On the Sandy Shoulders of Giants

In the spring of 1960, the largest of all recorded earthquakes rocked an idyllic seascape on the coast of Chile. Hundreds of locals, so 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 sister tsunami. All boats were lost.

Now, four decades later, a team of international seismologists have finally examined the stratigraphic tracks of the Chilean Giant and the devastating tsunami it caused. As revealed in last week’s Nature, their analysis of the layers of earth in the coastal region has shed light upon the long-misunderstood history of the 1960 monster.

Typically, the magnitude of an earthquake is directly related to 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. And 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.

The 1960 quake was caused when one piece of the earth’s crust—a tectonic plate—slid beneath another. Bordering the Chilean coast, the Nazca plate pushed under the South American plate to its east. Though it moved just three inches per year, over several centuries great amounts of energy built up, until May 22, when the earth finally succumbed to the pressure. And just fifteen minutes after the resulting quake—much like the ripples a dropped stone makes on the surface of a pond—the tectonic grind created a tsunami whose waves would reach as far as Japan.

Chilean eyewitnesses in 1960 said that even five miles inland, the tsunami coated the land with sand. Seismologists Marco Cisternas and Brian F. Atwater took advantage of this fact to clear up the contradictory historical evidence of the region’s earthquake timeline. As revealed in the Nature study, they dug up the earth’s settled layers of rock and sand. They assumed each layer of sand a footprint of a past tsunami, and thus a reliable indicator of a past earthquake. In this way, the team was 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.

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 standing dead 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.

But this isn’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—in ink, at least, if not in sand.