Silent witnesses seismic sagas: Chilean scientists are studying historic accounts of earthquakes for clues that will help them predict the size and timing of future eruptions.

• Author: Werner, Louis

"... and as God was pleased that on the sixteenth of December, two hours before sunset, within the time it takes to recite three credos, with a tremble and earthquake all this city fell down, without leaving any house where one could dwell or dare to enter ..."

Report to the Lieutenant Governor, Town Council of Imperial, Chile January 8,1576

This obscure report buried deep in Chile's colonial archives has recently become a key piece of seismological data. The Apostles' Creed in the Catholic mass takes roughly thirty seconds to recite, and the writer was presumably on his knees as soon as the shaking started. Thus we know that the Chilean earthquake of December 16, 1575, lasted about ninety seconds. This is of interest because duration of motion partly determines how much stored energy an earthquake releases, and therefore helps to predict the timing and magnitude of the next one.

In a study published last year in the prestigious scientific journal Nature, Marco Cisternas at the Catholic University in Valparaiso and his co-authors used this and other eyewitness accounts of the 1575 earthquake, along with reports of nearby earthquakes in 1737 and 1837, to help explain why the 1960 quake centered near Concepcion was so surprisingly powerful.

The 1960 quake's magnitude was the equivalent of an estimated 250-350 years of stored energy, yet that fault's previous earthquake had occurred only 123 years earlier. Why? Using both documents and field data, Cisternas determined that the previous two quakes in 1737 and 1837 had been insufficiently strong producing little if any land subsidence or tidal wave--to fully release the energy that the fault had stored until they struck. The fault line thus remained a half-cocked gun waiting additional centuries to be fired with full force in 1960.

Seismologists routinely sift through eyewitness records of earthquakes that occurred before the modern era in search of such tell-tale detail. According to Leonardo Seeber, a senior earthquake researcher at New York's Columbia University, the accuracy of seismic data actually decreased when recording instruments were first introduced because they were more error prone than word-of-mouth accounts. "Seismologists finally realized that there was still a lot of value in reports in newspapers, diaries, and the like--even of more recent events. Data does not have to be quantitative to be useful."

Although geologists do not have to read the Old Testament, for instance, to know that the Near East is seismically active, the prophets Samuel ("Then the earth shook and trembled, the foundations of heaven moved and shook" 22:8) and Isaiah ("The earth shall reel to and fro like a drunkard" 24:20) made this clear long ago. Useful scientific data can be gleaned even from laymen's reports of, say, in which direction buildings toppled, or how high the tide rose or fell, or in what orientation fissures opened in the ground.

Pliny the Younger, in a letter to the Roman historian Tacitus about the eruption of Mr. Vesuvius on August 24, A.D. 79--which killed his uncle, Pliny the Elder, the author--ironically--of the greatest scientific treatise of the classical era--wrote, "We saw the sea sucked away, apparently forced back by the earthquake; at any rate, it receded from the shore so that quantities of sea creatures were left stranded on dry sand." His description of the eruption's mushroom-shaped ash column--like an "umbrella pine"--holds valid today, as volcanologists still call such an ash cloud a pino, and any explosive...