In 1755, an earthquake strong enough to knock down chimneys and steeples hit the city of Boston, Massachusetts. The Cape Ann earthquake was the most damaging earthquake to ever occur in New England, and is estimated to be classified at modified Mercalli intensities of “strong” to “very strong” – based off contemporary descriptions of damage from Boston and surrounding areas. Now, researchers report in Seismological Research Letters that the signs of this earthquake can be seen in a sediment core taken from Sluice Pond in eastern Massachusetts.
Researcher Katrin Monecke of Wellesley College and her colleagues identified a layer of light brown organic-rich mud within the sediment core, which is estimated to have been deposited between 1740 and 1810 due to an underwater landslide. They believe this landslide was caused by the 1755 Cape Ann Earthquake.
Although the epicenter of the earthquake was likely located offshore in the Atlantic Ocean, the tremors could be felt along the eastern seaboard from Nova Scotia to South Carolina. New England is located within a tectonic plate, which means that it is not nearly as active an earthquake zone as a place like California, which is on an active tectonic plate margin. “There are zones of weakness mid-plate in New England and you do build up tectonic stress here, you just don’t build it up at the same rate that would occur at a plate boundary,” explains Monecke.
Since researchers in the less active areas don’t have many faults to study, they can look for signs of seismically-induced landslides, or the deformation of soft soils, in order to analyze the history and prehistoric record of earthquakes in these regions. For this particular sediment core taken from Sluice Pond, the researchers hope that it will give seismologists a way to assess the sedimentary record of earthquakes for other lakes in the region.
“It is important to see what an earthquake signature looks like in these sediments, so that we can start looking at deeper, older records in the region and then figure out whether 1755-type earthquakes take place for example, every 1000 years, or every 2000 years,” says Monecke.
The research team analyzed the sediment size and composition, as well as pollen and plant material and even industrial contaminants in order to identify changes in sediment layers over time in the core. Their attention was drawn to a light brown layer deposited at the time of the Cape Ann earthquake, since it had a coarser mix of sediments and slightly different plant microfossils.
“These were our main indicators that something had happened in the lake. We saw these near shore sediments and fragments of near-shore vegetation that appear to have been washed into the deep basin,” says Monecke.
The researchers plan to sample even older layers of New England lakes in hopes of building their record of past earthquakes. They are currently assessing Walden Pond, the famous site of Henry David Thoreau’s memoir Walden; or, Life in the Woods.
“It got slightly less ground shaking [than Sluice Pond] in 1755, but it might have been affected by a 1638 earthquake in southern New Hampshire,” Monecke says. “We already have sediment cores from that lake, and now we are unraveling its sedimentary history and trying to get an age model there as well.”