In this paper we present a three-dimensional velocity structure of the Kozani-Grevena region, a relatively low-seismicity area of the western margin of the Internal Hellenides, which was struck by a MS=6.6 earthquake on May 13, 1995. P wave arrival times from 656 aftershocks are inverted to resolve the velocity structure of the upper 12 km of the crust. The 1995, ruptured fault (Dheskati fault) lies at the southeastern end of a 35-km-long, normal fault (the Servia fault). The crustal heterogeneities identified by tomography help to constrain the geometry and structure of the 1995 fault and the rupture evolution during the mainshock. The aftershocks and the velocity structure define a fault dipping ~35¿ toward NW, which from 5 km depth splays upward into two nearly parallel shallow faults. The slip during the mainshock and the aftershocks all were concentrated along the deeper portion of the fault, while the limited slip observed at the surface occurred on the northern of the two shallower splays (the Paleochori fault). It appears that the lithological heterogeneities of the shallow crust are responsible for the splaying of the main fault upward. Our work stresses the importance of crustal structure in determining fault geometry and segmentation and rupture evolution.¿ 1997 American Geophysical Union