Egyptian pyramids were built to withstand earthquakes

On August 7, 1847, a major earthquake struck Egypt. With its epicenter in the Fayum region, 100 kilometers (62 miles) south of Cairo, historical records report dozens dead and hundreds of buildings destroyed. But the pyramids remained standing. Some, like the Great Pyramid of Giza — the tomb of Egyptian Pharaoh Khufu — have withstood tremors for about 4,600 years. An analysis of vibrations inside the tomb published Thursday in Scientific Reports shows how its design dampens outside vibrational frequencies, preventing amplification of quake effects.

“Historical accounts indicate the earthquake loosened many of the remaining outer casing stones of the pyramids of Giza, and some blocks fell off,” says Mohamed ElGabry, a researcher at Egypt’s National Research Institute of Astronomy and Geophysics (NRIAG) and the study’s lead author. “However, the main body of the Great Pyramid [and the other pyramids] remained largely intact and structurally sound,” he adds.

Architects and engineers have long noted the stability of Egypt’s pyramids; for millennia, they were the tallest structures humans had ever erected. Some reasons are obvious: a pyramidal shape — with each side of the base measuring 230.33 meters and tapering as it rises to an original height of 146.59 meters, topped originally by a tiny pyramidion (now lost, leaving a 9 by 9 meter summit) — concentrates most of the mass at the bottom. That design also lowers the center of gravity. But that does not explain everything: the Great Pyramid of Khufu, built from 2.3 million blocks joined (and not always) with some gypsum, behaves as a single unit.

“Imagine it like a swing: every structure has a preferred rhythm at which it vibrates most easily,” ElGabry says.

Using a technique called the seismic HVSR method (Horizontal-to-Vertical Spectral Ratio) or Nakamura method, the researchers measured natural vibrations at about 30 points in the Great Pyramid of Khufu, from the Queen’s Chamber to the King’s Chamber, including ventilation shafts and debris-filled passages. With very little variation, the vibration is always the same.

“We found that most of the Great Pyramid naturally vibrates at about 2.3 cycles per second [2.3 hertz, Hz]. This tells us it is extremely well built and uniform throughout; it behaves as a single solid structure rather than many separate pieces,” says ElGabry.

That means the pyramid vibrates very uniformly and evenly from base to summit and across its internal parts (chambers, passages, shafts). “There are practically no weak zones that behave very differently from the rest,” ElGabry says. This homogeneity, the Egyptian researcher adds, “is excellent for stability because it reduces the likelihood of dangerous internal stresses and cracks forming during an earthquake.”

The researchers observed something they consider very important during their measurements. Vibrations recorded around the pyramid differ from those inside it. The ground surrounding it vibrates at a much lower frequency — about 0.6 Hz — whereas the pyramid vibrates at 2.3 Hz. Because these two frequencies are quite different, the pyramid does not resonate with the ground during an earthquake, and that is crucial.

“Resonance is like pushing a child on a swing at exactly the right rhythm: even small pushes make them go very high,” explains ElGabry.

If the frequencies match, the shaking becomes much stronger and more destructive. “The large frequency difference between the pyramid and the ground prevents this dangerous amplification, which helps keep the monument stable,” he concludes.

“The fundamental natural vibration period of the pyramid is removed from the characteristic vibration period of the ground on which it sits,” explains Amadeo Benavent, principal investigator of the seismic engineering research group at the Technical University of Madrid. “That prevents the horizontal motion of the ground and of the pyramid from entering resonance when an earthquake occurs.”

As with sound when voices overlap, “the phenomenon of resonance is potentially very damaging because it causes the structure’s horizontal displacements to grow greatly,” says Benavent, who reviewed the research.

Beyond mass distribution, other factors also account for the Great Pyramid’s seismic resistance. One factor is the millions of joints between blocks, which help dissipate part of the energy introduced by an earthquake. Another is the base. “

“I recently visited the area while attending, as an invited member, a Mediterranean geology conference, and I was able to verify that the pyramid platform is formed by a thick layer of well-cemented limestones, which likely gives these structures a stable, rigid substrate,” says Juan Ignacio Soto, an expert on seismicity from the geodynamics department at the University of Granada in Spain.

The Egyptians believed in an afterlife. The building of pyramids — which are tombs — was justified by that belief system. If the pharaoh was to have eternal life, his pyramid should be eternal too. Everything suggests the Egyptians built with that idea of eternity in mind. But the architectural logic behind that durability is very different from today’s, at least regarding earthquakes.

As Mohamed ElGabry concludes, “the pyramid is extremely stiff compared with many modern tall buildings.” In fact, the modern approach is the opposite: “A skyscraper is intentionally designed to be relatively flexible.”

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