CT had an earthquake yesterday. Does that mean more could follow?

Photo of Nicole Funaro
An aerial view of central Greenwich.

An aerial view of central Greenwich.

File / Patrick Sikes / For Hearst Connecticut Media

There was a rumble underfoot in Greenwich on Thursday afternoon, and it wasn’t from a fleet of trucks or heavy construction work. It was from a 1.4 magnitude earthquake that occurred two kilometers north-northwest of Cos Cob at 12:38 p.m., according to the U.S. Geological Survey. Earthquakes under a 2.0 magnitude are considered micro earthquakes, according to Merriam-Webster Dictionary

A small tremor was last detected in the town back in 2016, but how common are earthquakes across Connecticut? Is there the potential for a significant earthquake? Here’s what to know about the phenomenon in Connecticut. 

How common are earthquakes in Connecticut?

According to the Connecticut Division of Emergency Management and Homeland Security, earthquakes are considered an “unusual occurrence” in the state. However, more 115 earthquakes have been documented in Connecticut since the first recorded instance in 1678, the Northeast States Emergency Consortium notes. 

Can Connecticut get a strong earthquake?

A severe earthquake is considered unlikely by the Connecticut Division of Emergency Management and Homeland Security. Geologist Terry Tullis told Brown University that hundreds of years of earthquake data in the state show a history of small earthquakes, with the largest in recent memory a 3.1 magnitude quake in Plainfield in 2015

According to Tullis, earthquakes result from “sliding on small sections of typically unrecognized faults,” or “places of weakness in the Earth’s crust left over from hundreds of millions of years ago when New England was an active plate boundary.” 

Most of these faults are now inactive, Tullis told Brown, and don’t connect with each other as much as those in the West Coast. This lack of “interconnected faults” reduces the chances of a significant earthquake of an 8.0 magnitude or higher. 

When do aftershocks occur?

Most aftershocks tend follow significant earthquakes, which are defined by the Southern California Earthquake Center as an earthquake that is large enough to produce significant damage. Michigan Technological University further clarifies that earthquakes of 5.5 magnitude or more are capable of causing slight to major damage. The largest earthquake in recent history in Connecticut occurred in 2015 when a 3.1 magnitude quake was felt in Plainfield.

Aftershocks happen as a result of the stress level changing on the main shock's fault, the Southern California Earthquake Center notes, and most of the aftershocks will occur on the same fault. 

The U.S. Geological Survey notes that 6 percent earthquakes are followed by a larger earthquake. In these situations, the first earthquake is considered a “foreshock.” In these instances, the Los Angeles Times reported that smaller earthquakes can sometimes precede quakes of at least 4.0 magnitude.

What was the strongest earthquake in state history?

According to Connecticut Humanities’ ConnecticutHistory.org, the largest earthquake in state history took place on May 16, 1791 in Moodus. The first earthquake to occur that day came on with “two heavy shocks in quick succession and a fissure measuring several meters long formed in the ground,” according to ConnecticutHistory.org. Thirty lighter shocks occurred shortly after and more than 100 lighter shocks continued overnight. 

Moodus derives its name from the Native American term “Machimoodus,” meaning the place of noises, and the town and the surrounding area around East Haddam have been known for earthquakes.

Is there a fault line in Connecticut?

The town of Moodus recording the strongest earthquake in history indicates that there are underground faults in the central part of Connecticut near the town and surrounding East Haddam area, according to ConnecticutHistory.org. In a study of microearthquakes (low-intensity quakes) starting in August 1981, the area around East Haddam recorded over 500 fault-shifting events in three months, according to ConnecticutHistory.org. 

In 2011, firefighters responded to a loud “boom” near Moodus that was later attributed to a “powerful underground fault shift,” according to ConnecticutHistory.org.