This is How Geologists Measure Movement of the Surface After an Earthquake

To analyze the deformation of the earth’s surface after an earthquake, Katleen creates an interferogram. (photo: Katleen Wils)
23/02/2017
This is How Geologists Measure Movement of the Surface After an Earthquake
post by
Jonas Van Boxel

Katleen Wils studies earthquake zones in Chile. She maps the tiniest movements of the surface with colourful satellite images. 

On Christmas day 2016 the island Chiloé, in South-Central Chile, was hit by an earthquake with  a magnitude of 7.6. Thousands were evacuated and there was a tsunami-alarm in effect. Fortunately, no fatalities or giant sea wave were reported.

Chile, one of the most tectonically active zones in the world, is a fascinating area for Katleen, a doctoral student from the University of Ghent. She studies the exact area where the quake hit on 25 December. To do so, she uses images from the ESA Sentinel-1, a radar satellite that flies over the area every 12 days.

Radar satellites transmit electromagnetic waves to illuminate an area of the surface. By registering and measuring the waves that ‘bounce back’, the satellite can deduct a lot of information about the surface.

Counting colours

In order to analyze the deformation of the earth’s surface after an earthquake, Katleen compares two satellite images – one before and one after the tremor – to create what is called an interferogram. The different colours indicate a phase shift: the closer the colours are together, the bigger the movement of the surface. “You have to count the fringes. One of those means that the surface has moved 2,83 cm towards or away from the satellite.”

The earthquake on Christmas day caused the southern part of Chiloé to move some tens of centimeters westwards, Katleen concludes from the interferogram. The south-western part of the island rose, while the south-eastern part went down.

The deformation map helps to measure the location, magnitude and type of earthquake, improve earthquake models, and investigate the future seismic hazard of an area.

Once a week, Katleen works at our Institute to make calculations on a specialized computer. Over the next four years, she studies every deformation that has occurred, starting around 1990. At RBINS, she is guided by geologists Jan Walstra and Vanessa Heyvaert.

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