Why the New Jersey earthquake was felt hundreds of miles away

More than 150,000 people reported feeling the New Jersey quake, some from hundreds of miles away, according to the U.S. Geological Survey, which collects reports of the shaking. While the number of reports reflects the region’s population density, it also highlights fundamental geological differences between the tectonically active west coast and the east coast, which is covered by old faults that occasionally reactivate. There is.

Because the underlying rocks on the East Coast are older, colder, and denser, and faults take longer to repair, seismic waves travel farther than on the West Coast, where faults have ruptured the Earth’s crust.

USGS seismologist Susan Huff said the magnitude 4.8 quake was “not large enough to cause general damage, but large enough to be widely felt.” “Once an earthquake occurs, waves travel more efficiently to the east than to the west, and the Earth’s crust is older, cooler, and less prone to breakage. We’ve seen this time and time again. You teeth [4.8] In California, it doesn’t feel this far away. ”

Several geologists said earthquakes larger than magnitude 4 are not common on the East Coast, but such earthquakes are common. Not unexpected. In 2017, a magnitude 4.1 earthquake occurred near Dover, Delaware. In 2011, a magnitude 5.8 earthquake was believed to have occurred near Mineral, Virginia. The earthquake was felt by more people than any other earthquake in U.S. history and caused significant damage, including to the Washington Monument and the Washington National Cathedral in Washington, DC. In 1994, a magnitude 4.6 earthquake occurred near Reading, Pennsylvania.

The New Jersey earthquake that occurred at 10:23 a.m. Friday was relatively shallow, just 3 miles below the surface. Research into the earthquake continues, but Christopher Carcedi, a seismologist at the Carnegie Institution for Science in Washington, said a common cause of earthquakes like this on the East Coast is changes in the Earth’s surface under the weight of ice sheets. He said that this is true. Ice age.

“Otherwise, this is an inactive fault that is likely readjusting itself in response to ice loss,” Carchedi said.

Hough said the region is generally considered a “passive rim” between North America and the Atlantic Ocean, meaning there are no active faults or plate boundaries. But she points out that one of her colleagues calls it “the margin of passive-aggressiveness, because sometimes it bites.”

“Moderate earthquakes in the East are always rare and interesting,” Hough said, noting that Friday’s quake will be eagerly studied.

As soon as he felt the shaking, Bohon started counting down. Earthquakes cause multiple types of seismic waves that travel through the Earth at different speeds. Measuring the time between the first tremor and the next series of waves can give you a rough idea of ​​how close the tremors came, similar to measuring the time between the flash of lightning and the rumble of thunder. .

Mr. Bohon reached 8 seconds, so the epicenter was The earthquake wasn’t that close. Then she started looking into local earthquake reports to make sure it was an earthquake she hadn’t imagined.

It has long been known that the shaking of an earthquake travels farther on the east coast than on the west coast. According to the USGS, an earthquake could be felt in an area 10 times larger than the East Coast.

On the West Coast, “because the rocks are warmer, they are more active, and because they have recently experienced activity, the rocks are warmer and some of the seismic energy will be attenuated or absorbed as it passes through that rock.” ” said Carcedi. .

In contrast, rocks on the east coast are colder and more brittle, so they transmit seismic energy better than on the west coast.

Friday’s quake is a reminder that while people think of earthquakes as a West Coast phenomenon, they can happen anywhere, Bohon said.