(Portland, OR)  --  An earthquake could cause problems for fuel tankers on the Willamette River.  Investigators in Portland are looking into the risk after a NuStar Energy facility near San Francisco experienced an explosion and fire triggered by an earthquake.  NuStar also has a location in Portland, and the tankers along a six-mile stretch store about 90-percent of Oregon's fuel.  If an earthquake affected the area, the soil that the tankers sit on could liquefy, causing the tanks to fail.