Array observations of microtremors and aftershocks were carried out near the permanent strong-motion observation sites and the damaged areas, after the Kocaeli, Turkey, earthquake of 17 August 1999. The major objectives were to determine S-wave velocity structures at the sites and to understand the site effects on strong motion or damage. Array observation of microtremors is a useful method for determining the S-wave velocity structures in a sedimentary basin, with less practical restriction than the other geotechnical methods. The spatial autocorrelation method (SPAC) was applied to array data of microtremors for determining S-wave velocity structures. The SPAC method generally provides an equivalent result with that of the frequency-wavenumber method, using fewer array sites and a smaller array size.