For nearly 1,500 years, the Hagia Sophia Grand Mosque has stood as a symbol of resilience in Istanbul, enduring the rise and fall of empires, powerful earthquakes and the passage of time. Yet, with the growing risk of a new major earthquake, concerns are mounting over structural vulnerabilities in the centuries-old monument and efforts are now underway to preserve it.
“Hagia Sophia is like a patient whose condition becomes more complex with age,” said architect Hasan Fırat Diker, a professor of architecture at the Fatih Sultan Mehmet Vakif University in the Turkish megacity.
Diker is a member of the Hagia Sophia Scientific Committee, a multidisciplinary team leading the most comprehensive restoration of the historic structure in the past hundred years. Working with precision and care, the committee aims to reinforce the building against future threats while respecting its architectural legacy.
Originally completed in the sixth century, the Hagia Sophia’s massive dome, one of the earliest constructed on pendentives, has collapsed and been rebuilt multiple times, including major failures in 558, 989 and 1346. Today, preservation experts are racing against time to ensure the landmark remains intact for generations to come.
Since then, the structure and dome have held firm, even as other buildings around it fell in the quakes of 1509, 1526, 1766 and 1894.
"Hagia Sophia is the heart of Istanbul,” said Diker, "Its continued preservation can be attributed to the efforts of every ruling power – from the Byzantines to the Ottomans and into the Turkish republic era – each doing their best within their economic means to protect it.”
The Hagia Sophia, a cathedral, museum and mosque at different times, is now undergoing its most extensive restoration since the founding of the Republic of Türkiye in 1923.
Initial work focused on surrounding Ottoman-era structures, but attention has now turned to the main building. "We know its most vulnerable points. Our reinforcement efforts will proceed accordingly,” said Diker.
One of the most significant issues lies in the four asymmetrical arches supporting the main dome – a result of earlier repairs and historic damage.
"Especially after the last two major collapses during the Byzantine era, the dome and arches had to be rebuilt. But they were made thinner than they should have been, which made the structure more fragile and in constant need of protection,” he explained.
One major threat during earthquakes is hammering, a phenomenon where two adjacent buildings or structural components collide with each other due to insufficient separation between them.
"During an earthquake, the two semi-domes built to support the main dome can start to sway. This movement may create hammering, which risks damaging the main dome instead of protecting it,” he said.
Engineers are now reinforcing connection points between the central dome and semi-domes to reduce this danger. "Even though Hagia Sophia has structural weaknesses, we’re not in a position to rebuild it from scratch. Instead, our goal is to intervene using the most appropriate methods for its current state and extend its life span,” he added.
The restoration includes cleaning the underground sections, restoring the dome and minarets and addressing long-standing structural vulnerabilities.
"In previous restorations, especially when Hagia Sophia was a museum, scaffolding around the whole building was often avoided to preserve the visitor experience,” he said. "Now, we’re taking a more holistic approach.”
Every phase of the project is being shaped by disaster scenarios, including a high-magnitude earthquake, he said.
"These earthquake simulations aren’t new – engineers have been studying this since the 1999 earthquake,” he said, adding that today, with advanced technology and data from major global quakes, they can model much more realistic interventions.
He emphasized that the project avoids unnecessary intervention that might compromise the building’s character. "We want to protect it, but we don’t want to alienate it,” he said.
Ilknur Türkoğlu, a cultural heritage expert, said Istanbul’s seismic risk is well established.
"Though Istanbul does not sit directly on a major fault line, its surrounding region is highly active, causing significant earthquakes that have historically caused damage to the city, like the 1999 Gölcük and Düzce earthquakes,” Turkoglu said.
"With such a large and dense population, the damage and casualties from a similar earthquake today would be far greater,” she warned.
As the capital of the Roman, Byzantine and Ottoman empires, Istanbul's cultural wealth adds another layer of risk. Many structures are centuries old and vulnerable.
"Knowing exactly what we have is the first step,” said Turkoglu, an assistant professor at the Istanbul Gelişim University’s Department of Architecture.
"Unfortunately, Istanbul still lacks a comprehensive inventory of its cultural assets, both above and below ground.”
She called for a systematic review of the earthquake resilience of such sites, and for immediate reinforcement where risks are identified.
Museums housed in historic structures also need urgent planning. "We need to conduct earthquake drills, ensure artifacts and display cases are secure and check that emergency systems like automatic shut-offs for electricity, gas and water are in place,” she said.
She added that even if walls remain standing, unsecured objects inside can still injure or kill.
The situation following the 1999 and February 2023 earthquakes offered a preview, she said, of what could happen without better preparedness, including plans to protect or relocate valuable artifacts.
