On Friday morning around 10:30 am local time, a magnitude 4.8 earthquake struck three miles below Whitehouse Station, New Jersey. While not approaching the magnitude of the monster quakes on the West Coast, the seismic waves traveled hundreds of miles, jolting not only New York City, but also Philadelphia, Boston and Washington DC. The United States Geological Survey is urging the region to prepare for smaller-scale aftershocks.
For a region not used to earthquakes, it was a shock. Its far-reaching impact appears not to be a whim, but a byproduct of the East Coast’s unique geology of ancient faults and rock composition.
“Earthquakes in this region are unusual, but not unexpected,” seismologist Paul Earle of the USGS National Earthquake Information Center said during a news conference Friday. “Earthquakes on the East Coast are felt much further – four or five times further – than a similar earthquake on the West Coast.”
For example, in 2011, people felt the shock of a 5.8 earthquake in Virginia as far as 600 miles away, while a 6.8 earthquake a few years later in Napa, California – which produced twice as much energy –less than half that distance. Considering that the East Coast is much more densely populated than the West Coast, this means that a lot of people over a much larger area will experience at least some shaking, even if the magnitude is considerably smaller than something like a Loma Prieta earthquake. that devastated the Bay Area in 1989.
Displaced East Coasters can blame the geology beneath their feet. On the west coast, a vast web of fault lines continually emerges along an active plate boundary, sending shocks through the landscape. “New faults form, old faults develop, become stressed, and rupture in large earthquakes,” says structural geologist Folarin Kolawole of Columbia University.
But when an earthquake occurs on a particular fault, there are neighboring faults through which the energy is distributed. Because the western US has so many fault lines along an active plate, the country basically has many channels to absorb the energy of earthquakes – a kind of underground shock absorber.
Although the USGS has not yet identified the exact fault responsible for today’s earthquake, it occurred in a region where the fault system is more static than on the West Coast. It appears an inactive fault reactivated Friday morning beneath New Jersey, somewhere in the Ramapo fault system.
The relative stability of the east coast fault system is due to its geological age: the rocks formed hundreds of millions of years before the west coast rocks. Geologically speaking, the East Coast is a quiet old man, while the West Coast is a rambunctious teenager.
“We don’t have that tectonic complexity on the East Coast,” said Gregory Mountain, a geophysicist at Rutgers University. ‘We had it in the geological past, hundreds of millions of years ago, but things have pretty well solidified – you might call it that – and stabilized. For that reason, seismic energy on the East Coast could likely travel a lot further and cause less energy loss as the distance increases.”