(Portland, OR)  --  A Portland State University geologist says a major earthquake hitting Oregon is not a matter of if, but when.  Scott Burns tells "KOIN-TV" that three faults run beneath the city of Portland, and the Juan de Fuca plate is located off the Oregon Coast.  Burns says the 6.0 quake that struck the San Francisco Bay area over the weekend is a reminder that a big temblor could just as easily hit Portland.  He says a large quake hits the Pacific Northwest about every 500 years on average, and the last big one struck the region more than 300 years ago.