Two major earthquakes occurred in Turkey along the North Anatolian fault in 1999. The first one, which occurred on 17 August 1999, had a moment magnitude of 7.4 and ruptured the 140-km segment of the fault in the Marmara region. Adapazari, a city with a population of 190,000, which is mostly located on a deep alluvial basin in the near field of the ruptured fault, was among the worst-affected urban areas in the earthquake-affected region. The distribution of damage over the city was highly nonuniform, indicating the variability of the ground response to the strong motion. Five- to six-story buildings located over deep alluvial soils were most adversely affected by the earthquake. Geotechnical characteristics of the alluvial basin of Adapazari are evaluated using the deep and shallow borehole Iogs and the measured parameters from the field and laboratory. This data is used for developing representative one-dimensional site-response models for various depths of alluvium and for liquefaction assessment in saturated surface soils. Recorded data from several aftershocks are utilized for model calibration. The results of soil-response studies are found to be highly Correlated with the general trends in the intensity and distribution of building damage. Based on in situ tests, soil liquefaction is determined to have occurred extensively in Adapazari which is in conformity with the postearthquake observations of widespread liquefaction-induced foundation displacements. Evidently, liquefied soil layers served as passive isolation mechanism for numerous buildings, some of which were subjected to excessive foundation displacements of various forms without signs of significant structural damage.