Massimo Barbieri - Geologist and remote sensing application specialist at sarmap s.a. (Switzerland)
Paolo Pasquali - Responsible for the SARscapeŽ technical development and engineering at sarmap s.a. (Switzerland)

The Afar region, spanning areas of Ethiopia, Eritrea and Djibouti, marks an active tectonic margin located on the Arabian, Nubian and Somali plate triple junction. This is an area, well known to geologists, where continental crust is splitting apart and the sea-floor spreading is beginning (fig. 1). This process is the same as that taking place during the Jurassic period (around 190 millions years ago) when the Mid Atlantic Ocean started opening.

Figure 1: Different stages of a new ocean formation. Modified from Ebinger, 2005

Dramatic evidence of ongoing opening on the Afar rift was recorded in September 2005, by scientists at Addis Ababa University, who observed intense seismic activity emanating from the area. The sequence of more than 160 powerful earthquakes lasted more than 1 week (fig. 2). At the same time, hundreds of crevices, volcanic vents and tectonic fissures opened up in the ground (fig. 3) along a 60-km-long zone.

The Ethiopian scientific team asked for the support of international scientists in studying this event, including those with experience in the analysis of Synthetic Aperture Radar (SAR) satellite data for mapping ground displacements. This aspect of the work was led by University of Leeds geophysicist, Dr. Tim Wright. The international team, which included experts from the UK, the US, New Zealand, France and Ethiopia, made also use of classical investigation means such as GPS, seismometers, and other geophysical and geochemical techniques to determine the properties of rock and magma below the surface and to monitor crustal movement.

The "multi technique" approach gave a unique chance to prove the reliability of SAR satellite data in monitoring and accurately measuring land deformations in tectonically active areas. "In its simplest form, you are taking two snapshots of the same place, separated by a period of time, to see how far they have moved apart", Dr. Wright said.

Figure 2 - Coloured and shaded relief map for northern Afar, and study area. Main figure shows the location of the dyke (red line) intruded along the entire Dabbahu magmatic segment, and Gabho (G) and Dabbahu (D) strato-volcanoes (red stars), superimposed on a coloured and shaded relief map derived from the Shuttle Radar Topographic Mission elevation model. Filled circles are earthquake epicentral locations (relative to the event marked by the yellow triangle) grouped before and after 24 September 2005. Most events occurred between 20 and 29 September 2005. The, 60-km-long Dabbahu, Alayta (A), Tat 'Ale (TA) and Erta 'Ale (EA) tectono-magmatic segments are the locus of Quaternary strain. To the Northeast, the Afar depression is bounded by the Northwest-Southeast trending Danakil horst (DH), and to the West, by the Western escarpment. Dashed box encloses the area shown in the SAR interferometric image (Fig. 4). The upper left inset shows the location of the study area with respect to East Africa, and the FURI seismic station near Addis Ababa (star). Modified from Wright et al., 2006

Figure 3 - Evidence of tectonic faulting induced by the intrusion of magmatic dikes. The small figure shows the volcanic vent where a small eruption took place. Photographs by (left) Liz Baker - Royal Holloway - and (right) Anthony Philpotts - University of Connecticut

The satellite data analysis consisted of the Interferometric processing (and further interpretation) of several Envisat ASAR (Advanced Synthetic Aperture Radar) acquisitions made over the area before and after the event.

Figure 4a - The image represents an interferogram generated from a pair of SAR acquisitions - made by the ASAR instrument onboard the ESA (European Space Agency) ENVISAT satellite - on May 6th and October 28th 2005, respectively before and after the seismic event occurred on September 26th. These data, which have been acquired in IS2 Mode, have a ground spatial resolution of around 25 meters and a scene coverage of around 100 X 100 Kilometres. Each colour cycle, from red to yellow, is called interferometric fringe. It corresponds to a displacement - in the satellite view direction - of half the observation wavelength (i.e. 2.8 cm for the C band operating onboard ENVISAT ASAR). The magnitude of the rift opening along the major rupture zone (thick red line), as measured by several ENVISAT ASAR interferometric pairs acquired along ascending and descending passes (respectively SE-NW and NE-SW satellite flying directions), was of around 8 meters and it probably occurred over the 10 day period of the seismic swarm. The area surrounded by the white box is shown in detail in fig. 4b.

Figure 4b

In order to increase both the temporal resolution (i.e reduced satellite revisiting time) and the imaged area coverage, a team from the University of Leeds and from sarmap s.a. (a Swiss company - and a cooperation partner of CREASO - specialised in software development and applications in the domain of remote sensing, particularly airborne and spaceborne SAR data) processed several tens of ENVISAT ASAR pairs acquired in ScanSAR - or Wide Swath - mode (scene coverage around 400 X 400 km).

The collaboration with the sarmap company was a unique opportunity for the team led by Dr. T. Wright, since it provided the chance to work with the SAR Interferometric processor SARscapeŽ, (copyright by sarmap s.a.; distributed by CREASO), which can also ingest data acquired in ScanSAR mode. The SARscapeŽ processor is the only software, which can be used to process the ASAR Wide Swath data starting from the level-0 (RAW product) format; it enabled us to have a much larger coverage in the satellite flight direction (i.e. more than thousand kilometres) per scene. The application of SAR Interferometry to Wide Swath data was the only way to observe the fast land displacements related to the initial post-rifting activity (fig. 5a and 5b, both generated with the help of SARscapeŽ).

"Because Envisat is routinely acquiring data in areas prone to earthquakes and volcanic eruptions, we were able to make very precise measurements of this rare phenomenon for the first time. The results from Envisat have been vital for guiding our ongoing field studies, and without the satellite data we would have had no idea of the scale of this event" Dr. Wright said. Special thanks go to the European Space Agency for the exceptional chance provided by making freely available large quota of ENVISAT ASAR data for scientific application purposes.

Figure 5a - ScanSAR Interferogram generated from the ENVISAT ASAR data pair acquired respectively on 13 October 2005 and 17 November 2005

Figure 5b - ScanSAR Interferogram generated from the ENVISAT ASAR data pair acquired respectively on 3 December 2005 and 7 January 2006

References:
1. Continental break-up: The East African perspective Astronomy & Geophysics 46 (2), 2.16-2.21 - Ebinger, 2005
2. Magma-maintained rift segmentation at continental rupture in the 2005 Afar dyking episode - Wright et al., 2006
3. Reviews by Massonnet and Feigl 1998, Burgmann et al, 2000)