InSAR: Nhìn thấy Động đất Từ không gian

What Is InSAR?

InSARA 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 — is a satellite remote sensing technique that uses radar signals to measure surface deformation with centimeter to millimeter precision over large areas. A radar satellite transmits microwave pulses toward Earth's surface and records the reflected signal. By comparing the phase of radar returns from two passes over the same area at different times, scientists can create an interferogram that maps the change in distance between the satellite and the ground surface between the two acquisition dates. Because radar waves have wavelengths of a few centimeters and phase can be measured to a fraction of a wavelength, the technique achieves extraordinary sensitivity to surface displacement — often better than 1 centimeter over areas of thousands of square kilometers. The technology has transformed observational earthquake science by making it possible to map the ground deformation caused by earthquakes, slow slip events, and volcanic unrest from space.

Advantages Over Ground-Based Networks

Traditional geodetic networks — GPS stations, leveling benchmarks, tiltmeters — provide precise measurements but only at discrete points. 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. provides spatially continuous coverage of the entire deformation field at tens-of-meter resolution, revealing the full spatial pattern of ground displacement rather than interpolations between sparse measurement points. This spatial completeness is essential for constraining the detailed slip distribution on 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. planes and for detecting displacement in areas without ground-based instrumentation. 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. is particularly valuable in remote or politically difficult regions where deploying 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. networks is impossible or impractical.

How Radar Interferometry Works

The underlying physics of 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. exploits the wave nature of radar signals. When a radar satellite illuminates the ground, each point on the surface reflects a signal with a phase that depends on the distance between the satellite and the ground. If the ground moves between two satellite acquisitions — say, because of an earthquake — the distance changes, and so does the phase of the reflected signal. By subtracting the phase maps from two acquisitions (forming an interferogram), the common phase contributions from the satellite orbit geometry and the stable topography cancel out, leaving a residual phase that represents the change in range (line-of-sight distance) between the satellite and the ground. This range change is displayed as a series of color fringes in the interferogram, with each full color cycle representing one-half wavelength of displacement (about 2.8 centimeters for the C-band radar used on ESA's Sentinel-1 satellites). Counting fringes and their sign reveals the magnitude and direction of ground deformation.

Decorrelation and Atmospheric Noise

Not all interferograms are clean. Temporal decorrelation occurs when the ground surface changes between acquisitions — due to vegetation growth, snow cover, agricultural plowing, or urban construction — causing the phase information to become random noise. Atmospheric delay noise is caused by differences in the water vapor content of the troposphere between the two acquisitions; water vapor slows radar signals and can introduce apparent deformation signals of several centimeters. Careful selection of interferometric pairs, stacking of multiple interferograms, and atmospheric correction using weather models or GPS measurements can mitigate these effects. Modern satellite missions like Sentinel-1 with 6-day repeat intervals, and planned future missions with even shorter revisit times, are dramatically improving the temporal resolution and reducing decorrelation issues.

Mapping Fault Slip After Earthquakes

The most immediate application of 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. in earthquake science is mapping co-seismic deformation — the displacement of the ground surface caused by a major earthquake. When the 1992 Landers earthquake (Mw 7.3) in California was captured in an interferogram from the ERS-1 satellite — the first such observation — the earthquake science community was electrified. The fringe pattern beautifully traced the displacement field across hundreds of square kilometers, consistent with the known Strike-Slip FaultA fault where blocks of rock move horizontally past each other. The San Andreas Fault and North Anatolian Fault are major strike-slip faults that produce destructive earthquakes. mechanism and directly constraining the slip distribution on the fault ruptureThe breakage of rock along a fault during an earthquake, releasing stored elastic energy as seismic waves. Rupture length can range from meters (small quakes) to 1,000+ km (great earthquakes). surface. Since then, thousands of earthquake 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. interferograms have been published, building a rich observational database of co-seismic deformation for events ranging from Mw 5 to Mw 9.

Inverting for Fault Slip

The surface deformation measured by 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. is related to the distribution of slip on the fault at depth through the equations of elasticity. Given a model of Earth's elastic properties and an assumed fault geometry, geodesists invert the 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. interferogram to find the slip distribution on the fault plane that best explains the observed surface deformation. These slip models reveal which portions of the fault ruptured most (asperities — regions of high slip) and which remained locked, information that is impossible to obtain from seismic waveforms alone. Combined with 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. displacement vectors, 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. inversions provide the most detailed pictures of fault rupture currently achievable. The 2016 Kaikōura earthquake in New Zealand is a famous example: 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. revealed that the earthquake involved simultaneous rupture of more than 20 fault segments in a geometrically complex pattern that was completely unexpected from pre-earthquake structural geology.

Detecting Silent Earthquakes

Beyond the dramatic co-seismic deformation of large earthquakes, 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. can detect more subtle, slow deformation phenomena. Fault creepThe slow, continuous movement along a fault without generating significant earthquakes. Some sections of the San Andreas Fault creep at 2-3 cm/year. — the slow, aseismic sliding of 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. — produces a characteristic linear fringe pattern in accumulating 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. time series. The creeping segment of the Haiyuan Fault in China was mapped in detail using ERS satellite data, revealing spatial variations in creep rate along the fault. Slow slip events on subduction zones, volcanic inflation and deflation, and land subsidence due to groundwater extraction, oil production, or urban construction are all detectable 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. time series analysis. The combination of spatial coverage and sensitivity makes 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. an indispensable tool for monitoring geodynamic processes that occur too slowly or too quietly to be detected by seismographs but too fast to be distinguished from long-term plate motion by classical geodetic surveys.

Combining InSAR with GPS Data

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 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. are highly complementary techniques that together provide much stronger constraints on earthquake source models than either alone. GPS provides three-dimensional displacement vectors at discrete points with sub-millimeter accuracy, while 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. provides one-dimensional (line-of-sight) measurements over a continuous two-dimensional surface with centimeter accuracy. By combining ascending and descending orbit interferograms from the same earthquake, and integrating GPS displacement vectors, scientists can solve for the full three-dimensional displacement field across the entire deformation zone. This combined approach has been applied to major earthquakes worldwide — including the 1999 Izmit (Turkey), 2003 Bam (Iran), 2010 El Mayor–Cucapah (Mexico), and 2011 Tohoku (Japan) events — producing slip models of unprecedented resolution and detail. These models feed directly into assessments of 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 to adjacent fault segments, informing post-earthquake hazard evaluations used by emergency managers and government agencies.