Saturday earthquake in Tilton is second in two months

TILTON — An earth tremor felt by residents in Tilton, Belmont, Franklin, Laconia, Northfield, and Sanbornton on Saturday is not something out of the ordinary for this part of New Hampshire.

The magnitude 2.1 earthquake with an epicenter near School Street, Tilton, which struck at 11:07 p.m. Saturday, is the second quake reported in the town in two months. A previous 2.1-magnitude quake was reported in the same area on Jan. 5.

The most recent earthquake reportedly reached a depth of 2.36 miles.

State Geologist Rick Chormann said the Central New Hampshire seismic zone that runs north from Concord through Sanbornton has a higher frequency of small earthquakes than other parts of the state. The Ossipee area is another locus of enhanced seismic activity, he said.

Geophysicist Don Blakeman, with the U.S. Geological Survey, said such earthquakes are not uncommon in the Northeast. While considered minor quakes, the older, rigid rock structure here carries the earthquakes’ energy further than less dense rocks.

In Central New Hampshire, there have been a number of earthquakes with epicenters at Gaza Corners in Sanbornton. In a 12-month period in 2017-18, Sanbornton experienced nine earthquakes of magnitude 1.5 or higher.

Similar quakes were felt during that same time period in Freedom, New Hampshire, and Harrison, Maine — towns that lie in a straight easterly line from Sanbornton. Asked whether that indicates a fault line, Blakeman explained that any earthquake, by definition, lies on a fault, but that New England’s faults are generally small, separate ones.

Chorman agreed, pointing out that Freedom lies in the Ossipee earthquake zone, and lies east of Sanbornton. “The orientation of the structural grain [in the rock surface] is northeast-southwest, so an east-west connection would be across the grain,” he said. “I think they’re independent phenomena, and probably not connected.”

Tectonics

“It’s more the tectonic regime that controls the earthquakes,” Blakeman explained.

Scientists say the Earth's outer shell consists of several plates that glide over the mantle, the rocky inner layer above the core. The plates provide a hard and rigid shell but they can overlap and separate, creating faults in the crust.

Most quakes occur on the boundaries of tectonic plates, but that is not the case in New Hampshire. “These are intraplate earthquakes, where there is stress within the plate, rather than on the edges,” Blakeman explained.

Chorman said there’s still not a lot known about the ultimate causes of New Hampshire’s earthquakes, because there is no obvious geological fault. “It’s a bit of an enigma,” he said.

Chorman said the explanations that scientists offer harken back the geologic history some 400 million years ago. “The collisions of the tectonic plates built slivers of other pieces that were part of North America, creating zones of weakness in the underlying rock,” he said. “Some of those zones are at great depth in the earth, and then later, 200 million years after the supercontinent of Pangaea formed, they were stretched and pulled apart, so there were a lot of forces in the opposite direction, creating additional weaknesses. So they’ve theorized that those underlying zones of weakness are the cause, but we really don’t know. There’s no smoking gun, per se.”

Seismologist John Ebel, with Boston University’s Weston Observatory, believes New Hampshire’s earthquakes are aftershocks from an earthquake recorded in 1638, not long after the Pilgrims landed at Plymouth Rock. It was a “significant” earthquake with no known epicenter, but Ebel postulates that it was centered in New Hampshire.

Predictability

Chorman said the earthquakes here are “very random in terms of predictability and in terms of magnitude. “There’s just as much a chance of a really big earthquake as a very small one,” he said. “New England in recorded time has had some damaging earthquakes, and if it happens once, it can happen again — but it could be thousands of years from now. In earth time, there’s a big discrepancy between that and our calendar.”

While the causal factors are variable, there’s a consistency in location. However, “It’s kilometers deep in the earth, where we don’t have a chance for good observation,” Chorman said.