German researchers say the main earthquake threat for İstanbul remains under the Sea of Marmara because a key part of the Main Marmara Fault is locked and storing strain.
The study reviewed the magnitude 6.2 earthquake in April and nearly 20 years of earthquake records from the same fault area. It said the April quake was the largest on that fault in more than 60 years but that it did not release enough strain to reduce the longer-term risk.
They found that earthquakes of magnitude 5 and above have moved east since 2011, ending closer to the “locked” section south of the city. In simple terms, parts of the fault to the west are slipping slowly, while the section closer to İstanbul is locked, meaning it is not sliding and is storing energy.
The researchers also pointed to an offshore area with little recent earthquake activity in the Avcılar segment, between the Kumburgaz Basin and the locked Princes Islands segment. They said aftershocks from the April quake ended near the edge of this quiet zone, leaving a section they say could be a candidate area for the next moderate or larger earthquake.
Another finding involves the direction earthquakes tend to break on this fault. The team said the April quake sent shorter, more energetic seismic waves toward the east, a pattern that can increase shaking in the direction the rupture travels. They said this matters for İstanbul if future earthquakes start west of the city and rupture eastward.
Lead author Patricia Martínez Garzón said the results do not predict timing. “This does not tell us when a major earthquake may happen,” but it points to parts of the fault that are becoming more stressed.
The GFZ Helmholtz Centre for Geosciences said the Sea of Marmara section of the North Anatolian Fault Zone has not produced a magnitude 7 or larger earthquake since 1766. It said written records in the region go back more than 2,000 years and suggest an average repeat time of about 250 years for large quakes.
Separate earthquake probability studies have estimated a 35 percent chance of a magnitude 7.3 or larger earthquake near İstanbul in a 30-year period, rising to 47 percent in models that include time dependence and stress transfer from the 1999 İzmit earthquake.
The 1999 İzmit earthquake, a magnitude 7.4 event on the North Anatolian fault system east of İstanbul, killed 17,127 people according to official estimates.
İstanbul is Turkey’s largest city. Official population data put it at 15,701,602 residents at the end of 2024.