Ground motion directivity inferred from minaret damage during the 1999 Kocaeli and Duzce, Turkey earthquakes

Forward directivity of ground motion is one of important characteristics in near-source ground motion study. Dominance of the fault normal (FN) component has been reported based on the observation records obtained in the source area (e.g. Somerville et al, 1996). The author performed the damage survey of the two Turkey earthquakes in 1999, the Kocaeli and the Duzce earthquakes, and realized that minaret is a good target to investigate the ground motion directivity since a minaret is a simple cantilever structure with no structural directivity. This paper describes the ground motion directivity based on survey of minaret damage during the two earthquakes in Turkey. It is found from the damage survey that the fault normal direction was dominant with a certain distance from the fault, say, less than 1 km. While, at the region with very shorter distance, the damage direction was not fault normal but was about 40°from the fault strike. The direction seems to be similar to the distortion of principal direction of the ground motion in the focal region of the 1997 Northwestern Kagoshima earthquake (Watanabe et.al, 1999). Discussion is addressed for the specific damage direction inferred from the minaret damage near the fault.