The physical cause of the 1960 quake had already been explained. One piece of the earth’s crust—the Nazca tectonic plate—slowly slid beneath the South American continent to its east. Though it moved just three inches per year, great amounts of energy built up over several centuries - until May 22 when the earth finally succumbed to the pressure. And just 15 minutes after the resulting quake—much like the ripples a dropped stone makes on the surface of a pond—the subterranean grind created a tsunami.
But how many centuries had passed between earthquakes? Chilean eyewitnesses in 1960 reported that even five miles inland, the tsunami coated the land with sand. By assuming that each past earthquake in the area had also left behind a sandy layer, Cisternas and Atwater could determine the number of earthquakes by counting layers of sand in the earth.
The team spent three years digging up the earth’s settled layers of rock and sand. In this way, they were able to piece together a 2,000-year record of seismic history. The results: the quake of 1575 appeared clearly in the stratigraphic record, while those of 1737 and 1837 did not.