The P wave velocity structure of the upper 9 km of the El Asnam, Algeria, fault zone, the site of the Ms7.3 earthquake (October 10, 1980), is obtained by inverting arrival times from 310 selected aftershocks. Computed crustal structure and relocation of aftershocks yield new insight on the structural geology of the area, and help to clarify the evolution of fault-related folding and the complexity of the rupture episodes during the El Asnam earthquake. The correlation between tomograms and geologic structures observed at the surface and inferred at depth delineates local heterogeneities of the upper crust. There is evidence for distinct large-scale structural trends, consisting of two main thrusts with antithetic reverse faults. Seismic tomography provides reasonable constraints on the El Asnam fault-related fold geometry, pointing out a complex preexisting structure along which the rupture evolution is controlled by the heterogeneous crust. We propose that the main shock originated along an inverted normal fault bordering the pre-Neogene basement. Lateral lithologic heterogeneities obtained by seismic tomography reflect the fault segmentation and geometrical discontinuities along strike, in agreement with the tectonic pattern.¿ 1997 American Geophysical Union