Photo showing Inca buildings in the foreground and mountains in the background; some cracks are visible in the stonework.
Ancient Cusco was the site of destructive earthquakes in Inca times. Credit: Andy Combey

Cusco, a UNESCO World Heritage Site in Peru, is one of the oldest continuously inhabited cities in the Western Hemisphere. Founded as early as 1000 CE and capital of the Inca Empire in the 1200s–1500s, Cusco’s ancient and highly decorated stonework abuts Spanish colonial and modern architecture.

But Cusco’s monuments tell of more than just ancient civilizations and fallen empires. Etched into the structures are tales of a turbulent geological past, punctuated by severe earthquakes.

Researchers studying earthquake damage to buildings across Cusco have cataloged thousands of displaced blocks and fractures, capturing evidence of two devastating earthquakes, one of which occurred in Inca times and was previously undocumented.

The study, led by researchers from the University of Grenoble Alpes in France and Peru’s Geological, Mining, and Metallurgical Institute (INGEMMET), sheds light on the frequency and scale of tremors in a region where documentation of earthquakes is sparse.

Gaps in the Record

“Every stone added to the mosaic helps to better estimate the seismic hazard of the area.”

“The Cusco Basin is particularly prone to destructive earthquakes, sitting inland from a major subduction zone and astride a network of faults,” said lead author Andy Combey of the University of Grenoble.

In 1650, Cusco was the epicenter of one of the most destructive earthquakes in Peru’s history. Another temblor in 1950 reportedly affected more than 30,000 people and damaged half the buildings in Cusco.

Modern-day Cusco is a continually urbanizing tourist hot spot, but the risk from earthquakes is largely unknown. “[Peru’s] earthquake catalog only includes information from the last 500 years,” said coauthor Carlos Benavente Escóbar of INGEMMET. “Geological faults in the Cusco Basin need further study; they represent a latent hazard to about half a million people.”

Researchers analyzed displaced blocks and fractures in Inca stonework as evidence of earthquake damage. Credit: Andy Combey

The archaeological survey mapped the orientation of thousands of fractures and blocks with broken corners, giving information on ground shaking direction indicative of different earthquakes.

“Every stone added to the mosaic helps to better estimate the seismic hazard of the area,” said Klaus Hinzen, a professor of earthquake geology at the University of Cologne who was not involved in the study.

As well as providing a fuller seismic record, the technique—known as archaeoseismology—can also help scientists understand how Inca and pre-Inca peoples lived or adapted to geological hazards, explained Benavente.

Researchers note that some of the archaeological damage could have been caused by weathering or landslides. For this reason, the researchers scored each broken block by the likelihood it was caused by a quake, then used statistical models to check for anomalies in wider damage patterns. They also examined photos taken soon after the 1950 earthquake to discount effects of more recent tremors and give an estimate of relative earthquake timing.

Two Severe Earthquakes

Their survey revealed two distinct patterns of deformation. Colonial buildings were damaged from ground shaking orientated east–west, whereas Inca buildings suffered north to south shaking. “This corroborates accounts of the 1650 earthquake impacts and hints to a previously unreported earthquake in Inca times,” said Combey.

Their data also showed the 1950 earthquake was, in fact, less damaging to Inca buildings than previously thought, causing only a handful of fractures out of those logged.

According to Laura Pecchioli of Humboldt University of Berlin, who was not involved in the study, archaeoseismology is a relatively new discipline and is still being developed. Although most research has focused on Europe, she explained, the Cusco results “could stimulate new avenues of research for the protection and conservation of cultural heritage in the future.”

Researchers from INGEMMET are continuing to build the earthquake catalog for the region using a range of methods, including archaeoseismology. Benavente is currently conducting fieldwork in the Ancash region of Peru, tracing ancient earthquakes and their relationship to landslides and debris flows.

The ultimate aim, explained Benavente, is to push for updates to building regulations in Peru, which currently consider earthquake hazards largely from offshore megathrusts and underestimate inland faults, which can, as the Cusco buildings show, cause destructive earthquakes.

—Erin Martin-Jones (@Erin_M_J), Science Writer

Citation: Martin-Jones, E. (2022), Written in stone: Inca buildings remember ancient earthquakes, Eos, 103, https://doi.org/10.1029/2022EO220452. Published on 20 September 2022.
Text © 2022. The authors. CC BY-NC-ND 3.0
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