Precision relocation of hundreds of small earthquakes occurring along the Imperial Fault in Southern California during the last two decades reveals parallel steaks of seismicity at 9-km depth. These strands are spaced about 0.5 km apart within a 2 km wide zone of earthquakes near the brittle-ductile transition between the shallow locked part of the fault and a creeping zone at depth. These results suggest that the lower crustal shear zone below the Imperial Fault, site of major earthquakes in 1940 and 1979, must be at least two kilometers wide.