Because no significant damage was reported, the little quake's big bang remains a mystery.
John Bellini, a USGS geophysicist, told the Daily News yesterday that such booms are often reported by people inside buildings during small earthquakes, and may be caused by a tremor causing sudden vibrations in a building's structure.
Bellini said Philly's temblor was a "really small" quake caused by a really small fault - maybe 100 feet long in contrast to the San Andreas Fault's 800 miles - and the chances of an encore are slim to none.
"These little earthquakes happen all the time on really small faults that are everywhere," Bellini said. "This one is not an active fault. It never will be studied. It never will be named. We're never going to know exactly where this fault is. It's so small, we're never going to find it."
So rest easy, Philadelphia.
"If you blinked, you missed it," said Edward Turzanski, 51, who didn't blink as the boom resounded in the parish school he was in near Knights and Fairdale roads.
"If you were inside, the loud bang sounded as if something very large fell over in the basement. If you were outside, it seemed worse. The kids out in the parking lot were really spooked because they felt a jolt right underneath their feet, heard the boom and couldn't imagine what made that kind of noise."
Turzanski, a senior fellow at the Foreign Policy Research Institute and a national-security analyst, said he was fascinated by how quickly his sons determined "the impact area from Bensalem through most of Torresdale by using social networking."
"Their friends were reporting nothing going on at Rhawn and the boulevard, but people were feeling it at Knights and Fairdale."
Turzanski said instant messaging - "Did your lights go out? No. Do you smell gas? No." - quickly ruled out a disaster.
"There were false rumors floating around - a transformer blew up, manhole covers blew sky high. One of my sons said his buddy heard that the gas station at Knights and Fairdale blew up. I said, 'I just drove past there. The gas station's fine.' End of rumor."