The 1979 Imperial Valley earthquake caused the Imperial fault to rupture 10 to 15 km southeast of the epicenter, in addition to the well documented rupture to the northwest. This is established from the properties of the initial S-wave arrival at strong-motion stations in Mexico at the southern end of the rupture zone. First, the amplitude of this S-velocity pulse is larger at stations 10 to 15 km southeast of the epicenter than at those within 2 km of the epicenter. Second, the pulse shapes and polarizations at three stations south of the epicenter are consistent with southeastward propagation. Third, travel time considerations rule out contributions to these pulses from faulting originating more than 4 km northwest of the epicenter.