado in 1882. Fault-scarp data on Sangre de Cristo Fault Zone in south-central CO suggests past events of magnitude 7 to 7.3. Despite this evidence for past seismic activity, the nature and rates of the tectonic processes responsible for creating this seismic hazard are not well understood. Geologic and primitive geodetic techniques produce conflicting results for the Rio Grande Rift, and no modern deformation rates have been measured for the Front Range area. Knowledge of the nature and orientation of tectonic stress regimes within the southern Rockies will provide a basis for assessing the capability of faults to slip seismically and produce earthquakes. 26 control points of the National Geodetic Survey’s High Accuracy Reference Network were occupied in August and September of 2001. Positions are calculated with respect to four permanent stations operated by the International GPS Service in order to tie into a global reference frame. The best stations show horizontal errors of 1 centimeter (vertical errors are greater due to the troposphere), which will allow 2 mm/yr precision in velocity estimates if repeated after ten years. HARN was established in 1991 for purpose of adjusting the geodetic mapping datum of North America to a space-based standard. Per standard NGS practice, the 1991 measurements consist of relatively short 3-6 hour sessions, resulting in a ~15 cm precision level more than adequate for their purpose of continental scale mapping datum adjustment. The raw data are currently being reprocessed using improved satellite orbits and state-of-the-art point positioning algorithms which may yield results of sufficient precision to calculate strain rates given the 10 year period between the two measurements. As the 1991 measurements are much shorter than the 2001 sessions and predate the establishment of the IGS precise satellite tracking network, it is likely that the errors in the 1991 positions will be on the order of 5 cm. Given the expected extension rates rates are less than 5 mm/year, it is unlikely that we will be able to definitevely determine any local effects, although some regional results may be possible due to some extended measurements taken at a few stations in 1991.