When New Jersey native Mariella C. Chilmaza ’05 woke up unusually early Saturday morning, it took a second for her to register what was happening.

“In the beginning I just thought my alarm was going off, but then I saw things on my desk start to shake. I’m from Jersey so I had no idea what was going on,” Chilmaza said.

What Chilmaza felt was an earthquake, which while moderate is still an unusual occurrence in the Northeast. It had a 5.1 magnitude at its epicenter and about a 3 intensity in the Boston area. Because the East coast does not lie between tectonic plates’ boundaries, there are relatively few earthquakes in the Northeast.

The earthquake, which started at 6:50 a.m. and lasted for about 30 seconds, was centered 15 miles southwest of Plattsburgh, N.Y. But tremors were felt in Boston, eastern Canada and even as far south as Baltimore, according to the United States Geological Survey.

Harvard was one of the first to know when the quake struck, since the University has a project which involves a nearly real-time analysis of larger earthquakes. The results taken by the University were sent around the world to help figure out the exact details of the earthquake.

Michael S. Antolik, a post-doctoral fellow in seismology in the Earth and Planetary Sciences Department, said the cause of Saturday’s earthquake is difficult to determine since the majority of the faults in the Northeast are inactive. But he said his preliminary calculations show the quake to be a thrust earthquake caused by compressional stresses.

A 5.1-magnitude earthquake is strong enough to cause significant damage but the reported damage was relatively low, aside from some moderate road damage in New York and Vermont. No fatalities or serious injuries have been reported.

In Cambridge, the tremors were felt with an intensity of three on a scale of 12 and were apparently strong enough to ring church bells around Boston and Cambridge.

The intensity of an earthquake refers to the amount of shaking at any particular location. Magnitude, which is what the Richter scale measures, refers to the size of the earthquake at its source. Thus, each earthquake has only one magnitude and many intensity readings.

Though the intensity of the earthquake was quite low, Cambridge received nine call-ins and reports regarding the tremors. Comparatively, most cities and suburbs within the shock wave radius received only one to three call-ins. It caused a flurry of e-mails on some House open e-mail lists as residents discussed the quake and Harvard’s earthquake preparedness.

Some residents of California, where earthquakes occur much more frequently because of the state’s location upon tectonic plates and active faults, said they were shocked by a quake in this part of the country.

Ryan J. Montoya ’05, who called Los Angeles home for much of his life, said he took it in stride.

“The earthquake didn’t even wake me up. People on the East are so weak when it comes to these things,” he said.