GPS基地局がプレート運動を追跡する方法

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測地学全地球測位システムの受信機を用いて、プレートの動きや地殻変動をミリメートル単位の精度で測定する手法。地震と地震の間に断層に歪みがどのように蓄積するかを明らかにする。 — the application of GPS measurements to precise ground position tracking — allows scientists to measure プレート（テクトニックプレート）移動し、浮遊し、時に破断する地球のリソスフェアの巨大な区画。7つの主要プレートと約8つの小規模プレートがあり、それらの相互作用がほとんどの地震の原因となる。 movements with millimeter-level accuracy, map the accumulation of strain on fault lines地表に現れた断層の痕跡で、線状または破砕された岩石の帯として視認できる。地質学者は活断層線を地図化し、周辺地域の地震リスクを評価する。, 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 プレート（テクトニックプレート）移動し、浮遊し、時に破断する地球のリソスフェアの巨大な区画。7つの主要プレートと約8つの小規模プレートがあり、それらの相互作用がほとんどの地震の原因となる。 Velocities

The most fundamental application of GPS測地学全地球測位システムの受信機を用いて、プレートの動きや地殻変動をミリメートル単位の精度で測定する手法。地震と地震の間に断層に歪みがどのように蓄積するかを明らかにする。 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 fault摩擦によって動きが妨げられ、応力が蓄積している断層区間。固着断層がついに破壊すると、大地震を引き起こすことがある。, 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 津波海底地震時の海底の急激な変位によって発生する一連の海の波。津波はジェット機並みの速度（時速700km以上）で海洋全域を伝わることがある。 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 クーロン応力伝達地震が周辺の断層の応力状態を変化させ、将来の地震を誘発または遅延させうる過程。どの断層が破壊に近づいたかを予測するために用いられる。 calculations that forecast 余震本震と同じ断層領域で本震の後に続く、より規模の小さい地震。余震活動は数週間から数年続くことがあり、最大の余震は通常本震よりマグニチュードが1.0〜1.2小さい。 sequences.

InSAR Complementing GPS

干渉SAR（InSAR）地震前後に撮影されたレーダー画像を比較することで、センチメートル単位の精度で地表変動を測定する衛星レーダー技術。断層のすべりパターンを明らかにする。 (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.

すべり速度断層に沿った変位の平均速度で、通常は年間ミリメートル単位で測定される。すべり速度が高いほど、一般に地震発生頻度とリスクが高いことを示す。 Determination from GPS

Long GPS time series allow direct measurement of すべり速度断層に沿った変位の平均速度で、通常は年間ミリメートル単位で測定される。すべり速度が高いほど、一般に地震発生頻度とリスクが高いことを示す。 — 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 すべり速度断層に沿った変位の平均速度で、通常は年間ミリメートル単位で測定される。すべり速度が高いほど、一般に地震発生頻度とリスクが高いことを示す。 to approximately 24 mm/year of right-lateral motion. Combining this GPS-derived すべり速度断層に沿った変位の平均速度で、通常は年間ミリメートル単位で測定される。すべり速度が高いほど、一般に地震発生頻度とリスクが高いことを示す。 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 (確率論的地震ハザード解析（PSHA）起こりうるすべての地震の発生源・マグニチュード・地震動レベルを考慮し、特定の揺れの水準を超える確率として結果を表現する、地震ハザードの定量評価手法。).

GPS in Earthquake Early Warning

GPS instruments now play a direct role in earthquake 緊急地震速報（EEW）地震を検知し、強い揺れが到達する前に人々やシステムに警報を送るシステム。数秒から数十秒の猶予を提供し、身を守る行動をとるのに十分な時間となる。 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測地学全地球測位システムの受信機を用いて、プレートの動きや地殻変動をミリメートル単位の精度で測定する手法。地震と地震の間に断層に歪みがどのように蓄積するかを明らかにする。 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 プレート（テクトニックプレート）移動し、浮遊し、時に破断する地球のリソスフェアの巨大な区画。7つの主要プレートと約8つの小規模プレートがあり、それらの相互作用がほとんどの地震の原因となる。 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 干渉SAR（InSAR）地震前後に撮影されたレーダー画像を比較することで、センチメートル単位の精度で地表変動を測定する衛星レーダー技術。断層のすべりパターンを明らかにする。 and traditional seismometry, geodetic GPS is indispensable to modern seismic hazard assessment and すべり速度断層に沿った変位の平均速度で、通常は年間ミリメートル単位で測定される。すべり速度が高いほど、一般に地震発生頻度とリスクが高いことを示す。 quantification.