The Philly region got a collective jolt Friday morning at 10:23 a.m. when a 4.8-magnitude earthquake centered near Lebanon, N.J. shook the region for a few seconds.
The feeling of the building around you shaking is undoubtedly jarring. There hasn’t been seismic activity this big in the area since August 2011’s tremors caused by a 5.8 magnitude quake in Richmond, Va. But experts — while much busier today — are not alarmed.
“It’s not something that we don’t anticipate, you just don’t know when one will happen,” said Shannon Graham, an assistant professor at the College of New Jersey.
The East Coast is in the middle of the North American plate on the Ramapo Fault Zone, a system of faults in the earth’s crust that formed when Africa and North America smashed into each other to form Pangea hundreds of millions of years ago, then when they broke away from each other.
Graham said that while the faults are no longer active, there are still “zones of weakness in the crust” which the current stress field in the crust can cause to reactivate, causing earthquakes that are typically 3.0 magnitude or smaller, but occasionally more like today’s.
Could there be aftershocks?
There already have been, according to the U.S. Geological Survey map. There have been several earthquakes in the area, ranging from 1.8 to 2.2 magnitude, which humans are less likely to feel. The USGS has listed these as an “earthquake sequence.”
Around 6 p.m. Friday, the area recorded a bigger aftershock that measured 4.0 magnitude 7 km southwest of Gladstone, N.J., which the USGS predicted a 16% chance of occurring in its aftershock forecast. Shortly after the event, the USGS raised the probability of a magnitude 3.0 aftershock to 78% from 46% and lowered the chance of a magnitude 5.0 to 2% from 3%.
Graham said that there’s a chance of aftershocks any time there’s an earthquake, in this instance for up to a week after. The aftershocks should also diminish over time.
“When an earthquake happens it basically influences the stress on the rocks around it, and it can promote failure on other little basic patches of the fault,” Graham said.
The USGS also has an aftershock forecast on its website.
Are earthquakes going to get bigger or more frequent here?
Probably not. Graham described today’s earthquake as “not common, but not unexpected to happen” for the region.
“We’re at a place where we have ancient zones of weakness, you could think of them as, where we used to be at a plate boundary and there are remnant cracks in the crust that can occasionally have an earthquake,” Graham said.
Places that fall on the boundary between two of the earth’s plates — like in California where the San Andreas Fault lies — have more frequent or larger seismic events because they are near a plate boundary, which creates a tectonically active transform plate boundary in the San Andreas Fault’s case.
“If this happened in California they probably wouldn’t even wake up,” Graham said. “But here’s it’s a big deal because they don’t happen very often.”
