On 30 October 2020, a powerful earthquake struck Turkey's western coast and the Greek island of Samos, causing extensive damage as well as casualties—the Copernicus Sentinel-1 mission was used to map the related co-seismic ground displacements within 24 hours, contributing to relief efforts.
Based on the Hellenic Unified Seismological Network (H.U.S.N.) the magnitude (M) 7.0 quake - which was felt as far away as Athens and Istanbul - struck at a depth of some 10-15 km. It was the biggest quake to have hit Samos since 1955, when the island was struck by a M6.9 earthquake along its southern coast, and very similar to the 1904 M6.8 earthquake, which also occurred along its northern coast.
Following the event, a mini-tsunami (max tsunami runup heights of 1.5-2 m and runup distances locally exceeding 20-30 m) flooded the port of Samos, amplifying the building damage along the northern Samos coast.
Just a few hours after the event, the Aristotle University of Thessaloniki (AUTh) started interferometric processing using several on-demand services (i.e. ESA SNAP DInSAR, CNES DIAPASON and CNR IREA P-SBAS) hosted on the Geohazards Exploitation Platform (GEP). This was used to map the related co-seismic ground displacements. Processing results were immediately made accessible to the scientific community (through the GEP community area).
The processing was executed within 24 hours after the event thanks to the availability of data acquired from the Sentinel-1 mission of the European Union's Copernicus Programme, on the same day as the earthquake and less than 12 hours apart.
With its 250 km-wide swath over land surfaces, the Copernicus Sentinel-1 satellites give scientists a broad view of displacements, allowing them to examine such ground displacements caused by earthquakes, further developing scientific knowledge of them.
Interferometric SAR (InSAR) exploits the phase difference between two complex radar SAR observations of the same area, taken from slightly different sensor positions, and extracts distance information about Earth's terrain.
By combining the phase of these two images after coregistration, an interferogram can be generated where phase is highly correlated to the terrain topography and deformation patterns can be mapped. If the phase shift related to topography is removed from the interferograms, the difference between the resulting products will show surface deformation patterns that occurred between the two acquisition dates. This methodology is called DInSAR.
As illustrated in the first image above, both interferometric measurements appear aligned showing a sinking pattern (up to 6-7 cm) concentrated along a narrow area at the northernmost part of Samos (Agios Konstantinos – Vourliotes region), whereas the W-NW part of Samos island shows significant uplift (locally exceeding 10 cm).
Professor Costas Papazachos, of AUTh, initially interpreted what happened stating, "The rupture started from the hypocentre (relocated depth ~13 km), possibly expanded bilaterally in both directions (East and West), rupturing an area with a length of 55-60 kilometres and a width of 15 kilometres (the fault, white polygon). The large fault displacements possibly occurred along the western fault segment (orange area), where fault may have reached the surface, significantly shifting the sea bottom (though this can be done only by the static displacement) and generating the observed tsunami."
This indication is suggested as an attempt to explain the significant uplift (over 10 cm) observed in W-NW of Samos island (typical for the fault footwall), corresponding to a large local fault slip (> 0.5-1 metre on the fault rupture surface), as well as the distribution of post-seismic activity, which now "breaks" the fault segments that slipped less during the mainshock, mainly east, but also west of the orange area (see Image 2 above).
Combining seismological data and spaceborne observations, the preliminary interpretation of the earthquake activity on Samos Island was prepared and communicated to Greek authorities. "We count another success of Earth Observation and platform-based solutions in supporting the timely interpretation of phenomena, improving our understanding of geohazards and their consequences" said Assistant Professor Michael Foumelis (AUTh), who supports scientific animation concerning advanced EO processing for geohazards within the CEOS activity called Geohazards Lab initiative.
About the Copernicus Sentinels
The Copernicus Sentinels are a fleet of dedicated EU-owned satellites, designed to deliver the wealth of data and imagery that are central to the European Union's Copernicus environmental programme.
The European Commission leads and coordinates this programme, to improve the management of the environment, safeguarding lives every day. ESA is in charge of the space component, responsible for developing the family of Copernicus Sentinel satellites on behalf of the European Union and ensuring the flow of data for the Copernicus services, while the operations of the Copernicus Sentinels have been entrusted to ESA and EUMETSAT.
Acknowledgements
This article was written with the contribution of Fabrizio Pacini of Terradue and Assistant Professor Michael Foumelis (AUTh).