New Jersey Earthquake Aftershock

Less than eight hours after a 4.8 magnitude earthquake was confirmed by the USGS in New Jersey, more shaking was felt as far as Philadelphia and Long Island just before 6 p.m. for 10 or 15 seconds.

The shaking was confirmed to be an aftershock, with a magnitude of 4.0 three miles from Bedminster at 5:59 p.m.

Aftershocks are smaller earthquakes that happen in the same general area during the days to years following a larger event or "mainshock," the USGS websie says.

"They occur within 1-2 fault lengths away and during the period of time before the background seismicity level has resumed," the site says.

"As a general rule, aftershocks represent minor readjustments along the portion of a fault that slipped at the time of the mainshock. The frequency of these aftershocks decreases with time."

The USGS said aftershocks can happen in the week following an earthquake, with a 50 percent probability of quakes with magnitudes above 3.

This is a developing story. Check back for more.

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