How InSAR works

We use InSAR to make surface deformation maps, or interferograms, like this one from the 2014 Napa earthquake in California.

Credit: Copernicus data (2014)/ESA/PPO.labs/Norut/COMET-SEOM Insarap study
Credit: Copernicus data (2014)/ESA/PPO.labs/Norut/COMET-SEOM Insarap study
How interferometry works

Most remote sensing satellites measure the sun’s radiation reflected back from the ground, usually in the visible and infra-red part of the electromagnetic spectrum.

Radar satellites are different. They transmit electromagnetic waves to illuminate an area of the Earth’s surface, and record the amplitude and phase of the waves that bounce back.

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Interferograms

The different colours in an interferogram show changes in the phase of radar waves before and after an earthquake. The coloured contours represent the interference fringes between the two sets of data.

This interferogram shows the area around Izmit, a town in Northern Turkey that was struck by an earthquake in 1999. It was created from two sets of radar data: the first was obtained before the earthquake, and the second 35 days afterwards.

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How we interpret interferograms


Interferogram of Izmit earthquake and cross-section of displacement

In this interferogram, contours (interference fringes) of the same colour represent one wavelength’s difference between the phases recorded before and after the earthquake.

The wavelength of the radar waves is 56mm. This means that the distance between adjacent fringes represents a ground movement of 28mm – half the radar wavelength, because each wave travels from the satellite to the ground and then back again.

The closer together the fringes, the greater the strain on the ground. The red lines show where the fringes are closest together and therefore the source of the earthquake (the North Anatolian Fault).

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Determining future seismic hazard

Interferograms help us to determine the location, magnitude and type of an earthquake. They can also help us to improve earthquake models, and investigate the future seismic hazard for an area.

After the Izmit earthquake, geophysicists at the US Geological  Survey calculated the seismic hazard for Istanbul. They analysed the extent of the slip from interferograms of the earthquake, and also studied historical records.

They found that the Izmit earthquake was the seventh in a series of earthquakes on the North Anatolian Fault since the 1930s, and that the earthquakes had been moving west along the fault towards Istanbul.

From this, along with the interferograms, they predicted a 62% chance that Istanbul would experience ‘strong shaking’ within the following 30 years.

Scientists also use interferograms to measure accumulation of interseismic strain. This technique offers most potential as a medium-range forecasting tool, although it doesn’t provide precise predictions along the lines of ‘there will be a catastrophic earthquake in Istanbul next Wednesday’. Instead, it gives the likelihood of an earthquake occurring within a certain period.

Centre for Observation and Modelling of Earthquakes, Volcanoes and Tectonics