With our own Won-Young Kim, quoted, here is the report from the Lehigh Valley Express-Times:
Recent vibrations didn't cross the border, but minor earthquakes remain possible in Warren, Hunterdon countiesTuesday, February 17, 2009By BILL WICHERTThe Express-Times
Residents in Warren and Hunterdon counties did not feel the second recent earthquake to hit nearby Morris County, but the northwestern part of the state remains just as vulnerable to a little ground shaking.
From Allamuchy Township to Milford, earthquakes have a minor history in the two counties. Preparing for earthquakes in this section of the country is difficult because the incidents tend to occur infrequently and on a small scale, according to seismology experts.
"People shouldn't lose any sleep over it," said Martha Oliver Withjack, a professor in the Rutgers University Department of Earth & Planetary Sciences.
A small earthquake hit about 5:30 p.m. Saturday with a magnitude of 2.4, according to the U.S. Geological Survey. The agency received 81 responses within 56 miles of the epicenter near Boonton from people who said they felt it.
Reports came from parts of New York and northern New Jersey but not Warren and Hunterdon counties, the agency reported.
An earthquake Feb. 2, with a magnitude of 3.0, was centered in the Morris County municipalities of Rockaway, Dover and Morris Plains. People reported feeling that one in Hackettstown, Blairstown Township, Belvidere, Hampton, Lebanon and Readington Township, according to the USGS.
Two earthquakes occurring in such a close time span is unusual, but Won-Young Kim, a seismologist at the Lamont-Doherty Earth Observatory in Palisades, N.Y., said he wouldn't be surprised to see another earthquake today.
Both earthquakes occurred along the Ramapo Fault, a roughly 200-million-year-old system of faults stretching from southeastern New York to eastern Pennsylvania. Most of the seismic activity in New Jersey occurs in that area, Kim said.
Parallel fault lines running through Warren and Hunterdon counties help explain the rare earthquakes that have occurred locally.
A 3.5-magnitude earthquake occurred in August 2003 in Milford, followed by a 2.1-magnitude earthquake last July in the borough. The last earthquake of that or greater strength in Warren County occurred in 1986 in the Allamuchy Township area.
Without more seismic stations in the western part of the state, smaller earthquakes might have gone unnoticed, Kim said.
"It's a quiet area," Kim added.
If an earthquake hits deeper underground, there will be less of an effect at the surface of the epicenter, but more people will feel the shaking across a larger distance, Kim said. The first of the two earthquakes was 5 kilometers deep, compared with 2 kilometers for Saturday's earthquake, according to the U.S. Geological Survey.
Unlike in California, the infrequency of earthquakes also makes them harder to predict, Withjack said. The rare occurrence of earthquakes leaves New Jersey residents unprepared to deal with a large-scale incident, she said.
"People prepare for what they're used to," Withjack said.