The Incan citadel of Machu Picchu in Cusco, Peru, is one of the world’s most important archaeological monuments. Machu Picchu is classified as a UNESCO World Heritage site and at risk from climatic change. However, the site could also be at significant risk from seismic hazards.
Peru has a record of large earthquakes generated both along a major subduction zone offshore (capable of producing earthquakes of magnitude 8 and above) and on active crustal faults along the Andean Cordillera. However due to the short historical earthquake catalogue (< 500 years) our knowledge about the historic and current seismic hazard in Machu Picchu is limited.
A new study by an international team of geoscientists, led by Dr Miguel Angel Rodriguez-Pascua from the Geological Survey of Spain (IGME) and including Oxford Earth Sciences Professor Richard Walker, has revealed that structures at the Machu Picchu site suffered through at least two earthquakes as they were being built. These events not only damaged walls, but also triggered a sudden change in construction techniques.
The study published this month in the Journal of Seismology, centred on a survey of three of Machu Picchu’s most significant temples, which revealed more than 140 examples of earthquake damage. These include large blocks of stone that have shifted or whose corners have been chipped. While some of this damage can be attributed to slumping rocks or soil beneath the temples, in other instances, movement of many of the damaged blocks, including substantial gaps between some formerly interlocking blocks of stone, was likely driven by seismic shaking from at least two major quakes. The study revealed that the type of damage seen on the corners of blocks embedded in the stone walls only occurs as they rhythmically clatter against each other during an earthquake.
Researchers believe the seismic events that rattled Machu Picchu likely occurred between 1438 and 1491 C.E., the period when the main parts of the city were developed and well before Europeans arrived in the area. A lack of written records or oral tradition make it difficult to narrow that window of time. Regardless of when those quakes occurred, construction methods thereafter shifted to a cheaper and easier scheme of merely stacking smaller blocks of rock, rather than carving them so that they interlocked.
It is hoped that studies such as this, which utilise earthquake archaeological effects (EAEs) data can be used in the future to understand and protect important archaeological sites.
This research came out of a joint project called CUSCO-PATA, which was funded through a scheme to promote joint UK-Peru Scientific research. The project was led by scientists from INGEMMET, and brought together an international team of researchers from France, Germany, Spain, and the UK.