How did the Great Pyramid of Giza survive Egyptian earthquakes for 4,500 years?

The Great Pyramid of Giza has survived in Egypt for more than 4,500 years. Earthquakes have repeatedly rocked the area, including the 5.8-magnitude Cairo earthquake in 1992, which dislodged some of the stones of the pyramid’s outer shell. However the main body remained essentially intact.

How well did she survive? A new study of the pyramid’s vibrations by Egyptian geophysicist Assem Salama and colleagues provides insight into its performance during earthquakes, and identifies some interesting features.

But we must be careful when we conclude that its impressive longevity is evidence of its makers’ knowledge of earthquake engineering.

What the research found

The researchers measured the pyramid’s vibrations under ambient conditions. They found that its natural frequencies — the frequencies at which it “prefers” to vibrate — range mostly between about 2.0 and 2.6 Hz (cycles per second). The surrounding soil has a much lower dominant frequency, around 0.6 Hz.

Every structure has a natural rhythm. Push the baby on the swing at the right moment and the movement develops; Push at the wrong moment and little happens.

Buildings and monuments behave similarly. If the vibration of an earthquake matches the natural frequency of the structure, the motion can be amplified. This is called resonance, and it can be disastrous.

A diagram of the internal structure of the Great Pyramid. Salama et al. /Scientific reports

The study also indicates reduced vibrations near the so-called vent chambers above the King’s Chamber. It is understood that these chambers redirect the enormous weight of the stone upward, and may also affect how the vibrational energy moves through the pyramid.

These results suggest some behaviors that may be useful during an earthquake, including frequency mismatch between the pyramid and the soil. But it does not prove in itself that people intentionally built the pyramid to be able to withstand earthquakes.

How did researchers measure it?

The study used a method called horizontal-to-vertical spectral ratio analysis, or HVSR. This records small background movements from wind, traffic, human activity and natural ground vibrations.

By comparing the horizontal and vertical components of these movements, researchers can estimate the dominant frequencies in the soil and structure. In this case, the tools were placed in 37 locations in and around the pyramid, including the interior corridors, exterior stones, and adjacent soil.

Researchers placed sensors in and around the Great Pyramid to measure its vibrations. Salama et al. /Scientific reports

This fits into the heritage structure. Engineers cannot drill into the Great Pyramid, load it ly, or place tools on it like a modern bridge.

The method provides useful information without harm. However, it only measures the response to small background vibrations, not the severe shaking caused by the earthquake.

The importance of frequency mismatch

When vibration from an earthquake occurs at a frequency that matches the natural frequency of the structure, it can cause resonance. Ringing can be disastrous.

The 1940 collapse of the Tacoma Narrows Bridge in the United States is often attributed to ringing during high winds. Wikimedia

So the size difference is important. If the ground and structure vibrate at different rates, the ground is unlikely to feed energy efficiently into the structure.

But this addresses only one possible mechanism of earthquake damage. There are many examples of structures that perform poorly during earthquakes, even though there is a frequency mismatch with the soil below.

The ability to resist earthquakes is more complex

Modern seismic design does not evaluate resilience by comparing a single frequency.

Instead, we look at the full list of questions. How severe is the expected shaking? On what ground is the structure located? How heavy and flexible is the structure? Can the structure deform and dissipate energy without sudden collapse? How serious is failure?

The natural period or rhythm of the structure (related to its natural frequency) is part of this assessment. But it sits alongside many other factors.

In practice, earthquake damage depends not only on the earthquake but on the structures that receive it. For example, the Newcastle earthquake that struck Australia in 1989 was not massive by global standards, but many buildings performed poorly and 13 people died.

The 1989 Newcastle earthquake in Australia was not massive, but it caused significant damage and killed 13 people. Australian Earthquake Engineering Society, CC BY

For the Great Pyramid, the behavior of the stonework is of particular importance. Ambient vibration testing measures behavior under very small movements. During strong earthquakes, masonry can crack, open joints, buckle, slide, and lose rigidity. Each of these changes alters the natural period of the structure, complicating the behavior.

Beware of survival bias

When assessing the longevity of a pyramid, we must also take survival bias into account.

It is known that in World War II, statistician Abraham Wald was asked where armor should be added to aircraft. The obvious answer was to reinforce the places with the greatest number of bullet holes in returning aircraft.

father (argue the opposite)[https://archive.org/details/statisticaldecis0000abra_t1e7]: Those planes survived. The aircraft that did not return was missing from the data.

This famous chart shows the pattern of bullet holes on returning World War II aircraft. Martin Grandjean/McGeddon (image)/US Air Force (strike plot concept)/Wikimedia, CC BY

Older structures pose a similar problem. We admire the ancient canals, temples and pyramids because they are still there. Failed structures, weak foundations, poor details and abandoned experiments have mostly disappeared.

This does not diminish the Great Pyramid. It simply means that looking at structures still standing today doesn’t tell us everything about the design intentions behind them.

What the pyramid teaches us

The pyramid may not have been intentionally designed to be able to withstand earthquakes. But its survival is not a coincidence either.

From an engineering point of view, it has many favorable features: a wide base, a low center of mass, a tapered shape, a symmetrical plan, a suitable limestone foundation, and a massive masonry load path. He is squat, solid and well-grounded rather than tall, lean and flexible.

The safest conclusion is that the builders made excellent engineering choices. These choices may have been driven by building experience, observation, structural necessity, or cultural intent. Its seismic benefits may be real without being the original purpose.

The survival of the Great Pyramid is not magic, nor is it evidence of ancient seismic design. The evidence for this is that this study is important and impressive, but incomplete.

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