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Coseismic and Postseismic Displacements From the Mozambique Earthquake of 22 February 2006 Detected by InSAR

Manabu Hashimoto(1), Yo Fukushima(1) and Taku Ozawa(2)

(1) Kyoto University, Gokasho, Uji, 611-0011, Japan
(2) NIED, Tennodai 3-1, Ibraki 305-0006, Japan

Abstract

A Mw7.0 (USGS) earthquake occurred on 22 February, 2006 (UT), in the southern part of Mozambique, Africa. Its epicenter was estimated to be (21.2S, 33.3E) by USGS(2006). Fenton and Bommer (2006) made a reconnaissance survey in the epicentral region and reported the distribution of surface ruptures of up to 2m. Unfortunately they could not survey the total extent of surface ruptures, since its southern part is located in an area of landmines.

The western branch of East African Rift reaches the northern Mozambique but its southern extension is unclear. There has been a long controversy on the southern extension of the rift, especially in relation to the triple junction in the Indian Ocean. CMT solution of this event is of normal fault type with the strike of nearly north-south (e.g. USGS, 2006), which is consistent with an east-west extension. This earthquake occurred close to the southern extension of the rift and is considered to be related to the activity of this rift. Therefore it is important to investigate the fault geometry and characteristics of slip distribution of this event.

We tried to detect coseismic and postseismic displacements from this event and collected Envisat SAR images acquired before and after its occurrence. We obtained two interferometric images of coseismic displacement fields by analyzing pairs of scenes taken on November 11, 2003 and May, 7 or June 11, 2004 (Figure 1). We detected at least 16 fringes (>44.8cm LOS displacement) near the southern extension of the presumed surface ruptures. The extent of fringes is much larger than that expected from the surface rupture. This observation suggests that the source fault is elongated in both the north and south direction from the surface rupture. Unfortunately there were no clear fringes in the northern part due to poor coherence. We also obtained an interferometric image of postseismic displacements from the pairs of scenes taken on May 7 and June 11, 2006, and June 11, 2006 and February 11, 2007. There is a sharp discontinuity in fringes longer than 50km in the epicentral region, which probably corresponds to the fault trace. We also recognized depression on the west side of the surface rupture during the postseismic periods.

We also analyzed ALOS/PALSAR data acquired on Dec. 26, 2006 and Feb. 10, 2007. Any significant deformation can not be recognized in the obtained interferogram. The first acquisition by ALOS/PALSAR was made 9 months after the quake. This observation suggests that postseismic deformation faded away within 9 months.

We tried to model the coseismic displacements using dislocation model on the basis of teleseismic body wave model by Yamanaka(2006) and coseismic rupture detected by Fenton and Bommer(2006). We adopted two segments with the dip angle of 75 degree which is concordant with CMT solutions. We also fixed rake to be -80 degree (normal faulting). By trail and error we estimated the total length of 27km, width of 20km, slip of 2.0-2.7m. The seismic moment is evaluated to be 3.9E19Nm (Mw=7.0).

 

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