The Pacific Northwest Geodetic Array (PANGA) uses real-time GPS measurements to monitor crustal deformation and mitigate natural hazards throughout the Pacific Northwest. These hazards arise from earthquakes, volcanic eruptions, landslides, and coastal sea-level encroachment. In addition, PANGA GPS measurements are used to monitor man-made structures such as Seattle's sagging Alaska Way Viaduct, 520 and I90 floating bridges and power-generation/drinking-supply dams throughout the Cascadia subduction zone, including the megadams along the Columbia River. GPS data are telemetered in real-time back to CWU, where they are processed in real-time using both JPL's RTG software as well as Trimble's RTKNet Integrity Manager software to provide relative positioning of several mm resolution. Here we provide an overview of real-time GPS data, CWU's PANGA analysis activities and data processing algorithms.
Real-time plot of GPS measurements at station SC02 relative to LSIG. Click here for all real-time data.
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Through its unique ability to resolve unforeseen modes of plate boundary deformation, continuous GPS measurements along the Cascadia subduction zone have revolutionized both our basic knowledge of interplate fault processes as well as how we assess earthquake hazards and strategize their mitigation. GPS has resolved several dozen Cascadia megathrust slow slip events, for instance, that indicate a wide variability in plate coupling along strike. The largest events typically last several weeks, propagate hundreds of kilometers, and have moment magnitudes of 6.3-6.8. Similar measurements on other plate boundaries have also forced the recognition that slow slip events are widespread and can both trigger and be triggered by conventional earthquakes. How the northern Cascadia transients interact with local seismicity remains an open question, but, averaged over many events, they appear to delineate a down-dip limit of the eventual megathrust rupture that lies far closer to the major population centers of the Pacific Northwest than is typically assumed in strong ground motion simulations. Traditional analyses (one position estimate per day, two-week latency with final orbits/clocks) are uniquely able to resolve these and many related questions about subduction zone geodynamics, and has pointed the way towards a measurable and dramatic reduction of seismic hazards in Cascadia.
GPS is no longer limited to daily positioning of steady-state deformation. Arguably the most stunning recent technological improvement is resolving dynamic ground motions at amplitudes down to a few centimeters and frequencies up to a 1 hz with high-sampling rate receivers.
Some applications of real-time GPS include but are definitely not limited to:
The scientific and hazards mitigation potential of high-rate GPS cannot be overstated, and in real-time, its ability to guide instrument deployment during tectonic/volcanic events, or to assist societies in mitigating damage from earthquakes, volcanic events, landslides, or tsunami generation, is unique. Unlike conventional seismometers, high-rate GPS positions are stable in extreme ground motion and maintain fidelity across the long period spectrum, out to and including static offsets. The signal to noise ratio of position resolution increases as motion amplitude increases towards arbitrarily large values. GPS displacements are estimated directly and are therefore not subject to tilt/acceleration ambiguities present in force-balance/strong motion seismometers. The ability to stay on scale and accurately measure near-field dynamic velocities of meters or 10's of meters per second without clipping or aliasing has no other instrumental analog and is unique to GPS. Today, real-time GPS data can be fed into seismic strong-ground motion analyses for emergency response purposes.
First observation with real-time GPS of transient westerly Cascadia forearc deformation due to the May, 2009 Episodic Tremor and Slip (ETS) event. Blue box denotes timing of the ETS event. Top two time series show final (non-real time) PBO positions for stations SC02 and P426 produced using merged GAMIT and GIPSY processing and IGS final orbits. Bottom two time series show station positions as they were observed in real-time using 60-minute position estimates and IGS predicted orbits. These time series have received no post-processing whatsoever. The transient westerly offsets of ~5mm agree well between the post-processed daily and real-time data products. Bottom panel shows hours per day of non-volcanic tremor recorded during this ETS event, and lower right hand panel shows locations of the tremor epicenters. Tremor data from www.pnsn.org.
These data are made possible in part by our affiliates and sponsors:
Rinex and other data files are available from our public archive via anonymous ftp.