to the data center where it is collected, archived and distributed. These data sets continue to be freely and openly available to the public, with equal access provided for all users. PBO data includes the raw data collected from each instrument, quality-checked data in formats commonly used by PBO's various user communities, and processed data such as calibrated time series, velocity fields, and error estimates. Some scientific questions that continue to be addressed by the EarthScope project and the PBO data 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? References Global Positioning System Plate tectonics