Re-evaluation of Fault Geometry and Slip Distribution of the 1944 Bolu-Gerede Earthquake Rupture, North Anatolian Fault System, Turkey

The successive earthquakes along the North Anatolian Fault System in 20th century provides us fundamental data on fault segmentation, characteristics faulting behavior, and dimensions and scaling laws of faulting. In this point of view, we re-evaluate geometry and slip-distribution of the Ms7.3 Bolu-Gerede earthquake rupture of 1944, which has not been studied since 1970. The 1944 rupture is traceable for about 185-km, from 30-km west of Bolu (40.6N, 31.4E) to 20-km west of Ilgaz (41.0N, 33.4E) almost continuously and straightly, trending N75E. Amount of slip along the rupture varies between 2 and 6 m right-laterally. In the middle-east section of the rupture, east of Gerede, the slip is as large as 4.5 to 6 m. Along the other sections, the amount of slip decreases to about 2-4 m. The rupture can be subdivided into 5 to 7 geometrical segments of 10 to 45-km-long, which are separated by small separation, bend, step, push-up and pull-apart structures. At 6-km east of Iametpasa (40.9N, 32.7E), a series of foundation of a stone-bridge, which is built approximately AD 680+190-90 (Ikeda, 1994), has been offset for about 20 m. The 3.5 to 4.0 m slip-per-event during the 1944 earthquakes suggests that the 20 m displacement has been accumulated by 5 to 6 faulting events with an average recurrence interval of about 200 to 350 years. Re-evaluated average slip of the 1944 rupture is about 3.5 m that is almost twice of the previously well-known amount. The straightness and continuance of the fault strands foresee that the 1944 earthquake had more simple rupture process and shorter source time than those of 1999 Izmit earthquake of Ms 7.4.