The comforting difference is that Eastern fault lines are “much, much, much, less active” than they are out West. “What they record in one year,” notes Ebel, “I have to wait 100 years to record the same number of earthquakes here.” Another factor is that, because there’s so little activity, it’s all but impossible to identify which faults are most active.
We do know that central New Hampshire and coastal Maine are relative hot spots. Kingston, Rhode Island, was hit by a 4.6 tremor in 1951. Middlebury, Vermont, was shaken by a 4.2 in 1962. As for Massachusetts, by far the worst was the Cape Ann Earthquake, on November 18, 1755, which struck just off the North Shore.
Although it occurred two centuries before the Richter scale was invented, seismologists have estimated it may have been as high as a 6.3 — strong enough that, as far north as Portland and as far south as Boston, chimneys toppled and falling debris left holes in roofs. The crew of a ship sailing off the coast assumed they’d hit a rock. In the aftermath, a fearful populace trembled in the face of fiery sermons with titles such as Earthquakes — the Works of God and Tokens of His Just Displeasure.
Just because that’s the biggest quake that’s happened here doesn’t mean it’s the biggest that could happen here, says Ebel. “In 1663, there was an earthquake along the St. Lawrence River in Canada that, in my opinion, was greater than a magnitude 7. And then in 1886, you have the Charlestown, South Carolina, earthquake [which may have exceeded 7.3]. Could that happen here? My opinion is we can’t rule it out.”
If a quake like that were to rock metropolitan Boston, Ebel hazards a guess that the damage would be comparable with the Loma Prieta earthquake. There, “you had pockets of severe damage on the poor soil areas and then you had sections of the city that had little or no damage,” he says. “I suspect that’s the scenario we would see in Boston.”
Ebel is especially worried about “places like the Back Bay and South Boston” since the loosely packed man-made ground in such areas could shake three times as violently as other, firmer landscape.
Compounding the risk? The buildings in such neighborhoods, so many of them dating from the 19th and early 20th centuries, are constructed of un-reinforced masonry, like brick, stone, and concrete — by far the most vulnerable type of material when it comes to seismic activity. Such construction could buckle, topple, or crumble like chalk in a strong enough quake. (Wood-frame houses, by contrast, are able to sway with an earthquake’s lateral force.)
Worse than San Fran?
Ten years ago, a civil engineering doctoral student at Stanford developed an Earthquake Disaster Risk Index, ranking 10 cities worldwide based on criteria such as the severity and frequency of earthquakes, size and density of the population, and how well each was equipped to respond to the aftermath. The higher the score, the bigger the risk. At 54, Tokyo was by far the most perilous; San Francisco scored 37; Boston, 39.
Our poor showing had everything to do with the fact that so many of our buildings predate the seismic requirements of the Massachusetts building code, which, having been modeled on California’s code, was adopted in 1975.