They point out, however, that few of the region’s faults “are known to have been active in the current geologic era,” adding that “few earthquakes east of the Rockies… have been definitely linked to mapped geologic faults.”
It's possible, of course, that it was caused by a fault that we have yet to discover.
“There are faults in the subsurface in a lot of places, and they’re not necessarily known,” geophysicist Randy Baldwin of the USGS National Earthquake Information Center in Denver, told IFLScience.
“These old basement faults can reactive from time to time; they get stressed and they can create periodic earthquakes,” he added.
One tantalizing additional possibility resides in a study published back in 2016. Seismic imaging techniques revealed that pieces of the underside of the North American Plate are breaking off from time to time and sinking into the lower mantle. This leaves the remaining plate thinner than it was before, which makes it less rigid and more prone to slipping.
This was touted as a possible explanation for the 5.8M earthquake that shook Virginia back in August 2011, a region that hasn’t been seismically active for a very long time. Could the same effect have caused the Delaware quake?
The USGS also mentions “induced earthquakes”, those caused by human activity. Fracking, and primarily wastewater disposal, are the reasons why there has been a huge uptick in induced tremors in the US recently. It’s hinted that this could be behind the recent Delaware quake, but there’s no evidence for this just yet.