The blast shook the 200 block of North Fulton about 1:00 p.m.
House #213 took the brunt of the blast; its glass shattered, much of its front porch buckled.
Nixy Correa was walking toward the house when the blast occurred.
Correa's daughter translated what her mother saw: "She seen them working and when she walked around the corner, she seen the whole porch come down."
The explosion brought other residents outside.
"I think everybody was in panic, not really knowing what was going on," Amy Vasquez of the Allentown Fire Department said.
Four people were injured, two residents and two workers for a utility contractor.
"We found some workers that we found out later were from Great Western, they were pretty much walking wounded at the time," Allentown Fire Department Chief Lee Laubach said.
The utility crews had been working on the block for days replacing aging gas lines runnning to home.
Allentown residents are sensitive to natural gas line safety issues.
Just last February, only blocks away, a massive natural gas blast killed five people. Today's explosion is minor by comparison.
As to what went wrong this afternoon, UGI Utilities, Inc. said it was too early to tell.
"I don't think at this time we can tell definitely what actually occurred to make that explosion happen," Daniel Adamo of UGI Utilities, Inc. said.
