Southwick had served as the No. 2 quarterback behind Moore for the past two seasons.
Petersen had an open competition to win the No. 24 Broncos' starting job, and it seemed apparent that the more experienced Southwick eventually would be named the starter. Indeed, his experience in the offense gave him the nod over Grant Hedrick and Jimmy Laughrea.
"He's prepared really hard the last three or four years, and he's earned this opportunity," Petersen said in Boise on Sunday.
Southwick faces a difficult challenge right out of the gate -- the season opener Friday at No. 13 Michigan State in yet another "statement" game for the Broncos. Moore, a four-year starter, thrived in these types of games en route to setting an NCAA record with 50 career victories, more than any other FBS quarterback.
As the backup at Boise State, Southwick has seen plenty of game action. Last year, he played in seven games and completed 76.7 percent of his passes for 198 yards, with one touchdown and one interception. Hedrick begins the season as the backup.
"It feels great," Southwick said. "To finally be named a starter, it's a great privilege. There's been a great line of quarterbacks here for a long time. To be one of those guys next in line is a great privilege and a lot of responsibility."