Roman amphitheaters act like seismic invisibility cloaks

Roman amphitheaters are among the most ancient human constructions on Earth. These structures are remarkably well preserved in various places across the ancient Roman empire.

That’s especially remarkable because much of this territory is seismically active: it sits on the tectonic boundary between the Eurasian and African plates and has experienced numerous earthquakes that have destroyed other types of buildings. So just how these amphitheaters have survived for 2,000 years is something of a puzzle.

Today we get a potential answer thanks to the work of Stéphane Brûlé and colleagues at Aix-Marseille University in southern France. These guys have studied the way that certain structures buried in the ground, or sitting on top of it, can modify the way seismic waves travel through the Earth. In particular, they have studied “seismic invisibility cloaks” that can steer seismic waves around specific regions and thereby protect them.

Their conclusion is that Roman amphitheaters may act as seismic invisibility cloaks thanks to their shape. This, they say, is the reason for their remarkable longevity.

First some background. Physicists have long known that certain regular patterns of objects can interact with waves in a way that steers them and modifies their behavior. A curious feature of this phenomenon is that the objects themselves are much smaller than the waves themselves.  But the combined effect of many objects arranged in a regular pattern has an important influence on the waves.

Back in 2006, physicists used this idea to create a pattern of metal resonators that steer microwaves around a region of space. To an outside observer looking with microwave eyes, this region of space, and anything in it, disappears. In effect, the team had built the world’s first invisibility cloak.

Since then, researchers have built invisibility cloaks for a wide range of different waves in the electromagnetic spectrum and beyond. In 2012, they suggested that seismic invisibility cloaks could protect power stations and dams from earthquakes. Next, Brûlé and colleagues actually built and tested one.

Since then the researchers have continued their studies of seismic metamaterials, which they say can take several forms. The early experiments involved underground structures or voids. But more recent work suggests that surface features like trees and buildings can also  influence seismic waves.

One idea is that seismic waves cause a skyscraper to vibrate. But this vibration itself sends waves through the ground. So if the two sets of waves could be made to influence or even cancel each other, then the building would have an important mitigating influence on the waves. 

Brûlé and colleagues have even performed proof-of-principle measurements on the waves generated by a skyscraper as a result of seismic noise. The building in question is the LatinoAmericana Tower, a 282-meter skyscraper in Mexico City that has survived several major earthquakes since it was constructed in 1956. 

The researchers developed a computer model to study how skyscrapers arranged in a circle could act as an invisibility cloak that creates a safe zone at its center. “The buildings within the annulus of the cloak and outside the cloak would be badly affected by the seismic wave, but the region in the center (e.g. a park) would be a safe zone where people could gather and remain safe during an earthquake,” they say.

During the course of these studies, they noted a likeness between the circular patterns they generated and the design of ancient amphitheaters. “There are striking similarities between an invisibility cloak tested for various types of waves and sky views of antique Gallo-Roman theatres,” they say. “Perhaps this is the reason why some of these megastructures, as amphitheatres, have remained mostly intact through the centuries.”

That’s an interesting idea that could have important implications for the design of future buildings and the study of ancient ones.

Ref: arxiv.org/abs/1904.05323 : Role of Nanophotonics in the Birth of Seismic Megastructures