Engineering assessment of seismic hazard and long period ground motions at the Bolu Viaduct Site following the November 1999 earthquake

Abstract

The Mw 7.1 earthquake of November 12, 1999 severely damaged a 2.5 km long viaduct on a stretch under construction of the Istanbul-Ankara motorway, 18 km W of Bolu. The fault rupture crossed the viaduct with an observed offset close to 1.5 m; the viaduct deck suffered large permanent displacements but did not collapse. Seismic hazard at the site appears to be governed by characteristic earthquakes occurring every few hundred years both on the Düzce fault and on the main stretch of North Anatolian fault (NAF). Smaller active faults near the site also contribute to the hazard. We investigate how a reliable design response spectrum of displacement can be constructed for the viaduct site using both probabilistic and deterministic tools. After checking the applicability of known attenuation relations against recorded data, we illustrate a hazard analysis that incorporates a characteristic earthquake model for the relevant faults, and accounts for statistical directivity effects on the Düzce fault. Constantrisk response spectra up to 3 s are thus obtained for different return periods. Synthetic motions at the site are generated next, using a known source model, in order to explore the features of the displacement response spectrum at vibration periods in the range between 1s and 6 s, in which the dominant response of the viaduct deck, uncoupled from the piers, occurs. A discussion of results follows.