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To understand the generation mechanism of the Barn earthquake (Mw 6.6), we studied three-dimensional V-P, V-S and Poisson’s ratio (sigma) structures in the Bam area by using the seismic tomography method. We inverted accurate arrival times of 19490 P waves and 19015 S waves from 2396 aftershocks recorded by a temporal high-sensitivity seismic network. The 3-D velocity structure of the seismogenic region was well resolved to a depth of 14km with significant velocity variations of up to 5%. The general pattern of aftershock distribution was relocated by using the 3-D structure to delineate a source fault for a length of approximately 20km along a line 4.5km west of the known geological Barn fault; this source fault dips steeply westward and strikes a nearly northsouth line. The main shallow cluster of aftershocks south of the city of Bam is distributed just under the minor surface ruptures in the desert. The 3-D velocity structure shows a thick layer of high V-S and low sigma (minimum: 0.20) at a depth range of 2-6km. The deeper layer, with a thickness of about 2 km, appears to have a low V-S and high or (maximum: 0.28) from 6 km depth beneath Bam to a depth of 9km south of the city. The inferred increase of Poisson’s ratio from 2 to 10km in depth may be associated with a change from rigid and SiO2-rich rock to more mafic rock, including the probable existence of fluids. The main seismic gap of aftershock distribution at the depth range of 2 to 7km coincides well with the large slip zone in the shallow thick layer of high Vs and low sigma. The large slip propagating mainly in the shallow rigid layer may be one of the main reasons why the Barn area suffered heavy damage. (c) 2006 Elsevier B.V. All rights reserved.