A rare earthquake shook the New York City region on Friday morning, surprising residents and sending officials scrambling to confirm the extent of the damage.

Then, on Friday evening, a strong aftershock was felt across the area, though it was less intense than the initial quake.

Here’s what we know so far about the series of seismic events.

It started just before 10:30 a.m. Friday

The U.S. Geological Survey, a federal agency that tracks seismic events, said the earthquake happened at 10:23 a.m. and lasted less than a minute. That was long enough for many in the area to notice the ground and built environment shaking.

People as far north as Boston and as far south as Philadelphia reported feeling the rumbling.

The earthquake was among the area’s biggest ever recorded

As of Friday night, the preliminary measurement of the earthquake’s magnitude was 4.8, according to the USGS. New York Gov. Kathy Hochul said 2011 was the last time such a powerful earthquake was felt in the state.

Geologists said the intensity was unusual but not unprecedented for the region. In the 1700s and 1800s, at least three stronger quakes were recorded locally.

“In the past hundred years, there have been a handful of earthquakes in the broader area of similar magnitudes,” said Paul Earle, a USGS seismologist. “You’re going to have a lot more people feeling this earthquake than you would a similar size earthquake in California.”

It originated in northwest New Jersey and was felt far away

The USGS initially identified the epicenter of the earthquake as Whitehouse Station, in New Jersey’s Hunterdon County, about 50 miles west of New York City. Columbia University professor John Mutter told WNYC the quake might have started along the Ramapo Fault, which was tectonically active about 200 million years ago.

“It felt like you were in the Tower of Terror,” said Karen Romano, borough administrator of Lebanon, New Jersey, about 11 miles from the epicenter. “Everything went up and down.”

Reported damage and disruption was limited

The shaking led to temporary travel disruptions, including the closure of the Holland Tunnel, grounding of flights at area airports and delays on NJ Transit, while officials inspected infrastructure for damage. There were no reports of catastrophic damage, but in Newark, dozens of residents were evacuated from several homes.

In New York City, officials urged contractors to inspect work sites, including buildings, cranes, scaffolding and other equipment. Minor damage can be reported to 311 and significant or emergency damage to 911.

Aftershocks and risk of additional ones

Several small aftershocks occurred in northwest New Jersey in the hours after the earthquake, according to the USGS. At 5:59 p.m., a 3.8-magnitude aftershock again shook the wider region. No major damage was immediately reported.

As of Friday night, federal officials estimated there was a 78% chance of another 3.0-or-greater aftershock within the next week, a 16% chance of a 4.0-or-greater aftershock and a 2% chance of a 5.0-or-greater aftershock.

Delayed official notifications

While the initial quake ended within a minute, it took New York City officials almost half an hour to issue a public alert via the Notify NYC system, which has more than 1 million subscribers, and a broader alert didn’t start hitting cell phones until after 11 a.m.

Mayor Eric Adams’ administration defended the city’s response, but some elected officials, including Manhattan Borough President Mark Levine, expressed concern about the time gaps. “This is unacceptable,” he wrote in an email to constituents, noting that if the earthquake had been worse, the delay in notifications could have left people in danger.

After the 3.8-magnitude aftershock just before 6 p.m., the city sent out a Notify NYC alert at 6:12 p.m., calling on people to remain indoors and contact 911 if injured. There was not a general cell phone alert in this instance.

How to stay safe in earthquakes

Experts say in case of future earthquakes or aftershocks, residents should generally avoid objects that could topple, should move out of traffic, and should get low and grab onto something stable.

USGS guidelines recommend different safety strategies for people who are indoors or outdoors, but in both situations people are advised to stay away from heavy and hazardous materials. You can read more about the guidance here.