Layers of sand in the earth have helped seismologists investigate the history of earthquakes in Chile.

by Virginia Cooper Hughes

Photo by Pierre St. Amand Earthquake damage to homes in the town of Valdivia in Chile, near the epicenter of the earthquake.

In May of 1960, the largest earthquake ever recorded rocked an idyllic seascape on the coast of Chile. Hundreds of locals, frightened by the violent shaking of the land, sought refuge in small boats. They thought they were safe - until the sea roared with the 75-foot waves of the quake’s daughter tsunami. And they weren’t the only ones who felt it – giant waves were triggered in Japan and Hawaii, killing people and destroying homes.

Typically, the magnitude of an earthquake increases with the number of years since the last one in the area. So considering its enormity, at least 350 years should have elapsed between the Chilean quake and its immediate predecessor. Sure enough, Spanish conquistadors wrote of a large quake in 1575. But here’s the big mystery: other documents suggest the 1575 quake wasn’t the last one before 1960. Two others are documented: one in 1737 and another in 1837. This left those who were studying the 1960 event scratching their heads to figure out how so much energy could have built up in just 123 years.

Four decades later, a team of international seismologists, led by Brian Atwater from the University of Washington and Marco Cisternas from the Pontificia Universidad Católica de Valparaíso in Chile, have finally examined the tracks of the Chilean giant and the devastating tsunami it caused. As revealed last September in Nature, their analysis of the layers of earth in the coastal region has shed light upon the long-misunderstood history of the disaster.

Photo courtesy of Brian Atwater Seismologists dug a trench in a field near the epicenter of the earthquake to study the layers of sand.

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.

Photo courtesy of Brian Atwater Seismologists analysed layers of soil to see if sand from past earthquakes may have been left behind.

So what about the documents of the latter two? The team suggests neither quake caused lasting ecological damage. After counting the trunk rings of 15 dead, standing trees—presumed to have died in 1960—they found ten were alive in 1837 and two in 1737. The forest damage from the earlier quakes, then, could not have been as extensive as the havoc wreaked in 1960.

These findings aren’t any too reassuring for those living on a fault line. The two smaller quakes, whose occurrences could not have been predicted based on the big one preceding them, were still devastating enough to go down in history— at least in ink, if not in sand.

For more info:

Surviving a Tsunami - Lessons from Chile, Hawaii, and Japan
http://pubs.usgs.gov/circ/c1187/

Giant Earthquake Predictions Aided by Historical Data
http://news.nationalgeographic.com/news/2005/09/0915_050915_earthquake_2.html