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InSAR analysis allowed to map the 2006 M7 Machaze surface rupture in Mozambique

Mozambique is a country which has suffered a civil war during more than 15 years (1976-1992), with extensive minefields. This prevented somewhat a complete post-seismic survey after the large M7 earthquake that occurred in 2006, February 22nd, in the south of the country.
However, a British team from the Imperial College (Fenton and Bommer, 2006, Seismo. Research Letters) could visit the area within a few weeks after the mainshock and identified a short section of the surface rupture.
Location of the 2006 earthquake (Raucoules eet al. 2010), with focal mechanism and surface rupture inferred from InSAR.

Fortunately, the whole area was covered by SAR data which helped in studying the coseismic + postseismic surface deformation (e.g. ENVISAT 6th June 2004 - 7th May 2006), as well as only post-seismic deformation  (e.g. ENVISAT May-June 2006). We here briefly document the results from two studies (Raucoules et al 2010, Remote Sensing and Environment; Copley et al 2012, J. Geophys. Research).

This normal-faulting earthquake occurred in a quite uninhabited region, then causing "only" 4 deaths and 36 injuries. The focal mechanism, together with the presence of several North-South morphostructures. This area, according to several authors, is in the southern continuation of the Western Branch of the Eastern African Rift Valley.
The InSAR data analyses, as performed by the two teams, came out with similar conclusions. The fault source is formed of two segments, with a total surface rupture length of about 40 km (see figure below). Major displacement occurred along the southern one, where field observations (Fenton and Bommer, 2006) observed 1-2 m of maximum vertical throw.
Epicentral region of the 2006 Machaze quake, from Copley et al., 2012. Green and red circles are aftershocks epicenters. Red star depicts the mainshock. The small focal mechanism are those of aftershocks (from various references), labeled with their hypocenter depth. The blue line shows the surface fault location, inferred from post-seismic InSAR analysis. White stars are the locations of main field observations by Fenton and Bommer (2006). The white arrows in the northeast show cumulative fault scarps visible in morphology.The coseismic interferogram is quite noisy, and the surface trace could only be interpreted with the post-seismic analysis of InSAR.
2004/04/06 - 2006/05/07 InSAR interferogram (ASAR ENVISAT data). One fringe represents 2.8 cm of displacement in the line of sight. The fault trace has been inferred from the post-seismic InSAR signal.
The InSAR source model is a 40 km long and 20 km wide fault plane, dipping of about 70° to the west . Maximum displacement occurred at depth (around 4-5 meters).
Slip distribution obtained from the InSAR data analysis (Copley et al., 2012). The fault plane extends over 40 km long and 20 km wide. Maximum displacement, around 4-5 meters, occurred at depth.
The surface trace can be followed on Google Earth over 7 km, in the area of the observation points at the northern tip of the southern segment, as reported by Fenton and Bommer (2006). There, the fault scarp facing to the West is still visible on the 2013 Google Earth image.


Illustration of the 1.5 m high scarp, facing to the west, from Google Earth image (top) and on the field (picture from Fenton and Bommer, 2006). Approximate coordinates: 21.172° S; 33.349°E.Around 5 km south of the above observation point, the Google Earth picture (2013) also depicts the surface rupture with several right-stepping strands.
Right-stepping fault strands in the northern part of the 2006 earthquake fault southern segment, as can be seen on 2013 Google Earth pictures. Approximate coordinates: 21.213° S; 33.357°E.



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