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The Great Earthquake
When The Great Earthquake struck New England, learned men blamed everything from God’s wrath to an overabundance of lightning rods in Boston. Two hundred and twenty-five years later, geologists are at last discovering the true causes.
August/September 1980 | Volume 31, Issue 5
Winthrop told his Harvard Chapel audience that, like many others, he was forced to stay in bed, listening to the beams of his house crack in the violence, for at least the first two minutes of the earthquake. When he arose to seek his watch, it read 4:15 A.M. In examining his mantel clock, which he had earlier synchronized with his watch, he found that it had stopped at 35 seconds past 4:11. A test tube that Winthrop had placed inside the clock case “for security” after an experiment had toppled onto the clock’s pendulum, probably with the first tremor. Therefore, Winthrop concluded, the earthquake of 1755 lasted at least four minutes.
To determine the speed of the sway of the region’s buildings in the earthquake, he relied on the travels of a key tossed from his mantel and measured the distance that one of his chimney bricks had been thrown—thirty feet from a thirty-two-foot-high chimney. By calibrating the known speed of a falling object—thanks to Newton—he could show that his brick had probably traveled twenty-one feet in one second. The clue of the key, which had apparently not traveled so forcefully, suggested to him that the velocity of moving objects during an earthquake varied, depending on height.
From the northwesterly direction that the key flew, he ascertained that the course of the earthquake had been from northwest to southeast and calculated that it had occurred “at some considerable distance from this place,” since he had heard the earthquake about thirty seconds before he had felt it. If the speed of sound is about thirteen miles an hour, he reasoned, then earthquake vibrations traveled at some speed slower than sound.
He arrived, correctly, at a conviction that earthquakes were emitted in an undulating motion. During an aftershock felt the night after the November 18 tremor, Winthrop was sitting at a hearth with his feet on the bricks. As the tremor passed, his feet were lifted directly upward by a series of individual bricks, moving one at a time. “It was not a motion of the whole hearth together,” he explained to his audience, “either from side to side, or up and down; but of each brick separately by itself. Now as the bricks were contiguous, the only motion, which could be communicated to them separately, was in perpendicular direction...and this shock, I apprehend, was occasioned by one small wave of earth rolling along.”
But if Winthrop was the consummate observer of an earthquake in practice, a lesser known compatriot proved the more insightful theoretician. John Perkins was a Boston physician who, in his personal journals, freely indulged in scientific speculation, some of it—as in the origins of coal from plant matter—startlingly accurate. In 1758 he published an anonymous tract in the New American Magazine on the causes and effects of the 1755 earthquake, which he suspected had originated in the White Mountains of New Hampshire. Perkins recognized features in the earth and earthquakes that would not emerge as commonplace for generations.
Perkins’ suggestions were disarmingly reasonable. It was apparent, he noted, that because earthquakes often occurred near volcanic activity, there may be some relationship between heat and earthquakes. But that evidence had created a popular assumption that fire and rarefied vapors were a universal cause of earthquakes. In nonvolcanic regions, the concept was often supported by the occasional spewing up of sulfurous material. But any heat at these nonvolcanic sites may be misleading, he cautioned; it might be the effect of friction, of the earthquake itself, and not be a cause at all.
Instead, he said, “the settlement of high lands may sometimes be the first moving cause of earthquakes.” Imagine, he proposed, “what might be the consequence, by the infinitely greater force produced by the weight of a continent land, upon any quantity of matter put in motion under it.” Such settling was more likely to occur, he said, where caverns or channels “weaken the stability of the foundation supporting the earth.” Noting that earthquakes often occurred on continental coasts, he conjectured that they took place when the higher lands settled and forced the emergence of new coast land. The agitation of the sea that one saw during earthquakes was the effect of the spreading of the coast. Perkins’ theories are surprisingly close to the now commonplace awareness that stress, faults, and the collision of expanding tectonic plates contribute to earthquakes.
The Cape Ann earthquake of 1755 remained the premier in the United States until 1811, when a series of earthquakes struck the Mississippi River embayment at New Madrid, Missouri, and was followed by more than one thousand aftershocks. But on the East Coast, no earthquake would match the Cape Ann tremor until the 1886 Charleston, South Carolina, tremor that severely damaged that community. No earthquake east of the Rockies in the twentieth century has matched the Cape Ann disturbance or those two that followed.