Authorities say a "catastrophic failure" of a 24-inch gas pipeline being tested by PG&E Sunday afternoon caused an explosion, which triggered a significant mudslide.
California Highway Patrol Capt. Mike Maskarich says the vapor that was released from the explosion is not harmful.
"There was a lot of pressure that had come through that line," Maskarich told ABC7 News, adding that the side of the hill near the freeway had "a significant amount of erosion."
Authorities are advising motorists to avoid the area and use U.S. Highway 101 as an alternate route.
Three lanes on I-280 have reopened, but a fourth lane remains closed while crews continue the cleanup.