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Machu Picchu was built over major fault zones. Now, researchers think they know why

Archaeologists and architects alike have long wondered why 15th century Incans built the grand citadel of Machu Picchu where they did, high in the remote Andes atop a narrow ridge in what is now Peru. One simple answer, researchers now suggest, is that that’s where building materials for the site—large amounts of already fractured rock—were readily available.

Both satellite images and recent field work reveal that the ground beneath Machu Picchu is crisscrossed with fault zones of various sizes, some of which control the orientation of river valleys in the region by providing weak zones that are more easily eroded by flowing water. Because some of these faults run from northeast to southwest and others trend from northwest to southeast, they collectively create an X where they intersect beneath the site, researchers reported this week at the annual meeting of the Geological Society of America in Phoenix.

When earthquakes along these fault zones cause rocks to shift, they generate prodigious quantities of fractured rock (large stones in foreground). But these fault zones also channel meltwater from ice and snow and rainwater, thus enabling residents to more effectively collect it. They also help drain it away during intense thunderstorms, preventing short-term damage and aiding long-term preservation of the site.

Further evidence that the site selection for Machu Picchu may not have been an accident: Similar analyses reveal other ancient Incan sites, including Ollantaytambo, Pisac, and Cusco were also built at the intersections of fault zones.