If you have ever experienced an earthquake, you know that the Earth literally moves beneath your feet. And now there’s satellite data to show just how much. Scientists studying satellite radar data from ESA’s Envisat and the Italian Space Agency’s COSMO-SkyMed, have been able analyze the movement of Earth during and after a recent earthquake in central Italy. A 6.3 earthquake shook the town of L’Aquila in on April 6, 2009, and satellite data is being used to map surface deformation in the Earth that took place after the quake and the numerous aftershocks that followed.
Using Synthetic Aperture Radar (SAR) data from these satellites, scientists took two or more radar images of the same ground location and compared them. The data is precise enough to show the differences in a scale of a few millimeters between images taken before and after the quake
Combining the before and after data, the scientists created ‘interferogram’ images that appear as rainbow-colored interference patterns. A complete set of colored bands, called ‘fringes’, represents ground movement relative to the spacecraft of half a wavelength, which is 2.8 cm in the case of the Envisat.
The Envisat interferogram shows nine fringes surrounding an area where the ground moved as much as 25 cm (along a line between the satellite’s orbital position and the earthquake area).
“By using available 3D ground displacements from five GPS location sites around the affected area, we were able to confirm the preliminary results obtained with Envisat data,” said Stefano Salvi from INGV’s Earthquake Remote Sensing Group.
The COSMO-SkyMed , which is a constellation of three satellites, can provide more frequent data. This means new interferograms can be calculated every few days.
The COSMO-SkyMed data together with the Envisat data and possibly radar data from other satellites will ensure a dense sampling of the ground deformation around the L’Aquila area in the next months, which could make this earthquake one of the most covered by SAR Interferometry measurements.
To ensure all scientists are able to contribute to the analysis of the earthquake, ESA is making its Earth observation dataset collected over the L’Aquila area freely accessible with an innovative fast data download mechanism. The dataset will be continuously updated with the newest Envisat acquisitions.
“We produced an interferogram just a few hours after the Envisat acquisition by combining these data with data acquired before the earthquake on 1 February. We were pleased that we were able to immediately see the pattern of the earthquake,” said Riccardo Lanari of IREA-CNR in Naples, Italy.