The Stable North American Reference Frame (SNARF) working group has developed for PBO a regional reference frame that is fixed to the stable interior of the North American tectonic plate. SNARF was developed to help address questions of where relative plate motion is being accommodated, how deeply plate boundary dynamics penetrate into the plate interior, and the nature of non-rigid interior plate behavior such as mid-continent rebound and coastal subsidence from the last glacial retreat. SNARF provides a common frame by which to compare results from different analysis groups. It is anticipated that the National Geodetic Survey will adopt SNARF as the successor to the current North American reference frame (NAD83).The figure to the left shows the SNARF 1.0 velocity field adjusted for glacial rebound. Blewitt et al. (2005)
Episodic Tremor and Slip (ETS), a process involving very slow slip and nearly silent tremors on deep subducting faults, was discovered in Cascadia by geodesists during the planning stage of EarthScope. PBO observations of surface deformation have been used to define areas where fault slip is released by ETS and to predict where strain accumulation will be released in the next great Cascadia earthquake, estimated to be of magnitude 9.
Plot B in the figure to the right shows the total slow slip from ETS events in white. The red line marks the lower limit of seismogenic coupling, and the area to the west (or left) of the line is above the expected rupture zone for the next large earthquake. These results suggest that the next subduction zone earthquake will rupture farther inland than previously thought, closer to the large population area of the Puget Sound. Chapman and Melbourne (2009)
PBO data yield new information about the creeping section of the central San Andreas Fault. A creeping fault section does not store seismic energy that would otherwise contribute to large earthquakes. Sequential radar satellite passes were used to image the accumulation of slip along the fault (figure, left) to confirm that the creeping segment experiences almost full fault slip each year, nearly equivalent to the deep slip rate between the North American and Pacific plates.
If the fault were locked, the seismic stress accumulation over 150 years would be sufficient to generate a M7.2 to M7.4 earthquake. To the north in the Bay Area, and to the south in Parkfield, more typical fault behavior shows an annual slip deficit reflecting stored seismic energy that will contribute to future earthquakes. The integrated and high-precision data from PBO have helped researchers observe these phenomena in unprecedented detail. Ryder and Burgmann (2008)
InSAR images acquired under PBO GeoEarthScope dramatically improved strain resolution and coverage of the 3-D deformation field in the western United States. The InSAR interferogram at right was generated using GeoEarthScope data and shows surface deformation associated with the Wells, Nevada, earthquake of February 21, 2008. InSAR data were used to determine the extent and location of the ruptured southeast-dipping fault bounding the eastern flank of the Snake Mountains, and the pattern of uplift and subsidence caused by the earthquake. The Wells earthquake was typical of others in the Basin and Range of the United States. Amelung and Bell (2008)
Many graduate students use PBO data in their research. For her PhD thesis, Christine Puskas integrated PBO Global Positioning System (GPS) data with other observations to create a snapshot of annual surface motion due to faulting and volcanic deformation in and around Yellowstone caldera (figure, left). In 2004, the caldera began to uplift and subside without accompanying earthquakes due to the accumulation and migration of fluids derived from a magma reservoir that is tens of kilometers
deep. Puskas' work revealed the contributions of volcanic forces, fault and plate boundary motions, and the gravitational force of high-standing mountain ranges to the GPS-determined regional deformation. Puskas and Smith (2009) Soil Moisture PBO data are being used in applications that reach beyond understanding the deformation of the
PBO data are being used in applications that reach beyond understanding the deformation of the solid earth. Kristine Larson and her colleagues at the University of Colorado at Boulder used GPS signal reflections (which are a source of unwanted noise in geodetic applications) to measure near-surface soil moisture and its change with time.
Soil moisture observations are critical for weather and climate forecasts, both to improve agricultural yields and to mitigate the impact of drought and extreme weather events. A GPS-based soil moisture network would complement planned satellite soil moisture missions, providing thousands of calibration points across the globe. At a PBO station in Marshall, Colorado, the reflected GPS signals (colored dots in figure, above) show a strong correlation with in situ soil moisture measurements (gray band); both display soil drying after discrete rainfall events (blue bars). Larson et al. (2008)