A fortress of moving stones
High above Cusco, at an altitude of 3,701 meters (12,142 feet), the massive stone walls of Saksaywaman (also spelled Sacsayhuamán) guard the ancient Inca capital. The complex is a polygonal masonry, where enormous stones are fitted together without mortar. This construction technique, known as ashlar masonry, involves cutting blocks of stone so precisely that they interlock perfectly. The joints are so tight that in many places, a piece of paper cannot be inserted between them.
The effectiveness of this design becomes apparent in an earthquake. Peru is a seismically active region, and its history is marked by powerful tremors. The devastating earthquake of March 31, 1650, which measured around 7.5 on the Richter scale, destroyed many Spanish colonial buildings in Cusco. However, the underlying Inca foundations and walls, including those at Saksaywaman, largely withstood the event.
The reason for this resilience is a phenomenon modern engineers call "dancing stones." During an earthquake, the precisely fitted, mortar-free blocks can move and shift with the seismic waves. This flexibility allows the structure to dissipate the energy of the quake rather than rigidly resisting it. Once the shaking stops, gravity pulls the stones, and they resettle back into their original, stable positions.
Engineering an earthquake response
The Inca builders employed several sophisticated techniques to achieve this effect. The walls lean inward by 3 to 5 degrees, lowering the structure's center of gravity and increasing stability. Individual stones are often polygonal and irregular, creating an interlocking, three-dimensional puzzle that resists separation. The largest limestone blocks at Saksaywaman are immense, with estimates of their weight varying from 125 to nearly 200 tonnes.
The construction, which likely began during the reign of Inca Pachacuti in the 15th century, involved shaping hard volcanic rocks like granite and andesite. Without iron tools, stonemasons used harder hammer stones and bronze tools to pound the blocks into shape. The process was a meticulous one of trial and error, repeatedly lifting and setting the huge stones to grind them into a perfect fit.
This ancient technology is more than a historical curiosity. Modern engineers study Inca masonry to develop better earthquake-resistant building practices. The principles of allowing controlled movement and dissipating seismic energy are now being incorporated into contemporary seismic engineering, showing the effectiveness of Inca construction methods.