North Anatolian Fault

Fault geometry and segmentation

The North Anatolian Fault is a mature, linear plate boundary, but its trace is not continuous. Instead, it is divided into a series of distinct fault segments separated by structural complexities such as step-overs, fault jogs, and bends. These geometric irregularities act as barriers that regulate the nucleation, propagation, and termination of large seismic ruptures along the fault system.

Releasing step-overs and rupture barriers

During large strike-slip earthquakes, dynamic ruptures propagate along individual segments but can be stopped or delayed when they encounter step-overs. Empirical observations and dynamic modeling along the fault indicate a critical step-over width of approximately 3 to 4 kilometers:

• Rupture propagation: Releasing step-overs with a width of less than 2 km are typically bypassed. During the 1999 Izmit earthquake, the dynamic rupture successfully jumped across the 1-to-2-km-wide Sapanca step-over to propagate from the Sapanca segment onto the Sakarya segment.
• Rupture arrest: Releasing step-overs wider than 4 km act as highly effective physical barriers. In the same 1999 event, the westward-propagating rupture was completely arrested by the 4-to-5-km-wide Karamürsel step-over basin, preventing the rupture from jumping onto the Yalova segment further west.

Multisegment ruptures

While individual segments of 30 to 100 kilometers in length often control smaller earthquakes, major seismic events on the fault typically involve multisegment ruptures that break through multiple geometric barriers. For example, the 1944 Bolu-Gerede earthquake ruptured approximately 180 km of the fault, breaking through five distinct structural segments (the Bolu, Yeniçağa, Gerede, İsmetpaşa, and Bayramören segments) separated by minor bends and jogs.