Rupture parameters of the 21 May 2003, Mw 6.8, Zemmouri (Northern Algeria) earthquake deduced from InSAR

We study the surface deformation associated with the 21 May, 2003 (Mw=6.8) Zemmouri (Algeria) earthquake, the strongest seismic event felt in the region since 1716. Global and NEIC focal mechanism solutions of the mainshock indicate reverse faulting with a ~N60E trending rupture dipping ~ 45° SE. The earthquake epicenter relocated along the shoreline caused an average 0.50 m coastal uplift along ~50-km-long coastline. We mapped the coseismic surface displacement field caused by the earthquake using the ENVISAT ASAR (IS2) and RADARSAT standard beam (ST5) data. We were able to obtain coseismic interferograms from both the ascending and descending orbits of ENVISAT satellite. The RADARSAT data proved useful only in the descending mode. While the two RADARSAT interferograms cover the entire area of coastal uplift, ENVISAT data cover only the western half of the epicentral zone. Although the InSAR coherence in the epicenter area is poor, deformation fringes are observed along the coast in different patches. In the Boumerdes area, the maximum deformation is indicated by the high gradient of fringes visible in all interferograms in agreement with field measurements (tape, DGPS and leveling). To constrain the earthquake rupture parameters, we model the interferograms and uplift measurements using elastic dislocations on triangular fault patches in an elastic and homogeneous half space. We invert the coseismic slip using a curved surface constructed from triangular elements using Poly3Dinv program that uses a damped least square minimization. The fault rupture is 65-km-long and dips ~40° to the south with a smooth change in strike north of Boumerdes from N 60°-65° to N 95°-105°.