InSAR:从太空看地震

What Is InSAR?

InSAR通过对比地震前后拍摄的雷达图像，以厘米级精度测量地表形变的卫星雷达技术，可揭示断层的滑动模式。 — 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. 干涉合成孔径雷达（InSAR）通过对比地震前后拍摄的雷达图像，以厘米级精度测量地表形变的卫星雷达技术，可揭示断层的滑动模式。 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 断层（地质学）岩石沿其发生位移的破裂面。断层长度从数毫米到数千公里不等。会引发地震的主要断层称为活动断层。 planes and for detecting displacement in areas without ground-based instrumentation. 干涉合成孔径雷达（InSAR）通过对比地震前后拍摄的雷达图像，以厘米级精度测量地表形变的卫星雷达技术，可揭示断层的滑动模式。 is particularly valuable in remote or politically difficult regions where deploying GPS大地测量利用全球定位系统接收机以毫米级精度测量构造板块运动和地壳变形的方法，可揭示地震之间断层上应变积累的过程。 networks is impossible or impractical.

How Radar Interferometry Works

The underlying physics of 干涉合成孔径雷达（InSAR）通过对比地震前后拍摄的雷达图像，以厘米级精度测量地表形变的卫星雷达技术，可揭示断层的滑动模式。 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 干涉合成孔径雷达（InSAR）通过对比地震前后拍摄的雷达图像，以厘米级精度测量地表形变的卫星雷达技术，可揭示断层的滑动模式。 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 走滑断层岩石块沿水平方向相互滑动错开的断层。圣安德烈亚斯断层和北安纳托利亚断层是引发破坏性地震的主要走滑断层。 mechanism and directly constraining the slip distribution on the fault rupture地震期间岩石沿断层发生破裂，将储存的弹性能量以地震波形式释放的过程。破裂长度小到数米（小地震），大到超过1,000公里（大地震）。 surface. Since then, thousands of earthquake 干涉合成孔径雷达（InSAR）通过对比地震前后拍摄的雷达图像，以厘米级精度测量地表形变的卫星雷达技术，可揭示断层的滑动模式。 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 干涉合成孔径雷达（InSAR）通过对比地震前后拍摄的雷达图像，以厘米级精度测量地表形变的卫星雷达技术，可揭示断层的滑动模式。 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 干涉合成孔径雷达（InSAR）通过对比地震前后拍摄的雷达图像，以厘米级精度测量地表形变的卫星雷达技术，可揭示断层的滑动模式。 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大地测量利用全球定位系统接收机以毫米级精度测量构造板块运动和地壳变形的方法，可揭示地震之间断层上应变积累的过程。 displacement vectors, 干涉合成孔径雷达（InSAR）通过对比地震前后拍摄的雷达图像，以厘米级精度测量地表形变的卫星雷达技术，可揭示断层的滑动模式。 inversions provide the most detailed pictures of fault rupture currently achievable. The 2016 Kaikōura earthquake in New Zealand is a famous example: 干涉合成孔径雷达（InSAR）通过对比地震前后拍摄的雷达图像，以厘米级精度测量地表形变的卫星雷达技术，可揭示断层的滑动模式。 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, 干涉合成孔径雷达（InSAR）通过对比地震前后拍摄的雷达图像，以厘米级精度测量地表形变的卫星雷达技术，可揭示断层的滑动模式。 can detect more subtle, slow deformation phenomena. Fault creep断层沿线缓慢而持续的运动，不产生显著地震。圣安德烈亚斯断层部分区段以每年2至3厘米的速度蠕滑。 — the slow, aseismic sliding of a 断层（地质学）岩石沿其发生位移的破裂面。断层长度从数毫米到数千公里不等。会引发地震的主要断层称为活动断层。 — produces a characteristic linear fringe pattern in accumulating 干涉合成孔径雷达（InSAR）通过对比地震前后拍摄的雷达图像，以厘米级精度测量地表形变的卫星雷达技术，可揭示断层的滑动模式。 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 干涉合成孔径雷达（InSAR）通过对比地震前后拍摄的雷达图像，以厘米级精度测量地表形变的卫星雷达技术，可揭示断层的滑动模式。 time series analysis. The combination of spatial coverage and sensitivity makes 干涉合成孔径雷达（InSAR）通过对比地震前后拍摄的雷达图像，以厘米级精度测量地表形变的卫星雷达技术，可揭示断层的滑动模式。 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

干涉合成孔径雷达（InSAR）通过对比地震前后拍摄的雷达图像，以厘米级精度测量地表形变的卫星雷达技术，可揭示断层的滑动模式。 and GPS大地测量利用全球定位系统接收机以毫米级精度测量构造板块运动和地壳变形的方法，可揭示地震之间断层上应变积累的过程。 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 干涉合成孔径雷达（InSAR）通过对比地震前后拍摄的雷达图像，以厘米级精度测量地表形变的卫星雷达技术，可揭示断层的滑动模式。 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 库仑应力传递地震改变邻近断层应力状态、从而可能触发或延迟未来地震的过程，用于预测哪些断层正被推向更接近破裂的状态。 transfer to adjacent fault segments, informing post-earthquake hazard evaluations used by emergency managers and government agencies.