How has Great Pyramid survived earthquakes for 4,600 years?

For more than 4,600 years, the Great Pyramid of Giza has stood almost unchanged on the edge of the Egyptian desert. A new scientific study suggests that its extraordinary resilience may stem not only from its immense size, but also from a combination of sophisticated construction, favourable geology, and a natural resistance to earthquake vibrations.

The study, published on 21 May in Scientific Reports, found that the Pyramid of King Khufu possesses dynamic and geological characteristics that may have protected it from severe earthquake damage over thousands of years.

At the centre of the research lies a simple but enduring question: why has the Great Pyramid remained remarkably stable despite millennia of earthquakes, environmental stress, and ageing?

To investigate, researchers used a non-destructive geophysical technique known as the horizontal-to-vertical spectral ratio (HVSR). The method measures tiny natural vibrations in the ground and structures, generated by wind, human activity, traffic or weak seismic movement. By analysing these vibrations, scientists can determine how buildings and the surrounding earth respond to shaking.

The research team conducted 37 measurements inside and around the pyramid, including readings from the King’s Chamber, the Queen’s Chamber, internal corridors and the upper relieving chambers above the King’s Chamber, as well as from the surrounding soil and bedrock.

The results showed that most sections of the pyramid vibrate at highly similar natural frequencies; mainly between 2 and 2.6 hertz, with an average of around 2.3 hertz. This indicates that the pyramid behaves less like a collection of individual stone blocks and more like a single, cohesive structure when subjected to vibration.

Assem Salama, associate professor in the seismology department at Egypt’s National Research Institute of Astronomy and Geophysics and co-author of the study, said the team was not surprised by the pyramid’s stability, given that it has already endured for nearly 5,000 years. What stood out, however, was the striking consistency of measurements across different parts of the monument.

That consistency, he explained, reflects a highly balanced dynamic response, an impressive characteristic from an engineering perspective.

The study also identified a crucial distinction between the pyramid and the ground beneath it. While the pyramid’s natural frequency is around 2.3 hertz, the surrounding soil vibrates at a much lower frequency, approximately 0.6 hertz.

This difference is significant because it reduces the likelihood of seismic resonance, a dangerous phenomenon that occurs when the ground and a structure vibrate at similar frequencies during an earthquake. In such cases, internal shaking can intensify dramatically, increasing the risk of severe damage or collapse.

In simpler terms, the pyramid and the ground beneath it do not “shake to the same rhythm”. That mismatch may have helped minimise the impact of earthquakes on the structure over centuries.

The study notes that the Giza region has experienced several earthquakes over time, including a powerful earthquake near Fayoum in 1847, estimated at magnitude 6.8, and the 1992 earthquake, which measured 5.8. The latter caused some outer stones to fall from the tops of the pyramids but did not trigger structural collapse in the Great Pyramid.

Another critical factor is location. The pyramid was constructed on a solid limestone foundation rather than softer soil. According to the study, this geological setting likely played a major role in preserving the monument’s long-term stability.

Salama said the choice of site may have been among the most important engineering decisions made by the ancient builders. A strong rocky foundation can reduce uneven pressure, limit the amplification of seismic waves and help massive structures remain stable over exceptionally long periods.

The findings do not suggest that the ancient Egyptians possessed modern seismic engineering knowledge. However, they do indicate that the pyramid’s design, materials and location created conditions that enabled it to withstand natural forces with remarkable success.

More than four and a half millennia after its construction, the Great Pyramid continues to provide new insights, not only into ancient Egyptian engineering, but also into how monumental structures can survive in earthquake-prone environments for thousands of years.

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المصدر: Daily News Egypt