It is well known that the mechanical and physical properties of subsoil layers as well as poor construction practices play an important role in determining the level of structural damage sustained from earthquakes. Thus, sufficient knowledge of these properties is essential for estimating the weak points of a region. The ground vulnerability index (K-g), an indicator of the weak points of a region, also has a substantial importance role in mitigating earthquake hazards in multidisciplinary studies. In this context, we have used engineering basement shear wave velocity (v(s30)), fundamental frequencies and peak amplitude values of the horizontal-to-vertical ratio (H/V) to calculate K-g values for the study area (Kocaeli Province, Turkey). Structural damage was investigated after the August 17, 1999 Kocaeli (M-w=7.4) Earthquake and compared with the calculated K-g values. The results indicate that the K-g values decrease towards the southern part of the study area, which is in good agreement with damage distribution from this earthquake. K-g values of >10 appear to be indicative of the most vulnerable areas in the study area. Structures built along the coastlines in Deirmendere area were the mostly severely damaged or collapsed entirely; we found that the subsoil beneath them have high K-g values of between 10 and 25 In contrast, K-g values in the southern districts, which sustained little or no damage, are relatively low (K-g<10). In our study, the K-g values and soil types showed good agreement. Potential vulnerable areas also have poor subsoil conditions. Therefore, it could be important to consider these results for the future planning and designing of urban areas. These results could also be used as a rapid way to detect potential vulnerable areas before the occurrence of a destructive earthquake.