GPS et Tremblements de Terre: Mesurer la Déformation du Sol

GPS Geodesy and Plate Motion Measurement

Before the Global Positioning System (GPS) was available for scientific geodesy, measuring the slow movement of 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.s required decades of precise surveying using optical instruments, with accuracies limited to millimeters at best over short distances. GPS transformed the field. Modern continuous GPS receivers at fixed monuments can measure horizontal positions to sub-millimeter precision and vertical positions to a few millimeters, recording data every second, every day, year after year. By analyzing the accumulated displacement of GPS monuments over years to decades, scientists can directly 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. velocities with remarkable accuracy — and detect the subtle deformation that precedes, accompanies, and follows earthquakes.

The GNSS Revolution

While "GPS" remains the common term, modern geodetic networks actually use signals from multiple Global Navigation Satellite System (GNSS) constellations: US GPS, Russian GLONASS, European Galileo, and Chinese BeiDou. Using multiple constellations improves positioning geometry, accuracy, and reliability. Dense GNSS networks like the Southern California Integrated GPS Network (SCIGN), the Pacific Northwest Geodetic Array (PANGA), and Japan's GEONET (over 1,300 stations) provide a continuous, high-resolution picture of surface deformation across entire tectonic regions.

Continuous GPS Networks Along Faults

Continuous GPS networks along active Fault LineThe 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.s are among the most powerful tools for understanding where seismic strain is accumulating and how fast. Along the San Andreas Fault (Geology)A fracture in rock along which movement has occurred. Faults range from millimeters to thousands of kilometers long. Major faults that produce earthquakes are called active faults. system in California, GPS monuments on either side of the fault move apart at rates of 20–35 millimeters per year — directly measuring 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. of the fault system. Where a fault is lockedA 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 GPS stations on either side of the fault converge (for a compressional fault) or diverge (for an extensional fault) slowly, building up the elastic strain that will eventually be released in an earthquake. Where a fault is creeping, the GPS stations move smoothly and continuously without the build-up of large locked-zone strain.

Interseismic, Coseismic, and Postseismic Deformation

Geodesists divide the deformation signal around a Fault (Geology)A fracture in rock along which movement has occurred. Faults range from millimeters to thousands of kilometers long. Major faults that produce earthquakes are called active faults. into three phases. Interseismic deformation is the slow build-up of elastic strain as the locked Fault (Geology)A fracture in rock along which movement has occurred. Faults range from millimeters to thousands of kilometers long. Major faults that produce earthquakes are called active faults. accumulates stress between earthquakes — GPS stations show steady, gradual motion that reflects both plate motion and the locking of the fault. Coseismic deformation is the sudden displacement that occurs during the earthquake itself — GPS receivers near the fault may jump by meters within seconds, with the displacement direction and pattern directly reflecting the sense of slip on the fault. Postseismic deformation continues for months to years after the earthquake as viscoelastic relaxation in the lower crust and mantle gradually redistributes the earthquake-induced stress, and as afterslip continues on the deeper part of the Fault (Geology)A fracture in rock along which movement has occurred. Faults range from millimeters to thousands of kilometers long. Major faults that produce earthquakes are called active faults..

Detecting Slow Slip Events

One of the most scientifically significant discoveries of the GPS era in geophysics is the existence of slow slip events (SSEs) — transient episodes of Fault (Geology)A fracture in rock along which movement has occurred. Faults range from millimeters to thousands of kilometers long. Major faults that produce earthquakes are called active faults. slip that release stress without producing significant seismic shaking. First detected in the late 1990s in the Cascadia Subduction Zone using GPS, SSEs appear as brief (days to weeks) reversals in the normally steady interseismic movement of GPS monuments. The Cascadia events occur at intervals of 12–18 months and release stress equivalent to Mw 6.5–6.8 earthquakes on the deeper portion of the subduction interface, just downdip of the locked megathrust zoneA section of a fault where friction prevents movement, causing stress to accumulate. When a locked fault finally ruptures, it can produce a major earthquake.. Similar slow slip events have been detected in Japan, New Zealand, and Central America. Understanding the relationship between slow slip on the downdip portion of a Subduction ZoneA region where one tectonic plate dives beneath another into the mantle. Subduction zones produce the world's largest earthquakes (M8.5+) and are associated with deep ocean trenches and volcanic arcs. and the potential for megathrust rupture on the updip locked zone is one of the central research questions in modern earthquake science.

Co-seismic and Post-seismic Deformation

Large earthquakes produce dramatic co-seismic displacements that are precisely measured by 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 2011 Tohoku earthquake (Mw 9.0) moved GPS stations on eastern Honshu up to 5.3 meters eastward and 0.5 meters downward — measurements that directly constrained the distribution of slip on the subduction interface and revealed that slip at the trench was much larger than models had anticipated. The 2010 Maule earthquake (Mw 8.8) in Chile moved coastal stations up to 3 meters horizontally. These co-seismic displacement fields, combined with seismic waveform inversion and 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. data, allow the detailed mapping of slip distribution on the fault — which patches slipped most, which remained locked, and what stress changes resulted for surrounding segments.

Afterslip and Viscoelastic Relaxation

In the months and years following a major earthquake, GPS networks detect continued deformation as the crust and mantle adjust to the sudden redistribution of stress. Afterslip — continued fault motion at depths below or adjacent to the main rupture — accounts for some of this post-seismic signal. Viscoelastic relaxation — the slow flow of the ductile lower crust and upper mantle in response to the earthquake-induced stress change — accounts for the rest. Distinguishing these two mechanisms from GPS time series requires sophisticated modeling that incorporates the rheological properties of the crust and mantle. Understanding post-seismic deformation is important for assessing how the stress on neighboring Fault (Geology)A fracture in rock along which movement has occurred. Faults range from millimeters to thousands of kilometers long. Major faults that produce earthquakes are called active faults. segments has changed following a major earthquake, directly informing 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. transfer calculations.

GPS in Earthquake Early Warning

The same GPS networks that measure long-term tectonic deformation can contribute to 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 for large earthquakes. Traditional seismic 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. relies on P-waveThe fastest seismic wave, traveling through both solid rock and liquid at 5-8 km/s. P-waves compress and expand material in the direction of travel, like a slinky. They arrive first at seismograph stations. detection, which suffers from magnitude saturation for the very largest events — the earthquakes most important to warn about. High-rate GPS (1–10 samples per second) can detect the large static displacements of major earthquakes within seconds of rupture initiation, providing magnitude estimates that do not saturate at high magnitudes. This GPS-based magnitude estimation is particularly important for Subduction ZoneA region where one tectonic plate dives beneath another into the mantle. Subduction zones produce the world's largest earthquakes (M8.5+) and are associated with deep ocean trenches and volcanic arcs. megathrust earthquakes like the 2011 Tohoku event, where early seismic estimates were significantly too low. Integration of high-rate 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. data into real-time earthquake monitoring and Seismic Alert SystemMexico's SASMEX, one of the world's first public earthquake early warning systems, operational since 1991. Provides up to 60 seconds of warning for Mexico City from coastal earthquakes. pipelines is an active area of development at geodetic and seismological agencies worldwide.

Strain Budgets and Seismic Hazard Implications

GPS geodesy has enabled the computation of geodetic strain budgets — quantitative estimates of how fast strain is accumulating on specific faults compared to how fast it is being released through seismic slip. On the Himalayan frontal thrust, GPS measurements show the Indian subcontinent converging with Asia at roughly 20 mm/yr, yet the rate of strain release in large earthquakes falls far short of the geodetically measured convergence rate, implying a large ongoing deficit. These strain budget analyses directly feed into probabilistic seismic hazard assessments by providing independent constraints on long-term fault slip rates, complementing the geological PaleoseismologyThe study of prehistoric earthquakes through geological evidence such as fault trenches, uplifted terraces, and tsunami deposits. Extends the earthquake record back thousands of years. evidence from trenching studies and Fault ScarpA cliff or steep slope formed by vertical displacement along a fault during an earthquake. Fault scarps can be meters high and provide visible evidence of past earthquake activity. mapping. Where geodetically measured strain rates are high and earthquake recurrence intervalsThe average time between major earthquakes on a particular fault. Estimated from paleoseismology and historical records. The Cascadia subduction zone has a recurrence interval of ~500 years. are long, the Seismic GapA section of an active fault that has not produced an earthquake for a long time compared to neighboring sections. Seismic gaps may indicate increased probability of a future earthquake. concept can be applied to identify segments most likely to host future large earthquakes.