The East Coast's ancient rocks allowed the Friday morning earthquake to rattle an estimated 42 million people in several states, giving it a much wider reach than its West Coast cousins, geologists said.
Why it matters: If an earthquake of the same magnitude happened on the West Coast, it's likely that it wouldn't have been felt as far away, because seismic waves in the eastern half of North America behave differently than those out West.
How it works: Scientists are still trying to precisely pin down the variety of factors that alter how earthquakes behave across the U.S., but they know geological differences between the East and the West are key.
- The rock in the Eastern U.S. is much older than rock in the West.
- For example, the Appalachian Mountains are some of the oldest mountains in the world, forming hundreds of millions of years before other North American ranges.
- The older, denser rock in the East allows seismic waves to cross them more effectively when an earthquake occurs, meaning the released energy can travel further distances and be felt over larger areas, according to USGS.
- When seismic waves meet the younger and more fractured rock in the West, the energy is absorbed in faults and dissipates faster.
Zoom out: A good example of how these differences can drastically alter seismic behavior is comparing the magnitude 5.8 quake that hit Virginia in 2011 to the magnitude 6 earthquake that shook Napa Valley in 2014.
- The Virginia earthquake is believed to be is believed to be the most widely felt earthquake in recorded North American history, with people across the Eastern Seaboard — from Vermont to parts of Florida — reporting shaking.
- On the other hand, even though a similar amount of energy was released, the Napa Valley quake was mostly contained in California.
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