Vibrating minarets reveal hidden earthquake damage in two Ottoman-era towers

When the ground beneath southern Türkiye began to shake on 6 February 2023, the twin earthquakes that struck Kahramanmaraş did more than topple modern apartment blocks. They also tested centuries-old masonry towers that had survived countless previous tremors. Among the structures standing in the shadow of the magnitude 7.7 and 7.6 events were the Yusuf Ziya Pasha and Sunullah Melik minarets, two historical masonry minarets whose slender stone bodies experienced the full force of one of the most destructive earthquake sequences in modern Turkish history. A new study published in the Bulletin of Earthquake Engineering has now traced, with remarkable precision, exactly what those hours of shaking did to the two towers, using nothing more exotic than the minarets’ own ambient vibrations and a carefully calibrated computer model.

The research, carried out by Onur Onat of the Department of Civil Engineering at Inonu University in Malatya, rests on a deceptively simple idea. Every structure, from a guitar string to a skyscraper, has a set of natural frequencies at which it prefers to oscillate. These frequencies depend on the structure’s mass and stiffness, so when an earthquake cracks masonry, opens bed joints, or otherwise softens a building, the natural frequencies drop. By measuring the minarets’ vibration signatures before and after the Kahramanmaraş sequence, engineers can effectively ask the towers themselves how much damage they sustained, even in places the eye cannot see.

Ambient vibration measurement, the technique at the heart of the study, requires no artificial shaking. Wind, distant traffic, and micro-tremors in the ground provide a continuous, gentle excitation that is more than enough to coax a tall minaret into swaying at its natural frequencies. Sensitive seismometers recorded these barely perceptible motions at both towers, and signal processing identified the natural frequencies, damping ratios, and mode shapes, the characteristic patterns of deformation associated with each frequency, for the first four vibration modes of each minaret. The pre-earthquake and post-earthquake modal vectors were then compared using the Modal Assurance Criterion, a standard statistical measure of how similar two vibration patterns are.

The results tell a consistent story of measurable but not catastrophic degradation. The first four identified frequencies decreased by an average of 5.46 percent for the Yusuf Ziya Pasha minaret and 5.98 percent for the Sunullah Melik minaret. A frequency drop of roughly six percent may sound modest, but in structural terms it corresponds to a meaningful loss of lateral stiffness, the property that allows a slender tower to resist horizontal earthquake forces. Intriguingly, the damping ratios, which describe how quickly vibrations die away, showed no uniform trend, a reminder that energy dissipation in cracked masonry is a far messier phenomenon than stiffness loss.

Field inspections corroborated what the vibration data implied. At the Yusuf Ziya Pasha minaret, observers documented bending-induced tensile cracking in the lower body of the tower, exactly where the flexural demands of earthquake shaking would be expected to concentrate in a cantilever-like masonry shaft. At the Sunullah Melik minaret, the damage took a different form: sliding and separation along the bed joints, the horizontal mortar lines between stone courses, around the pulpit region. These distinct crack patterns illustrate a crucial point about masonry behavior, namely that the same earthquake can produce qualitatively different failure mechanisms in structurally similar towers depending on their geometry, construction details, and local ground conditions.

To translate these observations into a quantitative assessment of seismic performance, the study employed finite element models, numerical representations of each minaret built from thousands of interconnected elements. Rather than assuming material properties from textbooks, the researcher independently calibrated separate pre-earthquake and post-earthquake models using the measured fundamental frequencies of each tower. The models were then verified against the higher-mode frequencies and the modal vectors, ensuring that the numerical replicas matched reality not just in their simplest vibration pattern but across the full measured spectrum. This two-stage approach, calibration followed by independent verification, guards against the common pitfall of a model that fits one number by accident while being wrong everywhere else.

With trustworthy models in hand, the study moved from diagnosis to prognosis. Nonlinear time-domain analyses subjected both the pre-earthquake and post-earthquake models to five unscaled ground-motion records, letting the computer simulate how the towers would respond to real earthquake shaking in their intact and damaged states. Nonlinear analysis is essential here because masonry does not behave elastically under strong shaking; cracks open and close, joints slide, and stiffness degrades progressively as the motion intensifies. The models incorporated material behavior consistent with established plasticity formulations for masonry, allowing the simulations to capture the initiation and propagation of the very cracking patterns observed in the field.

The comparison between intact and damaged simulations produced one of the study’s most consequential findings. The post-earthquake stiffness reductions were reflected far more clearly in the deformation demands than in the global force responses, particularly under the stronger ground motions. In other words, a damaged minaret may carry roughly similar peak forces as an intact one, yet it must bend, sway, and distort substantially more to do so. Since deformation, not force, is what ultimately tears masonry apart, this result carries a sobering implication for heritage managers: towers that appear to have weathered an earthquake without collapse may nonetheless have crossed into a regime where the next strong event produces disproportionately larger displacements and more severe cracking.

Perhaps the most methodologically valuable conclusion concerns what vibration monitoring can and cannot detect. The combined measurement and modeling framework demonstrated that mode-dependent dynamic changes, the frequency shifts distributed across the first four modes, can identify earthquake-related stiffness loss even when the principal global mode shapes remain broadly preserved. This is good news for the growing field of structural health monitoring of cultural heritage, because mode shapes are notoriously difficult to measure accurately in the field, while frequencies can be tracked reliably with modest instrumentation. The study suggests that a monitoring program need not capture perfectly resolved shape changes to flag damage; a careful reading of frequency trends across multiple modes, interpreted through a calibrated model, suffices.

The implications extend well beyond two minarets in Malatya. Türkiye holds an extraordinary inventory of historical masonry minarets, and the 2023 Kahramanmaraş sequence damaged or destroyed many mosques and their towers across the affected region, prompting widespread post-earthquake assessments. The approach demonstrated here, pairing ambient vibration surveys with independently calibrated nonlinear finite element models, offers a repeatable template for triaging that inventory: identifying which towers have silently lost stiffness, quantifying how much seismic reserve remains, and prioritizing retrofit resources accordingly. As climate extremes and seismic activity continue to threaten unreinforced masonry heritage worldwide, the quiet hum of a minaret vibrating in the wind may prove to be one of the most eloquent damage reports a historic structure can give.

Subject of Research: Structural health monitoring and seismic performance assessment of historical masonry minarets damaged by the 2023 Kahramanmaraş earthquake sequence

Article Title: Structural health monitoring and seismic performance assessment of two historical masonry minarets before and after the 2023 kahramanmaraş earthquake sequence

Article References: Structural health monitoring and seismic performance assessment of two historical masonry minarets before and after the 2023 kahramanmaraş earthquake sequence. (n.d.). https://doi.org/10.1007/s10518-026-02721-4

Image Credits: AI Generated

DOI: 10.1007/s10518-026-02721-4

Keywords: structural health monitoring, masonry minarets, Kahramanmaraş earthquakes, ambient vibration, modal analysis, finite element model updating, nonlinear seismic analysis, natural frequencies, damage assessment, cultural heritage, earthquake engineering, Türkiye