BETHLEHEM, Pa. — In the aftermath of a 4.8 magnitude earthquake in New Jersey that was felt throughout surrounding states, plenty of people are wondering exactly how they felt those shakes in the Lehigh Valley.
As it turns out, the momentary effects which were felt throughout Allentown, Bethlehem, Easton and the surrounding areas is the result of the age of the earth’s crust in the region.
Meteorologist and atmospheric scientist Matthew Cappucci said there's an exponential increase with each increase on the moment magnitude scale.
“Earthquakes, however, are felt over a wider area in the eastern United States, compared to the West. The reason being the rocks out here and the crust are older, you have older rock that has been heated, has been compressed.Meteorologist and atmospheric scientist Matthew Cappucci
Moment is a product of the distance a fault moved, and the force required to move it. The magnitude scale is based on the total moment release of the earthquake.
“Earthquakes, however, are felt over a wider area in the eastern United States, compared to the West," Cappucci said. "The reason being the rocks out here and the crust are older, you have older rock that has been heated, has been compressed.
“So over millions of years long, long ago, the rock was subjected to extreme temperatures and pressures, and so it's very dense.
"And because it's very dense, it's easier for that signal, that shaking, to spread over a broader area.”
Far fewer faults in the East
Cappucci said many of the faults along the East Coast of the country are “very old, so many have had time to heal, whereas in the western U.S. — California, Oregon, Washington — those faults are very active, they’re kind of crunching all the time.”
As such, when the West Coast experiences a quake, it can disperse quite strongly along those faults, while the East Coast has far fewer faults.
"And so shaking can hop those faults traveling through the dense crust and travel very far,” he said.
“When the stress on the edge overcomes the friction, there is an earthquake that releases energy in waves that travel through the earth's crust and cause the shaking that we feel."U.S. Geological Survey
The U.S. Geological Survey notes earthquakes are caused by sudden slips on a fault. Tectonic plates are in constant slow motion, but because of friction, they can become stuck at their edges.
“When the stress on the edge overcomes the friction, there is an earthquake that releases energy in waves that travel through the earth's crust and cause the shaking that we feel,” the USGS states.
'In real quick time'
Lehigh University seismologist Anne Meltzer explained that the U.S. Geological Survey functions as “the mission agency that monitors earthquakes in the U.S. and globally.”
Its national seismic network’s sensors pick up on ground motion and share that information across the nation, Meltzer said.
“So all that data goes in real time or near real time to a data center, and we know enough about earthquakes now and the ground motions associated with them that within seconds to minutes, it can be determined where the earthquake happened, and what the magnitude of that event was.”
Meltzer said that as more data comes in and human analysts review that information, it is updated for those in the network and the public.
“So for me this morning, I basically felt what I would have thought could have actually been a truck rumbling by, which is maybe what many people felt," she said. "It lasted a little longer than I thought a truck should last, but you never know.
"I got on my phone, and looked up real quick just to see if, indeed, it was an earthquake, because you can access that information really, really quickly."
Like ripples in a pond
Meltzer said the seismic wave generated at the center of the quake resemble the ripples in a pond when you throw a rock in the water.
"You see those rings moving out… except obviously, it’s in three dimensions," she said.
“So if you did a survey in the Lehigh Valley, people might describe a different sensation, depending on what they actually felt."Lehigh University seismologist Anne Meltzer
“And so as it moves, that initial energy is spreading out over a larger area, as it as it moves over a distance, the energy gets a little bit less.
"And then also, as the energy moves through the earth, the rocks absorb some of that energy as well. The farther away from the earthquake you are, the less you're going to feel the ground motion."
Those in New Jersey likely felt far more ground motion, Meltzer said, while in the Lehigh Valley, they likely felt less, depending upon where they stood at the moment and the type of ground beneath them.
“So if you did a survey in the Lehigh Valley, people might describe a different sensation, depending on what they actually felt.
"But the rocks in this part of the country in the mid-Atlantic region, the bedrock is pretty hard rock. It’s igneous rocks and metamorphic rocks, granites and things like that, they transmit energy very efficiently.”
Pa. quake locations, aftershocks
According to the Pennsylvania State Seismic Network, earthquakes in the commonwealth tend to happen in the northwestern and southeastern parts of the state, though they have happened sporadically in other locations.
PASEIS states that the majority of earthquakes in Pennsylvania’s southeastern region have happened along the Lancaster Seismic Zone, located along the western edge of the Newark Basin Seismic Zone, a broader zone extending from the LSZ through New Jersey and into New York.
The NBSZ borders the Newark Basin, which formed by Mesozoic rifting, PASEIS said. The western side of the NBSZ is marked by the Ramapo fault system, which stretches to the northeast, dips to the southeast, and goes from southeastern New York through southeastern Pennsylvania.
"Earthquakes have also occurred sporadically in other parts of Pennsylvania. While the majority of earthquakes are small, there have been moderate size earthquakes in the past."Pennsylvania State Seismic Network
“Earthquakes in Pennsylvania occur primarily in the northwestern and southeastern portions of the state," PASEIS states on its website.
"However, earthquakes have also occurred sporadically in other parts of Pennsylvania. While the majority of earthquakes are small, there have been moderate size earthquakes in the past, such as the September 25, 1998 magnitude 5.2 Pymatuning earthquake.”
Pennsylvania's Department of Conservation and National Resources says the Pymatuning quake was the largest earthquake registered in the commonwealth.
As of March 1999, there were 11 aftershocks, with the largest being a magnitude of 2.3.
Pennsylvania's DCNR page states a Virginia earthquake in 2011 registered at a magnitude of 5.8 and was felt in the commonwealth, including the Lehigh Valley.
Bigger shock 'not a zero chance'
Beyond earthquakes, areas of Pennsylvania near bituminous and anthracite coal mining regions of the state and in industrial mineral quarries have reported a large number of seismic events linked to mine blasts.
Cappucci said there is only about a 4% chance that the 10:25 a.m. quake is not the main quake, meaning there is an incredibly small chance a larger earthquake could happen down the road, which would make the Friday morning event a “foreshock.”
“There's a 46 percent chance of aftershocks of magnitude 3.0 or greater in the days ahead. However, I don't expect those to be felt in Pennsylvania."Meteorologist and atmospheric scientist Matthew Cappucci
“I don't think that's going to happen, but it's not a zero chance,” Cappucci said.
“There's a 46 percent chance of aftershocks of magnitude 3.0 or greater in the days ahead. However, I don't expect those to be felt in Pennsylvania.
"Those would be much more local, smaller scale aftershocks that would really only impact the immediate area of northern New Jersey and perhaps be felt somewhere over the southwestern New York Metro area.”
