Somehow, while the rest of the ancient Egyptian world has crumbled around it, the Great Pyramid of Khufu at Giza remains standing, even after 4,600 years.
Even through major earthquakes and the gradual loss of its gleaming white casing stones, the main structure of the pyramid endures, its granite and limestone blocks still firmly in place, almost as though the vast edifice were only just emerging from the throes of construction rather than slowly decaying.
Archaeologists are still trying to uncover the full complement of engineering ingenuities that led to the Great Pyramid becoming the sole surviving member of the Seven Wonders of the Ancient World.
A new revelation about the pyramid’s design could add another feather to Egyptian engineering caps.
According to new research, several properties of the structure could make it surprisingly earthquake resistant – whether the builders intended it or not. The fortifying features include the empty “relieving chambers” directly above the burial chamber of Pharaoh Khufu.
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The construction of the Great Pyramid was an absolute mammoth of an undertaking: It’s built of some 2.3 million stone blocks, weighing a total of around 6 million metric tons, some transported over a distance of hundreds of miles to get to the building site.
These blocks were then carefully placed and joined together to build a mostly solid structure standing about 147 meters (482 feet) high, with only a handful of hollow chambers hidden inside.
As a mostly solid structure, the pyramid is extremely sturdy, with its weight concentrated towards the ground and distributed across the base. That alone, however, would not make it immune to earthquake damage or the ravages of time.
Several pyramids have crumbled into at least partial ruin. The outer structure of the Meidum Pyramid, for example, collapsed dramatically in ancient times. The Pyramids of Userkaf, Sahure, and Unas resemble rough piles of rubble.
There is also evidence in Mesoamerica that pyramids – made of stone material that’s vulnerable to the shear stresses invoked by earthquakes – can be sundered by the rumbling of the ground beneath them.
Egypt is not especially prone to earthquakes, but at least two major ones have been recorded within an 80-kilometer (50-mile) radius of the Great Pyramid.
In 1847, an earthquake with an estimated magnitude of 6.8 rattled the region. In 1992, a magnitude 5.8 quake was recorded, shaking several casing stones loose from the top part of the Giza pyramid.
As the largest pyramid still standing and one of the oldest, the Great Pyramid got researchers wondering: Why has this enormous, ancient structure survived where others have fallen?
To investigate, a team led by seismologist Asem Salama of the National Research Institute of Astronomy and Geophysics in Egypt placed vibration sensors in and around the pyramid to determine how it hums in response to movements in the world around it.
They placed 37 portable accelerometers in various places: in the King’s Chamber and Queen’s Chamber, in the vertically stacked pressure-relieving chambers directly above the King’s Chamber, in passages and tunnels, on the outer stones, and on the ground around the pyramid.
These sensors measure tiny ambient vibrations from sources already omnipresent in the area – from distant traffic, the wind, ocean-wave energy traveling through Earth, and the imperceptible tremors constantly running through the crust.
In the ground surrounding the pyramid, these sources combined to create a consistent background frequency of around 0.6 hertz (Hz).
However, in most locations within the pyramid, the frequency was about 2.0 to 2.6 Hz.
This mismatch between the ground’s vibrational frequency and that of the pyramid itself could be part of the reason why earthquakes have done so little harm.
Because they don’t vibrate at the same frequency, seismic energy may transfer less efficiently between the ground and the structure, helping prevent the kind of resonance amplification that can severely damage buildings.
But, while the vibration was mostly consistent throughout the pyramid, with amplification increasing with height, there was one notable exception: the relieving chambers.
These are usually interpreted as a means of mitigating the weight bearing down on the King’s Chamber. In these chambers, the vibration amplification dropped sharply – suggesting that these chambers also redistribute stress and interrupt vibration.
Although their purpose may have been a load-bearing one, the finding suggests that these voids may also have inadvertently helped quake-proof the pyramid.
The pyramid – squat, weighty, and solid – behaves very differently from how buildings are designed to withstand earthquakes today; strategies that mostly revolve around flexibility.
Related: Mysterious Structure Found Buried Beneath an Ancient Egyptian City
The researchers are careful to note that any suggestion that earthquake resistance was an intentional part of the pyramid’s design is purely speculative at this point, but they seem keen to find evidence that this is the case.
“These findings present compelling quantitative evidence that ancient Egyptian architects possessed profound geotechnical understanding, optimizing structure design and site characterization to assure millennial-scale stability against seismic hazards,” the team writes in their paper.
In future works, the team plans to repeat some of the measurements at key locations which “gave little anomalies”, confident their findings “will affirm the Khufu Pyramid as both an architectural marvel and a testament to ancient seismic engineering principles relevant to modern geoheritage conservation.”
The findings have been published in Scientific Reports.
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