How the Great Pyramids of Giza survived 4,500 years of earthquakes, study reveals

The Great Pyramid of Giza has been around for more than 4500 years. It had survived many earthquakes, including the 5.8 high-magnitude earthquake of 1992, which had shaken some of the outer casing of the pyramid, but the core remained intact. For years, it has been a baffling question for researchers how these architectural monuments stood the test of time. A study published in the Scientific Reports, by Egyptian geophysicist Asem Salama and his colleagues, provides insights into its performance during earthquakes and identifies some interesting features.

Seismic frequency mismatch

Researchers found a seismic frequency mismatch. Researchers measured ambient vibrations at 37 internal and external locations. They found that its natural frequencies, the frequency at which it likes to vibrate, range between 2.0 and 2.6 hertz. At the same time, the surrounding soil has a dominant frequency of around 0.6 hertz. The study used a method called horizontal-to-vertical spectral ratio analysis, or HVSR, which records tiny background motion from wind, traffic, human activity and natural ground vibrations.

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Internal shock absorbers

The study showed that the Pyramid's internal structures actively manage vibration to dampen the energy before it can compromise the internal structure. Just above the King's chamber in the pyramid, there are relieving chambers which reported reduced vibrations and act as major shock absorbers. Beyond Seismic activity, the granite formation of the structure reflects waves instead of absorbing them. Because of the dense nature of the granites due to high concentration of quartz crystal and their dense molecular structure, they also create acoustic effects.

What does it imply?

Every structure has its natural frequency. If the surrounding frequency during the earthquake matches the frequency, then it could amplify the motion, and this is called resonance, which would be catastrophic. Because the frequencies of the ground and the monument are completely mismatched, earthquake energy cannot efficiently transfer into or be amplified within the structure, thereby protecting it from collapse. But it's also to be noted that modern earthquake design relies on much more than just frequency comparison. Like the intensity of the shaking, the weight and flexibility of the structure, and the centre of mass of the structure. The Pyramid does have a favourable structure, like a broad base, a low centre of mass, a tapering form and a symmetrical plan. It is fair to conclude that the makers of the pyramid made some efficient design and engineering choices.