Current Ground Deformation Derived from GPS Observations near PBO Station AC 55 along the Yentna River, South of Denali National Park Reserve, Alaska

The Plate Boundary Observatory (PBO) has a network of 1100 permanent continuously operating Global Positioning System (GPS) stations, of which approximately 200 are located in Alaska. GPS station AC55 was accidently installed on a slow moving landslide. In order to isolate the landslide motion, this study uses 20 stations within 100 km proximity of AC55 to separate regional ground motion associated with seasonal variability and postseismic deformation from the 2002, M 7.9 Denali earthquake. Time series of motion in the horizontal and a vertical component was plotted for the entire network spanning the observation period (2002-2014). GPS observations were originally processed in IGS08. This study then established a local reference frame, which allows for intra-regional deformation analysis on areas of interest located within the local reference frame. From these measurements, a baseline of the relative motion located outside of the landslide body was calculated. The seasonal variability due to hydrological loading and the regional tectonic motion were calculated and removed. Hence the pure relative motion associated with the landslide event was derived.

The relatively stable local reference frame is able to provide an accuracy of ± 1.5 mm/year for local ground motion. The results show that PBO station AC55 moves with a steady horizontal velocity of 5.5 cm/year toward N 75° E, and has a subsidence rate of 2.6 cm/year. The mechanism of the steady displacement velocity at AC55 is not fully understood and requires geotechnical field investigation to understand the kinematics of the landslide motion.

More broadly, this GPS signal processing method has wider application allowing researchers to conduct precise landslide monitoring in remote regions using long-history (>5 years) GPS stations.