Prehistoric Evidence of Crustal Earthquakes and Debris Flow in Archaeological Site of Pikillaqta in Cusco: Archaeological Implications

In the Cusco Valley, seismic activity resulted from reactivations of the Qoricocha, Tambomachay, and Pachatusan faults in 1986, 1950, and 1650 during the Inca and Wari cultures' settlement. However, the pre-1650 seismic record is scant, with few chroniclers mentioning Inca-era earthquakes lacking precise information. Filling this void, our study centers on paleoseismological and archaeoseismological research in the Cusco Valley, emphasizing the Pikillaqta Archaeological Park from the Wari Empire era (600–1000 AD). The mysterious abandonment circa 900 AD prompts an exploration of earthquake-related effects. Archaeological signs of seismic activity at Pikillaqta reveal a distinctive NW-SE orientation unrelated to local architectural or topographic factors. Additionally, we identified debris flow deposits, up to 2.5 meters thick, within structures. Drone imagery aids debris flow mapping, determining its origin. Radiocarbon dating places earthquakes and debris flow around 776–986 AD. Paleoseismic data on Pachatusan and Tambomachay faults, intersecting at Pikillaqta, reveal two earthquakes between 770 and 900 CalAD and 856–988 CalAD. Our inference suggests these events, occurring simultaneously or in a closely successive sequence, triggered a crisis, leading to Pikillaqta's abandonment.