DEVELOPMENT OF A 3-D TOPOGRAPHICAL BASIN STRUCTURE BASED ON SEISMIC AND GEOTECHNICAL DATA: CASE STUDY AT A HIGH SEISMICTY AREA OF GÖLYAKA, DÜZCE, TURKEY

The near-fault situation and high seismicity of an area makes the determination of the bedrock geometry more complex, and in return, it makes this area much more critical and important for the studies of site response analysis in account of seismic hazard assessment. Without a good model of topography and basin structure, any powerful calculation method would be in lack of a well-developed basin response. The study has been conducted in the Gölyaka basin that uniquely falls within the bifurcated section of the North Anatolian Fault System. The surface rupture of the 1999 Earthquakes of Kocaeli (Mw=7.4) and Düzce (Mw=7.2) bound this tectonically formed basin, respectively. In the study area, a 3-D basin structure of the model has been developed based on the geophysical and geotechnical data along with the geology. In particular, combined active and passive surface wave methods along with the H/V microtremor measurements at 29 locations, the vertical electrical sounding at 14 locations, the geotechnical boring data at about 30 locations and one deep boring data have been used to develop the structure of the sediment basin. The high-resolution 1-D shear wave velocity profile has been obtained through surface wave methods by using active Multichannel Analysis of Surface Waves (MASW) and passive Microtremor Array Method (MAM), respectively. The Schlumberger Vertical electrical sounding (VES) method has been applied to evaluate the bedrock of the basin depth. Furthermore, the H/V measurements have been carried out to be correlated with the fundamental period of the deepest point of the constructed basin. Finally, geotechnical and deep boring data have also been used along with the basin geology to be cross-referenced with the 1-D Vs profile. The result of this comprehensive survey led to a well-developed 3-D geometry model of the Gölyaka basin. The 3-D basin geometry model has been developed from the seismic surface wave results and later correlated with the results of the VES, deep Gölyaka basin fundamental periods and geotechnical boring data. Based on the surface wave results and the 2-D velocity profiles obtained, a bedrock level with an average velocity of 1100 m/s was accepted as the bedrock depth limit in the region. The implemented deep and geotechnical boring data with seismic results suggested that Gölyaka basin is composed of about five to seven sedimentary layers. It was concluded that the thicknesses of the alluvial sediments within the Gölyaka basin were estimated to be about 200-350 m, with irregular basin geometry and heterogeneity due to over-step faulting near the basin boundary. Furthermore, this in return, has resulted in inclined layering and nonlinearity in the velocity profile. The constructed model can successfully be used for site effects characterization and response studies in the region