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

The M_w 7.1 earthquake of November 12, 1999 severely damageda 2.5 km long viaduct on a stretch under construction of theIstanbul-Ankara motorway, 18 km W of Bolu. The fault rupture crossedthe viaduct with an observed offset close to 1.5 m; the viaduct decksuffered large permanent displacements but did not collapse.Seismic hazard at the site appears to be governed by characteristicearthquakes occurring every few hundred years both on the Düzce faultand on the main stretch of North Anatolian fault (NAF). Smaller activefaults near the site also contribute to the hazard.We investigate how a reliable design response spectrum of displacement canbe constructed for the viaduct site using both probabilistic and deterministictools. After checking the applicability of known attenuation relations againstrecorded data, we illustrate a hazard analysis that incorporates acharacteristic earthquake model for the relevant faults, and accounts forstatistical directivity effects on the Düzce fault. Constant-risk responsespectra up to 3 s are thus obtained for different return periods. Syntheticmotions at the site are generated next, using a known source model, inorder to explore the features of the displacement response spectrum atvibration periods in the range between 1s and 6 s, in which the dominantresponse of the viaduct deck, uncoupled from the piers, occurs. Adiscussion of results follows.