The Plate Boundary Observatory or (PBO) is one of three components of the Earthscope project, along with USArray and SAFOD (the San Andreas Fault Observatory at Depth). The PBO is exploring the structure and evolution of the North American continent and the processes related to earthquakes and volcanoes by installing an array of high precision Global Positioning System GPS monuments, borehole strainmeters, and tiltmeters throughout the western United States.
UNAVCO, a membership-governed consortium funded by the National Science Foundation (NSF) and NASA, installs the PBO. Upon completion in September 2008, there will be 875 GPS stations, 103 borehole strainmeters, and 5 laser strainmeters throughout the western United States and Alaska concentrated around the most active fault systems and volcanoes.
The GPS stations are categorized into five clusters. The transform cluster is in the vicinity of the San Andreas transform fault, which cuts through California; the subduction cluster is in the Cascadia subduction zone, which includes northern California, Oregon, Washington, and southern British Columbia, and is a result of the Juan de Fuca plate subducting under the North American plate; the extension cluster is in the Basin and Range region; the volcanic cluster is in volcanic regions such as the Yellowstone caldera, the Long Valley caldera, and the Cascade volcanoes; the backbone cluster is at 100–200 km across the United States to provide complete spatial coverage.
The GPS instrumentation is high-precision and continuously operating. High-precision means the units can detect sub-cm motion, and continuously operating means a 15 second sampling rate and one data file for every 24 hours. A completed PBO GPS station occupies about a three by three meter plot of land.
Some scientific questions that are being addressed by the EarthScope project and the PBO include:
- How does accumulated strain lead to earthquakes?
- Are there recognizable precursors to earthquakes?
- How does the evolution of the continent influence the motions that are happening today?
- What happens to geologic structures at depth?
- What influences the location of features such as faults and mountain ranges?
- Is it inherited from earlier tectonic events or related to deeper processes in the mantle?
- How is magma generated? How does it travel from the mantle to reach the surface?
- What are the precursors to a volcanic eruption?
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