VS30mapping and soil classification for seismic site effect evaluation in Dinar region, SW Turkey

The Dinar earthquake (MS= 6.1) of 1995 October 1 killed 90 people and destroyed more than 4000 buildings. Despite the moderate size of the earthquake, the level of damage was extremely high, which led to many studies that were carried out in the region. The majority of these studies concluded that the main reasons for the damage were the construction errors and the poor soil conditions. However, at that time no appropriate soil condition map based on extended, high density measurements was available. Shear wave velocity is an important parameter for evaluating the dynamic behaviour of soil in the shallow subsurface. Thus site characterization in calculating seismic hazards is usually based on the near surface shear wave velocity values. The average shear wave velocity for the top 30 m of soil is referred to as VS30. For earthquake engineering design purposes, both the Uniform Building Code (UBC) and Eurocode 8 (EC8) codes use VS30 to classify sites according to the soil type. The Vs30 values calculated by using multichannel analysis of surface waves (MASW) were used to create a new soil classification map of the Dinar region. Surface seismic measurements were carried out at 50 locations mostly in Dinar city and its surroundings. The dispersion data of the recorded Rayleigh waves were inverted using a Genetic Algorithm (GA) method to obtain shear wave velocity profiles of the investigated sites. Thus the derived Vs30 map of the Dinar region was transformed to the UBC and EC8 standards. Soil classification results show that most parts of the region, located in alluvial basin, have low shear wave velocity values. These values are within the range of 160–240 m s−1 and thus fall into the SD and SE categories according to the UBC and the C and D categories according to EC8. Within the region, some parts located on the hill zone and the transition zone have better soil conditions corresponding to SC (UBC) and B (EC8) categories and have comparatively high shear wave velocities in the range of 500–740 m s−1 and 350–450 m s−1, respectively. VS30 and soil classification maps were compared with the damage distribution associated with the earthquake. In possession of a detailed shear wave velocity map of Dinar City, in general, the results show that there is a correlation between the VS30 values and the damage distribution of the region. In addition to the low VS30 values, the likely causes of the damage were investigated, and it is observed that one of the major factors for high levels of damage is 3-D variations of geological structures.