From this article (emphasis mine):
Earthquakes in Delaware do not occur on the edge of a tectonic plate, as is more common in places such as California, where fault lines between plates generate earthquakes. Generations of California residents have been bracing for the so-called “Big One” along the San Andreas Fault.
Instead, the Delaware temblor occurred far from the edge of a plate, said Thomas Pratt, a research geophysicist at the U.S. Geological Survey who is based in Virginia.
“There’s a lot of speculation, but we don’t have a good answer for why these earthquakes are occurring in the middle of the plates,” Pratt said.
Boom: INTELLIGENT QUAKING, you guys!