EarthquakesEast U.S. vs. West Coast earthquakes

Why was an earthquake in Virginia felt at more than twice the distance than a similar-sized earthquake in California? The answer is one that many people may not realize. Earthquakes east of the Rocky Mountains can cause noticeable ground shaking at much farther distances than comparably-sized earthquakes in the West.

A magnitude 5.8 earthquake in 2011 in Mineral, Virginia, was felt up to 600 miles from the epicenter. Tens of millions of people in the eastern United States and southeastern Canada felt this earthquake.

For comparison, a magnitude 6.0 earthquake in 2014 in Napa, California, was felt only as far as 250 miles from the epicenter. Despite the Napa earthquake releasing about twice as much energy as the Virginia earthquake and causing much more damage near the epicenter, it wasn’t felt nearly as far away.

As another example, the magnitude 4.1 earthquake that occurred in December 2017 near Dover, Delaware, was felt approximately 200 miles from the epicenter. The region that felt this earthquake is about the same size as that of the much larger California event, which released about 700 times more energy.

Scientists are researching a variety of factors that influence regional differences in the intensity and effects of earthquakes. Some of the factors have to do with the nature of the underlying tectonic plates and their geologic history. Others are connected to the size and age of buildings.

Seismic waves can travel farther in the East
USGS says that eastern North America has older rocks, some of which formed hundreds of millions of years before those in the West. These older formations have been exposed to extreme pressures and temperatures, making them harder and often denser. Faults in these older rocks have also had more time to heal, which allows seismic waves to cross them more effectively when an earthquake occurs.