One mystery of the Great Pyramid’s longevity has finally been solved

Egypt’s Great Pyramid of Giza has resisted earthquake damage for nearly 5,000 years, and scientists finally know why.

The pyramid tends to vibrate at a different frequency compared with the surrounding soil, which prevents excessive shaking during an earthquake. Combined with the pyramid’s sturdy shape and mass-distributing internal design, this difference has kept the structure intact and stable, researchers write May 21 in Scientific Reports.

“It’s no surprise that they are very seismically resistant,” given that the pyramids have remained standing for this long, says Sherif El-Tawil, a civil engineer at the University of Michigan in Ann Arbor who was not involved in the research. But the new study offers important insight into why the pyramids are so resistant to seismic damage, he says.

The Great Pyramid, the last of the ancient Seven Wonders of the World still standing, was built in Giza around 2600 B.C. to serve as the tomb of the Pharaoh Khufu. It contains about 2.3 million stone blocks and took more than two decades to construct.

Though Egypt generally has low seismic activity, it does occasionally experience strong earthquakes, including a magnitude 6.8 in 1847 and a magnitude 5.8 in 1992. Despite these powerful tremors, the Great Pyramid has suffered only minimal damage.

To uncover the origins of this enduring stability, geophysicist Mohamed ElGabry and colleagues monitored subtle vibrations at 37 points within and around the pyramid. Intentionally shaking the pyramid could be damaging to it, so the team instead relied on tiny disturbances caused by far-off ocean waves, traffic or other human activities to set the structure vibrating.

At about three-quarters of the measurement sites inside the pyramid, the structure naturally vibrated back and forth at frequencies between 2 and 2.6 times per second. The narrow distribution of frequencies indicates that stress is evenly distributed throughout the pyramid. The surrounding soil, meanwhile, vibrated more slowly, oscillating a little more than once every two seconds.

Because the natural frequencies of the building materials and the soil are different, they are less likely to experience resonance, in which the pyramid absorbs energy from the soil’s vibrations. Resonance would significantly increase the strength of the vibrations — though not necessarily their frequency — and the risk of damage during an earthquake.

Spaces inside the pyramid also helped mitigate shaking. Ancient Egyptian builders constructed pressure-relieving chambers above the king’s chamber, where the pharaoh was entombed, to distribute the pyramid’s weight and protect the burial chamber in the event of a collapse. Those chambers also decreased the strength of the vibrations closer to the top of the pyramid, the team found.

Most buildings behave like upside-down pendulums, says ElGabry, of Egypt’s National Research Institute of Astronomy and Geophysics in Cairo. Their bases are anchored to the ground, but the tops have more room to sway and shake. In the Great Pyramid, this behavior meant that vibrations were amplified by a factor of four in the king’s chamber, which is situated well above the bedrock and near the center of the pyramid. But in the pressure-relieving chambers above the king’s chamber, the vibrations were amplified by only a factor of three. It’s not yet clear what factors of the chamber’s design contribute to this effect.

“All of this is really amazing to look at from today’s engineering point of view,” says ElGabry. “But it’s more amazing and more impressive when you look into the tools and available resources we had 4,600 years [ago].” However, the findings can’t confirm whether ancient Egyptians intentionally designed the pyramids with earthquakes in mind, the researchers write in the study.

Modern builders can use similar strategies to plan projects, choose materials and create enduring structures, ElGabry says. “When we design our buildings, we design for 100 years [or] for 500 years,” he says. To apply principles that make an edifice last far longer than that, “it’s important to understand how this building has survived.”