Residents of the house, which belongs to the Kappa Sigma fraternity, said they started smelling natural gas at 1 a.m. Sunday.
Police said someone deliberately leaked gas and then tried to ignite the home.
Officials said the explosion would have destroyed the house and damaged nearby properties.
About 50 people attended an invite-only Halloween party earlier Sunday, but most were gone by 11 p.m., the Los Angeles Times reported.
The house was unlocked overnight.
Investigators said they are looking into possible motives, and police are warning students to take notice of suspicious activity.
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