Comment les Stations GPS Suivent le Mouvement des Plaques Tectoniques

How GPS Became a Seismological Tool

Global Positioning System technology, originally developed for military navigation, has become one of the most powerful tools in modern seismology and geodynamics. GPS GeodesyThe use of Global Positioning System receivers to measure tectonic plate motion and crustal deformation with millimeter precision. Reveals how strain accumulates on faults between earthquakes. — the application of GPS measurements to precise ground position tracking — allows scientists to measure Tectonic PlateA massive segment of Earth's lithosphere that moves, floats, and sometimes fractures. There are 7 major and about 8 minor plates, and their interactions cause most earthquakes. movements with millimeter-level accuracy, map the accumulation of strain on fault linesThe trace of a fault on the Earth's surface, visible as a line or zone of broken rock. Active fault lines are mapped by geologists to assess earthquake hazard for nearby communities., detect volcanic inflation, and record the instantaneous co-seismic displacements produced by large earthquakes. The resulting data is foundational to understanding earthquake hazard in ways that seismometry alone cannot provide.

Principles of High-Precision GPS Measurement

Consumer GPS devices achieve accuracy of a few meters by measuring signal travel times from satellites. Geodetic GPS stations achieve millimeter-level accuracy through several additional techniques. Phase-based measurement uses the carrier wave phase (rather than just the coarse ranging signal), which contains far more precise timing information. Differential correction uses a network of reference stations at known positions to remove common atmospheric and clock errors. Long occupation times — continuous recordings over months and years — allow averaging that further reduces noise. The result is position time series accurate to 1–3 mm horizontally and 5–10 mm vertically.

Measuring Tectonic PlateA massive segment of Earth's lithosphere that moves, floats, and sometimes fractures. There are 7 major and about 8 minor plates, and their interactions cause most earthquakes. Velocities

The most fundamental application of GPS GeodesyThe use of Global Positioning System receivers to measure tectonic plate motion and crustal deformation with millimeter precision. Reveals how strain accumulates on faults between earthquakes. is measuring the velocity of tectonic plates relative to each other or to a stable reference frame. The ITRF (International Terrestrial Reference Frame) provides a globally consistent reference against which all GPS station velocities are measured. In plate interiors far from active deformation zones, GPS stations move at the steady plate velocity — typically 2–10 cm/year for major plates. The Pacific Plate moves northwest at approximately 8 cm/year relative to stable North America. The Indian Plate collides into Asia at roughly 5 cm/year, driving the Himalayan orogeny.

Velocity Fields and Strain Accumulation

Near active plate boundaries, the velocity field becomes complex because elastic strain accumulates in the crust rather than being released continuously. At a locked faultA section of a fault where friction prevents movement, causing stress to accumulate. When a locked fault finally ruptures, it can produce a major earthquake., the deep portion of the fault below the seismogenic zone slips freely at plate velocity, but the shallow locked section is held stationary by friction. GPS stations on either side of a locked fault move toward each other at reduced velocity compared to far-field plate motion, and the difference reveals how much strain is accumulating. This "interseismic coupling" measurement is a direct observational constraint on seismic hazard because larger coupling implies greater eventual earthquake potential.

Co-Seismic Displacement: Reading Earthquakes in the GPS Record

When a large earthquake ruptures, the elastic strain accumulated over decades is released instantaneously, and GPS stations near the fault jump to new positions. The 2011 Tohoku earthquake moved GPS stations in northeastern Japan as much as 5.3 meters eastward and 1.2 meters downward — the largest co-seismic displacement ever recorded. The spatial pattern of these displacements, mapped using dozens of stations, revealed the distribution of slip on the fault plane with unprecedented resolution. This slip model directly informs TsunamiA series of ocean waves generated by sudden displacement of the seafloor during an underwater earthquake. Tsunamis can travel across entire ocean basins at jet speed (700+ km/h). models because the seafloor deformation pattern determines the initial wave shape.

Post-Seismic Relaxation

Following a large earthquake, GPS stations continue to move for months to years as the earth responds to the abrupt stress change through afterslip and viscoelastic relaxation. Afterslip occurs when the fault continues to slip slowly below the seismogenic zone in response to the stress loaded by the main rupture. Viscoelastic relaxation occurs in the lower crust and upper mantle, which flow on geological timescales. GPS time series capture both processes, enabling discrimination between them based on their different spatial patterns and temporal decay rates. Understanding post-seismic deformation is important for Coulomb Stress TransferThe process by which an earthquake changes stress on nearby faults, potentially triggering or delaying future earthquakes. Used to forecast which faults are brought closer to failure. calculations that forecast AftershockA smaller earthquake that follows the mainshock in the same fault region. Aftershock sequences can last weeks to years, with the largest aftershock typically 1.0-1.2 magnitudes below the mainshock. sequences.

InSAR Complementing GPS

InSAR (Interferometric SAR)A satellite radar technique that measures ground deformation with centimeter accuracy by comparing radar images taken before and after an earthquake. Reveals fault slip patterns. (Interferometric Synthetic Aperture Radar) provides spatially dense deformation maps — effectively a GPS measurement at every pixel of a satellite image — but with lower temporal resolution than continuous GPS. GPS provides continuous temporal monitoring at discrete points; InSAR provides snapshot maps covering large areas. The two techniques are complementary: GPS validates and calibrates InSAR products, while InSAR provides spatial context between GPS stations. Together they enabled the first complete maps of interseismic strain accumulation on the San Andreas and other major fault systems.

Slip RateThe average rate of displacement along a fault, typically measured in millimeters per year. Higher slip rates generally indicate higher earthquake frequency and hazard. Determination from GPS

Long GPS time series allow direct measurement of Slip RateThe average rate of displacement along a fault, typically measured in millimeters per year. Higher slip rates generally indicate higher earthquake frequency and hazard. — the long-term velocity at which two sides of a fault move relative to each other. For the southern San Andreas Fault, GPS measurements constrain the Slip RateThe average rate of displacement along a fault, typically measured in millimeters per year. Higher slip rates generally indicate higher earthquake frequency and hazard. to approximately 24 mm/year of right-lateral motion. Combining this GPS-derived Slip RateThe average rate of displacement along a fault, typically measured in millimeters per year. Higher slip rates generally indicate higher earthquake frequency and hazard. with the known fault length and historical earthquake record allows calculation of the expected recurrence interval for large ruptures, linking GPS geodesy directly to probabilistic seismic hazard assessment (Probabilistic Seismic Hazard Analysis (PSHA)A method for quantifying earthquake hazard that considers all possible earthquake sources, magnitudes, and ground motion levels, expressing results as probability of exceeding specific shaking levels.).

GPS in Earthquake Early Warning

GPS instruments now play a direct role in earthquake Earthquake Early Warning (EEW)A system that detects an earthquake and sends alerts to people and systems before strong shaking arrives. Can provide seconds to tens of seconds of warning, enough to take protective action. systems. Real-time GPS streams can detect the large co-seismic displacement waves from great earthquakes within tens of seconds, providing magnitude estimates that are not subject to the saturation problems that affect seismometer-based magnitude scales for M 8+ events. The GPS-based magnitude estimate helps ensure that early warning systems issue appropriately scaled alerts for the largest earthquakes, where seismic magnitude underestimation is most consequential.

Continuous GPS Networks

Major continuous GPS networks include the USGS Continuously Operating Reference Stations (CORS), the Southern California Integrated GPS Network (SCIGN), the Japanese GEONET (1,300+ stations), and the UNAVCO Plate Boundary Observatory. Data from these networks is archived and freely available, enabling retrospective analysis of deformation events and routine updating of tectonic models. New constellations beyond GPS — including Russia's GLONASS, Europe's Galileo, and China's BeiDou — are being integrated into geodetic networks, improving coverage and redundancy.

Summary

GPS GeodesyThe use of Global Positioning System receivers to measure tectonic plate motion and crustal deformation with millimeter precision. Reveals how strain accumulates on faults between earthquakes. transformed seismology from a science that only observes earthquakes when they happen into one that can monitor the slow accumulation of strain that precedes them. By measuring Tectonic PlateA massive segment of Earth's lithosphere that moves, floats, and sometimes fractures. There are 7 major and about 8 minor plates, and their interactions cause most earthquakes. velocities, mapping interseismic coupling, recording co-seismic displacements, and tracking post-seismic relaxation, GPS provides the most complete observational picture of the earthquake cycle available. Combined with InSAR (Interferometric SAR)A satellite radar technique that measures ground deformation with centimeter accuracy by comparing radar images taken before and after an earthquake. Reveals fault slip patterns. and traditional seismometry, geodetic GPS is indispensable to modern seismic hazard assessment and Slip RateThe average rate of displacement along a fault, typically measured in millimeters per year. Higher slip rates generally indicate higher earthquake frequency and hazard. quantification.