Twenty seven buildings collapsed and 2076 buildings were heavily damaged during 17 August 1999 Izmit Earthquake in Avc1lar (west of Istanbul), which is nearly 100 km from the epicenter of the earthquake. The geology of Avc1lar consists of Upper Miocene poorly bounded conglomerate and sandstone (Cukurcesme formation), claystone with sandstone and limestone interbedding (Gungoren member) and limestone with sandstone and claystone interbedding (Bak1rkoy formation). Lithological and geotechnical parameters of these formations in Avc1lar are not different from non damaged parts of the western Istanbul such as Zeytinburnu, Bak1rkoy and Beylikduzu, but these formations were cut by several faults in the damaged area. Collapsed and damaged buildings are located on this fault zone. Thus, cause of large amplification and damage in Avc1lar might be related with this fault zone because the fault zone behaves as a waveguide trapping seismic energy.