Empirical Attenuation Relationships for Western Anatolia, Turkey

Seismic hazard studies have become progressively more important for earthquake engineering applications in western Anatolia, which contains one of the world's best examples of a rapidly extending intra-continental tectonic regime. A two-stage regression analysis was applied to peak ground acceleration and 5%-damped spectral acceleration values of 168 recordings from 49 earthquakes in order to develop empirical attenuation relationships which can be used to predict ground motion for western Anatolia. Moment magnitudes for earthquakes range between 4.0 and 6.4 while the hypocentral distances range between 15 and 200 km in our dataset. Site classifications, as one of the predictor variables for the regression analysis, were obtained using horizontal to vertical spectral ratio estimates. These estimates, together with empirical attenuation relationship predictions, have shown that soil amplification is significant in western Anatolia. Attenuation relationship models that are obtained explicitly account for nonlinear sediment effects for deep soil sites in the region. Nonlinear effects of deep soil sites at lower periods are significant at the higher levels of shaking and manifest over-prediction for acceleration values, while they manifest lower prediction values at lower levels of shaking. Both results from the horizontal to vertical ratio method and the regression analysis show that the number of strong motion stations located on the rock sites in the region should be increased and the present site classification of strong motion stations in Turkey should be re-evaluated in detail. When obtained attenuation relation models were compared with the attenuation relationships based on data from tectonically similar regions, the attenuation relations modelled for a specific region could not, in general, be used in engineering applications realized for another region. Our results also indicate that the spectral acceleration model defined in the Turkish Building Code cannot adequately explain magnitude and distance dependencies in western Anatolia