The Great Pyramid of Giza in Egypt has stood for more than 4,500 years. In many strong earthquakes, only the outer stone cladding was damaged, while the main structure was almost intact. This has attracted long-term academic attention to the mystery of its earthquake resistance. A new study by Egyptian geophysicist Asem Salama and colleagues reveals part of the pyramid's dynamic behavior under earthquakes by measuring its vibration characteristics in environmental noise, and proposes that a "natural frequency mismatch" may be one of the important factors for its long-term survival.
The research team tested the tiny vibrations of the pyramid in the natural environment without applying external force, and found that its "natural frequency" (that is, the frequency at which the structure "prefers" vibration) is mainly concentrated between about 2.0 and 2.6 Hz, while the dominant frequency of the foundation soil layer where it is located is about 0.6 Hz. There is a significant difference between the two. The researchers pointed out that this "frequency mismatch" may make it difficult for seismic waves to form effective resonance between the pyramid and the foundation, thereby weakening energy transfer and reducing the structural amplification effect.
The article uses the analogy of a swinging swing: if the pushing frequency is consistent with the inherent rhythm of the swing, its swing will rapidly amplify; otherwise, it will be difficult to produce significant effects. The response of buildings in earthquakes is similar. When the ground vibration frequency is close to the natural frequency of the building, it is easy to resonate and cause catastrophic damage. The Great Pyramid of Giza has a large frequency difference with the foundation soil, which reduces this risk to a certain extent.
The study also noted that relatively low vibration levels were measured near the multi-layered "decompression chamber" above the "King's Chamber" inside the pyramid. These stone chambers are thought to be used to disperse the weight of the massive stones above, and the structural arrangement seems to have affected the propagation path of vibration energy inside the pyramid, weakening the vibrations in some areas.
To obtain these data, the team used the so-called "horizontal-vertical spectral ratio" (HVSR) method, which analyzes the dominant frequencies of soil and structures by recording extremely small vibrations caused by wind, traffic, human activities, and natural seismic background noise. Because the pyramids are an important cultural heritage, engineers cannot drill, load, or deploy sensors on a large scale like modern bridges or tall buildings, so HVSR is a suitable tool to obtain critical information without destroying the cultural relics.
The researchers set up a total of 37 measuring points inside and outside the pyramid, covering internal passages, external stones, and surrounding soil layers to compare vibration characteristics at different locations. However, the author also emphasized that this method can only reflect the response under weak environmental vibrations and cannot be directly equivalent to the actual performance under strong earthquake conditions. The latter may trigger nonlinear behaviors such as stone cracking, joint opening, and block slippage, thereby changing the inherent period of the structure.
The article reminds the public that the longevity of the pyramids should not be simply regarded as direct evidence that "the ancient Egyptians mastered systematic earthquake-resistant engineering." Modern seismic design evaluates overall toughness rather than a single frequency parameter: Engineers need to consider multiple factors such as expected earthquake intensity, site soil quality, structural weight and flexibility, energy dissipation and deformation capacity, and the consequences of failure.
In contemporary engineering practice, even if there is a frequency mismatch between the foundation and the superstructure, buildings may still perform poorly in earthquakes. The actual damage to some buildings in the 1992 Cairo 5.8 magnitude earthquake and the 1989 Newcastle, Australia earthquake reflects this complexity. For stone buildings such as the Great Pyramid, the cracking, dislocation and stiffness degradation of the stones during strong earthquakes will dynamically change their frequency, making the structural behavior more difficult to predict.
The author also draws an analogy from the case of "survivor bias" in statistics: During World War II, if the location selection of fighter aircraft for armor installation was based only on the distribution of bullet holes on the returning aircraft, those aircraft that failed to return due to being shot in key parts would be ignored. Similarly, ancient pyramids, aqueducts and temples still standing today are often survivors of a large number of failed experiments, weak foundations and poor detailed designs that have been screened out by history. Therefore, the full design intentions of the ancients cannot be inferred from existing examples alone.
Nevertheless, the author believes that the survival of the Great Pyramid is by no means purely accidental, but a reflection of quite mature empirical engineering judgment. From an engineering point of view, it has a wide base, a low center of gravity, a layer-by-layer shape, a high degree of symmetry on the plane, solid limestone bedrock, and a continuous, thick masonry force transmission path. It generally presents the characteristics of short, rigid, and stable, rather than a tall, soft, and thin fragile form.
The article points out that the safest conclusion is that ancient Egyptian builders formed excellent structural choices in long-term practice. These choices may be due to construction experience, material limitations, religious and cultural needs or symbolic meanings, and their earthquake resistance benefits are unintentionally obtained "side benefits." Therefore, the survival of the Great Pyramid is neither the result of mysterious forces nor proof that ancient people have systematically mastered earthquake-resistant design in the modern sense. However, this research still provides an important and admirable empirical basis for understanding its structural behavior, while also leaving a series of open questions for further exploration.