Slip distribution, fault geometry, and fault segmentation of the 1944 Bolu-Gerede earthquake rupture, North Anatolian fault, Turkey

The 1944 Bolu-Gerede earthquake (M_s 7.3) occurred along the North Anatolian fault in northern Turkey and ruptured for about 180 km of the fault. The surface slip distribution and fault geometry, which provide fundamental data on detailed fault segmentation and recurrence of faulting during a multisegment rupture, are poorly known for this event. We conducted geomorphological and geological investigations and eyewitness interviews to determine cumulative offsets through several earthquake cycles. The 1944 rupture comprised five fault segments based on slip distribution and fault step-overs and bends. From west to east, they are called the Bolu, Yeniçaǧa, Gerede, İsmetpaşa, and Bayramören segments. The segments were 21 to 46 km long, and the average slip was 1.9-4.3 m, similar to well-examined segments of the 1999 İzmit rupture. In contrast, the fault jogs were relatively small, suggesting that the 1944 earthquake rupture process was relatively smooth and of short duration. Proposed fault segmentation is consistent with the extent of surface ruptures and damage intensity associated with historical earthquakes in 967, 1035, and 1050 A.D. Historical records indicate that the Bolu and Yeniçaǧa segments ruptured in 967 A.D., and the Gerede, İsmetpaşa, and possibly, Bayramören segments ruptured in 1035 A.D. Therefore, the 1944 rupture occurred as a consequence of multisegment rupture, which consists of at least two distinct earthquake segments. Along the Gerede segment, which slipped 3 to 6 m in 1944, cumulative offsets of 6 to 25 m were observed. These offsets are double, triple, and quadruple multiples of the 1944 slip, implying a similar slip distribution during the four most recent large earthquakes along this segment and supporting the concept of characteristic slip, even though the rupture length varied greatly during the last four earthquake cycles.