Why did the earthquake on the East Coast cover so much land?

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At about 10:30 a.m. local time Friday morning, a magnitude 4.8 earthquake struck three miles below Whitehouse Station, New Jersey. Although nowhere near the intensity of the West Coast's worst earthquakes, seismic waves traveled hundreds of miles, shaking not only nearby New York City, but Philadelphia and Boston and Washington, DC. The United States Geological Survey is urging the area to prepare for minor earthquakes.

For a region that was not accustomed to earthquakes, it was a shock. Its massive impact is no quirk, but a byproduct of the unique geology of the East Coast's ancient fault lines and rock structure.

“Earthquakes in this region are unusual, but not unexpected,” Paul Earley, a seismologist with the USGS National Earthquake Information Center, said in a press call Friday. “Earthquakes on the East Coast are felt much farther than earthquakes on the West Coast – four or five times farther.”

For example, a magnitude 5.8 earthquake in Virginia in 2011 was felt by people 600 miles away, while a magnitude 6.8 quake measured a few years later in California—which produced twice the energy—felt less than half the distance. Decided. Given how much more densely populated the East Coast is than the West Coast, that means a lot of people over a very large area will feel at least a little shaking, even if the intensity is much less than something like the Loma Prieta earthquake. , which devastated the Bay Area in 1989.

Frustrated East Coasters can blame the geology beneath them. On the west coast, a vast network of faults erupts all the time at the active plate boundary, causing tremors across the landscape. “We have new faults forming, we have old faults that are straining and breaking in large earthquakes,” says Folarin Kolawole, a structural geologist at Columbia University.

But when an earthquake occurs along a given fault, there are neighboring faults through which the energy is distributed. Basically, because the western US has a lot of faults along an active plate, it has a lot of channels to absorb earthquake energy – underground shock-absorbers, of sorts.

Although the USGS has not yet pinpointed the exact fault responsible for today's earthquake, it occurred in an area where the fault system is more stable than the West Coast. It appears that a dormant fault somewhere in the Ramapo Fault System under New Jersey was reactivated Friday morning.

The relative stability of the East Coast Fault System is due to its geological age: its rocks were formed millions of years earlier than those of the West Coast. Geologically speaking, the East Coast is a placid old man, while the West Coast is a rambunctious teenager.

“On the East Coast we don't have that tectonic complexity,” says Rutgers University geophysicist Gregory Mountain. “We had this in the geologic past, hundreds of millions of years ago, but things have become pretty solidified – that's one way of saying it – and stabilized. For this reason, on the East Coast, seismic energy can actually travel quite a distance and there is less energy loss with distance.