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The Lake Natron (Tanzania) July 2007 rifting event revealed by InSAR

Nicolas d'Oreye(1), Anneleen Oyen(1), Christelle Wauthier(2), Eric Calais(3), Valérie Cayol(4), Francois Kervyn(2), Evelyn Mbede(5) and Elifuraha Sariah(5)

(1) National Mus. of Nat. History, 19 rue Josy Welter, 7256 Walferdange, Luxembourg
(2) Royal Museum for Central Africa, Leuvensesteenweg 13, 3080 Tervuren, Belgium
(3) Purdue University, west lafayette, IN 47907-1397, United States
(4) Univ. Blaise Pascal, CNRS-UMR 6524, 63038 Clermont-Ferrand cedex, France
(5) Uni. Dar Es Salaam, P.O.BOX 35052, Dar Es Salaam, Tanzania

Abstract

Thanks to a systematic programming of SAR data acquisition over some active African volcanoes in the frame of ESA projects (ESA Cat-1 nr 3224, ESA/JAXA ALOS nr 3690), a major rifting event was captured in the area between Lakes Natron and Manyara, northern Tanzania in July 2007. The longest ruptures extend over more than 15 km and the deformation affects an area of about 60 x 60 km centered on the Southern flank of the Gelai volcano and extends up to the Lengai volcano (which entered in eruption in early September).

The main deformation pattern is observed on interferograms spanning a Mw 5.9 pure normal faulting Earthquake that struck Northern Tanzania on July 17th 2007 at 14:10:42 UTC (2.586°S; 36.281°E; USGS-NEIC). An ENVISAT interferogram (July–August 2007) shows 16 fringes with positive range change in the eastern half and 6 fringes with negative range change in the western half. The central part, bounded by NNE-SSW linear discontinuities that most likely represent normal faults traces at the surface, shows at least 20 fringes with negative range change indicative of subsidence. (One fringe represents a 2.8 cm satellite-ground range change at a nadir angle of about 40°).

Interestingly, an interferogram spanning a period prior to the main shock (April -July 2007) shows 4 fringes of negative range change in an elliptical pattern collocated with the “axial graben” identified above. And more interferograms spanning periods after the main event show also interesting deformations demonstrating that the area is still deforming at a significant rate and suggesting that a dyke intrusion has been progressing.

These InSAR results are consistent with field observations of open fractures (Mbede and Sariah, personal communication), seismicity (Déverchère, Ferdinand and co-workers, personal communication) and GPS measurements (Calais, personal communication) and are indeed consistent with a dyke intrusion beneath the southern flank of the Gelai volcano.

Preliminary modeling also confirms that hypothesis to explain the current seismo-magmatic crisis (Calais, personal communication). In particular, none of the forward dislocation models tested for a magnitude 6 Earthquake (i.e. equivalent to the sum of the seismically released moment from July 1 to August 30) could explain the observed deformation (by InSAR and GPS) but underestimate it by a factor of about 5. More inversion modeling based on the combination of a 3D Mixed Boundary Element method and a neighborhood inversion algorithm is currently under process.

If confirmed by further measurements and models, this is (one of) the first dyking event ever captured geodetically in a continental rift. This event holds important clues to our understanding of the role of magmatic processes in the dynamics of continental rifting.

 

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry