Plate tectonics posits that earthquakes arise from plates rubbing against one another, or moving under one another. New Jersey recently had an earthquake. In 1755 there was an estimated magnitude 6.0 tremblor just outside of Boston, MA. The northeastern United States is located in the middle of the North American plate. So why are there earthquakes outside of major fault zones?
Here’s a link to a map of earthquakes in the northeast from 1975-2017. They are small, but there are a lot of them, scattered throughout the region. I see a few larger ones on both sides of the St Lawrence River.
Nonetheless, the epicenter of the recent New Jersey earthquake was near Lebanon, New Jersey, closer to Pennsylvania than the Atlantic Ocean. The seismically active Mid-Atlantic ridge runs down the center of the Atlantic, over 1000 miles away. My link shows epicenters scattered around the Northeast, extending to the Great Lakes region.
Frederik J. Simons, a professor of geosciences at Princeton University tells NBC news that the latest earthquake occurred in the Ramapo fault system, “a boundary where the continental and oceanic tectonic plates meet and are stuck together.” That explanation puzzles me, as the epicenter was fairly far inland. Cite: Earthquake shakes U.S. East Coast, impacting New York, Pennsylvania, New Jersey and others
The general answer is: tectonic plates stretch and squeeze more than you might think. Most plates, for example, are moving in a somewhat different direction one one side than on the other – so, there must be some mid-plate stretching and squeezing going on.
Faults don’t only occur at plate boundaries. Intraplate faults are less common but can exist where there’s a failed rift or partial fracture within a continental plate that is subject to tectonic strain. The New Madrid fault system is a prominent example.