An Eastern earthquake: Rocking or rolling?

By Robert Titus

We certainly know how to properly celebrate the 200th anniversary of the infamous New Madrid earthquakes! Tuesday’s Eastern earthquake, registering a 5.8 on the Richter scale, hearkens back to the Mississippi Valley events of 1811. Those shocks, in New Madrid, Missouri, were about a hundred times worse than what was experienced this week in northern Virginia, but there are similarities. Both belong to a category of quake called the “Eastern earthquake.”

Eastern earthquakes are fundamentally different from those that sweep through our American Far West. Our quakes are simply not as strong. There are fewer great crustal stresses in eastern North America than in the West. California is colliding with the Pacific crust; nothing like that is happening here. Our rocks are older, harder and colder; they just don’t have much energy stored up within them. But when they do occur, they radiate across enormous stretches of landscape and that is what we saw on Tuesday.

Tuesday’s quake started in the town of Mineral in Virginia. The exact focus of the quake was several miles below the surface. That appears to place it within what is called the Piedmont Province of the Appalachians. Those are very old and very brittle rocks. It is quite possible that a geological fault, hundreds of millions of years old, was reawakened on Tuesday. Once generated, Eastern earthquake waves radiate out far faster, and they travel far more distantly than anything that occurs on the West Coast. That’s why we, hundreds of miles away, were able to sense this distant quake.

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