The Great Earthquake

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At its sea-covered epicenter, the Cape Ann event (scientists now believe) would have been ranked 8.0 or 9.0 on a scale of 1.0 through 12.0, known as the modified Mercalli scale that measures the visible effects of earthquakes. On land, the earthquake might have registered a Richter scale magnitude of 6.0. The 1886 Charleston earthquake has been given a Mercalli intensity of 9.0 to 10.0, while the San Francisco earthquake reached 8.3 on a Richter rating with a Mercalli index of 11.0

Geologists are still puzzled by the dynamics of the 1755 quake. Unlike California—where the Pacific plate meets the North American plate, and essentially dives under it—the Eastern area is not at the earthquake-prone edge of such plates. But more than two hundred years after colonial scientists initiated the investigation, answers may be starting to emerge.

A study of New England’s magnetic field recently revealed a highly magnetic and circular underwater geologic formation at Cape Ann, probably a mass of gabbro, a rock denser than the granite known to surround it. Scientists suggest that this is a pluton, a huge rock cylinder descending into the earth. Similar plutons had been identified ashore—in the White Mountains—and when geophysicists compared notes, they found that plutons with matching characteristics existed in the same sites as the major historic earthquakes of the region. It may be that stress at the boundaries of these pillars caused not only the Cape Ann earthquake but also many of those that followed in the eastern United States.

Were an earthquake of the same intensity to recur at Cape Ann today, geologists have told the U.S. Congress, the damage, especially in the filled areas that now make up half of Boston, would be considerable, with lives lost. But in 1755, more sparsely settled Americans could give thanks for having been spared, newly mindful of the wages of sin. Considering the paucity of earthquakes for the next one hundred and thirty years, at least a few of them must have mended their ways.