The rumbling felt by many Delawareans just before noon on Wednesday was not caused by an earthquake.
Delaware's State Geologist David Wunsch told WDEL he felt the shaking near his home in Pike Creek at about 11:30 a.m., and quickly started talking with people in his field.
"Looking at our sensors, and talking with some of our colleagues, we could not find any record of shaking that would be more diagnostic of an earthquake."
Wunsch said attention quickly turned to scheduled testing at Aberdeen Proving Ground in Maryland.
"There's reports now of shaking in Eastern Maryland, and the time fits. It was about 11:30 in the morning, and they set off a significant blast. And apparently, from what they are reporting, because we have this very low cloud bank, the sound echoed off of that."
Aberdeen posted on their Facebook page at about 1 p.m. that they were doing testing on Wednesday, about 90 minutes after the shaking.
APG often does testing, but Wunsch said it's not common to feel those in Delaware.
"It is unusual. I've been working here 11 years and I never recall this occurring before, but it is fairly common. For example, a lot of times small earthquakes are confused with quarry blasts, where they blast rocks in a quarry or mine, and nearby Pennsylvania and Maryland have quarries, so that was one of my first thoughts. It's kind of like hitting the funny bone, if you hit it at the right place, it can radiate along rocks that transmit it very well."
There was a 2.1 magnitude quake which struck in the Columbia, Maryland area near 2 a.m. Wednesday morning.
"If it was an aftershock, they usually occur near where the original earthquake was centered, and there's no record of a second earthquake near there from what I could tell from the monitoring sites."