Why do earthquakes occur in the middle of the continent far from plate boundaries?

Most earthquakes occur near tectonic plate boundaries, where the Earth's crust is composed of large fragments that interact with one another. These interactions can involve plates sliding past each other, colliding, or moving apart. However, a map of the U.S. shows that earthquakes also take place in the interior regions of the continent, though less frequently than those along the edges. This raises the question: why do earthquakes occur away from tectonic plate boundaries? 

Research into this phenomenon is limited, but studies indicate that ancient regional and local variations in the Earth's crust play a significant role. These variations reflect stress conditions that have persisted for millions of years, often stemming from long-ago tectonic activities such as mountain-building and continental rifting. 

Stress is continuously transmitted from the boundaries throughout the tectonic plates. When this stress encounters a pre-existing weakness in the crust—such as fractures or faults from older geological events—the likelihood of an earthquake occurring in that location increases.

Learn More:

• A Possible Cause of Earthquakes in the Continental Interior
• This Dynamic Planet - World Map of Volcanoes, Earthquakes, Impact Craters, and Plate Tectonics
• This Dynamic Earth: The Story of Plate Tectonics