Survey of Geological Structure at Adapazari, The Concentrated Damage Area During 1999 Kocaeli, Turkey, Earthquake

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The bedrock structure of the basin of Adapazari was studied on the basis of Bouguer gravity anomaly, SPAC and H/V analyses of microseisms, and seismic refraction survey. The main results are as follows :(1)the bedrock subsides stepwise at the edge of depressions ;(2)the basin consists of three narrow depressions of the basement of E-W or NE-SW trending. The depth to bedrock reaches 1000-1500 m ;(3)3D model of deep ground structure of Adapazari basin based on the results of gravity survey, SPAC and H/V analyses of microseisms, and seismic refraction surveys ;(4)simulation of Kocaeli earthquake was performed using Finite Difference method based on the 3D model. The results show that strong motions, which brought severe damage to the town, is considered to have been produced by amplification due to the specific configuration of bedrock, that is, focusing and/or so-called basin-edge effect.It is pointed out that investigation of 3D bedrock configuration is an urgent issue for microzoning an urban area on a sedimentary basin, and for this, comparative analysis of gravity, microseisms and efraction wave is highly prospective, because we can obtain more reliable structural models using different kinds of physical parameters: seismic impedance and density distribution.The surveys of shallow soil profile at the down town was also performed. The results are as follows ;(1)any soil layer of loose sand was not found. This fact shows that typical liquefaction was not considered to occur during the mainshock ;(2)thickness of very soft soil layers, at which probe of Swedish sounding sink by its own weight, varies in the town. It is considered to affect the difference of damage of building between inertia type damage and settlement type damage.