A fault in slow motion
The North Anatolian Fault (NAF) is a 1,500-kilometer-long, right-lateral strike-slip fault that slices across northern Turkey. It marks the boundary where the Anatolian Plate, to the south, grinds westward against the Eurasian Plate. This relentless motion, driven by the collision of the Arabian and Eurasian plates further east, averages about 20-25 millimeters per year. Most of this strain accumulates in locked sections of the fault, released suddenly and violently in major earthquakes.
Some segments, however, behave differently. The Ismetpaşa segment is an example where the fault moves through aseismic creep—a slow, steady slip that does not generate earthquakes. This creep releases a significant amount of the accumulated strain, with rates measured between 8 and 13 mm per year. This silent movement is continuously monitored by networks of high-precision GPS stations and satellite-based InSAR (Interferometric Synthetic Aperture Radar), which can detect surface deformation down to the millimeter scale. Data shows that even on this "creeping" segment, the slip is not uniform and may only occur in the upper 7 kilometers of the crust. Below this depth, the fault is likely locked, still accumulating stress.
The seismic gap and Istanbul
The study of creeping segments like Ismetpaşa helps researchers understand the hazard posed by locked segments. The most dangerous of these is the "seismic gap" located beneath the Sea of Marmara, just 20 kilometers south of Istanbul. This segment has not produced a major earthquake since 1766 and is currently locked, accumulating stress. Scientists estimate a slip deficit of several meters has built up, enough to generate an earthquake with a magnitude of 7.0 or greater.
The North Anatolian Fault is marked by a westward progression of major earthquakes. Since a devastating earthquake in Erzincan in 1939, a series of powerful ruptures has migrated west along the fault line. The magnitude 7.6 İzmit earthquake in 1999 was the last major event in this sequence, rupturing a segment just east of the Marmara Sea gap. This event, which lasted about 45 seconds, caused over 17,000 deaths and resulted in ground offsets as large as 5 meters. Analysis of stress changes after the İzmit quake indicates that it transferred additional strain to the already-stressed Marmara segment, increasing the probability of a future rupture.