New Haven Register (New Haven, CT)
Like New Haven pizza, secret to earthquake hidden in the crust
New England, and the Northeast broadly aren’t known for earthquakes. Our region lacks a famous fault line, and we aren’t at the boundary between two continental plates, both of which are hallmarks of earthquakes.
So where did the magnitude 4.8 New Jersey earthquake on Friday come from? And why do it, and others like it across Connecticut’s history, happen?
As it turns out, the Northeast’s geologic scars, the cracks and wounds caused by millions of years of plate tectonic activity might be partially blame.
“It’s all these hundreds of millions of years of geological processes layered on top of one another that give us this mosaic of faults in the crust, and these are the structures that enable earthquakes like the one we saw today,” said Maureen Long, a professor of earth and planetary sciences at Yale University.
Most earthquakes occur at or near the boundary between tectonic plates, the massive chunks of solid rock that float on the mantel underneath the Earth’s crust said John Elbe, senior researcher at the Weston Observatory in Massachusetts. The observatory has been an active earthquake monitoring site since the 1930s.
“Earthquakes are caused by stress and pressure building up on the rock due to the movement of the plates,” said Elbe, who added that “90 percent of the world’s earthquakes occur at the boundaries between plates — those are where most of the active faults are known today.”
But roughly 10 percent of earthquakes occur within a plate, said Elbe. Those intraplate earthquakes tend to happen in areas with lots of faults. One study called our boundary “passive-aggressive.”
This old crust
Elbe said that the northeast was riddled with thousands of ancient faults formed over millions of years. The continental collisions that built the Appalachian Mountains and folded them into their distinctive shape left our region with thousands of hidden continental scars.
“When pressure builds up on the entire interior of the plate it preferentially releases on older geologic structures, where there were basins that formed,” said Elbe. Such basins can be found in northeast New Jersey and central Connecticut — the Newark and Hartford basins respectively.
The breakup of the supercontinent Pangea, which left behind the dramatic rock formations of Sleeping Giant, East Rock, West Rock, and others up the middle of Connecticut, created even more faults and fractures. This same continental rifting led to the eventual formation of the Atlantic Ocean.
One of the largest named fault systems in our region, the Ramapo Fault, runs from Pennsylvania through New Jersey into New York.
These faults and fractures in the earth’s crust are more likely to experience earthquakes than solid bedrock. And some of the energy from the continental boundaries on the West Coast and MidAtlantic Ridge are transferred to our area. The pressure on both sides of the North American plate has to come out somewhere, and if it doesn’t come out at the plate boundaries it pops up in areas full of fractures, such as the Northeast.
“The faults don’t cause the earthquakes. It’s the pressure that causes the earthquakes,” said Elbe. “The earthquakes probably preferentially occur on old zones of weakness; the Ramapo fault is one.”
Jennifer Cooper Boemmels a professor of earth science at Southern Connecticut State University said the type of earthquake that Connecticut experienced on Friday morning felt consistent with this kind of intraplate pressure, the cracking smush of billions of tons of rock breaking in the middle.
“Given the fact that we’re in a region that’s under compression that’s fairly typical when an earthquake occurs in our region,” said Boemmels. “It’s typical to see that style of folding, reverse folding.”
Similar sorts of earthquakes occur in the center of Australia, far away from plate boundaries, because of the pressure pushing on the Australian plate, said Long.
One of the other things that might cause earthquakes in our region is that the Northeast has an unusual geological microclimate. Long said that some of our minor quakes may be related to the earth “de-stressing” thousands of years after the weight of the last glaciers retreated.
“It’s like putting your hand into a memory foam mattress and then watching it spring back up,” said Long. “So that’s another potential source of stress in the crust.”
Long studies an anomalous plume of hot molten rock that is rising up underneath New Hampshire and Vermont. She said that while she isn’t worried about eruption, local anomalies such as that might also contribute to earthquakes.
“It’s known as the Northern Appalachian Anomaly,” said Long. “Right now the answer is that we don’t know … I joke with my mom who lives up in New Hampshire right above the anomaly that she doesn’t need to sell her house or anything.”
The thing about this complex system of faults, fractures, forces and deep geologic time is that it makes it extremely difficult to pin down which fault is associated with any particular earthquake. Earthquakes here are weak and infrequent, and the faults themselves are dense and mostly unmapped. This means that attributing any single event to any single fault is much harder.
“Right now you can show me any fault you want,” said Elbe. “There are faults all over New England and I cannot tell you if those faults are active or not … the evidence is just not good enough for us to be able to determine that.”
Elbe said that active faults near continental boundaries produce tons more data than our faults, which makes it easier to build up enough data to know whether they are active. So, while the Ramapo Fault mentioned earlier has been blamed for several earthquakes, whether it was really associated with those earthquakes is still a matter of scientific investigation.
“In part it’s because we have so many old faults,” said Elbe. “In part it’s because we have so few earthquakes . ... What we get in one year of earthquake monitoring in California it takes us 100 years to get the same number of earthquakes.”
That environment of low rumblings and tons of cracks makes it exponentially harder to produce “smoking gun” evidence for an active fault, like a crack that breaches the surface, said Elbe.